Nervous System

Question 1

14.3 Check Yourself

Question 2

This is a hormone I secrete, which is located in the diencephalon.

People love me for secreting it… for insomnia and maybe even for regulating the onset of puberty.

Answer 2

1. The pineal gland
2. diencephalon
3. melatonin
   
   • *

Question 3

The myelin sheath • A lipid covering on long axons that acts to increase the speed of nerve impulse conduction, insulation for both CNS and PNS, and regeneration in the PNS • Schwann cells – neuroglia that make up the myelin sheath in the PNS • Oligodendrocytes- neuroglia that make up the myelin sheath in the CNS • Nodes of Ranvier – gaps between myelination on the axons • Saltatory conduction – conduction of the nerve impulse from node to node

Question 4

Blood Brain Barrier

lecture notes- keep reviewing them

Supplemental material not in book

Answer 4

1. Includes the least permeable capillaries of the body •
2. Excludes many potentially harmful substances •
3. Useless against some substances : Fats and fat soluble molecules •
4. Respiratory gases • Alcohol • Nicotine • Anesthesia

Question 5

Parasympathetic Division

Answer 5

1. 2. few cranial nerves (e.g., the vagus nerve), as well as
2. fibers that arise from the sacral (bottom) portion of the spinal cord. Therefore, this division is often referred to as the
3. craniosacral portion of the autonomic system.
   
   • In the parasympathetic division, the preganglionic fiber is long, and the postganglionic fiber is short because the ganglia lie near or within the organ.
4. the housekeeper division,
   
   • promotes all the internal responses we associate with a relaxed state.
     
     1. For example, it causes the pupil of the eye to contract,
     2. promotes digestion of food, and
     3. slows heart rate.
5. could be called the rest-and-digest system.
6. The neurotransmitter used by the parasympathetic division is acetylcholine (ACh).

Question 7

1. gray matter and white matter.
2. Gray matter contains cell bodies and short, nonmyelinated axons.
3. White matter contains myelinated axons that run together in bundles called tracts.

Answer 7

The nervous tissue composing the central nervous system.

Question 8

Spinal cord

Answer 8

• The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated.
• Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx).
• Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing
• . Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

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Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 10

reversedprompt

1. More than 100 substances are known or suspected to be neurotransmitters.
2. Common: acetylcholine, norepinephrine, dopamine, serotonin, glutamate, and GABA (gamma aminobutyric acid).
3. transmit signals between nerves; Nerve-muscle, nerve-organ, and nerve-gland synapses also communicate using neurotransmitters.
4. Acetylcholine (ACh) and norepinephrine are active in both the CNS and PNS.
   
   • In the PNS, these neurotransmitters act at synapses called neuromuscular junctions. We will explore the structure of the neuromuscular junctions in Section 13.2.
   1. In the PNS, ACh excites skeletal muscle but inhibits cardiac muscle. It has either an excitatory or inhibitory effect on smooth muscle or glands, depending on their location.

• Norepinephrine generally excites smooth muscle.
  
  • In the CNS, norepinephrine is important to dreaming, waking, and mood.
• Serotonin is involved in thermoregulation, sleeping, emotions, and perception.
• Many drugs that affect the nervous system act at the synapse. Some interfere with the actions of neurotransmitters, and other drugs prolong the effects of neurotransmitters (see Section 14.5).

1.

Answer 10

Neurotransmitter Molecules

Question 11

1) CNS
2) PNS- everything conveys information and takes instructions to different parts of body effected
- ganglia

Question 12

reversedprompt

Diencephalon

• a region that encircles the third ventricle.
  1. The hypothalamus
• forms the floor of the third ventricle.
• integrating center that helps maintain homeostasis.
  
  1. It regulates hunger, sleep, thirst, body temperature, and water balance.
• ***_controls the pituitary gland serves as a link between the nervous and endocrine systems*_**

2. The thalamus

• two masses of gray matter
• sides and roof of the third ventricle.
• receiving end for all sensory input except the sense of smell.
• Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord.
• integrates this information and sends it on to the appropriate portions of the cerebrum.
• The thalamus is involved in arousal of the cerebrum, and it participates in ** higher mental functions, such as memory and emotions.***

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Answer 12

hypothalamus and thalamus

are here

which is in the _______ ventricle

This is located on the floor of the ventricle and helps maintain homeostasis by

Question 13

The CNS: Spinal Cord

Answer 13

9 The CNS: Spinal cord • It extends from the base of the brain and along the length of the vertebral canal formed by the vertebrae. • The spinal cord functions to provide communication between the brain and most of the body. • It is the integrating center for reflex arcs. • Gray matter in the center is in a butterfly shape. • White matter surrounds the gray matter.

Question 14

Drug Abuse

Page 300

Answer 14

1. Like mental illness, drug abuse is linked to neurotransmitter levels

• dopamine is essential for mood regulation: working of the brain’s built-in reward circuit.
  
  1. The reward circuit is a collection of neurons that promotes healthy, pleasurable activities, such as consuming food.
  2. It’s possible to abuse behaviors such as eating, spending, or gambling because the behaviors stimulate the reward circuit and make us feel good. D
  3. Drug abusers take drugs that artificially affect the reward circuit to the point that they neglect their basic physical needs in favor of continued drug use.
• Drug abuse is
  
  1. apparent when a person takes a drug at a dose level and under circumstances that increase the potential for a harmful effect.
  2. Drug abusers are apt to display a psychological and/or physical dependence on the drug.
  3. Psychological dependence is apparent when a person craves the drug, spends time seeking the drug, and takes it regularly.
  4. With physical dependence, formerly called “addiction,” the person has become tolerant to the drug.
  5. More is needed to get the same effect, and withdrawal symptoms occur when he or she stops taking the drug.
  6. This is true for not only teenagers and adults but also newborn babies of mothers who abuse and are addicted to drugs. Alcohol, drugs, and tobacco can all adversely affect the developing embryo, fetus, or newborn.

Question 15

Anatomy of a Neuron

Answer 15

Classified according to function, the three types of neurons are sensory neurons, interneurons, and motor neurons (Fig. 14.3). Their functions are best described relative to the CNS. A sensory neuron takes nerve signals from a sensory receptor to the CNS. Sensory receptors are special structures that detect changes in the environment. An interneuron lies entirely within the CNS. Interneurons can receive input from sensory neurons and from other interneurons in the CNS. Thereafter, they sum up all the information received from other neurons before they communicate with motor neurons. A motor neuron takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland). Effectors carry out our responses to environmental changes, whether these are external or internal.

Question 16

reverse.prompt

Action Potential Propagation

In myelinated fibers, an action potential at one node of Ranvier causes an action potential at the next node, jumping over the entire myelin-coated portion of the axon. This type of conduction is called saltatory conduction (saltatio is a Latin word that means “to jump”) and is much faster. In thick, myelinated fibers, the rate of transmission is more than 100 m/s. Regardless of whether an axon is myelinated or not, its action potentials are self-propagating. Each action potential generates another, along the entire length of the axon.

Like the action potential itself, conduction of an action potential is an all-or-none event—either an axon conducts its action potential or it does not. The intensity of a message is determined by how many action potentials are generated within a given time. An axon can conduct a volley of action potentials very quickly, because only a small number of ions are exchanged with each action potential. Once the action potential is complete, the ions are rapidly restored to their proper place through the action of the sodium–potassium pump.

Neural Transmission: Action Potential Propagation

As soon as the action potential has passed by each successive portion of an axon, that portion undergoes a short refractory period, during which it is unable to conduct an action potential. This ensures the one-way direction of a signal from the cell body down the length of the axon to the axon terminal.

It is interesting to note that all functions of the nervous system, from our deepest emotions to our highest reasoning abilities, are dependent on the conduction of nerve signals.

Question 17

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

Answer 17

What are the hemispheres?

Question 18

heroine

Answer 18

smoking

injection

snorting

Question 19

Reverse

• CNS

Slides

Neuroaglia

Answer 19

1. Astrocytes
2. Microglia
3. Ependymal cells
4. Oligodendrocytes • PNS – Schwann cells – Satellite cells 1

Question 20

reverse.prompt

Amygdala—fight-or-flight; hippocampus—learning and memory. The hippocampus acts as a bridge between the sensory association areas of the cerebral cortex where memories are stored long term and the prefrontal areas of the cortex where memories are used.

Answer 20

List what limbic system structures are involved in the fight-or-flight reaction, learning, and long-term memory.

Question 21

Reverse

Synaptic Integration

A single neuron has a cell body and may have many dendrites (Fig. 14.6a). All can have synapses with many other neurons. Therefore, a neuron is on the receiving end of many signals, Page 286which can either be excitatory or inhibitory. Recall that an excitatory neurotransmitter produces an excitatory signal by opening sodium gates at a synapse. This drives the neuron closer to its threshold (illustrated by the green line in Fig. 14.6b). If threshold is reached, an action potential is inevitable. On the other hand, an inhibitory neurotransmitter drives the neuron farther from an action potential (red line in Fig. 14.6b) by opening the gates for potassium.

Neurons integrate these incoming signals. Integration is the summing up of excitatory and inhibitory signals. If a neuron receives enough excitatory signals (either from different synapses or at a rapid rate from a single synapse) to outweigh the inhibitory ones, chances are the axon will transmit a signal. On the other hand, if a neuron receives more inhibitory than excitatory signals, summing these signals may prohibit the axon from reaching threshold and then depolarizing (the solid black line in Fig. 14.6b).

Answer 21

Synaptic Integration

Question 22

Reverse

1. 100 known neurotransmitters.
   
   • The most widely studied neurotransmitters to date are
     
     1. acetylcholine: essential CNS neurotransmitter for memory circuits in the limbic system.
     2. norepinephrine, important to dreaming, waking, and mood
     3. dopamine: brain’s regulation of mood. Dopamine is also the basal nuclei neurotransmitter that helps organize coordinated movements.
     4. serotonin, and thermoregulation, sleeping, emotions, and perception
     5. gamma-aminobutyric acid (GABA).
     6. • 1. an abundant inhibitory neurotransmitter in the CNS.
     7. Acetylcholine is an Norepinephrine is .
     8. 9. Neuromodulators are naturally occurring molecules that block the release of a neurotransmitter or modify a neuron’s response to a neurotransmitter. Two well-known neuromodulators are substance P and endorphins. Substance P is a neuropeptide that is released by sensory neurons when pain is present. Endorphins block the release of substance P and serve as natural painkillers. Endorphins are produced by the brain during times of physical and/or emotional stress. They are associated with the “runner’s high” of joggers.

Both pharmaceuticals and illegal drugs have several basic modes of action:

They promote the action of a neurotransmitter, usually by increasing the amount of neurotransmitter at a synapse. Examples include drugs such as alprazolam (Xanax) and diazepam (Valium), which increase GABA. These medications are used for panic attacks and anxiety. Reduced levels of norepinephrine and serotonin are linked to depression. Drugs such as fluoxetine (Prozac), paroxetine (Paxil), and duloxetine (Cymbalta) allow norepinephrine and/or serotonin to accumulate at the synapse, which explains their effectiveness as antidepressants. Alzheimer disease causes a slow, progressive loss of memory (see Section 18.5). Drugs used for Alzheimer disease allow acetylcholine to accumulate at synapses in the limbic system.

They interfere with or decrease the action of a neurotransmitter. For instance, antipsychotic drugs used for the treatment of schizophrenia decrease the activity of dopamine. The caffeine in coffee, chocolate, and tea keeps us awake by interfering with the effects of inhibitory neurotransmitters in the brain.

They replace or mimic a neurotransmitter or neuromodulator. The opiates—namely, codeine, heroin, and morphine—bind to endorphin receptors and in this way reduce pain and produce a feeling of well-being.

Ongoing research into neurophysiology and neuropharmacology (the study of nervous system function and the way drugs work in the nervous system) continues to provide evidence that mental illnesses are caused by imbalances in neurotransmitters. These studies will undoubtedly improve treatments for mental illness, as well as provide insight into the problem of drug abuse.

Answer 22

Drug Mode of Action

Question 23

reversedprompt

two types of cells:

1. neurons: cells that transmit nerve impulses between parts of the nervous system: Classified according to function in CNS

• 3 types: functions relative to CNS
  
  1. sensory neurons: takes nerve signals from a sensory receptor– special structures that detect changes in the environment–to the CNS
  2. interneurons: can receive input from sensory neurons and from other interneurons in the CNS; sum up all the information received from other neurons before they communicate with motor neurons.
  3. motor neurons A sensory neuron An interneuron lies entirely within the CNS: takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland)–carry out our responses to environmental changes, whether these are external or internal. Interneurons A motor neuron

2. neuroglia (sometimes referred to as glial cells): support and nourish neurons
   * greatly outnumber neurons
   * several types of neuroglia in the CNS, each with specific functions:
   
   • Microglia are phagocytic cells that help remove bacteria and debris, whereas
   • astrocytes provide metabolic and structural support directly to the neurons.
   • The myelin sheath is formed from the membranes of tightly spiraled neuroglia.
     
     • In the PNS, Schwann cells perform this function, leaving gaps called nodes of Ranvier.
     • In the CNS, neuroglia cells called oligodendrocytes form the myelin sheath.
     • Neuroglia (see Section 4.4) Anatomy of a Neuron

Answer 23

Nervous Tissue cells:

The three types of ______________________ which are classified by function

The types of _________________________ which are _________________ abundant than ____________.

(Fig. 14.3).

Question 24

Reverse

Classified according to function, the three types of neurons are sensory neurons, interneurons, and motor neurons (Fig. 14.3). Their functions are best described relative to the CNS. A sensory neuron takes nerve signals from a sensory receptor to the CNS. Sensory receptors are special structures that detect changes in the environment.

Answer 24

This is what sensory neuron does, taking signals from a sensory ________________ to the _____________. They are special structures that ________________________.

Question 25

reverse.prompt

Memory on a cellular level

Long Term Potentiation

Answer 25

• for curing mental disorders: important understanding memory on the cellular level
• After synapses have been used intensively for a short time, they release more neurotransmitters than before.
• This phenomenon, called long-term potentiation, may be involved in memory storage.

Question 26

reversedprompt

What are basal nuclei?

What is Parkinson disease (see Section 18.5)

Answer 26

i

• Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter.
• integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited.
• Integration ensures that movements are coordinated and smooth.
• is believed to be caused by degeneration of specific neurons in the basal nuclei.

Question 27

Reversed prompt

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 28

Functional Classification of the Peripheral Nervous System  Sensory (afferent) division  Nerve fibers that carry information to the central nervous system  Motor (efferent) division  Nerve fibers that carry impulses away from the central nervous system  Somatic nervous system = voluntary (skeletal muscles)  Autonomic nervous system = involuntary (cardiac and smooth muscles, glands)

Question 29

Reverse

Reticular Formation

Answer 29

Question 30

reversedprompt

• 14.2 The Central Nervous System 26 The limbic system • It joins primitive emotions (i.e., fear, pleasure) with higher functions, such as reasoning. • It can cause strong emotional reactions to situations but conscious thought can override and direct our behavior. • Includes • Amygdala – imparts emotional overtones • Hippocampus – important to learning and memory

Answer 30

Limbic System

Question 31

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

Answer 31

These are the primary Motor and Sensory Areas of the Cortex

Question 32

Resting Potential

Answer 32

Resting Potential

• Think of all the devices, such as your cell phone and laptop, that are battery-powered. Every battery is an energy source manufactured by separating positively charged ions across a membrane from negative ions. The battery’s potential energy can be used to perform work—for example, using your phone or lighting a flashlight. A resting neuron also has potential energy, much like a fully charged battery. This energy, called the resting potential, Page 283exists because the plasma membrane is polarized: Positively charged ions are stashed outside the cell, with negatively charged ions inside.
• As Figure 14.4a shows, the outside of the cell is positive because positively charged sodium ions (Na+) gather around the outside of the plasma membrane. At rest, the neuron’s plasma membrane is permeable to potassium, but not to sodium. Thus, positively charged potassium ions (K+) contribute to the positive charge by diffusing out of the cell to join the sodium ions. The inside of the cell is negative in relation to the exterior of the cell because of the presence of large, negatively charged proteins and other molecules that remain inside the cell because of their size.
• Figure 14.4 Generation of an action potential. a. Resting potential occurs when a neuron is not conducting a nerve impulse. During an action potential, (b) the stimulus causes the cell to reach its threshold. c. Depolarization is followed by (d) repolarization. e. A graph depicting the generation of an action potential.
• Tutorial: Neuron Action Potentials
• Like a battery, the neuron’s resting potential energy can be measured in volts. Whereas a D-size flashlight battery has 1.5 volts, a nerve cell typically has 0.070 volt, or 70 millivolts (mV), of stored energy (Fig. 14.4a). By convention, the voltage measurement is always a negative number. This is because scientists compare the inside of the cell—where negatively charged proteins and other large molecules are clustered—to the outside of the cell—where positively charged sodium and potassium ions are gathered.
• Just like rechargeable batteries, neurons must maintain their resting potential to be able to work. To do so, neurons actively transport sodium ions out of the cell and return potassium ions to Page 284the cytoplasm. A protein carrier in the membrane, called the sodium–potassium pump, pumps sodium ions (Na+) out of the neuron and potassium ions (K+) into the neuron (see Section 3.3). This action effectively “recharges” the cell so that, like a fresh battery, it can perform work.

Question 33

Reverse

contains two types of cells: neurons and neuroglia (sometimes referred to as glial cells). Neurons are the cells that transmit nerve impulses between parts of the nervous system; neuroglia support and nourish neurons.

Neuroglia (see Section 4.4) greatly outnumber neurons in the brain. There are several types of neuroglia in the CNS, each with specific functions. Microglia are phagocytic cells that help remove bacteria and debris, whereas astrocytes provide metabolic and structural support directly to the neurons. The myelin sheath is formed from the membranes of tightly spiraled neuroglia. In the PNS, Schwann cells perform this function, leaving gaps called nodes of Ranvier. In the CNS, neuroglia cells called oligodendrocytes form the myelin sheath. We will focus our attention on the anatomy and physiology of neurons.

Answer 33

Nervous tissue

Question 34

Reverse

4.2 The Central Nervous System 19 Prefrontal Cortex • “CEO of the brain” • Where you control and plan your actions • Working memory • Organization • Modulate your mood • Conscience • Personality • Not fully developed until at least 25 years of age– maybe even later! (You can blame your bad decisions on this if you are younger than this– ha!– or flip it around: drugs, alcohol, excessive videogaming, etc. can really have a permanent negative impact on this developing brain area even if you are of ‘legal age’…) 14.2 The Central Nervous System 20 1. The brain: Cerebrum – The cerebral cortex Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. salivation vocalization mastication longitudinal fissure facial expression swallowing thumb, fingers, and hand forearm arm trunk pelvis thigh leg foot and toes lips upper face

Question 35

Wave of depolarization/repolarization travels down the axon. • Resting potential is restored by moving potassium inside and sodium outside

Question 36

Reverse

Drugs and drug abuse • Both legal pharmaceuticals and illegal drugs of abuse have certain basic modes of action that are similar. They: – promote the action of a neurotransmitter. – interfere with or decrease the action of a neurotransmitter. – replace or mimic a neurotransmitter or neuromodulator.

Answer 37

14.4 The Peripheral Nervous System

LEARNING OUTCOMES

Upon completion of this section you should be able to

Describe the series of events during a spinal reflex.

Distinguish between the somatic and autonomic divisions of the peripheral nervous system.

Distinguish between the sympathetic and parasympathetic divisions of the autonomic division.

The peripheral nervous system (PNS), which lies outside the central nervous system, contains the nerves. Nerves are designated as cranial nerves when they arise from the brain and are termed spinal nerves when they arise from the spinal cord. In any case, all nerves carry signals to and from the CNS. So right now, your eyes are sending messages by way of a cranial nerve to the brain, allowing you to read this text. When you’re finished, your brain will direct the muscles in your fingers, by way of the spinal cord and a spinal nerve, to proceed to the next chapter.

Figure 14.15 illustrates the anatomy of a nerve. The cell body and the dendrites of neurons are in either the CNS or the ganglia. Ganglia (sing., ganglion) are collections of nerve cell bodies outside the CNS. The axons of neurons project from the CNS and form the spinal cord. In other words, nerves, whether cranial or spinal, are composed of axons, the long part of neurons.

Figure 14.15 The structure of a nerve. The peripheral nervous system consists of the cranial nerves and the spinal nerves. A nerve is composed of bundles of axons separated from one another by connective tissue.

(photo): ©Pasieka/Science Photo Library/Science Source

Humans have 12 pairs of cranial nerves attached to the brain. By convention, the pairs of cranial nerves are referred to by Roman numerals (Fig. 14.16). Some cranial nerves are sensory nerves—they contain only sensory fibers; some are motor nerves that contain only motor fibers; others are mixed nerves that contain both sensory and motor fibers. Cranial nerves are largely concerned with the head, neck, and facial regions of the body. However, the vagus nerve (X) has branches not only to the pharynx and larynx but also to most of the internal organs. It arises from the brain stem—specifically, the medulla oblongata, which communicates with the hypothalamus. These two parts of the brain control the internal organs.

Figure 14.16 The cranial nerves. Overall, cranial nerves receive sensory input from, and send motor outputs to, the head region. The spinal nerves receive sensory input from, and send motor outputs to, the rest of the body. Two important exceptions are the vagus nerve, X, which communicates with internal organs, and the spinal accessory nerve, XI, which controls neck and back muscles.

The spinal nerves of humans emerge from either side of the spinal cord (see Fig. 14.8). There are 31 pairs of spinal nerves. The roots of a spinal nerve physically separate the axons of sensory neurons from the axons of motor neurons, forming an arrangement resembling a letter Y. The posterior root of a spinal nerve contains sensory fibers that direct sensory receptor information inward (toward the spinal cord). The cell body of a sensory neuron is in a posterior-root ganglion (also termed a dorsal-root ganglion). The anterior (also termed ventral) root of a spinal nerve contains motor fibers that conduct impulses outward (away from the cord) to the effectors. Observe in Figure 14.8 that the anterior and posterior roots join to form a spinal nerve. All spinal nerves are called mixed nerves, because they contain both sensory and motor fibers. Each spinal nerve serves the particular region of the body in which it is located. For example, the intercostal muscles of the rib cage are innervated by thoracic nerves.

The Somatic System

The PNS has divisions: the somatic system and the autonomic system. The nerves in the somatic system serve the skin, skeletal muscles, and tendons (see Fig. 14.2). The somatic system sensory nerves take sensory information from external sensory receptors to the CNS. Motor commands leaving the CNS travel to skeletal muscles via somatic motor nerves.

Not all somatic motor actions are voluntary. Some are automatic. Automatic responses to a stimulus in the somatic system are called reflexes. A reflex occurs quickly, without your even having to think about it. For example, a reflex may cause you to blink your eyes in response to a flash of light, without your willing it. We will study the path of a reflex, because it allows us to study in detail the path of nerve signals to and from the CNS.Page 296

The Reflex Arc

Figure 14.17 illustrates the path of a reflex that involves only the spinal cord. If your hand touches a sharp pin, sensory receptors in the skin generate nerve signals that move along sensory fibers through the posterior (dorsal) root ganglia toward the spinal cord. Sensory neurons that enter the cord posteriorly pass signals on to many interneurons. Some of these interneurons synapse with motor neurons whose short dendrites and cell bodies are in the spinal cord. Nerve signals travel along these motor fibers to an effector, which brings about a response to the stimulus. In this case, the effector is a muscle, which contracts so that you withdraw your hand from the pin. Various other reactions are also possible—you will most likely look at the pin, wince, and cry out in pain. This whole series of responses occurs because some of the interneurons involved carry nerve signals to the brain. The brain makes you aware of the stimulus and directs these other reactions to it. In other words, you don’t feel pain until the brain receives the information and interprets it.

Figure 14.17 A spinal reflex arc. A stimulus (e.g., a sharp pin) causes sensory receptors in the skin to generate nerve signals that travel in sensory axons to the spinal cord. Interneurons integrate data from sensory neurons and then relay signals to motor neurons, causing contraction of a skeletal muscle and movement of the hand away from the stimulus.

SCIENCE IN YOUR LIFE

How does aspirin work?

Aspirin is made of a chemical called acetylsalicylic acid (ASA). Damaged tissue produces large amounts of a type of fatty acid called prostaglandin. Prostaglandin acts as a signal to the peripheral nervous system that tissue damage has occurred, which the brain interprets as pain. Prostaglandins are manufactured in the cell by an enzyme called COX (cyclooxygenase). ASA reduces the capabilities of this enzyme, thus lowering the amount of prostaglandin produced and the perception of pain.

The Autonomic System

The autonomic system is also in the PNS (see Fig. 14.2). The autonomic system regulates the activity of cardiac and smooth muscles, organs, and glands. The system is divided into the sympathetic and parasympathetic divisions (Fig. 14.18). Activation of these two systems generally causes opposite responses.

Figure 14.18 The two divisions of the autonomic nervous system. Sympathetic preganglionic fibers (left) arise from the thoracic and lumbar portions of the spinal cord; parasympathetic preganglionic fibers (right) arise from the cranial and sacral portions of the spinal cord. Each system innervates the same organs but has contrary effects.

Although their functions are different, the two divisions share some features: (1) They usually function in an involuntary manner; (2) they innervate all internal organs; and (3) they use two neurons and one ganglion for each impulse. The first neuron has a cell body within the CNS and a preganglionic fiber that enters the ganglion. The second neuron has a cell body within a ganglion and a postganglionic fiber that leaves the ganglion.

Reflex actions, such as those that regulate blood pressure and breathing rate, are especially important to the maintenance of homeostasis. These reflexes begin when the sensory neurons in contact with internal organs send messages to the CNS. They are completed by motor neurons within the autonomic system.

Sympathetic Division

Most preganglionic fibers of the sympathetic division arise from the middle portion of the spinal cord. They terminate almost immediately in ganglia that lie near the cord.

The sympathetic division is especially important during emergency situations when you might be required to fight or take flight. It accelerates the heartbeat and dilates the bronchi—active muscles, after all, require a ready supply of glucose and oxygen. Page 297Sympathetic neurons inhibit the digestive organs, as well as the kidneys and urinary bladder; the activities of these organs—digestion, defecation, and urination—are not immediately necessary if you’re under attack. The neurotransmitter released by the postganglionic axon is primarily norepinephrine (NE). The structure of NE is like that of epinephrine (adrenaline), an adrenal medulla hormone that usually increases heart rate and contractility.

Parasympathetic Division

The parasympathetic division includes a few cranial nerves (e.g., the vagus nerve), as well as fibers that arise from the sacral (bottom) portion of the spinal cord. Therefore, this division is often referred to as the craniosacral portion of the autonomic system. In the parasympathetic division, the preganglionic fiber is long, and the postganglionic fiber is short because the ganglia lie near or within the organ.

The parasympathetic division, sometimes called the housekeeper division, promotes all the internal responses we associate with a relaxed state. For example, it causes the pupil of the eye to contract, promotes digestion of food, and slows heart rate. It has been suggested that the parasympathetic system could be called the rest-and-digest system. The neurotransmitter used by the parasympathetic division is acetylcholine (ACh).

The Somatic Versus the Autonomic Systems

Recall that the PNS includes the somatic system and the autonomic system. Table 14.1 compares the features and functions of the somatic motor pathway with the motor pathways of the autonomic system.

Table 14.1Comparison of Somatic Motor and Autonomic Motor Pathways

Table Summary: Columns are for somatic motor pathway and autonomic motor pathways. Rows are for different points of comparison. Autonomic motor pathways are grouped into sympathetic and parasympathetic, as the other column-headers.

Autonomic Motor Pathways

Somatic Motor PathwaySympatheticParasympathetic

Type of controlVoluntary/involuntaryInvoluntaryInvoluntary

Number of neurons per messageOneTwo (preganglionic shorter than postganglionic)Two (preganglionic longer than postganglionic)

Location of motor fiberMost cranial nerves and all spinal nervesThoracolumbar spinal nervesCranial (e.g., vagus) and sacral spinal nerves

NeurotransmitterAcetylcholineNorepinephrineAcetylcholine

EffectorsSkeletal musclesSmooth and cardiac muscle, glands, and organsSmooth and cardiac muscle, glands, and organs

CHECK YOUR PROGRESS 14.4

Contrast cranial and spinal nerves.

Answer

The 12 pairs of cranial nerves receive sensory input from and send motor outputs primarily to the head region. The 31 pairs of spinal nerves receive sensory input from and send motor outputs to the rest of the body.

Detail the fastest way for you to react to a stimulus.

Answer

A reflex action is fastest when it involves just the reflex arc that passes only through the spinal cord, not the brain.

Predict what could happen to homeostasis if the autonomic nervous system failed.

Answer

Without the autonomic nervous system, activities of the cardiac muscles, smooth muscles, and glands would have to be regulated voluntarily. Maintaining homeostasis would be an overwhelming task.

CONNECTING THE CONCEPTS

For more on the interaction of the PNS with the other systems of the body, refer to the following discussions:

Section 5.3 explores how the divisions of the autonomic system regulate the heart rate and help maintain homeostasis.

Section 10.5 examines how signals between the brain and the diaphragm control the rate of breathing.

Section 15.1 provides an overview of the types of sensory inputs processed by the peripheral nervous system.

Question 38

Reverse

acetylcholine

matches

Choice, essential for memory circuits in the limbic system

essential for memory circuits in the limbic system

dopamine

matches

Choice, in the basal nuclei it helps coordinate movements, also plays a role in mood regulation

in the basal nuclei it helps coordinate movements, also plays a role in mood regulation

GABA

matches

Choice, abundant inhibitory neurotransmitter in the CNS

abundant inhibitory neurotransmitter in the CNS

norepinephrine

matches

Choice, important to dreaming, waking, and mood

important to dreaming, waking, and mood

serotonin

matches

Choice, thermoregulation, sleeping, emotions and perception

thermoregulation, sleeping, emotions and perception

Answer 38

neurotransmitters

Question 39

14.4 The Peripheral Nervous System

LEARNING OUTCOMES

Upon completion of this section you should be able to

Describe the series of events during a spinal reflex.

Distinguish between the somatic and autonomic divisions of the peripheral nervous system.

Distinguish between the sympathetic and parasympathetic divisions of the autonomic division.

The peripheral nervous system (PNS), which lies outside the central nervous system, contains the nerves. Nerves are designated as cranial nerves when they arise from the brain and are termed spinal nerves when they arise from the spinal cord. In any case, all nerves carry signals to and from the CNS. So right now, your eyes are sending messages by way of a cranial nerve to the brain, allowing you to read this text. When you’re finished, your brain will direct the muscles in your fingers, by way of the spinal cord and a spinal nerve, to proceed to the next chapter.

Figure 14.15 illustrates the anatomy of a nerve. The cell body and the dendrites of neurons are in either the CNS or the ganglia. Ganglia (sing., ganglion) are collections of nerve cell bodies outside the CNS. The axons of neurons project from the CNS and form the spinal cord. In other words, nerves, whether cranial or spinal, are composed of axons, the long part of neurons.

Figure 14.15 The structure of a nerve. The peripheral nervous system consists of the cranial nerves and the spinal nerves. A nerve is composed of bundles of axons separated from one another by connective tissue.

(photo): ©Pasieka/Science Photo Library/Science Source

Humans have 12 pairs of cranial nerves attached to the brain. By convention, the pairs of cranial nerves are referred to by Roman numerals (Fig. 14.16). Some cranial nerves are sensory nerves—they contain only sensory fibers; some are motor nerves that contain only motor fibers; others are mixed nerves that contain both sensory and motor fibers. Cranial nerves are largely concerned with the head, neck, and facial regions of the body. However, the vagus nerve (X) has branches not only to the pharynx and larynx but also to most of the internal organs. It arises from the brain stem—specifically, the medulla oblongata, which communicates with the hypothalamus. These two parts of the brain control the internal organs.

Figure 14.16 The cranial nerves. Overall, cranial nerves receive sensory input from, and send motor outputs to, the head region. The spinal nerves receive sensory input from, and send motor outputs to, the rest of the body. Two important exceptions are the vagus nerve, X, which communicates with internal organs, and the spinal accessory nerve, XI, which controls neck and back muscles.

The spinal nerves of humans emerge from either side of the spinal cord (see Fig. 14.8). There are 31 pairs of spinal nerves. The roots of a spinal nerve physically separate the axons of sensory neurons from the axons of motor neurons, forming an arrangement resembling a letter Y. The posterior root of a spinal nerve contains sensory fibers that direct sensory receptor information inward (toward the spinal cord). The cell body of a sensory neuron is in a posterior-root ganglion (also termed a dorsal-root ganglion). The anterior (also termed ventral) root of a spinal nerve contains motor fibers that conduct impulses outward (away from the cord) to the effectors. Observe in Figure 14.8 that the anterior and posterior roots join to form a spinal nerve. All spinal nerves are called mixed nerves, because they contain both sensory and motor fibers. Each spinal nerve serves the particular region of the body in which it is located. For example, the intercostal muscles of the rib cage are innervated by thoracic nerves.

The Somatic System

The PNS has divisions: the somatic system and the autonomic system. The nerves in the somatic system serve the skin, skeletal muscles, and tendons (see Fig. 14.2). The somatic system sensory nerves take sensory information from external sensory receptors to the CNS. Motor commands leaving the CNS travel to skeletal muscles via somatic motor nerves.

Not all somatic motor actions are voluntary. Some are automatic. Automatic responses to a stimulus in the somatic system are called reflexes. A reflex occurs quickly, without your even having to think about it. For example, a reflex may cause you to blink your eyes in response to a flash of light, without your willing it. We will study the path of a reflex, because it allows us to study in detail the path of nerve signals to and from the CNS.Page 296

The Reflex Arc

Figure 14.17 illustrates the path of a reflex that involves only the spinal cord. If your hand touches a sharp pin, sensory receptors in the skin generate nerve signals that move along sensory fibers through the posterior (dorsal) root ganglia toward the spinal cord. Sensory neurons that enter the cord posteriorly pass signals on to many interneurons. Some of these interneurons synapse with motor neurons whose short dendrites and cell bodies are in the spinal cord. Nerve signals travel along these motor fibers to an effector, which brings about a response to the stimulus. In this case, the effector is a muscle, which contracts so that you withdraw your hand from the pin. Various other reactions are also possible—you will most likely look at the pin, wince, and cry out in pain. This whole series of responses occurs because some of the interneurons involved carry nerve signals to the brain. The brain makes you aware of the stimulus and directs these other reactions to it. In other words, you don’t feel pain until the brain receives the information and interprets it.

Figure 14.17 A spinal reflex arc. A stimulus (e.g., a sharp pin) causes sensory receptors in the skin to generate nerve signals that travel in sensory axons to the spinal cord. Interneurons integrate data from sensory neurons and then relay signals to motor neurons, causing contraction of a skeletal muscle and movement of the hand away from the stimulus.

SCIENCE IN YOUR LIFE

How does aspirin work?

Aspirin is made of a chemical called acetylsalicylic acid (ASA). Damaged tissue produces large amounts of a type of fatty acid called prostaglandin. Prostaglandin acts as a signal to the peripheral nervous system that tissue damage has occurred, which the brain interprets as pain. Prostaglandins are manufactured in the cell by an enzyme called COX (cyclooxygenase). ASA reduces the capabilities of this enzyme, thus lowering the amount of prostaglandin produced and the perception of pain.

The Autonomic System

The autonomic system is also in the PNS (see Fig. 14.2). The autonomic system regulates the activity of cardiac and smooth muscles, organs, and glands. The system is divided into the sympathetic and parasympathetic divisions (Fig. 14.18). Activation of these two systems generally causes opposite responses.

Figure 14.18 The two divisions of the autonomic nervous system. Sympathetic preganglionic fibers (left) arise from the thoracic and lumbar portions of the spinal cord; parasympathetic preganglionic fibers (right) arise from the cranial and sacral portions of the spinal cord. Each system innervates the same organs but has contrary effects.

Although their functions are different, the two divisions share some features: (1) They usually function in an involuntary manner; (2) they innervate all internal organs; and (3) they use two neurons and one ganglion for each impulse. The first neuron has a cell body within the CNS and a preganglionic fiber that enters the ganglion. The second neuron has a cell body within a ganglion and a postganglionic fiber that leaves the ganglion.

Reflex actions, such as those that regulate blood pressure and breathing rate, are especially important to the maintenance of homeostasis. These reflexes begin when the sensory neurons in contact with internal organs send messages to the CNS. They are completed by motor neurons within the autonomic system.

Sympathetic Division

Most preganglionic fibers of the sympathetic division arise from the middle portion of the spinal cord. They terminate almost immediately in ganglia that lie near the cord.

The sympathetic division is especially important during emergency situations when you might be required to fight or take flight. It accelerates the heartbeat and dilates the bronchi—active muscles, after all, require a ready supply of glucose and oxygen. Page 297Sympathetic neurons inhibit the digestive organs, as well as the kidneys and urinary bladder; the activities of these organs—digestion, defecation, and urination—are not immediately necessary if you’re under attack. The neurotransmitter released by the postganglionic axon is primarily norepinephrine (NE). The structure of NE is like that of epinephrine (adrenaline), an adrenal medulla hormone that usually increases heart rate and contractility.

Parasympathetic Division

The parasympathetic division includes a few cranial nerves (e.g., the vagus nerve), as well as fibers that arise from the sacral (bottom) portion of the spinal cord. Therefore, this division is often referred to as the craniosacral portion of the autonomic system. In the parasympathetic division, the preganglionic fiber is long, and the postganglionic fiber is short because the ganglia lie near or within the organ.

The parasympathetic division, sometimes called the housekeeper division, promotes all the internal responses we associate with a relaxed state. For example, it causes the pupil of the eye to contract, promotes digestion of food, and slows heart rate. It has been suggested that the parasympathetic system could be called the rest-and-digest system. The neurotransmitter used by the parasympathetic division is acetylcholine (ACh).

The Somatic Versus the Autonomic Systems

Recall that the PNS includes the somatic system and the autonomic system. Table 14.1 compares the features and functions of the somatic motor pathway with the motor pathways of the autonomic system.

Table 14.1Comparison of Somatic Motor and Autonomic Motor Pathways

Table Summary: Columns are for somatic motor pathway and autonomic motor pathways. Rows are for different points of comparison. Autonomic motor pathways are grouped into sympathetic and parasympathetic, as the other column-headers.

Autonomic Motor Pathways

Somatic Motor PathwaySympatheticParasympathetic

Type of controlVoluntary/involuntaryInvoluntaryInvoluntary

Number of neurons per messageOneTwo (preganglionic shorter than postganglionic)Two (preganglionic longer than postganglionic)

Location of motor fiberMost cranial nerves and all spinal nervesThoracolumbar spinal nervesCranial (e.g., vagus) and sacral spinal nerves

NeurotransmitterAcetylcholineNorepinephrineAcetylcholine

EffectorsSkeletal musclesSmooth and cardiac muscle, glands, and organsSmooth and cardiac muscle, glands, and organs

CHECK YOUR PROGRESS 14.4

Contrast cranial and spinal nerves.

Answer

The 12 pairs of cranial nerves receive sensory input from and send motor outputs primarily to the head region. The 31 pairs of spinal nerves receive sensory input from and send motor outputs to the rest of the body.

Detail the fastest way for you to react to a stimulus.

Answer

A reflex action is fastest when it involves just the reflex arc that passes only through the spinal cord, not the brain.

Predict what could happen to homeostasis if the autonomic nervous system failed.

Answer

Without the autonomic nervous system, activities of the cardiac muscles, smooth muscles, and glands would have to be regulated voluntarily. Maintaining homeostasis would be an overwhelming task.

CONNECTING THE CONCEPTS

For more on the interaction of the PNS with the other systems of the body, refer to the following discussions:

Section 5.3 explores how the divisions of the autonomic system regulate the heart rate and help maintain homeostasis.

Section 10.5 examines how signals between the brain and the diaphragm control the rate of breathing.

Section 15.1 provides an overview of the types of sensory inputs processed by the peripheral nervous system.

Question 40

Reverse

Basal Nuclei

Answer 40

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

Question 41

Cerebrum- Brain

This is the color of matter.

These are the parts

The function of the cerebrum

Answer 41

• cerebral cortex: thin, outer layer of gray matter

1. Primary motor area – voluntary control of skeletal muscle •
2. Primary somatosensory area – for sensory information from skeletal muscle and skin
3. Association areas – integration occurs here
4. Processing centers – perform higher level analytical functions including Wernicke’s and Broca’s areas, both involved in speech.

Question 42

Blood Brain Barrier • Includes the least permeable capillaries of the body • Excludes many potentially harmful substances • Useless against some substances • Fats and fat soluble molecules • Respiratory gases • Alcohol • Nicotine • Anesthesia Supplemental material not in book 14.2 The Central Nervous System 8 The central nervous system • Both the brain and spinal cord are made up of 2 types of nervous tissue: • Gray matter – contains cell bodies and nonmyelinated fibers • White matter – contains myelinated axons 14.2 The Central Nervous System

Question 43

Dura mater •

Double-layered external covering

• Periosteum – dense connective tissue attached to surface of the skull
• Meningeal layer – outer covering of the brain
• Folds inward in several areas

Question 44

Reverse

Integration is the summation (adding up) of the inhibitory and excitatory signals received by a postsynaptic neuron. • This occurs because a neuron receives many signals at once.

Answer 44

Synaptic Integration

Question 45

Reversed prompt

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Answer 45

The Spinal Cord

Question 46

resting neuron- charge difference between inside and outside, maintained by K and Na pumps- while other channels- sodium – can’t get back in- exterior net positive, net negative interior- resting membrane potential- nerve impulse begins when stiumulus disturbs on dendrite- Na ions float into cell- charge reduced, change is enough, will cause nearby Na channels to open- depolarize-d local region- positively charged on inside and negative on the outside- neighboring channels open- depolarization on membrane- action potential- changes occur - to restore resting membrane potential, Na closes, K opens- allows K to float out repolarization membrane

Answer 46

Action Potential - Polarization

Question 47

Spinal Cord Functions and Injuries– Motor Control

1. Page 288

Answer 47

1. The brain coordinates the voluntary control of our limbs.
2. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows).
   
   • Therefore, if the spinal cord is severed, we suffers
     
     1. a loss of sensation and a
     2. loss of voluntary control—paralysis.
     3. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.
        3.

Answer 48

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

Question 49

reverse.prompt

Sensory receptors in skin and other organs respond to external and internal stimuli by generating nerve signals that travel by way of the PNS to the CNS. For example, if you smell baking cookies, olfactory (smell) receptors in the nose use the PNS to transmit that information to the CNS.The CNS performs information processing and integration, summing up the input it receives from all over the body. The CNS reviews the information, stores the information as memories, and creates the appropriate motor responses. The smell of those baking cookies evokes memories of their taste.

The CNS generates motor output. Nerve signals from the CNS go by way of the PNS to the muscles, glands, and organs, all in response to the cookies. Signals to the salivary glands make you salivate. Your stomach generates the acid and enzymes Page 281needed to digest the cookies—even before you’ve had a bite. The CNS also coordinates the movement of your arms and hands as you reach for the cookies.

Answer 49

The nervous system receives sensory input.

Question 50

Chapter Review

SUMMARIZE

14.1Overview of the Nervous System

The nervous system

Is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

Has three functions: (1) reception of input, (2) integration of data, and (3) generation of motor output.

Nervous Tissue

Nervous tissue contains the following two types of cells: neurons and neuroglia:

Neurons transmit nerve signals using action potentials.

Neuroglia nourish and support neurons.

Anatomy of a Neuron

A neuron is composed of dendrites, a cell body, and an axon. The axons of neurons may be clustered into nerves. There are three types of neurons:

Sensory neurons take nerve signals from sensory receptors to the CNS.

Interneurons occur within the CNS.

Motor neurons take nerve signals from the CNS to effectors (muscles or glands).

Myelin Sheath

Long axons are covered by a myelin sheath, which is formed by the neuroglia cells. Gaps in the myelin sheath are called nodes of Ranvier.

Multiple sclerosis (MS) occurs when the myelin sheath breaks down, causing a short-circuiting of nerve signals.

Physiology of a Neuron

Nerve signals move information within the nervous system. The generation of a nerve signal is based on the polarity across the membrane of the neuron.

Resting potential: There is more Na+ outside the axon and more K+ inside the axon. The axon does not conduct a signal. The resting potential is maintained by active transport using the sodium–potassium pump.

Action potential: On receipt of a stimulus strong enough to overcome the threshold, a change in polarity across the axonal membrane as a nerve signal occurs: When Na+ gates open, Na+ moves to the inside Page 303of the axon, and a depolarization occurs. When K+ gates open, K+ moves to the outside of the axon, and a repolarization occurs.

Signal propagation: The presence of the myelin sheath speeds the movement of the nerve signal by saltatory conduction. After the action potential has passed, a refractory period occurs during which no additional action potentials may be processed.

The Synapse

At the end of each axon is an axon terminal, which borders the synapse between another neuron or target cell.

When a neurotransmitter is released into a synaptic cleft, transmission of a nerve signal occurs.

Binding of the neurotransmitter to receptors in the receiving membrane causes excitation or inhibition.

Enzymes, such as acetylcholinesterase (AChE), assist in removing the neurotransmitter from the synaptic cleft.

Neurotransmitters

Neurotransmitters, such as acetylcholine, norepinephrine, and serotonin, are used to convey signals across the synapses.

Integration is the summing of excitatory and inhibitory signals.

14.2The Central Nervous System

The CNS receives and integrates sensory input and formulates motor output. The CNS consists of the spinal cord and brain. The CNS is protected by the meninges, which are filled with cerebrospinal fluid. The same fluid fills the four ventricles of the brain. In the CNS, gray matter contains cell bodies and nonmyelinated fibers. White matter contains myelinated axons organized as tracts.

The Spinal Cord

The spinal cord is responsible for conduction of information to and from the brain and carries out reflex actions.

The Brain

The cerebrum: The cerebrum has two cerebral hemispheres connected by the corpus callosum.

Sensation, reasoning, learning and memory, and language and speech take place in the cerebrum.

The cerebral cortex of each cerebral hemisphere has four lobes: frontal, parietal, occipital, and temporal.

The primary motor area in the frontal lobe sends out motor commands to lower brain centers, which pass them on to motor neurons.

The primary somatosensory area in the parietal lobe receives sensory information from lower brain centers in communication with sensory neurons.

Association areas are located in all the lobes. The prefrontal area in the frontal lobe is involved in reasoning and planning of actions.

Wernicke’s area and Broca’s area are two processing centers involved in speech.

Basal nuclei: The basal nuclei integrate commands to the muscles to coordinate movement. Parkinson disease is associated with the degradation of neurons in this area.

The diencephalon: The diencephalon contains both the hypothalamus and the thalamus. The hypothalamus controls homeostasis. The thalamus sends sensory input to the cerebrum.

The cerebellum: The cerebellum coordinates skeletal muscle contractions.

The brain stem: The brain stem includes the midbrain, the pons, and the medulla oblongata.

The medulla oblongata and pons have centers for breathing and the heartbeat.

The midbrain serves as a relay station between the cerebrum and spinal cord or cerebellum.

The reticular formation is part of the reticular activating system (RAS), which transfers sensory signals to higher processing centers in the brain.

14.3The Limbic System and Higher Mental Functions

The limbic system, located deep in the brain, is involved in determining emotions and higher mental functions, such as learning.

The amygdala determines when a situation deserves the emotion we call “fear.”

The hippocampus is particularly involved in storing and retrieving memories.

A memory may be processed as either short-term memory or long-term memory. Long-term memory may be classified as either semantic memory or episodic memory. Skill memory is involved with processes such as riding a bike.

14.4The Peripheral Nervous System

The PNS contains only nerves and ganglia (sing., ganglion).

Cranial nerves take impulses to and from the brain.

Spinal nerves take impulses to and from the spinal cord.

The PNS is divided into the somatic system and the autonomic system.

The Somatic System

The somatic system serves the skin, skeletal muscles, and tendons.

Some actions are due to reflexes, which are automatic and involuntary.

Other actions are voluntary and originate in the cerebral cortex.

The Autonomic System

The autonomic system is further divided into the sympathetic division and the parasympathetic division.

Sympathetic division: responses that occur during times of stress

Parasympathetic division: responses that occur during times of relaxation

Actions in these divisions are involuntary and automatic.

These divisions innervate internal organs.

Two neurons and one ganglion are used for each impulse.Page 304

14.5Drug Therapy and Drug Abuse

Neurotransmitters, such as acetylcholine, norepinephrine, dopamine, and serotonin, play an important role in moving signals within the nervous system.

Neuromodulators block the release of a neurotransmitter.

Neurological drugs promote, prevent, or mimic the action of a particular neurotransmitter.

Drugs, such as alcohol, nicotine, and marijuana, may have depressant, stimulant, or psychoactive effects.

Dependency occurs when the body compensates for the presence of neurological drugs.

ASSESS

TESTING YOURSELF

Choose the best answer for each question.

14.1Overview of the Nervous System

Which of the following neuron parts receive(s) signals from sensory receptors of other neurons?

cell body

axon

dendrites

Both a and c are correct.

The neuroglia cells that form myelin sheaths in the CNS are called

oligodendrocytes.

ganglionic cells.

Schwann cells.

astrocytes.

microglia.

Which of these correctly describes the distribution of ions on either side of an axon when it is not conducting a nerve signal?

more sodium ions (Na+) outside and more potassium ions (K+) inside

more K+ outside and more Na+ inside

charged protein outside and Na+ and K+ inside

Na+ and K+ outside and water only inside

chloride ions (Cl−) outside and K+ and Na+ inside

When the action potential begins, sodium gates open, allowing Na+ to cross the membrane. This causes the charge on the inside of the neuron to become

more negative.

more positive.

neutral.

None of these are correct.

Repolarization of an axon during an action potential is produced by

inward diffusion of Na+.

outward diffusion of K+.

inward active transport of Na+.

active extrusion of K+.

Transmission of the nerve signal across a synapse is accomplished by the

movement of Na+ and K+.

release of a neurotransmitter by a dendrite.

release of a neurotransmitter by an axon.

release of a neurotransmitter by a cell body.

All of these are correct.

14.2The Central Nervous System

Which of the following cerebral areas is not correctly matched with its function?

occipital lobe—vision

parietal lobe—somatosensory area

temporal lobe—primary motor area

frontal lobe—Broca’s motor speech area

Which of the following brain regions is not correctly described?

The medulla oblongata regulates heartbeat, breathing, and blood pressure.

The cerebellum coordinates voluntary muscle movements.

The thalamus secretes melatonin, which regulates daily body rhythms.

The midbrain acts as a reflex center for visual, auditory, and tactile responses.

This part of the brain forms the link between the nervous system and the endocrine system.

corpus callosum

reticular formation

amygdala

hypothalamus

14.3The Limbic System and Higher Mental Functions

The regulation of the information that is to be relayed to memory is the function of the

reticular formation.

hippocampus.

hypothalamus.

cerebellum.

pons.

Memories are stored in the sensory association areas of the

cerebral cortex.

spinal cord.

brain stem.

hypothalamus.

14.4The Peripheral Nervous System

Label this diagram.

Page 305Which of the following is correct regarding the autonomic nervous system?

The action of its two divisions tends to have opposite effects on its target organs.

It is divided into sympathetic and parasympathetic divisions.

It is involved in reflect responses.

Major responsibilities are regulation of cardiac muscle, smooth muscles, organs, and glands.

All of these are correct.

The sympathetic division of the autonomic system does not cause

the liver to release glycogen.

dilation of bronchioles.

the gastrointestinal tract to digest food.

an increase in the heart rate.

14.5Drug Therapy and Drug Abuse

This neurotransmitter plays an important role in sleeping, emotions, and perception.

dopamine

acetylcholine

GABA

seratonin

Which of the following is a depressant of the CNS?

cocaine

methamphetamine

ecstasy

alcohol

ENGAGE

THINKING CRITICALLY

Demyelinating disorders, such as multiple sclerosis (discussed in the chapter opener), are the subject of numerous research projects. Many investigations focus on the cells that create myelin: the Schwann cells of the PNS and oligodendrocytes in the CNS. Other studies focus on immune system cells that attack this myelin sheath. The goal of this research is to determine how to restore lost myelin, which might help (or possibly cure) people living with MS and other demyelinating diseases. Investigations into the role played by the sheath in nerve regeneration may offer hope to victims of spinal cord injury.

Why are impulses transmitted more quickly down a myelinated axon than down an unmyelinated axon?

A buildup of very-long-chain saturated fatty acids is believed to be the cause of myelin loss in adrenoleukodystrophy. This rare disease is a demyelinating disorder like MS. It is the subject of the film Lorenzo’s Oil. This real-life drama focuses on Lorenzo Odone, whose parents successfully developed a diet that helped their son.

From your study of chemistry in Chapter 2, a fatty acid is a part of what type of molecule?

What distinguishes a saturated fatty acid from an unsaturated fatty acid?

From your study of nutrition in Chapter 9, what types of foods contain saturated fatty acids?

Why would you expect the motor skills of a child to improve as myelination continued during early childhood development?

Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC

ANSWER KEY

Testing Yourself

Click here for the answers to the Testing Yourself questions.

Answer

Testing Yourself: 1. c; 2. a; 3. a; 4. b; 5. b; 6. c; 7. c; 8. c; 9. d; 10. b; 11. a; 12. a. central canal; b. gray matter; c. white matter; d. cell body of interneuron; e. cell body of sensory neuron; 13. e; 14. c; 15. d; 16. d

Thinking Critically

Click here for the answers to the Thinking Critically questions.

Answer

Thinking Critically: 1. Myelin enables the signal to jump from node to node quickly, because the depolarization process occurs only at the node of Ranvier. 2a. Triglycerides, phospholipids. 2b. Unsaturated fatty acids (usually liquid at room temperature) are characterized by one or more double bonds between carbons, whereas saturated fatty acids (usually solid at room temperature) have all single bonds. 2c. Animal fat, butter, fatty cuts of meat. 3. Myelination enables signals to travel through axons more quickly, which helps coordinate motor skills.

Question 51

Reverse

Physiology of a Neuron

Nerve signals are the electrochemical changes that convey information within the nervous system. In the past, nerve signals could be studied only in neurons that had been removed from the body or from other organisms. More advanced techniques now enable researchers to study nerve signals in single, intact nerve cells.

Answer 51

Physiology of a Neuron

Question 52

How does transmission across the synapse occur? • Nerve impulse reaches the axon terminal. • Calcium ions enter the axon terminal and stimulate the synaptic vesicles to fuse with the presynaptic membrane; the axon terminal membrane of the first neuron. • Neurotransmitters are released and diffuse across the synapse, where they bind with the postsynaptic membrane; the dendrite/cell body membrane on the second neuron to inhibit or excite the neuron.

Question 53

reversedprompt

1. from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4).
   * From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord (Visuals)

1. central canal: cerebrospinal fluid, as do the
   
   • dorsal and ventral roots join before spinal nerve leaves vertebral canal forming mixed nerve
2. meninges that protect the spinal cord.
3. gray matter:
   
   • centrally located and shaped like the letter H
   • Portions of sensory neurons,
   • motor neurons,
   • interneurons that communicate with these two types of neurons.
   • spinal nerve dorsal root sensory fibers enter here
   • ventral root of spinal nerve motor fibers exit the gray matter
4. white matter
   
   • in areas around the gray matter
   • ascending tracts taking information to the brain (primarily located posteriorly)
   • and descending tracts taking information from the brain (primarily located anteriorly).
   • Many tracts cross just after they enter and exit the brain, so the
     
     • left side of the brain controls the right side of the body.
     • right side of the brain controls the left side of the body.
5. individual vertebra protects .
6. spinal nerves
   
   • dorsal root of a spinal nerve contains sensory fibers entering the gray matter.
   • ventra root spinal nerve motor fibers exit gray matter
   • both dorsal and ventra roots join before spinal nerve exits canal
   • part of PNS
7. intervertebral foramina
8. Fibrocartilage intervertebral discs separate the vertebrae.
   
   • If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

• • • • The white matter of the spinal cord occurs. The white matter contains

Answer 53

The Spinal Cord

Download, attach image and study it, 14.8 a Figure 14.8b

Figure 14.8

(Fig. 14.8a–c).

The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

Answer 54

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Question 55

reverse.prompt

The peripheral nervous system (PNS)

Answer 55

1. , which lies outside the central nervous system, contains the nerves.
2. Nerves are designated as cranial nerves when they arise from the brain and are termed spinal nerves when they arise from the spinal cord. In any case, all nerves carry signals to and from the CNS. So right now, your eyes are sending messages by way of a cranial nerve to the brain, allowing you to read this text. When you’re finished, your brain will direct the muscles in your fingers, by way of the spinal cord and a spinal nerve, to proceed to the next chapter.
3. Figure 14.15 illustrates the anatomy of a nerve. The cell body and the dendrites of neurons are in either the CNS or the ganglia. Ganglia (sing., ganglion) are collections of nerve cell bodies outside the CNS. The axons of neurons project from the CNS and form the spinal cord. In other words, nerves, whether cranial or spinal, are composed of axons, the long part of neurons.

Question 56

• Though the majority of each cerebral hemisphere is composed of tracts,
• there are masses of gray matter deep within the white matter
• integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited: Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

Answer 56

Basal Nuclei

What is integration?

What is Parkinson’s disease?

Question 57

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Answer 58

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Question 59

Figure 14.17 A spinal reflex arc. A stimulus (e.g., a sharp pin) causes sensory receptors in the skin to generate nerve signals that travel in sensory axons to the spinal cord. Interneurons integrate data from sensory neurons and then relay signals to motor neurons, causing contraction of a skeletal muscle and movement of the hand away from the stimulus.

SCIENCE IN YOUR LIFE

How does aspirin work?

Aspirin is made of a chemical called acetylsalicylic acid (ASA). Damaged tissue produces large amounts of a type of fatty acid called prostaglandin. Prostaglandin acts as a signal to the peripheral nervous system that tissue damage has occurred, which the brain interprets as pain. Prostaglandins are manufactured in the cell by an enzyme called COX (cyclooxygenase). ASA reduces the capabilities of this enzyme, thus lowering the amount of prostaglandin produced and the perception of pain.

The Autonomic System

The autonomic system is also in the PNS (see Fig. 14.2). The autonomic system regulates the activity of cardiac and smooth muscles, organs, and glands. The system is divided into the sympathetic and parasympathetic divisions (Fig. 14.18). Activation of these two systems generally causes opposite responses.

Figure 14.18 The two divisions of the autonomic nervous system. Sympathetic preganglionic fibers (left) arise from the thoracic and lumbar portions of the spinal cord; parasympathetic preganglionic fibers (right) arise from the cranial and sacral portions of the spinal cord. Each system innervates the same organs but has contrary effects.

Although their functions are different, the two divisions share some features: (1) They usually function in an involuntary manner; (2) they innervate all internal organs; and (3) they use two neurons and one ganglion for each impulse. The first neuron has a cell body within the CNS and a preganglionic fiber that enters the ganglion. The second neuron has a cell body within a ganglion and a postganglionic fiber that leaves the ganglion.

Reflex actions, such as those that regulate blood pressure and breathing rate, are especially important to the maintenance of homeostasis. These reflexes begin when the sensory neurons in contact with internal organs send messages to the CNS. They are completed by motor neurons within the autonomic system.

Sympathetic Division

Most preganglionic fibers of the sympathetic division arise from the middle portion of the spinal cord. They terminate almost immediately in ganglia that lie near the cord.

The sympathetic division is especially important during emergency situations when you might be required to fight or take flight. It accelerates the heartbeat and dilates the bronchi—active muscles, after all, require a ready supply of glucose and oxygen. Page 297Sympathetic neurons inhibit the digestive organs, as well as the kidneys and urinary bladder; the activities of these organs—digestion, defecation, and urination—are not immediately necessary if you’re under attack. The neurotransmitter released by the postganglionic axon is primarily norepinephrine (NE). The structure of NE is like that of epinephrine (adrenaline), an adrenal medulla hormone that usually increases heart rate and contractility.

Parasympathetic Division

The parasympathetic division includes a few cranial nerves (e.g., the vagus nerve), as well as fibers that arise from the sacral (bottom) portion of the spinal cord. Therefore, this division is often referred to as the craniosacral portion of the autonomic system. In the parasympathetic division, the preganglionic fiber is long, and the postganglionic fiber is short because the ganglia lie near or within the organ.

The parasympathetic division, sometimes called the housekeeper division, promotes all the internal responses we associate with a relaxed state. For example, it causes the pupil of the eye to contract, promotes digestion of food, and slows heart rate. It has been suggested that the parasympathetic system could be called the rest-and-digest system. The neurotransmitter used by the parasympathetic division is acetylcholine (ACh).

The Somatic Versus the Autonomic Systems

Recall that the PNS includes the somatic system and the autonomic system. Table 14.1 compares the features and functions of the somatic motor pathway with the motor pathways of the autonomic system.

Table 14.1Comparison of Somatic Motor and Autonomic Motor Pathways

Table Summary: Columns are for somatic motor pathway and autonomic motor pathways. Rows are for different points of comparison. Autonomic motor pathways are grouped into sympathetic and parasympathetic, as the other column-headers.

Autonomic Motor Pathways

Somatic Motor PathwaySympatheticParasympathetic

Type of controlVoluntary/involuntaryInvoluntaryInvoluntary

Number of neurons per messageOneTwo (preganglionic shorter than postganglionic)Two (preganglionic longer than postganglionic)

Location of motor fiberMost cranial nerves and all spinal nervesThoracolumbar spinal nervesCranial (e.g., vagus) and sacral spinal nerves

NeurotransmitterAcetylcholineNorepinephrineAcetylcholine

EffectorsSkeletal musclesSmooth and cardiac muscle, glands, and organsSmooth and cardiac muscle, glands, and organs

CHECK YOUR PROGRESS 14.4

Contrast cranial and spinal nerves.

Answer

The 12 pairs of cranial nerves receive sensory input from and send motor outputs primarily to the head region. The 31 pairs of spinal nerves receive sensory input from and send motor outputs to the rest of the body.

Detail the fastest way for you to react to a stimulus.

Answer

A reflex action is fastest when it involves just the reflex arc that passes only through the spinal cord, not the brain.

Predict what could happen to homeostasis if the autonomic nervous system failed.

Answer

Without the autonomic nervous system, activities of the cardiac muscles, smooth muscles, and glands would have to be regulated voluntarily. Maintaining homeostasis would be an overwhelming task.

CONNECTING THE CONCEPTS

For more on the interaction of the PNS with the other systems of the body, refer to the following discussions:

Section 5.3 explores how the divisions of the autonomic system regulate the heart rate and help maintain homeostasis.

Section 10.5 examines how signals between the brain and the diaphragm control the rate of breathing.

Section 15.1 provides an overview of the types of sensory inputs processed by the peripheral nervous system.

Question 60

Science in your life: aspirin

Answer 60

SCIENCE IN YOUR LIFE

How does aspirin work?

Aspirin is made of a chemical called acetylsalicylic acid (ASA). Damaged tissue produces large amounts of a type of fatty acid called prostaglandin. Prostaglandin acts as a signal to the peripheral nervous system that tissue damage has occurred, which the brain interprets as pain. Prostaglandins are manufactured in the cell by an enzyme called COX (cyclooxygenase). ASA reduces the capabilities of this enzyme, thus lowering the amount of prostaglandin produced and the perception of pain.

Question 61

Cerebellum, CNS,

Answer 61

• 14.2 The Central Nervous System 23 3. The brain: Cerebellum • Receives and integrates sensory input from the eyes, ears, joints, and muscles about the current position of the body • Functions • Maintains posture • Coordinates voluntary movement • Allows learning of new motor skills (i.e., playing the piano or hitting a baseball) 1

Question 62

Myelin Sheath

Answer 62

A lipid covering on long axons that acts to increase the speed of nerve impulse conduction, insulation for both CNS and PNS, and regeneration in the PNS • Schwann cells – neuroglia that make up the myelin sheath in the PNS • Oligodendrocytes- neuroglia that make up the myelin sheath in the CNS • Nodes of Ranvier – gaps between myelination on the axons • Saltatory conduction – conduction of the nerve impulse from node to node

Question 63

Drug Influence in the CNS

Drug Influence on the CNS

Table Summary: Table lists the names of different substances in column 1. Other information related to these substances appears in columns 2 and 3.

SubstanceEffectMode of Transmission

AlcoholDepressantDrink

NicotineStimulantSmoked or smokeless tobacco

CocaineStimulantSniffed/snorted, injected, or smoked

Methamphetamine/EcstasyStimulantSmoked or pill form

HeroinDepressantSniffed/snorted, injected, or smoked

Marijuana/K2PsychoactiveSmoked or consumed

Answer 63

Table 14.2Drug Influence on the CNS

Table Summary: Table lists the names of different substances in column 1. Other information related to these substances appears in columns 2 and 3.

SubstanceEffectMode of Transmission

AlcoholDepressantDrink

NicotineStimulantSmoked or smokeless tobacco

CocaineStimulantSniffed/snorted, injected, or smoked

Methamphetamine/EcstasyStimulantSmoked or pill form

HeroinDepressantSniffed/snorted, injected, or smoked

Marijuana/K2PsychoactiveSmoked or consumed

Beginning in about 2005, several manufacturers began selling alcoholic energy drinks. With names like Four Loko, JOOSE, and Sparks, these drinks combine fairly high levels of alcohol with caffeine and other ingredients. Although interactions between drugs can be complex, the stimulant effects of caffeine can counteract some of the depressant effects of alcohol, so users feel able to drink more. Because caffeine does not reduce the intoxicating effects of alcohol, many state legislatures are banning these products, and in November 2010 the U.S. Food and Drug Administration warned several manufacturers that they would no longer be allowed to mix caffeine with alcohol in their products.

Nicotine

Although the numbers have been decreasing since 2011 according to the CDC, in 2015, 25.3% of high school students and 7.4% of middle school students reported using a tobacco product. When tobacco is smoked or chewed, nicotine is rapidly delivered throughout the body. It causes a release of epinephrine from the adrenal glands, increasing blood sugar and causing the initial feeling of stimulation. As blood sugar falls, depression and fatigue set in, causing the user to seek more nicotine. In the CNS, nicotine stimulates neurons to release dopamine, a neurotransmitter that promotes a temporary sense of pleasure, and reinforces dependence on the drug. About 70% of people who try smoking become addicted.

As mentioned in earlier chapters, smoking is strongly associated with serious diseases of the cardiovascular and respiratory systems. Once addicted, however, only 10–20% of smokers are able to quit. Most medical approaches to quitting smoking involve the administration of nicotine in safer forms, such as skin patches, gum, or a newly developed nicotine inhaler, so that withdrawal symptoms can be minimized while dependence is gradually reduced. Several antinicotine vaccines (such as NicVAX) are currently in development or in early clinical trials. These vaccines stimulate the production of antibodies that prevent nicotine from entering the brain.

Cocaine and Crack

Cocaine is an alkaloid derived from the shrub Erythroxylon coca. Approximately 35 million Americans have used cocaine by sniffing/snorting, injecting, or smoking. Cocaine is a powerful stimulant in the CNS that interferes with the reuptake of dopamine at synapses, increasing overall brain activity. The result is a rush of a sense of well-being that lasts from 5 to 30 minutes. However, long-term use of cocaine causes a loss of metabolic functions in the brain (Fig. 14.19).

Figure 14.19 Cocaine use. Brain activity before and after the use of cocaine.

(both photos): ©Science Source

“Crack” is the street name given to cocaine that is processed to a free-base form for smoking. The term crack refers to the crackling sound heard when the drug is smoked. Smoking allows high doses of the drug to reach the brain rapidly, providing an intense and immediate high, or “rush.” Approximately 8 million Americans use crack.

A cocaine binge is a period in which a user takes the drug at ever-higher doses. The user is hyperactive, with little desire for food or sleep, but has an increased sex drive. This is followed by a crash period, characterized by fatigue, depression, irritability, and a lack of interest in sex. In fact, men who use cocaine often become impotent.

Cocaine is highly addictive; with continued use, the brain makes less dopamine to compensate for a seemingly endless supply. The user experiences withdrawal symptoms and an intense craving for cocaine. Overdosing on cocaine can cause cardiac and/or respiratory arrest.Page 301

Methamphetamine and Ecstasy

Methamphetamine and ecstasy are considered club, or party, drugs. Methamphetamine (commonly called meth or crank) is a powerful CNS stimulant. Meth is often produced in makeshift home laboratories, usually starting with ephedrine or pseudoephedrine, common ingredients in many cold and asthma medicines. As a result, many states have passed laws making these medications more difficult to purchase. The number of toxic chemicals used to prepare the drug makes a former meth lab site hazardous to humans and to the environment. Over 9 million people in the United States have used methamphetamine at least once. It is available as a powder that can be snorted or as crystals (crystal meth or ice) that can be smoked.

The structure of methamphetamine is similar to that of dopamine, and the most immediate effect of taking meth is a rush of euphoria, energy, alertness, and elevated mood. However, this is typically followed by a state of agitation that, in some individuals, leads to violent behavior. Chronic use can result in what is called an amphetamine psychosis, characterized by paranoia, hallucinations, irritability, and aggressive, erratic behavior.

Ecstasy is the street name for MDMA (methylenedioxymethamphetamine), which is chemically similar to methamphetamine. Many users say that “X,” taken as a pill that looks like an aspirin or candy, increases their feelings of well-being and love for other people. However, it has many of the same side effects as other stimulants, plus it can interfere with temperature regulation, leading to hyperthermia, high blood pressure, and seizures. Although deaths from alcohol abuse are more common, ecstasy is identified as a cause of accidental death in young adults each year.

Drugs with sedative effects, known as date rape or predatory drugs, include flunitrazepam (Rohypnol, or roofies), gamma-hydroxybutyric acid (GHB), and ketamine (special K). Ketamine is actually a drug that veterinarians sometimes use to perform surgery on animals. Any of these drugs can be given to an unsuspecting person, who may fall into a dreamlike state in which he or she is unable to move and thus is vulnerable to sexual assault.

Heroin

Heroin is derived from the resin or sap of the opium poppy plant, which is widely grown in a region from Turkey to Southeast Asia and in parts of Latin America. Drugs derived from opium are called opiates, or more commonly, opioids. This class also includes morphine and codeine. After heroin is injected, snorted, or smoked, a feeling of euphoria, along with relief of any pain, occurs within a few minutes. It is estimated that 4 million Americans have used heroin sometime in their lives, and over 300,000 people use heroin annually.

As with other drugs of abuse, addiction is common. Heroin and opioids bind to receptors meant for the endorphins, naturally occurring neurotransmitters that kill pain and produce feelings of tranquility. With repeated use, the body’s production of endorphins decreases. Tolerance develops, so the user needs to take more of the drug just to prevent withdrawal symptoms (tremors, restlessness, cramps, vomiting), and the original euphoria is no longer felt. In the case of heroin, long-term users commonly acquire hepatitis, HIV/AIDS, and various bacterial infections due to the use of shared needles, and heavy users may experience convulsions and death by respiratory arrest.

Heroin addiction can be treated with synthetic opiate compounds, such as methadone or buprenorphine and naloxone (Suboxone), that decrease withdrawal symptoms and block heroin’s effects. However, methadone itself can be addictive, and methadone-related deaths are on the rise.

Marijuana and K2

Marijuana is the most commonly used illegal drug in the United States. Surveys vary, but in 2015, about 52% of young adults reported using marijuana in their lifetime, and 46% of the U.S. population had tried it at least once. It is derived from the dried flowering tops, leaves, and stems of the marijuana plant, Cannabis sativa, which contain and are covered by a resin that is rich in THC (tetrahydrocannabinol). The names cannabis and marijuana apply to either the plant or THC. Marijuana can be ingested, but usually it is smoked in a cigarette called a “joint.” Beginning with California in 1996, several states have legalized its use for medical purposes, such as in treating cancer, AIDS, and glaucoma. In 2012, Colorado became the first state to legalize recreational use. As of 2018, 8 states had joined Colorado in legalizing recreational use, and 22 additional states had authorized the use of marijuana for medicinal purposes. However, in 2005, the Supreme Court ruled that patients prescribed medical marijuana can still be prosecuted by federal agencies.

Page 302Researchers have found that THC binds to a receptor for anandamide, a naturally occurring neurotransmitter that is important for short-term memory processing, and perhaps for feelings of contentment. The occasional marijuana user experiences mild euphoria, along with alterations in vision and judgment. Heavy use can cause hallucinations, anxiety, depression, paranoia, and psychotic symptoms. Research is underway to identify the effects of long-term marijuana use on the brain, as well as on the effects of secondhand marijuana smoke on the respiratory system.

In recent years, awareness has been increasing about a synthetic compound called K2, or spice. Originally synthesized by an organic chemist at Clemson University, K2 is about ten times as potent as THC. The chemical is typically sprayed onto a mixture of other herbal products and smoked. However, because there is no regulation of how it is produced, the amount of K2 itself, or contaminants, can vary greatly. This may account for the several reports of serious medical problems and even deaths among K2 users.

CHECK YOUR PROGRESS 14.5

Contrast drug therapy and drug abuse.

Answer

Drug therapy is used to treat a disease or disorder. Drug abuse is using drugs without symptoms of disease or disorder.

List how the abuse of drugs, including alcohol and nicotine, affects the nervous system.

Answer

Alcohol and heroin are depressants; nicotine, cocaine, and methamphetamines are stimulants; marijuana produces euphoria.

Detail several modes of action of pharmaceutical and illegal drugs.

Answer

Alcohol increases the action of GABA and increases the release of endorphins in the hypothalamus. Nicotine stimulates dopamine release. Cocaine inhibits dopamine reuptake. Methamphetamine mimics the action of cocaine. Heroin is converted to morphine in the brain and binds to opioid receptors. Marijuana stimulates anandamide receptors.

CONNECTING THE CONCEPTS

For more on the long-term effects of drug use on the systems of the body, refer to the following discussions:

Section 5.7 explores the negative long-term effects of smoking on the cardiovascular system.

Section 11.4 provides information on how alcohol acts as a diuretic in the urinary system.

Section 20.2 examines the relationship between smoking and alcohol use and the increased risk of cancer.

CONCLUSION

The cause of multiple sclerosis (MS) is still unknown, but most researchers agree that there are most likely many contributing factors, including environmental influences, genetics, and a faulty immune system. Many individuals with MS are able to control their symptoms by using immunosuppressive medications, such as beta interferons. The fact that this treatment works suggests that, in many cases, MS is caused by the immune system incorrectly identifying the myelin sheaths as foreign material. The breakdown of the myelin can be detected using both MRI and SSEP tests (discussed in the chapter opener). However, environmental conditions are also suspected to cause MS. Studies have shown that the risk of contracting MS is influenced in part by where in the world you live, although the specific environmental factor or pollutant has not yet been identified. Genetics is also believed to play a role in some cases. But most researchers believe that a defect in a single gene is unlikely. Rather, it is more likely that a certain combination of genetic factors places an individual at a higher risk of contracting MS. Though there is no cure for MS, researchers have been very successful in developing disease-modifying drugs that reduce the symptoms and allow the individual to lead a normal life.

Question 64

has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

Answer 64

This is the human brain

Question 65

Reverse

• white matter. Myelination occurs and white matter develops as a child grows.
• Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech.
• Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area.
• Tracts take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Answer 65

Central White Matter

Question 67

Cerebellum

Answer 67

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

Answer 68

• 2 divisions – Central nervous system (CNS): –Brain and spinal cord –Peripheral nervous system (PNS): Nerves and ganglia (collections of cell bodies)

Question 69

Language and Speech

and Attending Issues

Explore imagery

The functions of the visual cortex, Wernicke’s area, and Broca’s area are shown in Figure 14.14.

Figure 14.14 The areas of the brain involved in reading. These functional images were captured by a high-speed computer during a PET (positron-emission tomography) scan of the brain. A radioactively labeled solution is injected into the subject, and then the subject is asked to perform certain activities. Cross-sectional images of the brain generated by the computer reveal where activity is occurring because the solution is preferentially taken up by active brain tissue and not by inactive brain tissue. These PET images show the cortical pathway for reading words and then speaking them. Red indicates the most active areas of the brain, and blue indicates the least active areas.

Review

Answer 69

1. Language:
   
   • semantic memory;
   • Seeing and hearing words depends on sensory centers in the occipital and temporal lobes, respectively.
     
     1. Damage to Wernicke’s area: the inability to comprehend speech.
     2. Damage to Broca’s area: the inability to speak and write.
     3. *
     4. re: language & speech: left brain and the right brain may have different functions.
     • Recall that the left hemisphere contains both Broca’s area and Wernicke’s area (see Section 14.2).
     • As you might expect, it appears that the left hemisphere plays a role of great importance in language functions.
     • The role of the isolated left hemisphere can be studied in patients after surgery to sever the corpus callosum. This procedure is used for seizure control in patients with epilepsy. After surgery, the patient is termed “split brain,” because there is no longer direct communication between the two cerebral hemispheres. If a split-brain individual views an object with only the right eye, its image will be sent only to the right hemisphere. This person will be able to choose the proper object for a particular use—scissors to cut paper, for example—but will be unable to name that object.
     • • Researchers now believe that the hemispheres process the same information differently. The left hemisphere is more global, whereas the right hemisphere is more specific in its approach. However, research also indicates that the classification of “right-brained” versus “left-brained” for individuals is probably not an accurate indication of an individual’s brain activity

Question 70

Reverse

What is an interneuron?

Answer 70

lies entirely within the CNS. Interneurons can receive input from sensory neurons and from other interneurons in the CNS. Thereafter, they sum up all the information received from other neurons before they communicate with motor neurons.

Question 71

reverse.prompt

What are neuromodulators?

Answer 71

• naturally occurring molecules that block the release of a neurotransmitter or modify a neuron’s response to a neurotransmitter.
• Two well-known neuromodulators are :

1. Substance P: neuropeptide that is released by sensory neurons when pain is present.
2. Endorphins.

*

Question 72

reversedprompt

• The brainstem
  
  1. Midbrain – relay station between the cerebrum and spinal cord or cerebellum; reflex center
  2. Pons – a bridge between cerebellum and the CNS; regulates breathing rate; reflex center for head movements
  3. Medulla oblongata – contains reflex centers for regulating breathing, heartbeat, and blood pressure

Answer 72

Brain

Brain Stem

14.2 Lecture notes

Question 73

reversedprompt

a. A sensory neuron has a long axon covered by a myelin sheath that takes nerve impulses all the way from dendrites to the CNS. b. In the CNS, some interneurons, such as this one, have a short axon that is not covered by a myelin sheath. c. A motor neuron has a long axon covered by a myelin sheath that takes nerve impulses from the CNS to an effector.

Answer 73

See, download, review and study the following diagrams:

Figure 14.3 The structure of sensory neurons, interneurons, and motor neurons.

Question 74

reversedprompt

difference in electron potential inside and outside of axon of neuron

across membrane about 40 …, inside the axon is negative- resting potential- no impulse -

structure of membrane- cell membranes channel proteins- neurons- 2 channels Na + or K+ channel- resting - Sodium is greater, outside than inside, and K is greater inside than outside

3 sodium out for every 2 K into the cell

when nerve impulse or action potential reaches center of membrane- stiumuls causes membraine to depolarize-

action potential- all or none event- if depolarize certain level- threshold- action potential occurs

gates open first, potential begins, sodium float into axon- positive charge- once moved- membrane potental- -70 to +35, threshold- overcomes, all or nothing event- depolarization- sudden rush of sodium axons into-

negative to positive,

K- channels open- float down to oustide of axon- concentration gradient- also + = repolarization- resumes negative charge as potassium exits- travels down axon- one membrane at a time- depolarization- stiumulus to neighboring - sections of membrane

refractory period- sodium gates- unpoen- action potential can’t move backwards and always moves in same direction

• K and Na pump- restores previous ion distribution Na outside and K inside- each small segment depolarization and repolarization takes just a few miliseconds, ready to transmit another action potential

Answer 74

Action potential animation

Answer 75

Time to download all photographs, review slides, do PLQ again and review adaptive learning along with flash cards and making study guide for Chapter Nervous System!

Question 76

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

Answer 76

(Fig. 14.8c, d)

Question 77

The nerves in the somatic system serve the skin, skeletal muscles, and tendons (see Fig. 14.2). The somatic system sensory nerves take sensory information from external sensory receptors to the CNS. Motor commands leaving the CNS travel to skeletal muscles via somatic motor nerves.

Not all somatic motor actions are voluntary. Some are automatic. Automatic responses to a stimulus in the somatic system are called reflexes. A reflex occurs quickly, without your even having to think about it. For example, a reflex may cause you to blink your eyes in response to a flash of light, without your willing it. We will study the path of a reflex, because it allows us to study in detail the path of nerve signals to and from the CNS.Page 296

The Reflex Arc

Figure 14.17 illustrates the path of a reflex that involves only the spinal cord. If your hand touches a sharp pin, sensory receptors in the skin generate nerve signals that move along sensory fibers through the posterior (dorsal) root ganglia toward the spinal cord. Sensory neurons that enter the cord posteriorly pass signals on to many interneurons. Some of these interneurons synapse with motor neurons whose short dendrites and cell bodies are in the spinal cord. Nerve signals travel along these motor fibers to an effector, which brings about a response to the stimulus. In this case, the effector is a muscle, which contracts so that you withdraw your hand from the pin. Various other reactions are also possible—you will most likely look at the pin, wince, and cry out in pain. This whole series of responses occurs because some of the interneurons involved carry nerve signals to the brain. The brain makes you aware of the stimulus and directs these other reactions to it. In other words, you don’t feel pain until the brain receives the information and interprets it.

Question 78

Propogation of an Action Potential

Answer 78

Propagation of an Action Potential

If an axon is unmyelinated, an action potential at one locale stimulates an adjacent part of the axon membrane to produce an action potential. Conduction along the entire axon in this fashion can be rather slow—approximately 1 meter/second (1 m/s) in thin axons—because each section of the axon must be stimulated.

Action Potential Propagation

In myelinated fibers, an action potential at one node of Ranvier causes an action potential at the next node, jumping over the entire myelin-coated portion of the axon. This type of conduction is called saltatory conduction (saltatio is a Latin word that means “to jump”) and is much faster. In thick, myelinated fibers, the rate of transmission is more than 100 m/s. Regardless of whether an axon is myelinated or not, its action potentials are self-propagating. Each action potential generates another, along the entire length of the axon.

Like the action potential itself, conduction of an action potential is an all-or-none event—either an axon conducts its action potential or it does not. The intensity of a message is determined by how many action potentials are generated within a given time. An axon can conduct a volley of action potentials very quickly, because only a small number of ions are exchanged with each action potential. Once the action potential is complete, the ions are rapidly restored to their proper place through the action of the sodium–potassium pump.

Neural Transmission: Action Potential Propagation

As soon as the action potential has passed by each successive portion of an axon, that portion undergoes a short refractory period, during which it is unable to conduct an action potential. This ensures the one-way direction of a signal from the cell body down the length of the axon to the axon terminal.

It is interesting to note that all functions of the nervous system, from our deepest emotions to our highest reasoning abilities, are dependent on the conduction of nerve signals.

Question 79

CHECK YOUR PROGRESS 14.3

Summarize the function of the limbic system.

Answer 79

A group of brain structures that blends primitive emotions and higher mental functions into a united whole.

Question 80

Reversed prompt

1. In common:

• (1) They usually function in an involuntary manner;
• (2) they innervate all internal organs; and
• (3) they use two neurons and one ganglion for each impulse.
  
  • The first neuron has a cell body within the CNS and a preganglionic fiber that enters the ganglion.
  • The second neuron has a cell body within a ganglion and a postganglionic fiber that leaves the ganglion.

1. Reflex actions: such as those that

• regulate blood pressure
• and breathing rate,
• are especially important to the maintenance of homeostasis.

1. These reflexes begin when the sensory neurons in contact with internal organs send messages to the CNS.
2. They are completed by motor neurons within the autonomic system.

Answer 80

CNS

Autonomic Nervous System

Sympathic Nervous System and Parasympathetic Nervous System

Question 81

14.4 Peripheral Nervous System Learning Outcomes

Answer 81

14.4 The Peripheral Nervous System

LEARNING OUTCOMES

Upon completion of this section you should be able to

Describe the series of events during a spinal reflex.

Distinguish between the somatic and autonomic divisions of the peripheral nervous system.

Distinguish between the sympathetic and parasympathetic divisions of the autonomic division.

Question 82

reversedprompt

• The resting potential energy of the neuron can be used to perform the work of the neuron: conduction of nerve signals.
• The process of conduction is termed an action potential, and it occurs in the axons of neurons:

1. A stimulus activates the neuron and begins the action potential.
   
   • For example, a stimulus for pain neurons in the skin would be the prick of a sharp pin.
   • However, the stimulus must be strong enough to cause the cell to reach threshold, the voltage that will result in an action potential. In Figure 14.4b, the threshold voltage is around −55 mV.

• An action potential is an all-or-nothing event.
  
  • ​Once threshold is reached, the action potential happens automatically and completely. On the other hand, if the threshold voltage is never reached, the action potential does not occur. Increasing the strength of a stimulus (such as pressing harder with the pin) does not change the strength of an action potential. However, it may cause more action potentials to occur in a given period. As a result, the person may perceive that pain has increased.

Answer 82

This is action potential of a neuron.

Another word for action potential is:

List the steps in the process of action potential:

See, Download and Study Figure 14.4b

Question 83

Reverse

Overview of brain and

Cerebrum with its 4 lobes and their functions:

Slides 14.2

Lecture notes

14.2 The Central Nervous System 15 1.

Upload/Download and Study image of brain

Answer 83

1. Cerebrum •
   
   • • The cerebrum is the largest portion of the brain •
   • Divided into 4 lobes:
   • 1. Frontal lobe: primary motor area and conscious thought
   • 2. Temporal lobe: primary auditory, smell, and speech area
   • 3. Parietal lobe: primary somatosensory and taste area
   • 4. Occipital lobe: primary visual area
   • • 14.2 The Central Nervous System 16 1.
   • 2. Cerebral hemispheres •
2. Cerebral cortex •
3. Primary motor and sensory areas of the cortex •
4. Association areas •
5. Processing centers •
6. Central white matter
   8.

Question 84

reversedprompt

Processing Centers

in the Cortex

(see Fig. 14.10). - download and see

Answer 84

• 1. receive information from the other association areas
  2. and perform higher-level analytical functions.
• The prefrontal area,
  
  • ​ frontal lobe, receives information from the other association areas and uses to reason and plan our actions.
• Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.
• The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex.
  
  1. Wernicke’s area is located in the posterior part of the left temporal lobe: helps us understand both the written and the spoken word and sends the information to Broca’s area.
  2. Broca’s area is located in the left frontal lobe.
  • anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth)
  • adds grammatical refinements
  • and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

1.

Question 85

reverse.prompt

• outside the central nervous system
• contains the nerves.
  
  1. cranial nerves: brain and are termed
  2. spinal nerves: spinal cord.
  3. all nerves carry signals to and from the CNS.
  • So right now, your eyes are sending messages by way of a cranial nerve to the brain, allowing you to read this text.
  • When you’re finished, your brain will direct the muscles in your fingers, by way of the spinal cord and a spinal nerve, to proceed to the next chapter.
    *

Answer 85

Peripheral Nervous System (PNS)

• Figure 14.15 illustrates the anatomy of a nerve. The cell body and the dendrites of neurons are in either the CNS or the ganglia. Ganglia (sing., ganglion) are collections of nerve cell bodies outside the CNS. The axons of neurons project from the CNS and form the spinal cord. In other words, nerves, whether cranial or spinal, are composed of axons, the long part of neurons.

Figure 14.15 The structure of a nerve. The peripheral nervous system consists of the cranial nerves and the spinal nerves. A nerve is composed of bundles of axons separated from one another by connective tissue.

** Download, review and look

Question 86

reverse.prompt

• evolutionary ancient group of linked structures deep within the cerebrum.
• It is a functional grouping rather than an anatomical one

1. blends primitive emotions w/
2. higher mental functions into a united whole.
   
   • sexual behavior and eating seem pleasurable
   • unpleasant sensations or emotions (pain, frustration, hatred, despair) are translated by the limbic system into a stress response.

• Structurally: 2 significant
  
  1. amygdala
  • ​ particular emotional overtones, creating sensation of fear.
  • use past knowledge fed to it by association areas to assess a current situation.
  • if necessary, trigger fight-or-flight reaction
    
    • So if you are out late at night and you turn to see someone in a ski mask following you, the amygdala may immediately cause you to start running.
    • The frontal cortex can override the limbic system, cause you to rethink the situation, and prevent you from acting out strong reactions

2. The hippocampus

• learning and memory
• information gateway: determines what information about the world is to be sent to memory and how this information is to be encoded and stored by other regions in the brain.
• Most likely, the hippocampus can communicate with the frontal cortex, because we know that memories are an important part of our decision-making processes.

Answer 86

Limbic System (14.2)

Function

Structurally

(Fig. 14.13) Page 293

The regions of the brain associated with the limbic system. In the limbic system (purple), structures deep within each cerebral hemisphere and surrounding the diencephalon join higher mental functions, such as reasoning, with more primitive feelings, such as fear and pleasure. Therefore, primitive feelings can influence our behavior, but reason can also keep them in check.

Question 87

• means of communication between
  
  • the brain and the
  • peripheral nerves that leave the cord.
    
    • When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Answer 87

Functions of the Spinal Cord

(see Fig. 14.2b, red arrows).

Question 88

Primary Motor and Sensory Area of the Cerebral Cortex

See and download Figure 14.11, Page 290

Answer 88

1. contains motor areas and sensory areas
2. association areas

• The primary motor area is in the

1. frontal lobe just anterior to (before) the central sulcus
   
   1. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section
   2. (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements.
      
      1. ​Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

Question 89

reversedprompt

The Synapse

Axon terminal

See, download study Figure 14.5

See and Study Figure 14.5 Signal transmission at the synapse. Transmission across a synapse from one neuron to another occurs when a neurotransmitter is released, diffuses across a synaptic cleft, and binds to a receptor in the membrane of the receiving neuron.

• Tutorial: Synaptic Cleft
• Transmission across a synapse is carried out by molecules called neurotransmitters, stored in synaptic Page 285vesicles in the axon terminals. (See Section 3.4 for a review of vesicle function.) The events (Fig. 14.5) at a synapse are (1) nerve signals traveling along an axon to reach an axon terminal; (2) calcium ions entering the terminal and stimulating synaptic vesicles to merge with the sending membrane; and (3) neurotransmitter molecules releasing into the synaptic cleft and diffusing across the cleft to the receiving membrane; there, neurotransmitter molecules bind with specific receptor proteins.

Figure 14.6 Integration of excitatory and inhibitory signals at the synapse. a. Inhibitory signals and excitatory signals are summed up in the dendrite and cell body of the postsynaptic neuron. Only if the combined signals cause the membrane potential to rise above threshold does an action potential occur. b. In this example, threshold was not reached.

Answer 89

• axon branches into many fine endings, each tipped by a small swelling called an axon terminal.
  
  • Each terminal lies very close to either the dendrite or the cell body of another neuron, area called synapse (Fig. 14.5). At a synapse, a small gap called the synaptic cleft
    
    1. separates the sending neuron from the receiving neuron. The nerve signal is unable to jump the cleft. Therefore:
    2. The nerve signal is unable to jump the cleft. Therefore, another means is needed to pass the nerve signal from the sending neuron to the receiving neuron.

Question 90

Reverse

Peripheral Nervous System

Answer 90

The peripheral nervous system (PNS) consists of nerves. Nerves lie outside the CNS. The division between the CNS and the PNS is arbitrary. The two systems work together and are connected to each other (Fig. 14.2).

Question 91

Summarize the major regions of the brain and describe the general function of each.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Answer 91

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Question 92

Reverse

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Answer 92

Spinal Cord

Question 93

Neurotransmitter Molecules

Answer 93

Neurotransmitter Molecules

More than 100 substances are known or suspected to be neurotransmitters. Some of the more common ones in humans are acetylcholine, norepinephrine, dopamine, serotonin, glutamate, and GABA (gamma aminobutyric acid). Neurotransmitters transmit signals between nerves. Nerve-muscle, nerve-organ, and nerve-gland synapses also communicate using neurotransmitters.

Acetylcholine (ACh) and norepinephrine are active in both the CNS and PNS. In the PNS, these neurotransmitters act at synapses called neuromuscular junctions. We will explore the structure of the neuromuscular junctions in Section 13.2.

In the PNS, ACh excites skeletal muscle but inhibits cardiac muscle. It has either an excitatory or inhibitory effect on smooth muscle or glands, depending on their location.

Norepinephrine generally excites smooth muscle. In the CNS, norepinephrine is important to dreaming, waking, and mood. Serotonin is involved in thermoregulation, sleeping, emotions, and perception. Many drugs that affect the nervous system act at the synapse. Some interfere with the actions of neurotransmitters, and other drugs prolong the effects of neurotransmitters (see Section 14.5).

Question 94

These are the differences amongst neuron types insofar as structures

Answer 94

1. In interneurons and motor neurons
   
   • multiple dendrites take signals to the cell body, and then an
   • axon conducts nerve signals away from the cell body.
2. ​In sensory neurons,

• a very long axon carries nerve signals from the dendrites associated with a sensory receptor to the CNS, and this
• axon is interrupted by the cell body.

Question 95

Limbic System 14.3

Answer 95

1. integrates our emotions (fear, joy, sadness)
2. with our higher mental functions (reason, memory).
3. Because of the limbic system, activities such as sexual behavior and eating seem pleasurable, and mental stress can cause high blood pressure.

Question 96

Reverse

What is a motor neuron?

Answer 96

This neuron takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland). Effectors carry out our responses to environmental changes, whether these are external or internal.

Question 97

An exchange of Na+ and K+ ions

Answer 97

1. generates an action potential
   * that moves along the length of an axon.
2. An action potential in one location

• stimulates the production of an action potential in an adjacent part of the axon membrane.
• If the nerve is myelinated, the action potential moves more quickly, “jumping” from one node of Ranvier to the next.

Question 98

reversedprompt

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

1. largest portion of the brain in mammals, including humans.
2. last center to receive sensory input and carry out integration before commanding voluntary motor responses.
3. It communicates with and coordinates the activities of the other parts of the brain.

Answer 98

This part of the brain is the largest and this is what it’s function is

Question 99

reversedprompt

1. communication between the brain and the peripheral nerves that leave the cord.

• When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).
• The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

1. The brain coordinates the voluntary control of our limbs.
2. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows).

• Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.
• Page 288*

1. Reflex Actions
   
   • The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17).
   • A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord.
   • Interneurons integrate the incoming data and relay signals to motor neurons.
   • A response to the stimulus occurs when motor axons cause skeletal muscles to contract.
   • Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.
   • Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

Answer 99

The Spinal Cord has these functions:

These can result from severed spinal cords

Answer 100

14.3 The Limbic System and Higher Mental Functions

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the limbic system.

Explain how the limbic system is involved in memory, language, and speech.

Summarize the types of memory associated with the limbic system.

The limbic system integrates our emotions (fear, joy, sadness) with our higher mental functions (reason, memory). Because of the limbic system, activities such as sexual behavior and eating seem pleasurable, and mental stress can cause high blood pressure.

Limbic System

The limbic system is an evolutionary ancient group of linked structures deep within the cerebrum. It is a functional grouping rather than an anatomical one (Fig. 14.13). The limbic system blends primitive emotions and higher mental functions into a united whole. As already noted, it accounts for why activities such as sexual behavior and eating seem pleasurable. Conversely, unpleasant sensations or emotions (pain, frustration, hatred, despair) are translated by the limbic system into a stress response.

Figure 14.13 The regions of the brain associated with the limbic system. In the limbic system (purple), structures deep within each cerebral hemisphere and surrounding the diencephalon join higher mental functions, such as reasoning, with more primitive feelings, such as fear and pleasure. Therefore, primitive feelings can influence our behavior, but reason can also keep them in check.

Two significant structures in the limbic system are the amygdala and the hippocampus. The amygdala, in particular, can cause experiences to have emotional overtones, and it creates the sensation Page 293of fear. This center can use past knowledge fed to it by association areas to assess a current situation. If necessary, the amygdala can trigger the fight-or-flight reaction. So if you are out late at night and you turn to see someone in a ski mask following you, the amygdala may immediately cause you to start running. The frontal cortex can override the limbic system, cause you to rethink the situation, and prevent you from acting out strong reactions.

The hippocampus is believed to play a crucial role in learning and memory. The hippocampal region acts as an information gateway during the learning process. It determines what information about the world is to be sent to memory and how this information is to be encoded and stored by other regions in the brain. Most likely, the hippocampus can communicate with the frontal cortex, because we know that memories are an important part of our decision-making processes.

Higher Mental Functions

As in other areas of biological research, brain research has progressed due to technological breakthroughs. Neuroscientists now have a wide range of techniques at their disposal for studying the human brain, including modern technologies that allow us to record its functioning.

Memory and Learning

Just as the connecting tracts of the corpus callosum are evidence that the two cerebral hemispheres work together, so the limbic system indicates that cortical areas may work with lower centers to produce learning and memory. Memory is the ability to hold a thought in mind or to recall events from the past, ranging from a word we learned only yesterday to an early emotional experience that has shaped our lives. Learning takes place when we retain and use past memories.

Types of Memory

We have all tried to remember a seven-digit telephone number for a short time. If we say we are trying to keep it in the forefront of our brain, we are exactly correct. The prefrontal area, active during short-term memory, lies just posterior to our forehead! There are some telephone numbers that we have memorized. In other words, they have gone into long-term memory. Think of a telephone number you know by heart, and try to bring it to mind without also thinking about the place or person associated with that number. Most likely you cannot. Typically, long-term memory is a mixture of what is called semantic memory (numbers, words, etc.) and episodic memory (persons, events, etc.).

Skill memory is another type of memory that can exist independent of episodic memory. Skill memory is involved in performing motor activities such as riding a bike or playing ice hockey. When a person first learns a skill, more areas of the cerebral cortex are involved than after the skill is perfected. In other words, you have to think about what you are doing when you learn a skill, but later the actions become automatic. Skill memory involves all the motor areas of the cerebrum below the level of consciousness.

Long-Term Memory Storage and Retrieval

Our long-term memories are apparently stored in bits and pieces throughout the sensory association areas of the cerebral cortex. Visual perceptions are stored in the vision association area, sounds are stored in the auditory association area, and so forth. As previously mentioned, the hippocampus serves as a bridge between the sensory association areas (where memories are stored) and the prefrontal area (where memories are used). The prefrontal area communicates with the hippocampus when memories are stored and when these memories are brought to mind. Some memories are emotionally charged, because the amygdala seems to be responsible for fear conditioning and associating danger with sensory stimuli received from various parts of the brain.Page 294

SCIENCE IN YOUR LIFE

What is amnesia?

Amnesia results from disruption of the memory pathways and can be temporary or permanent. In anterograde amnesia, injury to the limbic system separates long-term memories of events that occurred prior to the injury from events that occur in the here and now. An affected person might carry on a conversation about past events (memories of a long-ago birthday) but be unable to recall a breakfast menu from that morning. In retrograde amnesia, a blow to the head or similar injury abolishes all memories for a variable time before the injury. For example, a head injury occurring during a car accident may abolish all memories from hours to days prior to the accident.

Long-Term Potentiation

Though it is helpful to know the memory functions of various portions of the brain, an important step toward curing mental disorders is understanding memory on the cellular level. After synapses have been used intensively for a short time, they release more neurotransmitters than before. This phenomenon, called long-term potentiation, may be involved in memory storage.

Language and Speech

Language depends on semantic memory. Therefore, we would expect some of the same areas in the brain to be involved in both memory and language. Any disruption of these pathways could contribute to an inability to comprehend our environment and use speech correctly.

Seeing and hearing words depends on sensory centers in the occipital and temporal lobes, respectively. Damage to Wernicke’s area (see Section 14.2) results in the inability to comprehend speech. Damage to Broca’s area, on the other hand, results in the inability to speak and write. The functions of the visual cortex, Wernicke’s area, and Broca’s area are shown in Figure 14.14.

Figure 14.14 The areas of the brain involved in reading. These functional images were captured by a high-speed computer during a PET (positron-emission tomography) scan of the brain. A radioactively labeled solution is injected into the subject, and then the subject is asked to perform certain activities. Cross-sectional images of the brain generated by the computer reveal where activity is occurring because the solution is preferentially taken up by active brain tissue and not by inactive brain tissue. These PET images show the cortical pathway for reading words and then speaking them. Red indicates the most active areas of the brain, and blue indicates the least active areas.

(all): ©Marcus Raichle

One interesting aside pertaining to language and speech is the recognition that the left brain and the right brain may have different functions. Recall that the left hemisphere contains both Broca’s area and Wernicke’s area (see Section 14.2). As you might expect, it appears that the left hemisphere plays a role of great importance in language functions. The role of the isolated left hemisphere can be studied in patients after surgery to sever the corpus callosum. This procedure is used for seizure control in patients with epilepsy. After surgery, the patient is termed “split brain,” because there is no longer direct communication between the two cerebral hemispheres. If a split-brain individual views an object with only the right eye, its image will be sent only to the right hemisphere. This person will be able to choose the proper object for a particular use—scissors to cut paper, for example—but will be unable to name that object.

Researchers now believe that the hemispheres process the same information differently. The left hemisphere is more global, whereas the right hemisphere is more specific in its approach. However, research also indicates that the classification of “right-brained” versus “left-brained” for individuals is probably not an accurate indication of an individual’s brain activity.

CHECK YOUR PROGRESS 14.3

Summarize the function of the limbic system.

Answer

A group of brain structures that blends primitive emotions and higher mental functions into a united whole.

List what limbic system structures are involved in the fight-or-flight reaction, learning, and long-term memory.

Answer

Amygdala—fight-or-flight; hippocampus—learning and memory. The hippocampus acts as a bridge between the sensory association areas of the cerebral cortex where memories are stored long term and the prefrontal areas of the cortex where memories are used.

Describe the relationship between the left and right sides of the brain and language and speech.

Answer

Wernicke’s area and Broca’s area in the left hemisphere are related to speech, comprehension, and writing. The right hemisphere is associated with more nonverbal and creative functions.

CONNECTING THE CONCEPTS

For more information on these topics, refer to the following discussion:

Section 18.5 examines the effects of aging on the body, including the nervous system.

Question 101

Arachnoid mater • Middle layer • Web-like • Pia mater • Internal layer • Clings to the surface of the brain • Many blood vessels

Question 102

Reverse

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

Answer 102

Reflexes

Question 103

14.2 The Central Nervous System 22 2. The brain: Diencephalon Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. skull meninges pituitary gland fourth ventricle spinal cord Cerebrum Diencephalon Cerebellum hypothalamus midbrain pons Brain stem a. Parts of brain b. Cerebral hemispheres lateral ventricle third ventricle pineal gland corpus callosum thalamus (surrounds the third ventricle) medulla oblongata Figure 14.8 The human brain.

Answer 103

Check out the pic

Question 105

Reverse

14.4 The Peripheral Nervous System

LEARNING OUTCOMES

Upon completion of this section you should be able to

Describe the series of events during a spinal reflex.

Distinguish between the somatic and autonomic divisions of the peripheral nervous system.

Distinguish between the sympathetic and parasympathetic divisions of the autonomic division.

Answer 105

14.4 Peripheral Nervous System Learning Outcomes

Question 106

Reverse

role in CNS

Answer 106

Neurons are best classified according to:

Answer 107

14.4 The Peripheral Nervous System

LEARNING OUTCOMES

Upon completion of this section you should be able to

Describe the series of events during a spinal reflex.

Distinguish between the somatic and autonomic divisions of the peripheral nervous system.

Distinguish between the sympathetic and parasympathetic divisions of the autonomic division.

Question 108

Reverse

Association Areas

Answer 108

• integration occurs and
• where memories are stored.
• Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time.
• Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs.
• The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Question 109

reverse.prompt

14.2 The Central Nervous System 23 3. The brain: Cerebellum • Receives and integrates sensory input from the eyes, ears, joints, and muscles about the current position of the body • Functions • Maintains posture • Coordinates voluntary movement • Allows learning of new motor skills (i.e., playing the piano or hitting a baseball)

Question 110

Reverse

reception and processing of sensory information from both the external and the internal environments.

The nervous system hao major divisions.s tw The major structures of the nervous system are shown in Figure 14.1.

Answer 110

The nervous system

Question 111

Diencephalon

Answer 111

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

Question 112

Limbic System

Answer 112

• amygdala hippocampus olfactory bulb olfactory tract hypothalamus corpus thalamus callosum Figure 14.12 The regions of the brain associated with the limbic system. 14.3 The Limbic System and Higher Mental Functions 28 • Learning – what happens when we recall and use past memories • Memory – ability to hold a thought or to recall past events • Short-term memory – retention of information for only a few minutes 14.3 The Limbic System and Higher Mental Functions Higher mental functions 29 Higher mental functions • Long-term memory – retention of i

Question 113

Reverse

Overall: Master controlling and communicating system of the body

1. Sensory input – gathering information : To monitor changes (stimuli) occurring inside and outside the body
2. Integration : To process and interpret sensory input and decide if action is needed
3. Motor output : A response to integrated stimuli: the response activates muscles or glands

Answer 113

3 functions of the nervous systen

14.1 Overview of Nervous System Slides

Look at them and review diagrams

Question 114

Reverse

somatic system

Answer 114

The PNS has divisions: the somatic system and the autonomic system. The nerves in the somatic system serve the skin, skeletal muscles, and tendons (see Fig. 14.2). The somatic system sensory nerves take sensory information from external sensory receptors to the CNS. Motor commands leaving the CNS travel to skeletal muscles via somatic motor nerves.

Not all somatic motor actions are voluntary. Some are automatic. Automatic responses to a stimulus in the somatic system are called reflexes. A reflex occurs quickly, without your even having to think about it. For example, a reflex may cause you to blink your eyes in response to a flash of light, without your willing it. We will study the path of a reflex, because it allows us to study in detail the path of nerve signals to and from the CNS.Page 296

Question 116

1. , which lies outside the central nervous system, contains the nerves.
2. Nerves are designated as cranial nerves when they arise from the brain and are termed spinal nerves when they arise from the spinal cord. In any case, all nerves carry signals to and from the CNS. So right now, your eyes are sending messages by way of a cranial nerve to the brain, allowing you to read this text. When you’re finished, your brain will direct the muscles in your fingers, by way of the spinal cord and a spinal nerve, to proceed to the next chapter.
3. Figure 14.15 illustrates the anatomy of a nerve. The cell body and the dendrites of neurons are in either the CNS or the ganglia. Ganglia (sing., ganglion) are collections of nerve cell bodies outside the CNS. The axons of neurons project from the CNS and form the spinal cord. In other words, nerves, whether cranial or spinal, are composed of axons, the long part of neurons.

Answer 116

The peripheral nervous system (PNS)

Question 117

reversedprompt

CHECK YOUR PROGRESS 14.1

Sensory neurons take nerve signals from a sensory receptor to the CNS. Interneurons lie entirely within the CNS and communicate with other neurons. Motor neurons move nerve impulses away from the CNS to an effector. The parts are cell body, dendrites, and axon.

Answer 117

Describe the three types of neurons, and list the three main parts of a neuron.

Question 118

Reverse

Oligodendrocytes – branched cells that wrap CNS nerve fibers – Produce fatty insulating coverings (myelin sheath) around nerve fibers in the CNS – Can coil around as many as 60 different fibers at one time

Question 119

This is what sensory neuron does, taking signals from a sensory ________________ to the _____________. They are special structures that ________________________.

Answer 119

Classified according to function, the three types of neurons are sensory neurons, interneurons, and motor neurons (Fig. 14.3). Their functions are best described relative to the CNS. A sensory neuron takes nerve signals from a sensory receptor to the CNS. Sensory receptors are special structures that detect changes in the environment.

Question 120

• Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft.
• In some synapses, the receiving membrane contains enzymes that rapidly inactivate the neurotransmitter.
  
  • For example, the enzyme acetylcholinesterase (AChE) breaks down the neurotransmitter acetylcholine.
• In other synapses, the sending membrane rapidly reabsorbs the neurotransmitter, possibly for repackaging in synaptic vesicles or for molecular breakdown.
• The short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of receiving membranes.
• The receiving cell needs to be able to respond quickly to changing conditions.
• If the neurotransmitter were to linger in the cleft, the receiving cell would be unable to respond to a new signal from a sending cell.
• Neural Transmission: Synapse
  *

Answer 120

Last steps after Neurotransmitter released in Synaptic Cleft-

** Follow process beginning to end **

List steps and print out and understand photos

Question 121

reverse.prompt

The central nervous system • The CNS consists of the brain and spinal cord. •

Both are protected by •

Scalp and skin • Bones – skull and vertebral column • Meninges – 3 protective membranes that wrap around CNS • Cerebral spinal fluid (CSF) – space between meninges is filled with this fluid that cushions and protects the CNS • Blood brain barrier (BBB)

Question 122

1. a thin, highly convoluted outer layer of _____________ that covers the cerebral hemispheres.
2. _______________consists of neurons whose axons are not_________________
3. over 1 billion cell bodies
4. **accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness**
   5.

Answer 122

The Cerebral Cortex, gray matter, myelinated.

Question 123

reversedprompt

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

Answer 123

Relate how the RAS aids in homeostasis.

Question 124

reverse.prompt

Drugs

Answer 124

1. As you are reading these words, synapses throughout your brain are organizing, integrating, and cataloging the information you take in.
2. Neurotransmitters at these synapses control the firing of countless action potentials, thus creating a network of neural circuits.
3. It is amazing to realize that all thoughts, feelings, and actions of a human are dependent on neurotransmitters in the CNS and PNS.
4. By modifying or controlling synaptic transmission, a wide variety of drugs with neurological activity, both legal pharmaceuticals and illegal drugs of abuse, can alter mood, emotional state, behavior, and personality.

Answer 126

Identify the structures of the brain and provide a function for each.

Question 128

reverse.prompt

A group of brain structures that blends primitive emotions and higher mental functions into a united whole.

Answer 128

CHECK YOUR PROGRESS 14.3

Summarize the function of the limbic system.

Question 129

How does transmissions across a synapse occur?

** Steps ** Processe

Answer 129

1. Nerve impulse reaches the axon terminal.
2. Calcium ions enter the axon terminal and stimulate the synaptic vesicles to fuse with the presynaptic membrane; the axon terminal membrane of the first neuron.
3. Neurotransmitters are released and diffuse across the synapse, where they bind with the postsynaptic membrane;
4. the dendrite/cell body membrane on the second neuron to inhibit or excite the neuron.

Question 130

The brain and its component parts

Answer 130

1. 1. The brain: Cerebrum
2. • Cerebral hemispheres
3. • Cerebral cortex •
4. Primary motor and sensory areas of the cortex •
5. Association areas •
6. Processing centers •
7. Central white matter

• 14.2 The Central Nervous System 15
  *

Question 131

The PNS: Somatic division • The somatic system serves the skin, skeletal muscles and tendons. • Automatic responses are called reflexes. • Reflexes consist of sensory receptorsensory neuron interneuron  motor neuron effector organ (we talked about this in 14.1)

Question 132

reverse.prompt

• Prefrontal Cortex
• “CEO of the brain”
• Where you control and plan your actions
• Working memory
• Organization
• Modulate your mood
• Conscience
• Personality • Not fully developed until at least 25 years of age– maybe even later! (You can blame your bad decisions on this if you are younger than this– ha!– or flip it around: drugs, alcohol, excessive videogaming, etc. can really have a permanent negative impact on this developing brain area even if you are of ‘legal age’…)

Answer 132

CNS 14.2 Lecture Notes

This is called the CEO of the brain and controls working memory.

Question 133

reversedprompt

1. A single neuron has a cell body and may have many dendrites (Fig. 14.6a). All can have synapses with many other neurons. Therefore, a neuron is on the receiving end of many signals,which can either be excitatory or inhibitory.
2. Recall that an excitatory neurotransmitter produces an excitatory signal by opening sodium gates at a synapse. This drives the neuron closer to its threshold. If threshold is reached, an action potential is inevitable. On the other hand, an inhibitory neurotransmitter drives the neuron farther from an action potential (red line in Fig. 14.6b) by opening the gates for potassium.

Neurons integrate these incoming signals. Integration is the summing up of excitatory and inhibitory signals. If a neuron receives enough excitatory signals (either from different synapses or at a rapid rate from a single synapse) to outweigh the inhibitory ones, chances are the axon will transmit a signal. On the other hand, if a neuron receives more inhibitory than excitatory signals, summing these signals may prohibit the axon from reaching threshold and then depolarizing (the solid black line in Fig. 14.6b).

Answer 133

Synaptic Integration

Page 286

(illustrated by the green line in Fig. 14.6b)

What is integration?

What are excitatory and inhibitory signals?

Answer 134

Review slides 14.2:

The brain: Cerebrum – The cerebral cortex Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. salivation vocalization mastication longitudinal fissure facial expression swallowing thumb, fingers, and hand forearm arm trunk pelvis thigh leg foot and toes lips upper face

Question 135

Reverse

1. brain
2. spinal cord. •

• Both are protected by
• • Scalp and skin •
• Bones – skull and vertebral column •
• Meninges – 3 protective membranes that wrap around CNS
• • Cerebral spinal fluid (CSF) – space between meninges is filled with this fluid that cushions and protects the CNS
• • Blood brain barrier (BBB)
  
  • 2 Meninges •
  • Dura mater •
  • Double-layered external covering
  • • Periosteum – dense connective tissue attached to surface of the skull • Meningeal layer – outer covering of the brain
  • • Folds inward in several areas
  • The Central Nervous System: 3 Meninges •
  • Arachnoid mater • Middle layer •
  • Web-like • Pia mater •
  • Internal layer • Clings to the surface of the brain •
  • Many blood vessels

Answer 135

The central nervous system •

Protection for the system

Slides fron Class

14.2 The Central Nervous System

Supplemental material not in book 14.2

• http://droualb.faculty.mjc.edu/Lecture%20Notes/Unit%205/Meninges_peeled_away.jpg
• Supplemental material not in book 14.2

Question 136

Reverse

2. The brain: Diencephalon • Includes the • Hypothalamus – helps maintain homeostasis (hunger, sleep, thirst, body temperature, and water balance) and controls pituitary gland • Thalamus – 2 masses of gray matter that receive all sensory input except smell; involved in memory and emotions • Pineal gland – secretes melatonin, the hormone that controls our daily rhythms

Question 137

Science in your life? Strokes

Answer 137

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Question 138

Nervous Tissue cells:

The three types of ______________________ which are classified by function

The types of _________________________ which are _________________ abundant than ____________.

(Fig. 14.3).

Answer 138

two types of cells:

1. neurons: cells that transmit nerve impulses between parts of the nervous system: Classified according to function in CNS

• 3 types: functions relative to CNS
  
  1. sensory neurons: takes nerve signals from a sensory receptor– special structures that detect changes in the environment–to the CNS
  2. interneurons: can receive input from sensory neurons and from other interneurons in the CNS; sum up all the information received from other neurons before they communicate with motor neurons.
  3. motor neurons A sensory neuron An interneuron lies entirely within the CNS: takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland)–carry out our responses to environmental changes, whether these are external or internal. Interneurons A motor neuron

2. neuroglia (sometimes referred to as glial cells): support and nourish neurons
   * greatly outnumber neurons
   * several types of neuroglia in the CNS, each with specific functions:
   
   • Microglia are phagocytic cells that help remove bacteria and debris, whereas
   • astrocytes provide metabolic and structural support directly to the neurons.
   • The myelin sheath is formed from the membranes of tightly spiraled neuroglia.
     
     • In the PNS, Schwann cells perform this function, leaving gaps called nodes of Ranvier.
     • In the CNS, neuroglia cells called oligodendrocytes form the myelin sheath.
     • Neuroglia (see Section 4.4) Anatomy of a Neuron

Question 139

Reverse

Table 14.2Drug Influence on the CNS

Table Summary: Table lists the names of different substances in column 1. Other information related to these substances appears in columns 2 and 3.

SubstanceEffectMode of Transmission

AlcoholDepressantDrink

NicotineStimulantSmoked or smokeless tobacco

CocaineStimulantSniffed/snorted, injected, or smoked

Methamphetamine/EcstasyStimulantSmoked or pill form

HeroinDepressantSniffed/snorted, injected, or smoked

Marijuana/K2PsychoactiveSmoked or consumed

Beginning in about 2005, several manufacturers began selling alcoholic energy drinks. With names like Four Loko, JOOSE, and Sparks, these drinks combine fairly high levels of alcohol with caffeine and other ingredients. Although interactions between drugs can be complex, the stimulant effects of caffeine can counteract some of the depressant effects of alcohol, so users feel able to drink more. Because caffeine does not reduce the intoxicating effects of alcohol, many state legislatures are banning these products, and in November 2010 the U.S. Food and Drug Administration warned several manufacturers that they would no longer be allowed to mix caffeine with alcohol in their products.

Nicotine

Although the numbers have been decreasing since 2011 according to the CDC, in 2015, 25.3% of high school students and 7.4% of middle school students reported using a tobacco product. When tobacco is smoked or chewed, nicotine is rapidly delivered throughout the body. It causes a release of epinephrine from the adrenal glands, increasing blood sugar and causing the initial feeling of stimulation. As blood sugar falls, depression and fatigue set in, causing the user to seek more nicotine. In the CNS, nicotine stimulates neurons to release dopamine, a neurotransmitter that promotes a temporary sense of pleasure, and reinforces dependence on the drug. About 70% of people who try smoking become addicted.

As mentioned in earlier chapters, smoking is strongly associated with serious diseases of the cardiovascular and respiratory systems. Once addicted, however, only 10–20% of smokers are able to quit. Most medical approaches to quitting smoking involve the administration of nicotine in safer forms, such as skin patches, gum, or a newly developed nicotine inhaler, so that withdrawal symptoms can be minimized while dependence is gradually reduced. Several antinicotine vaccines (such as NicVAX) are currently in development or in early clinical trials. These vaccines stimulate the production of antibodies that prevent nicotine from entering the brain.

Cocaine and Crack

Cocaine is an alkaloid derived from the shrub Erythroxylon coca. Approximately 35 million Americans have used cocaine by sniffing/snorting, injecting, or smoking. Cocaine is a powerful stimulant in the CNS that interferes with the reuptake of dopamine at synapses, increasing overall brain activity. The result is a rush of a sense of well-being that lasts from 5 to 30 minutes. However, long-term use of cocaine causes a loss of metabolic functions in the brain (Fig. 14.19).

Figure 14.19 Cocaine use. Brain activity before and after the use of cocaine.

(both photos): ©Science Source

“Crack” is the street name given to cocaine that is processed to a free-base form for smoking. The term crack refers to the crackling sound heard when the drug is smoked. Smoking allows high doses of the drug to reach the brain rapidly, providing an intense and immediate high, or “rush.” Approximately 8 million Americans use crack.

A cocaine binge is a period in which a user takes the drug at ever-higher doses. The user is hyperactive, with little desire for food or sleep, but has an increased sex drive. This is followed by a crash period, characterized by fatigue, depression, irritability, and a lack of interest in sex. In fact, men who use cocaine often become impotent.

Cocaine is highly addictive; with continued use, the brain makes less dopamine to compensate for a seemingly endless supply. The user experiences withdrawal symptoms and an intense craving for cocaine. Overdosing on cocaine can cause cardiac and/or respiratory arrest.Page 301

Methamphetamine and Ecstasy

Methamphetamine and ecstasy are considered club, or party, drugs. Methamphetamine (commonly called meth or crank) is a powerful CNS stimulant. Meth is often produced in makeshift home laboratories, usually starting with ephedrine or pseudoephedrine, common ingredients in many cold and asthma medicines. As a result, many states have passed laws making these medications more difficult to purchase. The number of toxic chemicals used to prepare the drug makes a former meth lab site hazardous to humans and to the environment. Over 9 million people in the United States have used methamphetamine at least once. It is available as a powder that can be snorted or as crystals (crystal meth or ice) that can be smoked.

The structure of methamphetamine is similar to that of dopamine, and the most immediate effect of taking meth is a rush of euphoria, energy, alertness, and elevated mood. However, this is typically followed by a state of agitation that, in some individuals, leads to violent behavior. Chronic use can result in what is called an amphetamine psychosis, characterized by paranoia, hallucinations, irritability, and aggressive, erratic behavior.

Ecstasy is the street name for MDMA (methylenedioxymethamphetamine), which is chemically similar to methamphetamine. Many users say that “X,” taken as a pill that looks like an aspirin or candy, increases their feelings of well-being and love for other people. However, it has many of the same side effects as other stimulants, plus it can interfere with temperature regulation, leading to hyperthermia, high blood pressure, and seizures. Although deaths from alcohol abuse are more common, ecstasy is identified as a cause of accidental death in young adults each year.

Drugs with sedative effects, known as date rape or predatory drugs, include flunitrazepam (Rohypnol, or roofies), gamma-hydroxybutyric acid (GHB), and ketamine (special K). Ketamine is actually a drug that veterinarians sometimes use to perform surgery on animals. Any of these drugs can be given to an unsuspecting person, who may fall into a dreamlike state in which he or she is unable to move and thus is vulnerable to sexual assault.

Heroin

Heroin is derived from the resin or sap of the opium poppy plant, which is widely grown in a region from Turkey to Southeast Asia and in parts of Latin America. Drugs derived from opium are called opiates, or more commonly, opioids. This class also includes morphine and codeine. After heroin is injected, snorted, or smoked, a feeling of euphoria, along with relief of any pain, occurs within a few minutes. It is estimated that 4 million Americans have used heroin sometime in their lives, and over 300,000 people use heroin annually.

As with other drugs of abuse, addiction is common. Heroin and opioids bind to receptors meant for the endorphins, naturally occurring neurotransmitters that kill pain and produce feelings of tranquility. With repeated use, the body’s production of endorphins decreases. Tolerance develops, so the user needs to take more of the drug just to prevent withdrawal symptoms (tremors, restlessness, cramps, vomiting), and the original euphoria is no longer felt. In the case of heroin, long-term users commonly acquire hepatitis, HIV/AIDS, and various bacterial infections due to the use of shared needles, and heavy users may experience convulsions and death by respiratory arrest.

Heroin addiction can be treated with synthetic opiate compounds, such as methadone or buprenorphine and naloxone (Suboxone), that decrease withdrawal symptoms and block heroin’s effects. However, methadone itself can be addictive, and methadone-related deaths are on the rise.

Marijuana and K2

Marijuana is the most commonly used illegal drug in the United States. Surveys vary, but in 2015, about 52% of young adults reported using marijuana in their lifetime, and 46% of the U.S. population had tried it at least once. It is derived from the dried flowering tops, leaves, and stems of the marijuana plant, Cannabis sativa, which contain and are covered by a resin that is rich in THC (tetrahydrocannabinol). The names cannabis and marijuana apply to either the plant or THC. Marijuana can be ingested, but usually it is smoked in a cigarette called a “joint.” Beginning with California in 1996, several states have legalized its use for medical purposes, such as in treating cancer, AIDS, and glaucoma. In 2012, Colorado became the first state to legalize recreational use. As of 2018, 8 states had joined Colorado in legalizing recreational use, and 22 additional states had authorized the use of marijuana for medicinal purposes. However, in 2005, the Supreme Court ruled that patients prescribed medical marijuana can still be prosecuted by federal agencies.

Page 302Researchers have found that THC binds to a receptor for anandamide, a naturally occurring neurotransmitter that is important for short-term memory processing, and perhaps for feelings of contentment. The occasional marijuana user experiences mild euphoria, along with alterations in vision and judgment. Heavy use can cause hallucinations, anxiety, depression, paranoia, and psychotic symptoms. Research is underway to identify the effects of long-term marijuana use on the brain, as well as on the effects of secondhand marijuana smoke on the respiratory system.

In recent years, awareness has been increasing about a synthetic compound called K2, or spice. Originally synthesized by an organic chemist at Clemson University, K2 is about ten times as potent as THC. The chemical is typically sprayed onto a mixture of other herbal products and smoked. However, because there is no regulation of how it is produced, the amount of K2 itself, or contaminants, can vary greatly. This may account for the several reports of serious medical problems and even deaths among K2 users.

CHECK YOUR PROGRESS 14.5

Contrast drug therapy and drug abuse.

Answer

Drug therapy is used to treat a disease or disorder. Drug abuse is using drugs without symptoms of disease or disorder.

List how the abuse of drugs, including alcohol and nicotine, affects the nervous system.

Answer

Alcohol and heroin are depressants; nicotine, cocaine, and methamphetamines are stimulants; marijuana produces euphoria.

Detail several modes of action of pharmaceutical and illegal drugs.

Answer

Alcohol increases the action of GABA and increases the release of endorphins in the hypothalamus. Nicotine stimulates dopamine release. Cocaine inhibits dopamine reuptake. Methamphetamine mimics the action of cocaine. Heroin is converted to morphine in the brain and binds to opioid receptors. Marijuana stimulates anandamide receptors.

CONNECTING THE CONCEPTS

For more on the long-term effects of drug use on the systems of the body, refer to the following discussions:

Section 5.7 explores the negative long-term effects of smoking on the cardiovascular system.

Section 11.4 provides information on how alcohol acts as a diuretic in the urinary system.

Section 20.2 examines the relationship between smoking and alcohol use and the increased risk of cancer.

CONCLUSION

The cause of multiple sclerosis (MS) is still unknown, but most researchers agree that there are most likely many contributing factors, including environmental influences, genetics, and a faulty immune system. Many individuals with MS are able to control their symptoms by using immunosuppressive medications, such as beta interferons. The fact that this treatment works suggests that, in many cases, MS is caused by the immune system incorrectly identifying the myelin sheaths as foreign material. The breakdown of the myelin can be detected using both MRI and SSEP tests (discussed in the chapter opener). However, environmental conditions are also suspected to cause MS. Studies have shown that the risk of contracting MS is influenced in part by where in the world you live, although the specific environmental factor or pollutant has not yet been identified. Genetics is also believed to play a role in some cases. But most researchers believe that a defect in a single gene is unlikely. Rather, it is more likely that a certain combination of genetic factors places an individual at a higher risk of contracting MS. Though there is no cure for MS, researchers have been very successful in developing disease-modifying drugs that reduce the symptoms and allow the individual to lead a normal life.

Answer 139

Drug Influence in the CNS

Drug Influence on the CNS

Table Summary: Table lists the names of different substances in column 1. Other information related to these substances appears in columns 2 and 3.

SubstanceEffectMode of Transmission

AlcoholDepressantDrink

NicotineStimulantSmoked or smokeless tobacco

CocaineStimulantSniffed/snorted, injected, or smoked

Methamphetamine/EcstasyStimulantSmoked or pill form

HeroinDepressantSniffed/snorted, injected, or smoked

Marijuana/K2PsychoactiveSmoked or consumed

Question 141

Reverse

The PNS: Autonomic division • The autonomic system regulates the activity of involuntary muscles (cardiac and smooth) and glands.

Question 143

Reverse

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

Answer 143

Reticular Formation

Question 144

Do you know these?

Answer 144

SUMMARIZE

14.1Overview of the Nervous System

The nervous system

Is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

Has three functions: (1) reception of input, (2) integration of data, and (3) generation of motor output.

Question 146

14.3 The Limbic System and Higher Mental Functions

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the limbic system.

Explain how the limbic system is involved in memory, language, and speech.

Summarize the types of memory associated with the limbic system.

The limbic system integrates our emotions (fear, joy, sadness) with our higher mental functions (reason, memory). Because of the limbic system, activities such as sexual behavior and eating seem pleasurable, and mental stress can cause high blood pressure.

Limbic System

The limbic system is an evolutionary ancient group of linked structures deep within the cerebrum. It is a functional grouping rather than an anatomical one (Fig. 14.13). The limbic system blends primitive emotions and higher mental functions into a united whole. As already noted, it accounts for why activities such as sexual behavior and eating seem pleasurable. Conversely, unpleasant sensations or emotions (pain, frustration, hatred, despair) are translated by the limbic system into a stress response.

Figure 14.13 The regions of the brain associated with the limbic system. In the limbic system (purple), structures deep within each cerebral hemisphere and surrounding the diencephalon join higher mental functions, such as reasoning, with more primitive feelings, such as fear and pleasure. Therefore, primitive feelings can influence our behavior, but reason can also keep them in check.

Two significant structures in the limbic system are the amygdala and the hippocampus. The amygdala, in particular, can cause experiences to have emotional overtones, and it creates the sensation Page 293of fear. This center can use past knowledge fed to it by association areas to assess a current situation. If necessary, the amygdala can trigger the fight-or-flight reaction. So if you are out late at night and you turn to see someone in a ski mask following you, the amygdala may immediately cause you to start running. The frontal cortex can override the limbic system, cause you to rethink the situation, and prevent you from acting out strong reactions.

The hippocampus is believed to play a crucial role in learning and memory. The hippocampal region acts as an information gateway during the learning process. It determines what information about the world is to be sent to memory and how this information is to be encoded and stored by other regions in the brain. Most likely, the hippocampus can communicate with the frontal cortex, because we know that memories are an important part of our decision-making processes.

Higher Mental Functions

As in other areas of biological research, brain research has progressed due to technological breakthroughs. Neuroscientists now have a wide range of techniques at their disposal for studying the human brain, including modern technologies that allow us to record its functioning.

Memory and Learning

Just as the connecting tracts of the corpus callosum are evidence that the two cerebral hemispheres work together, so the limbic system indicates that cortical areas may work with lower centers to produce learning and memory. Memory is the ability to hold a thought in mind or to recall events from the past, ranging from a word we learned only yesterday to an early emotional experience that has shaped our lives. Learning takes place when we retain and use past memories.

Types of Memory

We have all tried to remember a seven-digit telephone number for a short time. If we say we are trying to keep it in the forefront of our brain, we are exactly correct. The prefrontal area, active during short-term memory, lies just posterior to our forehead! There are some telephone numbers that we have memorized. In other words, they have gone into long-term memory. Think of a telephone number you know by heart, and try to bring it to mind without also thinking about the place or person associated with that number. Most likely you cannot. Typically, long-term memory is a mixture of what is called semantic memory (numbers, words, etc.) and episodic memory (persons, events, etc.).

Skill memory is another type of memory that can exist independent of episodic memory. Skill memory is involved in performing motor activities such as riding a bike or playing ice hockey. When a person first learns a skill, more areas of the cerebral cortex are involved than after the skill is perfected. In other words, you have to think about what you are doing when you learn a skill, but later the actions become automatic. Skill memory involves all the motor areas of the cerebrum below the level of consciousness.

Long-Term Memory Storage and Retrieval

Our long-term memories are apparently stored in bits and pieces throughout the sensory association areas of the cerebral cortex. Visual perceptions are stored in the vision association area, sounds are stored in the auditory association area, and so forth. As previously mentioned, the hippocampus serves as a bridge between the sensory association areas (where memories are stored) and the prefrontal area (where memories are used). The prefrontal area communicates with the hippocampus when memories are stored and when these memories are brought to mind. Some memories are emotionally charged, because the amygdala seems to be responsible for fear conditioning and associating danger with sensory stimuli received from various parts of the brain.Page 294

SCIENCE IN YOUR LIFE

What is amnesia?

Amnesia results from disruption of the memory pathways and can be temporary or permanent. In anterograde amnesia, injury to the limbic system separates long-term memories of events that occurred prior to the injury from events that occur in the here and now. An affected person might carry on a conversation about past events (memories of a long-ago birthday) but be unable to recall a breakfast menu from that morning. In retrograde amnesia, a blow to the head or similar injury abolishes all memories for a variable time before the injury. For example, a head injury occurring during a car accident may abolish all memories from hours to days prior to the accident.

Long-Term Potentiation

Though it is helpful to know the memory functions of various portions of the brain, an important step toward curing mental disorders is understanding memory on the cellular level. After synapses have been used intensively for a short time, they release more neurotransmitters than before. This phenomenon, called long-term potentiation, may be involved in memory storage.

Language and Speech

Language depends on semantic memory. Therefore, we would expect some of the same areas in the brain to be involved in both memory and language. Any disruption of these pathways could contribute to an inability to comprehend our environment and use speech correctly.

Seeing and hearing words depends on sensory centers in the occipital and temporal lobes, respectively. Damage to Wernicke’s area (see Section 14.2) results in the inability to comprehend speech. Damage to Broca’s area, on the other hand, results in the inability to speak and write. The functions of the visual cortex, Wernicke’s area, and Broca’s area are shown in Figure 14.14.

Figure 14.14 The areas of the brain involved in reading. These functional images were captured by a high-speed computer during a PET (positron-emission tomography) scan of the brain. A radioactively labeled solution is injected into the subject, and then the subject is asked to perform certain activities. Cross-sectional images of the brain generated by the computer reveal where activity is occurring because the solution is preferentially taken up by active brain tissue and not by inactive brain tissue. These PET images show the cortical pathway for reading words and then speaking them. Red indicates the most active areas of the brain, and blue indicates the least active areas.

(all): ©Marcus Raichle

One interesting aside pertaining to language and speech is the recognition that the left brain and the right brain may have different functions. Recall that the left hemisphere contains both Broca’s area and Wernicke’s area (see Section 14.2). As you might expect, it appears that the left hemisphere plays a role of great importance in language functions. The role of the isolated left hemisphere can be studied in patients after surgery to sever the corpus callosum. This procedure is used for seizure control in patients with epilepsy. After surgery, the patient is termed “split brain,” because there is no longer direct communication between the two cerebral hemispheres. If a split-brain individual views an object with only the right eye, its image will be sent only to the right hemisphere. This person will be able to choose the proper object for a particular use—scissors to cut paper, for example—but will be unable to name that object.

Researchers now believe that the hemispheres process the same information differently. The left hemisphere is more global, whereas the right hemisphere is more specific in its approach. However, research also indicates that the classification of “right-brained” versus “left-brained” for individuals is probably not an accurate indication of an individual’s brain activity.

CHECK YOUR PROGRESS 14.3

Summarize the function of the limbic system.

Answer

A group of brain structures that blends primitive emotions and higher mental functions into a united whole.

List what limbic system structures are involved in the fight-or-flight reaction, learning, and long-term memory.

Answer

Amygdala—fight-or-flight; hippocampus—learning and memory. The hippocampus acts as a bridge between the sensory association areas of the cerebral cortex where memories are stored long term and the prefrontal areas of the cortex where memories are used.

Describe the relationship between the left and right sides of the brain and language and speech.

Answer

Wernicke’s area and Broca’s area in the left hemisphere are related to speech, comprehension, and writing. The right hemisphere is associated with more nonverbal and creative functions.

CONNECTING THE CONCEPTS

For more information on these topics, refer to the following discussion:

Section 18.5 examines the effects of aging on the body, including the nervous system.

Question 147

Table 14.1Comparison of Somatic Motor and Autonomic Motor Pathways

Table Summary: Columns are for somatic motor pathway and autonomic motor pathways. Rows are for different points of comparison. Autonomic motor pathways are grouped into sympathetic and parasympathetic, as the other column-headers.

Autonomic Motor Pathways

Somatic Motor PathwaySympatheticParasympathetic

Type of controlVoluntary/involuntaryInvoluntaryInvoluntary

Number of neurons per messageOneTwo (preganglionic shorter than postganglionic)Two (preganglionic longer than postganglionic)

Location of motor fiberMost cranial nerves and all spinal nervesThoracolumbar spinal nervesCranial (e.g., vagus) and sacral spinal nerves

NeurotransmitterAcetylcholineNorepinephrineAcetylcholine

EffectorsSkeletal musclesSmooth and cardiac muscle, glands, and organsSmooth and cardiac muscle, glands, and organs

Answer 147

Somatic and Parasympathetic Pathways

Question 148

reverse.prompt

True

Answer 148

Lecture Notes

True or False

The prefontal cortex is also a processing center in the cerebrum?

Question 149

Reticular Formation

Answer 149

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

Question 150

*** Very important card ***The central nervous system

14.2 The Central Nervous System

slides

***Contains supplemental material not in the book ***

Answer 150

1. consists of the brain and spinal cord. •
2. Both are protected by •
   
   • Scalp and skin •
   • Bones – skull and vertebral column •
   1. Meninges – 3 protective membranes that wrap around CNS •
      
      1. Cerebral spinal fluid (CSF) – space between meninges is filled with this fluid that cushions and protects the CNS •
      2. Blood brain barrier (BBB)
      3. 2 Meninges
         
         1. • Dura mater • Double-layered external covering Periosteum – dense connective tissue attached to surface of the skull
         2. • Meningeal layer – outer covering of the brain • Folds inward in several areas

***Supplemental material not in book

3 Meninges •

Arachnoid mater •

Middle layer • Web-like • Pia mater • Internal layer •

Clings to the surface of the brain •

Many blood vessels

http://droualb.faculty.mjc.edu/Lecture%20Notes/Unit%205/Meninges_peeled_away.jpg

Supplemental material not in book 14.2 The Central Nervous System 4 Cerebrospinal Fluid (CSF) •

Similar to blood plasma composition, as it is formed from blood plasma •

Forms a watery cushion to protect the brain •

Circulated in subarachnoid space, ventricles, and central canal of the spinal cord

Question 151

reverse.prompt

Describe the relationship between the left and right sides of the brain and language and speech.

Answer 151

Wernicke’s area and Broca’s area in the left hemisphere are related to speech, comprehension, and writing. The right hemisphere is associated with more nonverbal and creative functions.

Question 152

Describe the three types of neurons, and list the three main parts of a neuron.

Answer 152

CHECK YOUR PROGRESS 14.1

Sensory neurons take nerve signals from a sensory receptor to the CNS. Interneurons lie entirely within the CNS and communicate with other neurons. Motor neurons move nerve impulses away from the CNS to an effector. The parts are cell body, dendrites, and axon.

Question 153

1. It includes cranial nerves (12 pairs),
2. spinal nerves (31 pairs), and
3. ganglia (neuronal cell bodies) outside the CNS.
4. • Spinal nerves conduct impulses to and from the spinal cord. - Cranial nerves conduct impulses to and from the brain. •

Answer 153

PNS Slides Peripheral Nervous System

The PNS is divided into 2 systems. - Somatic division - Autonomic division

Question 154

Identify the structures of the spinal cord and provide a function for each.

Question 155

Oligodendrocyte

Answer 155

s – branched cells that wrap CNS nerve fibers – Produce fatty insulating coverings (myelin sheath) around nerve fibers in the CNS – Can coil around as many as 60 different fibers at one time

Question 156

Reversed prompt

(Fig. 14.8c, d)

Answer 156

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

Question 157

Reverse

Master controlling and communicating system of the body • Sensory input – gathering information • To monitor changes (stimuli) occurring inside and outside the body • Integration • To process and interpret sensory input and decide if action is needed • Motor output • A response to integrated stimuli • The response activates muscles or glands 14.1 Overview of the nervous system T

Question 158

List the functions of the spinal cord.

Answer 158

1. Provides a means of communication between the brain and the peripheral nerves
2. center for reflex actions.

Answer 159

• 2 divisions – Central nervous system (CNS): –Brain and spinal cord –Peripheral nervous system (PNS): Nerves and ganglia (collections of cell bodies)

Question 160

Stroke

Answer 160

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

1. Descending motor tracts (from the primary motor area)
2. and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla.
3. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over.
4. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere.
5. _**Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side._

Question 161

CNS

Neurons:

Resting Potential

Page 283- See, Download and Understand and Study 14.4 a

As Figure 14.4a

See Animation Tutorial: Neuron Action Potentials

How the Sodium–Potassium Pump Works

Like a battery, the neuron’s resting potential energy can be measured in volts. Whereas a D-size flashlight battery has 1.5 volts, a nerve cell typically has 0.070 volt, or 70 millivolts (mV), of stored energy (Fig. 14.4a). By convention, the voltage measurement is always a negative number. This is because scientists compare the inside of the cell—where negatively charged proteins and other large molecules are clustered—to the outside of the cell—where positively charged sodium and potassium ions are gathered.

Figure 14.4 Generation of an action potential. a. Resting potential occurs when a neuron is not conducting a nerve impulse. During an action potential, (b) the stimulus causes the cell to reach its threshold. c. Depolarization is followed by (d) repolarization. e. A graph depicting the generation of an action potential.shows,

Answer 161

• A resting neuron’s potential energy, much like a fully charged batter, exists because the plasma membrane is polarized: Positively charged ions are stashed outside the cell, with negatively charged ions inside.

oteins and other molecules that remain inside the cell because of their size.

• Study Figure 14.4a outside of the cell is positive because positively charged sodium ions (Na+) gather around the outside of the plasma membrane.

1. ***Process***At rest, the neuron’s plasma membrane is permeable to potassium, but not to sodium.
2. Thus, positively charged potassium ions (K+) contribute to the positive charge by diffusing out of the cell to join the sodium ions. The inside of the cell is negative in relation to the exterior of the cell because of the presence of large, negatively charged pr

Just like rechargeable batteries, neurons must maintain their resting potential to be able to work. To do so, neurons actively transport sodium ions out of the cell and return potassium ions to Page 284the cytoplasm. A protein carrier in the membrane, called the sodium–potassium pump, pumps sodium ions (Na+) out of the neuron and potassium ions (K+) into the neuron (see Section 3.3). This action effectively “recharges” the cell so that, like a fresh battery, it can perform work.

Question 162

Functional Classification of the Peripheral Nervous System

Answer 162

1. Sensory (afferent) division 
   
   • Nerve fibers that carry information to the central nervous system 
2. Motor (efferent) division 
   
   1. Nerve fibers that carry impulses away from the central nervous system 
3. Somatic nervous system = voluntary (skeletal muscles) 
4. Autonomic nervous system = involuntary (cardiac and smooth muscles, glands)

Question 163

reversedprompt

1. Much of the rest of the cerebrum
2. Myelination occurs and white matter develops as a child grows. ​

• Progressive myelination
  
  • enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area.
  • Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10.
  • The corpus callosum contains tracts that join the two cerebral hemispheres

Answer 163

What is Central White Matter?

Where is it found?

What process enables the brain to grow in size and complexity

What occurs?

Question 164

Microglia

Answer 164

Small, ovoid cells with spiny processes, “spider-like cells” –Phagocytes from red bone marrow that monitor the health of neurons – Dispose of dead cells, invading microorganisms M

Question 165

• naturally occurring molecules that block the release of a neurotransmitter or modify a neuron’s response to a neurotransmitter.
• Two well-known neuromodulators are :

1. Substance P: neuropeptide that is released by sensory neurons when pain is present.
2. Endorphins.

*

Answer 165

What are neuromodulators?

Question 167

Reverse

Why do drug abusers take drugs and what do the drugs do?

Answer 167

1. Most drug abusers take drugs that affect the neurotransmitter dopamine and thus artificially affect this reward circuit to the point that they ignore basic physical needs in favor of the drug.

Question 168

Reverse

1.  Sensory (afferent) division 
   
   • Nerve fibers that carry information to the central nervous system 
2. Motor (efferent) division 
   
   • Nerve fibers that carry impulses away from the central nervous system 
3. Somatic nervous system = voluntary (skeletal muscles) 
4. Autonomic nervous system = involuntary (cardiac and smooth muscles, glands)

Answer 168

Functional Classification of the Peripheral Nervous System

Question 169

reverse.prompt

1) thalamus
2) cranial nerve
3) cerebrum

Answer 169

• I consist of two masses of gray matter
• I live in the sides and roof of the third ventricle.
• I receive end all sensory input except the sense of smell.
• Visual, auditory, and somatosensory information arrives via my friends the and tracts from the spinal cord.
• I integrate this information and sends it on to the appropriate portions of the ______________
• In fact, I am very involved in arousal of _______ and
• I am quite fancy as I participate in higher mental functions, such as memory and emotions.

Question 170

Myelin Sheath

Answer 170

A lipid covering on long axons that acts to
increase the speed of nerve impulse conduction,
insulation for both CNS and PNS, and
regeneration in the PNS
• Schwann cells – neuroglia that make up the
myelin sheath in the PNS
• Oligodendrocytes- neuroglia that make up the
myelin sheath in the CNS
• Nodes of Ranvier – gaps between myelination
on the axons
• Saltatory conduction – conduction of the nerve
impulse from node to node

Question 171

Reverse

The PNS: Somatic division • The somatic system serves the skin, skeletal muscles and tendons. • Automatic responses are called reflexes. • Reflexes consist of sensory receptorsensory neuron interneuron  motor neuron effector organ (we talked about this in 14.1) 14.4 The Peripheral Nervous System 5 The PNS: Somatic division

Question 172

reversedprompt

1. infant, the brain enlarges due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”).
2. If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Answer 172

Cerebrospinal fluid abnormalities in children and adults

Question 173

reversedprompt

• A resting neuron’s potential energy, much like a fully charged batter, exists because the plasma membrane is polarized: Positively charged ions are stashed outside the cell, with negatively charged ions inside.

oteins and other molecules that remain inside the cell because of their size.

• Study Figure 14.4a outside of the cell is positive because positively charged sodium ions (Na+) gather around the outside of the plasma membrane.

1. ***Process***At rest, the neuron’s plasma membrane is permeable to potassium, but not to sodium.
2. Thus, positively charged potassium ions (K+) contribute to the positive charge by diffusing out of the cell to join the sodium ions. The inside of the cell is negative in relation to the exterior of the cell because of the presence of large, negatively charged pr

Just like rechargeable batteries, neurons must maintain their resting potential to be able to work. To do so, neurons actively transport sodium ions out of the cell and return potassium ions to Page 284the cytoplasm. A protein carrier in the membrane, called the sodium–potassium pump, pumps sodium ions (Na+) out of the neuron and potassium ions (K+) into the neuron (see Section 3.3). This action effectively “recharges” the cell so that, like a fresh battery, it can perform work.

Answer 173

CNS

Neurons:

Resting Potential

Page 283- See, Download and Understand and Study 14.4 a

As Figure 14.4a

See Animation Tutorial: Neuron Action Potentials

How the Sodium–Potassium Pump Works

Like a battery, the neuron’s resting potential energy can be measured in volts. Whereas a D-size flashlight battery has 1.5 volts, a nerve cell typically has 0.070 volt, or 70 millivolts (mV), of stored energy (Fig. 14.4a). By convention, the voltage measurement is always a negative number. This is because scientists compare the inside of the cell—where negatively charged proteins and other large molecules are clustered—to the outside of the cell—where positively charged sodium and potassium ions are gathered.

Figure 14.4 Generation of an action potential. a. Resting potential occurs when a neuron is not conducting a nerve impulse. During an action potential, (b) the stimulus causes the cell to reach its threshold. c. Depolarization is followed by (d) repolarization. e. A graph depicting the generation of an action potential.shows,

Question 174

Brain Stem

Answer 174

The Brain Stem

• The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a).
• The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses.
• The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS.
• In addition, the pons functions with the medulla oblongata to regulate breathing rate.
• Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.
• The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

Question 175

What’s the Cerebral Cortex?

Answer 175

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Answer 176

14.3 The Limbic System and Higher Mental Functions

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the limbic system.

Explain how the limbic system is involved in memory, language, and speech.

Summarize the types of memory associated with the limbic system.

The limbic system integrates our emotions (fear, joy, sadness) with our higher mental functions (reason, memory). Because of the limbic system, activities such as sexual behavior and eating seem pleasurable, and mental stress can cause high blood pressure.

Limbic System

The limbic system is an evolutionary ancient group of linked structures deep within the cerebrum. It is a functional grouping rather than an anatomical one (Fig. 14.13). The limbic system blends primitive emotions and higher mental functions into a united whole. As already noted, it accounts for why activities such as sexual behavior and eating seem pleasurable. Conversely, unpleasant sensations or emotions (pain, frustration, hatred, despair) are translated by the limbic system into a stress response.

Figure 14.13 The regions of the brain associated with the limbic system. In the limbic system (purple), structures deep within each cerebral hemisphere and surrounding the diencephalon join higher mental functions, such as reasoning, with more primitive feelings, such as fear and pleasure. Therefore, primitive feelings can influence our behavior, but reason can also keep them in check.

Two significant structures in the limbic system are the amygdala and the hippocampus. The amygdala, in particular, can cause experiences to have emotional overtones, and it creates the sensation Page 293of fear. This center can use past knowledge fed to it by association areas to assess a current situation. If necessary, the amygdala can trigger the fight-or-flight reaction. So if you are out late at night and you turn to see someone in a ski mask following you, the amygdala may immediately cause you to start running. The frontal cortex can override the limbic system, cause you to rethink the situation, and prevent you from acting out strong reactions.

The hippocampus is believed to play a crucial role in learning and memory. The hippocampal region acts as an information gateway during the learning process. It determines what information about the world is to be sent to memory and how this information is to be encoded and stored by other regions in the brain. Most likely, the hippocampus can communicate with the frontal cortex, because we know that memories are an important part of our decision-making processes.

Higher Mental Functions

As in other areas of biological research, brain research has progressed due to technological breakthroughs. Neuroscientists now have a wide range of techniques at their disposal for studying the human brain, including modern technologies that allow us to record its functioning.

Memory and Learning

Just as the connecting tracts of the corpus callosum are evidence that the two cerebral hemispheres work together, so the limbic system indicates that cortical areas may work with lower centers to produce learning and memory. Memory is the ability to hold a thought in mind or to recall events from the past, ranging from a word we learned only yesterday to an early emotional experience that has shaped our lives. Learning takes place when we retain and use past memories.

Types of Memory

We have all tried to remember a seven-digit telephone number for a short time. If we say we are trying to keep it in the forefront of our brain, we are exactly correct. The prefrontal area, active during short-term memory, lies just posterior to our forehead! There are some telephone numbers that we have memorized. In other words, they have gone into long-term memory. Think of a telephone number you know by heart, and try to bring it to mind without also thinking about the place or person associated with that number. Most likely you cannot. Typically, long-term memory is a mixture of what is called semantic memory (numbers, words, etc.) and episodic memory (persons, events, etc.).

Skill memory is another type of memory that can exist independent of episodic memory. Skill memory is involved in performing motor activities such as riding a bike or playing ice hockey. When a person first learns a skill, more areas of the cerebral cortex are involved than after the skill is perfected. In other words, you have to think about what you are doing when you learn a skill, but later the actions become automatic. Skill memory involves all the motor areas of the cerebrum below the level of consciousness.

Long-Term Memory Storage and Retrieval

Our long-term memories are apparently stored in bits and pieces throughout the sensory association areas of the cerebral cortex. Visual perceptions are stored in the vision association area, sounds are stored in the auditory association area, and so forth. As previously mentioned, the hippocampus serves as a bridge between the sensory association areas (where memories are stored) and the prefrontal area (where memories are used). The prefrontal area communicates with the hippocampus when memories are stored and when these memories are brought to mind. Some memories are emotionally charged, because the amygdala seems to be responsible for fear conditioning and associating danger with sensory stimuli received from various parts of the brain.Page 294

SCIENCE IN YOUR LIFE

What is amnesia?

Amnesia results from disruption of the memory pathways and can be temporary or permanent. In anterograde amnesia, injury to the limbic system separates long-term memories of events that occurred prior to the injury from events that occur in the here and now. An affected person might carry on a conversation about past events (memories of a long-ago birthday) but be unable to recall a breakfast menu from that morning. In retrograde amnesia, a blow to the head or similar injury abolishes all memories for a variable time before the injury. For example, a head injury occurring during a car accident may abolish all memories from hours to days prior to the accident.

Long-Term Potentiation

Though it is helpful to know the memory functions of various portions of the brain, an important step toward curing mental disorders is understanding memory on the cellular level. After synapses have been used intensively for a short time, they release more neurotransmitters than before. This phenomenon, called long-term potentiation, may be involved in memory storage.

Language and Speech

Language depends on semantic memory. Therefore, we would expect some of the same areas in the brain to be involved in both memory and language. Any disruption of these pathways could contribute to an inability to comprehend our environment and use speech correctly.

Seeing and hearing words depends on sensory centers in the occipital and temporal lobes, respectively. Damage to Wernicke’s area (see Section 14.2) results in the inability to comprehend speech. Damage to Broca’s area, on the other hand, results in the inability to speak and write. The functions of the visual cortex, Wernicke’s area, and Broca’s area are shown in Figure 14.14.

Figure 14.14 The areas of the brain involved in reading. These functional images were captured by a high-speed computer during a PET (positron-emission tomography) scan of the brain. A radioactively labeled solution is injected into the subject, and then the subject is asked to perform certain activities. Cross-sectional images of the brain generated by the computer reveal where activity is occurring because the solution is preferentially taken up by active brain tissue and not by inactive brain tissue. These PET images show the cortical pathway for reading words and then speaking them. Red indicates the most active areas of the brain, and blue indicates the least active areas.

(all): ©Marcus Raichle

One interesting aside pertaining to language and speech is the recognition that the left brain and the right brain may have different functions. Recall that the left hemisphere contains both Broca’s area and Wernicke’s area (see Section 14.2). As you might expect, it appears that the left hemisphere plays a role of great importance in language functions. The role of the isolated left hemisphere can be studied in patients after surgery to sever the corpus callosum. This procedure is used for seizure control in patients with epilepsy. After surgery, the patient is termed “split brain,” because there is no longer direct communication between the two cerebral hemispheres. If a split-brain individual views an object with only the right eye, its image will be sent only to the right hemisphere. This person will be able to choose the proper object for a particular use—scissors to cut paper, for example—but will be unable to name that object.

Researchers now believe that the hemispheres process the same information differently. The left hemisphere is more global, whereas the right hemisphere is more specific in its approach. However, research also indicates that the classification of “right-brained” versus “left-brained” for individuals is probably not an accurate indication of an individual’s brain activity.

CHECK YOUR PROGRESS 14.3

Summarize the function of the limbic system.

Answer

A group of brain structures that blends primitive emotions and higher mental functions into a united whole.

List what limbic system structures are involved in the fight-or-flight reaction, learning, and long-term memory.

Answer

Amygdala—fight-or-flight; hippocampus—learning and memory. The hippocampus acts as a bridge between the sensory association areas of the cerebral cortex where memories are stored long term and the prefrontal areas of the cortex where memories are used.

Describe the relationship between the left and right sides of the brain and language and speech.

Answer

Wernicke’s area and Broca’s area in the left hemisphere are related to speech, comprehension, and writing. The right hemisphere is associated with more nonverbal and creative functions.

CONNECTING THE CONCEPTS

For more information on these topics, refer to the following discussion:

Section 18.5 examines the effects of aging on the body, including the nervous system.

Question 177

Reversed prompt

The Spinal Cord (Book)

Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

• Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.
• (a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

Answer 177

• • extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4).
• From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.
• • Structure of the Spinal Cord
• A cross-section of the spinal cord (Fig. 14.8a) shows a

1. central canal,
2. gray matter, and
3. white matter.

*

Question 178

Action Potential Propogation

Answer 178

1. In myelinated fibers, an action potential at one node of Ranvier causes an action potential at the next node, jumping over the entire myelin-coated portion of the axon.
   
   1. This type of conduction is called saltatory conduction (saltatio is a Latin word that means “to jump”) and is much faster.
   2. In thick, myelinated fibers, the rate of transmission is more than 100 m/s.
   3. Regardless of whether an axon is myelinated or not, its action potentials are self-propagating.
   4. Each action potential generates another, along the entire length of the axon.
2. Like the action potential itself, conduction of an action potential is an all-or-none event—either an axon conducts its action potential or it does not.
   
   • The intensity of a message is determined by how many action potentials are generated within a given time.
   • An axon can conduct a volley of action potentials very quickly, because only a small number of ions are exchanged with each action potential.
   • Once the action potential is complete, the ions are rapidly restored to their proper place through the action of the sodium–potassium pump.
3. As soon as the action potential has passed by each successive portion of an axon, that portion undergoes a short refractory period, during which it is unable to conduct an action potential.

• This ensures the one-way direction of a signal from the cell body down the length of the axon to the axon terminal.

It is interesting to note that all functions of the nervous system, from our deepest emotions to our highest reasoning abilities, are dependent on the conduction of nerve signals.

Question 179

Nerve Impulse Resting Potential

Answer 179

The nerve impulse: Resting potential (RP)
• Resting potential – when the axon is not
conducting a nerve impulse (when the axon is “at
rest”)
• More positive ions outside than inside the
membrane
• Negative charge of -70 mV inside the axon
• More Na+ outside than inside
• More K+ inside than outside
14.1 Overview of the Nervous System

Question 180

• I consist of two masses of gray matter
• I live in the sides and roof of the third ventricle.
• I receive end all sensory input except the sense of smell.
• Visual, auditory, and somatosensory information arrives via my friends the and tracts from the spinal cord.
• I integrate this information and sends it on to the appropriate portions of the ______________
• In fact, I am very involved in arousal of _______ and
• I am quite fancy as I participate in higher mental functions, such as memory and emotions.

Answer 180

1) thalamus
2) cranial nerve
3) cerebrum

Question 181

14.2 The Central Nervous System 22 2. The brain: Diencephalon Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. skull meninges pituitary gland fourth ventricle spinal cord Cerebrum Diencephalon Cerebellum hypothalamus midbrain pons Brain stem a. Parts of brain b. Cerebral hemispheres lateral ventricle third ventricle pineal gland corpus callosum thalamus (surrounds the third ventricle) medulla oblongata Figure 14.8 The human brain.

Question 182

occipital

Answer 182

Visual

Question 183

Identify the lobes and major areas of the human brain.

Answer 184

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Question 185

The Spinal Cord

Answer 185

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Question 186

Reverse

Primary somatosensory area – for sensory information from skeletal muscle and skin •

Answer 186

Primary Somatosensory Areas

Question 187

The spinal cord and brain are protected by ______________, with the spinal cord being surrounded by _______________, and the brain enclosed by ____________.

They are both also wrapped in _________________.

Answer 187

1. protected by bone
   
   • The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull.
2. Also, both the spinal cord and the brain are wrapped in protective membranes known as meninges (sing., meninx).

Question 188

The nervous system

Answer 188

reception and processing of sensory information from both the external and the internal environments.

The nervous system hao major divisions.s tw The major structures of the nervous system are shown in Figure 14.1.

Question 189

Important concepts to focus on • What are the divisions of the nervous system? • What are the functions of the nervous system? • What are the three types of neurons? • What are neuroglia? • What is the structure of a neuron? • What is the myelin sheath? Saltatory conduction? Schwann cell? Node of Ranvier? • Explain the resting and action potential as they relate to a nerve impulse. • How does the nerve impulse traverse the synapse? • What are the 4 parts of the brain and their functions? • What structures protect the CNS? • What are the 2 parts of the peripheral nervous system? • Describe the actions of some drugs of abuse

Question 190

hypothalamus and thalamus

are here

which is in the _______ ventricle

This is located on the floor of the ventricle and helps maintain homeostasis by

Answer 190

Diencephalon

• a region that encircles the third ventricle.
  1. The hypothalamus
• forms the floor of the third ventricle.
• integrating center that helps maintain homeostasis.
  
  1. It regulates hunger, sleep, thirst, body temperature, and water balance.
• ***_controls the pituitary gland serves as a link between the nervous and endocrine systems*_**

2. The thalamus

• two masses of gray matter
• sides and roof of the third ventricle.
• receiving end for all sensory input except the sense of smell.
• Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord.
• integrates this information and sends it on to the appropriate portions of the cerebrum.
• The thalamus is involved in arousal of the cerebrum, and it participates in ** higher mental functions, such as memory and emotions.***

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 191

reversedprompt

• The tracts cross containing:
  1. midbrain:
• relay station for tracts between the cerebrum and the spinal cord or cerebellum.
• reflex centers for visual, auditory, and tactile responses.

1. pons:

• (“bridge” in Latin)
• contains bundles of axons traveling
• between the cerebellum and the rest of the CNS.
• functions with the medulla oblongata to regulate breathing rate.
• Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

3. medulla oblongata

• reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure).
• It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing.
• superior to the spinal cord
• • groups of axons that travel together;
    
    • Between brain and higher level brain centers
      
      1. Ascending tracts convey sensory information.
      2. Motor information is transmitted on descending tracts.

Answer 191

What are the tracts crossing in the ___________________________ that contain the midbrain, _________________, and ___________________.

See and download diagram 14.9,

(see Fig. 14.9a).

Page 292

What do ascending and descending tracts do?

What are the functions of each part of the brain stem?

Question 192

As mentioned in Section 14.1, there are more than 100 known neurotransmitters. The most widely studied neurotransmitters to date are acetylcholine, norepinephrine, dopamine, serotonin, and gamma-aminobutyric acid (GABA). Acetylcholine is an essential CNS neurotransmitter for memory circuits in the limbic system. Norepinephrine is important to dreaming, waking, and mood. The neurotransmitter dopamine plays a central role in the brain’s regulation of mood. Dopamine is also the basal nuclei neurotransmitter that helps organize coordinated movements. Serotonin is involved in thermoregulation, sleeping, emotions, and perception. GABA is an abundant inhibitory neurotransmitter in the CNS.

Neuromodulators are naturally occurring molecules that block the release of a neurotransmitter or modify a neuron’s response to a neurotransmitter. Two well-known neuromodulators are substance P and endorphins. Substance P is a neuropeptide that is released by sensory neurons when pain is present. Endorphins block the release of substance P and serve as natural painkillers. Endorphins are produced by the brain during times of physical and/or emotional stress. They are associated with the “runner’s high” of joggers.

Both pharmaceuticals and illegal drugs have several basic modes of action:

They promote the action of a neurotransmitter, usually by increasing the amount of neurotransmitter at a synapse. Examples include drugs such as alprazolam (Xanax) and diazepam (Valium), which increase GABA. These medications are used for panic attacks and anxiety. Reduced levels of norepinephrine and serotonin are linked to depression. Drugs such as fluoxetine (Prozac), paroxetine (Paxil), and duloxetine (Cymbalta) allow norepinephrine and/or serotonin to accumulate at the synapse, which explains their effectiveness as antidepressants. Alzheimer disease causes a slow, progressive loss of memory (see Section 18.5). Drugs used for Alzheimer disease allow acetylcholine to accumulate at synapses in the limbic system.

They interfere with or decrease the action of a neurotransmitter. For instance, antipsychotic drugs used for the treatment of schizophrenia decrease the activity of dopamine. The caffeine in coffee, chocolate, and tea keeps us awake by interfering with the effects of inhibitory neurotransmitters in the brain.

They replace or mimic a neurotransmitter or neuromodulator. The opiates—namely, codeine, heroin, and morphine—bind to endorphin receptors and in this way reduce pain and produce a feeling of well-being.

Ongoing research into neurophysiology and neuropharmacology (the study of nervous system function and the way drugs work in the nervous system) continues to provide evidence that mental illnesses are caused by imbalances in neurotransmitters. These studies will undoubtedly improve treatments for mental illness, as well as provide insight into the problem of drug abuse.

Question 193

neurotransmitters

Answer 193

• acetylcholine:
  1. essential for memory circuits in the limbic system
• dopamine
  1. in the basal nuclei it helps coordinate movements, also plays a role in mood regulation
• GABA: abundant inhibitory neurotransmitter in the CNS
• norepinephrine: important to dreaming, waking, and mood
• serotonin: thermoregulation, sleeping, emotions and perception.

Question 194

Reverse

BIOLOGY TODAY Science

Nerve Regeneration and Stem Cells

In humans, axons outside the brain and spinal cord can regenerate—but axons inside these organs cannot (Fig. 14A). After injury, axons in the human central nervous system (CNS) degenerate, resulting in permanent loss of nervous function. Interestingly, about 90% of the cells in the brain and the spinal cord are not even neurons. They are neuroglia cells. In nerves outside the brain and spinal cord, the neuroglia cells are Schwann cells that help axons regenerate. The neuroglia cells in the CNS include microglial cells, oligodendrocytes, and astrocytes, and they inhibit axon regeneration.

Figure 14A Regeneration of nerve cells. Outside the CNS, nerves regenerate because new neuroglia called Schwann cells form a pathway for axons to reach a muscle. In the CNS, comparable neuroglia called oligodendrocytes do not have this function.

The spinal cord contains its own stem cells. When the spinal cord is injured in experimental animals, these stem cells proliferate. But instead of becoming functional neurons, they become neuroglia cells. Researchers are trying to understand the process that triggers the stem cells to become neuroglia cells. In the future, this understanding would allow manipulation of stem cells into neurons.

In early experiments with neural stem cells in the laboratory, scientists at Johns Hopkins University caused embryonic stem (ES) cells to differentiate into spinal cord motor neurons, the type of nerve cell that causes muscles to contract. The motor neurons then produced axons. When grown in the same dish with muscle cells, the motor neurons formed neuromuscular junctions and even caused muscle contractions. The cells were then transplanted into the spinal cords of rats with spinal cord injuries. Some of the transplanted cells survived for longer than a month within the spinal cord. However, no improvement in symptoms was seen and no functional neuron connections were made.

In later experiments by the same research group, paralyzed rats were first treated with drugs and nerve growth factors to overcome inhibition from the central nervous system. These techniques significantly increased the success of the transplanted neurons. Amazingly, axons of transplanted neurons reached the muscles, formed neuromuscular junctions, and provided partial relief from the paralysis. Research is being done on the use of both the body’s own stem cells and laboratory-grown stem cells to repair damaged CNS neurons. Though many questions remain, the current results are promising.

Questions to Consider

What is the likely reason neurons cannot simply be transplanted from other areas of the body?

How might this research also help patients who suffer from neurodegenerative diseases, such as Parkinson disease?

Long axons tend to have a myelin sheath, but short axons do not. The gray matter of the CNS is gray because it contains no myelinated axons; the white matter of the CNS is white because it does. In the PNS, myelin gives nerve fibers their white, glistening appearance and serves as an excellent insulator. When the myelin breaks down, as happens in multiple sclerosis (MS) (see chapter opener), then it becomes more difficult for the neurons to transmit information. In effect, MS “short-circuits” the nervous system. The myelin sheath also plays an important role in nerve regeneration within the PNS. If an axon is accidentally severed, the myelin sheath remains and serves as a passageway for new fiber growth.

Answer 194

Biology Today Nerve Regeneration and Stem Cells

Question 196

Reverse

Processing Centers

Answer 196

These centers

of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Question 198

Reflexes

Answer 198

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

Question 199

Nerve Impulse Action Potential

Answer 199

5
The nerve impulse: action potential
• Action potential – rapid change in the axon membrane; a nerve impulse– threshold is -55mV
• Sodium gates open letting Na+ in
• Depolarization occurs (-70mV to threshold-55mV)
• Interior of axon loses negative charge (+35mV)
• Potassium gates open letting K+ out
• Repolarization occurs
• Interior of axon regains negative charge (-70mV)
• Wave of depolarization/repolarization travels down the axon.
• Resting potential is restored by moving potassium inside and sodium outside
14.1 Overview of the nervous system

Question 200

1. I regulate :

• hunger,
• sleep,
• thirst,
• body temperature,
• and water balance.

Answer 200

What is the hypothalamus?

Question 201

Relate how the RAS aids in homeostasis.

Answer 201

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

Question 202

reverse.prompt

1. Language:
   
   • semantic memory;
   • Seeing and hearing words depends on sensory centers in the occipital and temporal lobes, respectively.
     
     1. Damage to Wernicke’s area: the inability to comprehend speech.
     2. Damage to Broca’s area: the inability to speak and write.
     3. *
     4. re: language & speech: left brain and the right brain may have different functions.
     • Recall that the left hemisphere contains both Broca’s area and Wernicke’s area (see Section 14.2).
     • As you might expect, it appears that the left hemisphere plays a role of great importance in language functions.
     • The role of the isolated left hemisphere can be studied in patients after surgery to sever the corpus callosum. This procedure is used for seizure control in patients with epilepsy. After surgery, the patient is termed “split brain,” because there is no longer direct communication between the two cerebral hemispheres. If a split-brain individual views an object with only the right eye, its image will be sent only to the right hemisphere. This person will be able to choose the proper object for a particular use—scissors to cut paper, for example—but will be unable to name that object.
     • • Researchers now believe that the hemispheres process the same information differently. The left hemisphere is more global, whereas the right hemisphere is more specific in its approach. However, research also indicates that the classification of “right-brained” versus “left-brained” for individuals is probably not an accurate indication of an individual’s brain activity

Answer 202

Language and Speech

and Attending Issues

Explore imagery

The functions of the visual cortex, Wernicke’s area, and Broca’s area are shown in Figure 14.14.

Figure 14.14 The areas of the brain involved in reading. These functional images were captured by a high-speed computer during a PET (positron-emission tomography) scan of the brain. A radioactively labeled solution is injected into the subject, and then the subject is asked to perform certain activities. Cross-sectional images of the brain generated by the computer reveal where activity is occurring because the solution is preferentially taken up by active brain tissue and not by inactive brain tissue. These PET images show the cortical pathway for reading words and then speaking them. Red indicates the most active areas of the brain, and blue indicates the least active areas.

Review

Question 203

14.3 The Limbic System and Higher Mental Functions

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the limbic system.

Explain how the limbic system is involved in memory, language, and speech.

Summarize the types of memory associated with the limbic system.

The limbic system integrates our emotions (fear, joy, sadness) with our higher mental functions (reason, memory). Because of the limbic system, activities such as sexual behavior and eating seem pleasurable, and mental stress can cause high blood pressure.

Limbic System

The limbic system is an evolutionary ancient group of linked structures deep within the cerebrum. It is a functional grouping rather than an anatomical one (Fig. 14.13). The limbic system blends primitive emotions and higher mental functions into a united whole. As already noted, it accounts for why activities such as sexual behavior and eating seem pleasurable. Conversely, unpleasant sensations or emotions (pain, frustration, hatred, despair) are translated by the limbic system into a stress response.

Figure 14.13 The regions of the brain associated with the limbic system. In the limbic system (purple), structures deep within each cerebral hemisphere and surrounding the diencephalon join higher mental functions, such as reasoning, with more primitive feelings, such as fear and pleasure. Therefore, primitive feelings can influence our behavior, but reason can also keep them in check.

Two significant structures in the limbic system are the amygdala and the hippocampus. The amygdala, in particular, can cause experiences to have emotional overtones, and it creates the sensation Page 293of fear. This center can use past knowledge fed to it by association areas to assess a current situation. If necessary, the amygdala can trigger the fight-or-flight reaction. So if you are out late at night and you turn to see someone in a ski mask following you, the amygdala may immediately cause you to start running. The frontal cortex can override the limbic system, cause you to rethink the situation, and prevent you from acting out strong reactions.

The hippocampus is believed to play a crucial role in learning and memory. The hippocampal region acts as an information gateway during the learning process. It determines what information about the world is to be sent to memory and how this information is to be encoded and stored by other regions in the brain. Most likely, the hippocampus can communicate with the frontal cortex, because we know that memories are an important part of our decision-making processes.

Higher Mental Functions

As in other areas of biological research, brain research has progressed due to technological breakthroughs. Neuroscientists now have a wide range of techniques at their disposal for studying the human brain, including modern technologies that allow us to record its functioning.

Memory and Learning

Just as the connecting tracts of the corpus callosum are evidence that the two cerebral hemispheres work together, so the limbic system indicates that cortical areas may work with lower centers to produce learning and memory. Memory is the ability to hold a thought in mind or to recall events from the past, ranging from a word we learned only yesterday to an early emotional experience that has shaped our lives. Learning takes place when we retain and use past memories.

Types of Memory

We have all tried to remember a seven-digit telephone number for a short time. If we say we are trying to keep it in the forefront of our brain, we are exactly correct. The prefrontal area, active during short-term memory, lies just posterior to our forehead! There are some telephone numbers that we have memorized. In other words, they have gone into long-term memory. Think of a telephone number you know by heart, and try to bring it to mind without also thinking about the place or person associated with that number. Most likely you cannot. Typically, long-term memory is a mixture of what is called semantic memory (numbers, words, etc.) and episodic memory (persons, events, etc.).

Skill memory is another type of memory that can exist independent of episodic memory. Skill memory is involved in performing motor activities such as riding a bike or playing ice hockey. When a person first learns a skill, more areas of the cerebral cortex are involved than after the skill is perfected. In other words, you have to think about what you are doing when you learn a skill, but later the actions become automatic. Skill memory involves all the motor areas of the cerebrum below the level of consciousness.

Long-Term Memory Storage and Retrieval

Our long-term memories are apparently stored in bits and pieces throughout the sensory association areas of the cerebral cortex. Visual perceptions are stored in the vision association area, sounds are stored in the auditory association area, and so forth. As previously mentioned, the hippocampus serves as a bridge between the sensory association areas (where memories are stored) and the prefrontal area (where memories are used). The prefrontal area communicates with the hippocampus when memories are stored and when these memories are brought to mind. Some memories are emotionally charged, because the amygdala seems to be responsible for fear conditioning and associating danger with sensory stimuli received from various parts of the brain.Page 294

SCIENCE IN YOUR LIFE

What is amnesia?

Amnesia results from disruption of the memory pathways and can be temporary or permanent. In anterograde amnesia, injury to the limbic system separates long-term memories of events that occurred prior to the injury from events that occur in the here and now. An affected person might carry on a conversation about past events (memories of a long-ago birthday) but be unable to recall a breakfast menu from that morning. In retrograde amnesia, a blow to the head or similar injury abolishes all memories for a variable time before the injury. For example, a head injury occurring during a car accident may abolish all memories from hours to days prior to the accident.

Long-Term Potentiation

Though it is helpful to know the memory functions of various portions of the brain, an important step toward curing mental disorders is understanding memory on the cellular level. After synapses have been used intensively for a short time, they release more neurotransmitters than before. This phenomenon, called long-term potentiation, may be involved in memory storage.

Language and Speech

Language depends on semantic memory. Therefore, we would expect some of the same areas in the brain to be involved in both memory and language. Any disruption of these pathways could contribute to an inability to comprehend our environment and use speech correctly.

Seeing and hearing words depends on sensory centers in the occipital and temporal lobes, respectively. Damage to Wernicke’s area (see Section 14.2) results in the inability to comprehend speech. Damage to Broca’s area, on the other hand, results in the inability to speak and write. The functions of the visual cortex, Wernicke’s area, and Broca’s area are shown in Figure 14.14.

Figure 14.14 The areas of the brain involved in reading. These functional images were captured by a high-speed computer during a PET (positron-emission tomography) scan of the brain. A radioactively labeled solution is injected into the subject, and then the subject is asked to perform certain activities. Cross-sectional images of the brain generated by the computer reveal where activity is occurring because the solution is preferentially taken up by active brain tissue and not by inactive brain tissue. These PET images show the cortical pathway for reading words and then speaking them. Red indicates the most active areas of the brain, and blue indicates the least active areas.

(all): ©Marcus Raichle

One interesting aside pertaining to language and speech is the recognition that the left brain and the right brain may have different functions. Recall that the left hemisphere contains both Broca’s area and Wernicke’s area (see Section 14.2). As you might expect, it appears that the left hemisphere plays a role of great importance in language functions. The role of the isolated left hemisphere can be studied in patients after surgery to sever the corpus callosum. This procedure is used for seizure control in patients with epilepsy. After surgery, the patient is termed “split brain,” because there is no longer direct communication between the two cerebral hemispheres. If a split-brain individual views an object with only the right eye, its image will be sent only to the right hemisphere. This person will be able to choose the proper object for a particular use—scissors to cut paper, for example—but will be unable to name that object.

Researchers now believe that the hemispheres process the same information differently. The left hemisphere is more global, whereas the right hemisphere is more specific in its approach. However, research also indicates that the classification of “right-brained” versus “left-brained” for individuals is probably not an accurate indication of an individual’s brain activity.

CHECK YOUR PROGRESS 14.3

Summarize the function of the limbic system.

Answer

A group of brain structures that blends primitive emotions and higher mental functions into a united whole.

List what limbic system structures are involved in the fight-or-flight reaction, learning, and long-term memory.

Answer

Amygdala—fight-or-flight; hippocampus—learning and memory. The hippocampus acts as a bridge between the sensory association areas of the cerebral cortex where memories are stored long term and the prefrontal areas of the cortex where memories are used.

Describe the relationship between the left and right sides of the brain and language and speech.

Answer

Wernicke’s area and Broca’s area in the left hemisphere are related to speech, comprehension, and writing. The right hemisphere is associated with more nonverbal and creative functions.

CONNECTING THE CONCEPTS

For more information on these topics, refer to the following discussion:

Section 18.5 examines the effects of aging on the body, including the nervous system.

Question 204

Reverse

The synapse • The synapse is the junction between the sending neuron (presynaptic membrane) and the receiving neuron (postsynaptic membrane). • Transmission is accomplished chemically across a small gap between the two neurons (synaptic cleft) by a neurotransmitter (e.g., ACh, dopamine, or serotonin). • Neurotransmitters are stored in synaptic vesicles in the axon terminals

Answer 204

Nerve Impulse

Question 205

1. Protein channels specific for sodium ions are located in the plasma membrane of the axon. When an action potential begins in response to a threshold stimulus, these protein channels open and sodium ions rush into the cell.
2. Adding positively charged sodium ions causes the inside of the axon to become positive compared to the outside (Fig. 14.4c). This change is called depolarization, because the charge (polarity) inside the axon changes from negative to positive.
3. Almost immediately after depolarization, the channels for sodium close and a separate set of potassium protein channels opens. Potassium flows rapidly from the cell. As positively charged potassium ions exit the cell, the inside of the cell becomes negative again because of the presence of large, negatively charged ions trapped inside the cell. This change in polarity is called repolarization, because the inside of the axon resumes a negative charge as potassium exits the axon
4. Finally, the sodium–potassium pump completes the action potential. Potassium ions are returned to the inside of the cell and sodium ions to the outside, and resting potential is restored.

Answer 205

• Nerve Impulse
• Sodium Gates Open

See download and study 14.4c

(Fig. 14.4d).

1. Neural Transmission: Resting Membrane Potential and Propagation
2. Graph of an Action Potential
3. To visualize such rapid fluctuations in voltage across the axonal membrane, researchers generally find it useful to plot the voltage changes over time (Fig. 14.4e). During depolarization, the voltage increases from −70 mV to −55 mV to between +30 and +35 mV as sodium ions move to the inside of the axon. In repolarization, the opposite change occurs when potassium ions leave the axon. The entire process is very rapid, requiring only 3 to 4 milliseconds (ms) to complete.

***Proccess, know steps- short answers** study slides

Question 206

Reverse

Range in shape from squamous to columnar, many are ciliated • They line the ventricles of the brain and central canal of the spinal cord to form a permeable barrier between cerebrospinal fluid (CSF) and tissue fluid bathing cells of CNS, as well as from blood • Functionally, ependymal cells produce, possibly monitor, and assist in the circulation of cerebrospinal fluid – Help circulate CNS with their cilia

Answer 206

Ependymal Cells

Question 207

The nervous system receives sensory input.

Answer 207

Sensory receptors in skin and other organs respond to external and internal stimuli by generating nerve signals that travel by way of the PNS to the CNS. For example, if you smell baking cookies, olfactory (smell) receptors in the nose use the PNS to transmit that information to the CNS.The CNS performs information processing and integration, summing up the input it receives from all over the body. The CNS reviews the information, stores the information as memories, and creates the appropriate motor responses. The smell of those baking cookies evokes memories of their taste.

The CNS generates motor output. Nerve signals from the CNS go by way of the PNS to the muscles, glands, and organs, all in response to the cookies. Signals to the salivary glands make you salivate. Your stomach generates the acid and enzymes Page 281needed to digest the cookies—even before you’ve had a bite. The CNS also coordinates the movement of your arms and hands as you reach for the cookies.

Question 208

reversedprompt

1. integration occurs
2. memories are stored.
3. premotor area.
   
   • ​​Anterior to the primary motor area
   • organizes motor functions for skilled motor activities,
     
     • such as walking and talking at the same time.
       
       • Next, the primary motor area sends signals to the cerebellum, which integrates them.
       • A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs.
4. The visual association area in the occipital lobe

• processes and analyzes sensory information from the skin and muscles.
• just posterior to the primary somatosensory area,​​​​
  
  • associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before.

1. The auditory association area in the temporal lobe performs the same functions with regard to sounds

Answer 208

Association Areas are where

1)

2)

These are the association centers:

Question 209

Somatic Sensory System

PNS has divisions:

1) Somatic System- (see Fig. 14.2).
2) Autonomic System

Answer 209

the somatic system and the autonomic nervous system

Somatic System

• The nerves serve the
  
  • skin,
  • skeletal muscles, and
  • tendons
• nerves take sensory information from external sensory receptors to the CNS.
• Motor commands leaving the CNS travel to skeletal muscles via somatic motor nerves.

1. Not all somatic motor actions are voluntary. Some are automatic.
2. Automatic responses to a stimulus in the somatic system are called reflexes. A reflex occurs quickly, without your even having to think about it.
   
   1. For example, a reflex may cause you to blink your eyes in response to a flash of light, without your willing it. We will study the path of a reflex, because it allows us to study in detail the path of nerve signals to and from the CNS.Page 296

Question 210

reversedprompt

1. posterior to the central sulcus in the parietal lobe.
2. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented
3. . Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation.
4. Once again, the face and hands require the largest proportion of the sensory cortex.
5. Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10).
   
   1. The primary taste area in the parietal lobe (pink) accounts for taste sensations.
   2. Visual information is received by the primary visual cortex (blue) in the occipital lobe.
   3. The primary auditory area in the temporal lobe (green) accepts information from our ears.
   4. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Answer 210

primary somatosensory area

see and download: (Fig. 14.11b), Page 291

Question 211

reverse.prompt

Drug Abuse

Like mental illness, drug abuse is linked to neurotransmitter levels. As mentioned previously, the neurotransmitter dopamine is essential for mood regulation. Dopamine plays a central role in the working of the brain’s built-in reward circuit. The reward circuit is a collection of neurons that, under normal circumstances, promotes healthy, pleasurable activities, such as consuming food. It’s possible to abuse behaviors such as eating, spending, or gambling Page 300because the behaviors stimulate the reward circuit and make us feel good. Drug abusers take drugs that artificially affect the reward circuit to the point that they neglect their basic physical needs in favor of continued drug use.

Drug abuse is apparent when a person takes a drug at a dose level and under circumstances that increase the potential for a harmful effect. Drug abusers are apt to display a psychological and/or physical dependence on the drug. Psychological dependence is apparent when a person craves the drug, spends time seeking the drug, and takes it regularly. With physical dependence, formerly called “addiction,” the person has become tolerant to the drug. More is needed to get the same effect, and withdrawal symptoms occur when he or she stops taking the drug. This is true for not only teenagers and adults but also newborn babies of mothers who abuse and are addicted to drugs. Alcohol, drugs, and tobacco can all adversely affect the developing embryo, fetus, or newborn.

Alcohol

With the exception of caffeine, alcohol (ethanol) consumption is the most socially accepted form of drug use in the United States. According to a 2015 national survey, 26.9% of high school students reported drinking some alcohol (down from 37.4% in 2014), and 7% binge drank (five-plus drinks in one setting) during the 30 days preceding the survey. Among adults, 86.4% reported they had consumed alcohol during their lifetime, with 56% stating they had used alcohol in the past month.

Alcohol acts as a depressant on many parts of the brain (Table 14.2) by increasing the action of GABA, an inhibitory neurotransmitter. Depending on the amount consumed, the effects of alcohol on the brain can lead to a feeling of relaxation, lowered inhibitions, impaired concentration and coordination, slurred speech, and vomiting. If the blood level of alcohol becomes too high, coma or death can occur.

Table 14.2Drug Influence on the CNS

Table Summary: Table lists the names of different substances in column 1. Other information related to these substances appears in columns 2 and 3.

SubstanceEffectMode of Transmission

AlcoholDepressantDrink

NicotineStimulantSmoked or smokeless tobacco

CocaineStimulantSniffed/snorted, injected, or smoked

Methamphetamine/EcstasyStimulantSmoked or pill form

HeroinDepressantSniffed/snorted, injected, or smoked

Marijuana/K2PsychoactiveSmoked or consumed

Beginning in about 2005, several manufacturers began selling alcoholic energy drinks. With names like Four Loko, JOOSE, and Sparks, these drinks combine fairly high levels of alcohol with caffeine and other ingredients. Although interactions between drugs can be complex, the stimulant effects of caffeine can counteract some of the depressant effects of alcohol, so users feel able to drink more. Because caffeine does not reduce the intoxicating effects of alcohol, many state legislatures are banning these products, and in November 2010 the U.S. Food and Drug Administration warned several manufacturers that they would no longer be allowed to mix caffeine with alcohol in their products.

Nicotine

Although the numbers have been decreasing since 2011 according to the CDC, in 2015, 25.3% of high school students and 7.4% of middle school students reported using a tobacco product. When tobacco is smoked or chewed, nicotine is rapidly delivered throughout the body. It causes a release of epinephrine from the adrenal glands, increasing blood sugar and causing the initial feeling of stimulation. As blood sugar falls, depression and fatigue set in, causing the user to seek more nicotine. In the CNS, nicotine stimulates neurons to release dopamine, a neurotransmitter that promotes a temporary sense of pleasure, and reinforces dependence on the drug. About 70% of people who try smoking become addicted.

As mentioned in earlier chapters, smoking is strongly associated with serious diseases of the cardiovascular and respiratory systems. Once addicted, however, only 10–20% of smokers are able to quit. Most medical approaches to quitting smoking involve the administration of nicotine in safer forms, such as skin patches, gum, or a newly developed nicotine inhaler, so that withdrawal symptoms can be minimized while dependence is gradually reduced. Several antinicotine vaccines (such as NicVAX) are currently in development or in early clinical trials. These vaccines stimulate the production of antibodies that prevent nicotine from entering the brain.

Cocaine and Crack

Cocaine is an alkaloid derived from the shrub Erythroxylon coca. Approximately 35 million Americans have used cocaine by sniffing/snorting, injecting, or smoking. Cocaine is a powerful stimulant in the CNS that interferes with the reuptake of dopamine at synapses, increasing overall brain activity. The result is a rush of a sense of well-being that lasts from 5 to 30 minutes. However, long-term use of cocaine causes a loss of metabolic functions in the brain (Fig. 14.19).

Figure 14.19 Cocaine use. Brain activity before and after the use of cocaine.

(both photos): ©Science Source

“Crack” is the street name given to cocaine that is processed to a free-base form for smoking. The term crack refers to the crackling sound heard when the drug is smoked. Smoking allows high doses of the drug to reach the brain rapidly, providing an intense and immediate high, or “rush.” Approximately 8 million Americans use crack.

A cocaine binge is a period in which a user takes the drug at ever-higher doses. The user is hyperactive, with little desire for food or sleep, but has an increased sex drive. This is followed by a crash period, characterized by fatigue, depression, irritability, and a lack of interest in sex. In fact, men who use cocaine often become impotent.

Cocaine is highly addictive; with continued use, the brain makes less dopamine to compensate for a seemingly endless supply. The user experiences withdrawal symptoms and an intense craving for cocaine. Overdosing on cocaine can cause cardiac and/or respiratory arrest.Page 301

Methamphetamine and Ecstasy

Methamphetamine and ecstasy are considered club, or party, drugs. Methamphetamine (commonly called meth or crank) is a powerful CNS stimulant. Meth is often produced in makeshift home laboratories, usually starting with ephedrine or pseudoephedrine, common ingredients in many cold and asthma medicines. As a result, many states have passed laws making these medications more difficult to purchase. The number of toxic chemicals used to prepare the drug makes a former meth lab site hazardous to humans and to the environment. Over 9 million people in the United States have used methamphetamine at least once. It is available as a powder that can be snorted or as crystals (crystal meth or ice) that can be smoked.

The structure of methamphetamine is similar to that of dopamine, and the most immediate effect of taking meth is a rush of euphoria, energy, alertness, and elevated mood. However, this is typically followed by a state of agitation that, in some individuals, leads to violent behavior. Chronic use can result in what is called an amphetamine psychosis, characterized by paranoia, hallucinations, irritability, and aggressive, erratic behavior.

Ecstasy is the street name for MDMA (methylenedioxymethamphetamine), which is chemically similar to methamphetamine. Many users say that “X,” taken as a pill that looks like an aspirin or candy, increases their feelings of well-being and love for other people. However, it has many of the same side effects as other stimulants, plus it can interfere with temperature regulation, leading to hyperthermia, high blood pressure, and seizures. Although deaths from alcohol abuse are more common, ecstasy is identified as a cause of accidental death in young adults each year.

Drugs with sedative effects, known as date rape or predatory drugs, include flunitrazepam (Rohypnol, or roofies), gamma-hydroxybutyric acid (GHB), and ketamine (special K). Ketamine is actually a drug that veterinarians sometimes use to perform surgery on animals. Any of these drugs can be given to an unsuspecting person, who may fall into a dreamlike state in which he or she is unable to move and thus is vulnerable to sexual assault.

Heroin

Heroin is derived from the resin or sap of the opium poppy plant, which is widely grown in a region from Turkey to Southeast Asia and in parts of Latin America. Drugs derived from opium are called opiates, or more commonly, opioids. This class also includes morphine and codeine. After heroin is injected, snorted, or smoked, a feeling of euphoria, along with relief of any pain, occurs within a few minutes. It is estimated that 4 million Americans have used heroin sometime in their lives, and over 300,000 people use heroin annually.

As with other drugs of abuse, addiction is common. Heroin and opioids bind to receptors meant for the endorphins, naturally occurring neurotransmitters that kill pain and produce feelings of tranquility. With repeated use, the body’s production of endorphins decreases. Tolerance develops, so the user needs to take more of the drug just to prevent withdrawal symptoms (tremors, restlessness, cramps, vomiting), and the original euphoria is no longer felt. In the case of heroin, long-term users commonly acquire hepatitis, HIV/AIDS, and various bacterial infections due to the use of shared needles, and heavy users may experience convulsions and death by respiratory arrest.

Heroin addiction can be treated with synthetic opiate compounds, such as methadone or buprenorphine and naloxone (Suboxone), that decrease withdrawal symptoms and block heroin’s effects. However, methadone itself can be addictive, and methadone-related deaths are on the rise.

Marijuana and K2

Marijuana is the most commonly used illegal drug in the United States. Surveys vary, but in 2015, about 52% of young adults reported using marijuana in their lifetime, and 46% of the U.S. population had tried it at least once. It is derived from the dried flowering tops, leaves, and stems of the marijuana plant, Cannabis sativa, which contain and are covered by a resin that is rich in THC (tetrahydrocannabinol). The names cannabis and marijuana apply to either the plant or THC. Marijuana can be ingested, but usually it is smoked in a cigarette called a “joint.” Beginning with California in 1996, several states have legalized its use for medical purposes, such as in treating cancer, AIDS, and glaucoma. In 2012, Colorado became the first state to legalize recreational use. As of 2018, 8 states had joined Colorado in legalizing recreational use, and 22 additional states had authorized the use of marijuana for medicinal purposes. However, in 2005, the Supreme Court ruled that patients prescribed medical marijuana can still be prosecuted by federal agencies.

Page 302Researchers have found that THC binds to a receptor for anandamide, a naturally occurring neurotransmitter that is important for short-term memory processing, and perhaps for feelings of contentment. The occasional marijuana user experiences mild euphoria, along with alterations in vision and judgment. Heavy use can cause hallucinations, anxiety, depression, paranoia, and psychotic symptoms. Research is underway to identify the effects of long-term marijuana use on the brain, as well as on the effects of secondhand marijuana smoke on the respiratory system.

In recent years, awareness has been increasing about a synthetic compound called K2, or spice. Originally synthesized by an organic chemist at Clemson University, K2 is about ten times as potent as THC. The chemical is typically sprayed onto a mixture of other herbal products and smoked. However, because there is no regulation of how it is produced, the amount of K2 itself, or contaminants, can vary greatly. This may account for the several reports of serious medical problems and even deaths among K2 users.

CHECK YOUR PROGRESS 14.5

Contrast drug therapy and drug abuse.

Answer

Drug therapy is used to treat a disease or disorder. Drug abuse is using drugs without symptoms of disease or disorder.

List how the abuse of drugs, including alcohol and nicotine, affects the nervous system.

Answer

Alcohol and heroin are depressants; nicotine, cocaine, and methamphetamines are stimulants; marijuana produces euphoria.

Detail several modes of action of pharmaceutical and illegal drugs.

Answer

Alcohol increases the action of GABA and increases the release of endorphins in the hypothalamus. Nicotine stimulates dopamine release. Cocaine inhibits dopamine reuptake. Methamphetamine mimics the action of cocaine. Heroin is converted to morphine in the brain and binds to opioid receptors. Marijuana stimulates anandamide receptors.

CONNECTING THE CONCEPTS

For more on the long-term effects of drug use on the systems of the body, refer to the following discussions:

Section 5.7 explores the negative long-term effects of smoking on the cardiovascular system.

Section 11.4 provides information on how alcohol acts as a diuretic in the urinary system.

Section 20.2 examines the relationship between smoking and alcohol use and the increased risk of cancer.

CONCLUSION

The cause of multiple sclerosis (MS) is still unknown, but most researchers agree that there are most likely many contributing factors, including environmental influences, genetics, and a faulty immune system. Many individuals with MS are able to control their symptoms by using immunosuppressive medications, such as beta interferons. The fact that this treatment works suggests that, in many cases, MS is caused by the immune system incorrectly identifying the myelin sheaths as foreign material. The breakdown of the myelin can be detected using both MRI and SSEP tests (discussed in the chapter opener). However, environmental conditions are also suspected to cause MS. Studies have shown that the risk of contracting MS is influenced in part by where in the world you live, although the specific environmental factor or pollutant has not yet been identified. Genetics is also believed to play a role in some cases. But most researchers believe that a defect in a single gene is unlikely. Rather, it is more likely that a certain combination of genetic factors places an individual at a higher risk of contracting MS. Though there is no cure for MS, researchers have been very successful in developing disease-modifying drugs that reduce the symptoms and allow the individual to lead a normal life.

Question 212

3 functions of the nervous systen

Answer 212

Master controlling and communicating system of the body • Sensory input – gathering information • To monitor changes (stimuli) occurring inside and outside the body • Integration • To process and interpret sensory input and decide if action is needed • Motor output • A response to integrated stimuli • The response activates muscles or glands 14.1 Overview of the nervous system T

Question 213

Reverse

The gate control theory of pain proposes that the

• tracts in the spinal cord have “gates” and that
• these gates control the flow of pain messages from the peripheral nerves to the brain.
• Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain.
• Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

Answer 213

Gate Control Theory of Pain - Spinal Cord Functions

Question 215

BIOLOGY TODAY Science

Nerve Regeneration and Stem Cells

(Fig. 14A)- See Figure 14A Regeneration of nerve cells. Outside the CNS, nerves regenerate because new neuroglia called Schwann cells form a pathway for axons to reach a muscle. In the CNS, comparable neuroglia called oligodendrocytes do not have this function.

Answer 215

1. axons outside the brain and spinal cord can regenerate—but axons inside these organs cannot After injury, axons in the human central nervous system (CNS) degenerate, resulting in permanent loss of nervous function.
2. Interestingly, about 90% of the cells in the brain and the spinal cord are not even neurons. They are neuroglia cells.
3. In nerves outside the brain and spinal cord, the neuroglia cells are Schwann cells that help axons regenerate. The neuroglia cells in the CNS include microglial cells, oligodendrocytes, and astrocytes, and they inhibit axon regeneration
4. 5. The spinal cord contains its own stem cells. When the spinal cord is injured in experimental animals, these stem cells proliferate. But instead of becoming functional neurons, they become neuroglia cells. Researchers are trying to understand the process that triggers the stem cells to become neuroglia cells. In the future, this understanding would allow manipulation of stem cells into neurons.
5. In early experiments with neural stem cells in the laboratory, scientists at Johns Hopkins University caused embryonic stem (ES) cells to differentiate into spinal cord motor neurons, the type of nerve cell that causes muscles to contract. The motor neurons then produced axons. When grown in the same dish with muscle cells, the motor neurons formed neuromuscular junctions and even caused muscle contractions. The cells were then transplanted into the spinal cords of rats with spinal cord injuries. Some of the transplanted cells survived for longer than a month within the spinal cord. However, no improvement in symptoms was seen and no functional neuron connections were made.
6. In later experiments by the same research group, paralyzed rats were first treated with drugs and nerve growth factors to overcome inhibition from the central nervous system. These techniques significantly increased the success of the transplanted neurons. Amazingly, axons of transplanted neurons reached the muscles, formed neuromuscular junctions, and provided partial relief from the paralysis. Research is being done on the use of both the body’s own stem cells and laboratory-grown stem cells to repair damaged CNS neurons. Though many questions remain, the current results are promising.
7. Questions to Consider
8. What is the likely reason neurons cannot simply be transplanted from other areas of the body?
9. How might this research also help patients who suffer from neurodegenerative diseases, such as Parkinson disease?
10. Long axons tend to have a myelin sheath, but short axons do not. The gray matter of the CNS is gray because it contains no myelinated axons; the white matter of the CNS is white because it does. In the PNS, myelin gives nerve fibers their white, glistening appearance and serves as an excellent insulator. When the myelin breaks down, as happens in multiple sclerosis (MS) (see chapter opener), then it becomes more difficult for the neurons to transmit information. In effect, MS “short-circuits” the nervous system. The myelin sheath also plays an important role in nerve regeneration within the PNS. If an axon is accidentally severed, the myelin sheath remains and serves as a passageway for new fiber growth.

Question 216

basal nuclei

Answer 216

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

Question 217

reversedprompt

Overview of human brain

Answer 217

1. last great frontier of biology.
2. parts of the brain with reference to the
   
   1. cerebrum- 2 lateral ventricles
   2. the diencephalon- third ventricle
   3. the cerebellum, fourth ventricle
   4. brain stem- 4th
3. Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

Question 218

Physiology of a Neuron

Answer 218

Physiology of a Neuron

Nerve signals are the electrochemical changes that convey information within the nervous system. In the past, nerve signals could be studied only in neurons that had been removed from the body or from other organisms. More advanced techniques now enable researchers to study nerve signals in single, intact nerve cells.

Question 220

1. center for thousands of reflex arcs
2. A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord.
3. Interneurons integrate the incoming data and relay signals to motor neurons.
4. A response to the stimulus occurs when motor axons cause skeletal muscles to contract.
5. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands.
6. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Answer 220

Reflex Actions

and the Spinal Cord

Reflex Arcs

(see Fig. 14.17).

Question 221

Reverse

The central nervous system (CNS) consists of the brain and spinal cord. The brain is completely surrounded and protected by the skull. It connects directly to the spinal cord, similarly protected by the vertebral column.

Answer 221

Central nervous system

Question 222

Reverse

The PNS: Autonomic division • 2 divisions 1. Sympathetic division: coordinates the body for the “fight or flight” response by speeding up metabolism, heart rate, and breathing while slowing down and regulating other functions. 2. Parasympathetic division: counters the sympathetic system by bringing up a relaxed or the “rest and digest” state by slowing down metabolism, heart rate, and breathing, and returning other functions to normal. 14.4 The Peripheral Nervous System 8 The PNS: Autonomic division

Question 223

• 2 divisions – Central nervous system (CNS): –Brain and spinal cord –Peripheral nervous system (PNS): Nerves and ganglia (collections of cell bodies)

Question 224

Peripheral Nervous System (PNS)

• Figure 14.15 illustrates the anatomy of a nerve. The cell body and the dendrites of neurons are in either the CNS or the ganglia. Ganglia (sing., ganglion) are collections of nerve cell bodies outside the CNS. The axons of neurons project from the CNS and form the spinal cord. In other words, nerves, whether cranial or spinal, are composed of axons, the long part of neurons.

Figure 14.15 The structure of a nerve. The peripheral nervous system consists of the cranial nerves and the spinal nerves. A nerve is composed of bundles of axons separated from one another by connective tissue.

** Download, review and look

Answer 224

• outside the central nervous system
• contains the nerves.
  
  1. cranial nerves: brain and are termed
  2. spinal nerves: spinal cord.
  3. all nerves carry signals to and from the CNS.
  • So right now, your eyes are sending messages by way of a cranial nerve to the brain, allowing you to read this text.
  • When you’re finished, your brain will direct the muscles in your fingers, by way of the spinal cord and a spinal nerve, to proceed to the next chapter.
    *

Question 225

reversedprompt

Last steps after Neurotransmitter released in Synaptic Cleft-

** Follow process beginning to end **

List steps and print out and understand photos

Answer 225

• Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft.
• In some synapses, the receiving membrane contains enzymes that rapidly inactivate the neurotransmitter.
  
  • For example, the enzyme acetylcholinesterase (AChE) breaks down the neurotransmitter acetylcholine.
• In other synapses, the sending membrane rapidly reabsorbs the neurotransmitter, possibly for repackaging in synaptic vesicles or for molecular breakdown.
• The short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of receiving membranes.
• The receiving cell needs to be able to respond quickly to changing conditions.
• If the neurotransmitter were to linger in the cleft, the receiving cell would be unable to respond to a new signal from a sending cell.
• Neural Transmission: Synapse
  *

Question 226

Reverse

• This part of the Nervous System

1. performs information processing and integration–summing up the input it receives from all over the body.
2. reviews the information
3. stores the information as memories
4. creates the appropriate motor responses.
   
   • The smell of those baking cookies evokes memories of their taste.

Answer 226

What does the CNS do?

Question 227

Reverse

1. Master controlling and communicating system of the body •
2. Sensory input – gathering information •
3. To monitor changes (stimuli) occurring inside and outside the body •
4. Integration • To process and interpret sensory input and decide if action is needed •
5. Motor output • A response to integrated stimuli •
6. The response activates muscles or glands

Answer 227

14.1 Overview of the nervous system and its primary Functions

Question 228

1. has been called the last great frontier of biology.
2. parts of the brain with reference to the
   
   1. cerebrum- 2 lateral ventricles
   2. the diencephalon- third ventricle
   3. the cerebellum, fourth ventricle
   4. brain stem- 4th
3. Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

Answer 228

Overview of human brain

Answer 229

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 230

Primary Motor Area

Answer 230

• Primary motor area – voluntary control of skeletal muscle • P

Question 231

Structure Spinal Cord

Answer 231

• A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord.
• The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.
• Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.
• (a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections
• The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c).
• Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter.
• The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.
• The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly).
• Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Question 232

1. contain cerebrospinal fluid, as do the meninges that protect the spinal cord.
2. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c) contains:
   
   • Portions of sensory neurons and
   • motor neurons
   • interneurons that communicate with these two types of neurons.
3. spinal nerve

• The dorsal root contains sensory fibers entering the gray matter.
• The ventral root of a spinal nerve contains motor fibers exiting the gray matter. ​
• The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal forming a mixed nerve.

4. The white matter of the spinal cord occurs in areas around the gray matter. ​

• ascending tracts taking information to the brain (primarily located posteriorly)
• and descending tracts taking information from the brain (primarily located anteriorly).
• Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Answer 232

Spinal Cord Structures

Central Canal/ Vertebral Canal

Spinal nerves are a part of the PNS.

(Fig. 14.8c, d),

Question 233

The PNS has divisions: the somatic system and the autonomic system. The nerves in the somatic system serve the skin, skeletal muscles, and tendons (see Fig. 14.2). The somatic system sensory nerves take sensory information from external sensory receptors to the CNS. Motor commands leaving the CNS travel to skeletal muscles via somatic motor nerves.

Not all somatic motor actions are voluntary. Some are automatic. Automatic responses to a stimulus in the somatic system are called reflexes. A reflex occurs quickly, without your even having to think about it. For example, a reflex may cause you to blink your eyes in response to a flash of light, without your willing it. We will study the path of a reflex, because it allows us to study in detail the path of nerve signals to and from the CNS.Page 296

Answer 233

somatic system

Question 234

The brain has 4 of these.

This is where they are found

This is where they join - this number ventricle

the third and fourth ventricle join ____________

The _______________ joins the 4th ventricle ________________

All are filled with ________________________.

See/Download image

Answer 234

1. four ventricles.
2. A lateral ventricle is found on each side of the brain.
3. They join at the third ventricle.
4. The third ventricle connects with the fourth ventricle superiorly;
5. the central canal of the spinal cord joins the fourth ventricle inferiorly.
6. All structures are filled with cerebrospinal fluid.
7. 8. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

Question 235

1. Meninges • Arachnoid mater • Middle layer • Web-like •
2. Pia mater • Internal layer • Clings to the surface of the brain • Many blood vessels

Answer 235

The three layers of the meringes

layers 2 and 3 after Dura Mata

Question 236

PNS Autonomic Division

Answer 236

• The PNS: Autonomic division •
• 2 divisions
• 1. Sympathetic division: coordinates the body for the “fight or flight” response by speeding up metabolism, heart rate, and breathing while slowing down and regulating other functions
• . 2. Parasympathetic division: counters the sympathetic system by bringing up a relaxed or the “rest and digest” state by slowing down metabolism, heart rate, and breathing, and returning other functions to normal. 14.4 The Peripheral Nervous System 8 The PNS: Autonomic division

Question 237

Reverse

This division of the nervous system includes a few cranial nerves and is often referred to as the craniosacral portion of the autonomic nervous system, as well as the housekeeper division.

Answer 237

Parasympathetic Division

Question 238

These are the functions of the nervous system. (slides)

Answer 238

1. Master controlling and communicating system of the body •
2. Sensory input – gathering information •
3. To monitor changes (stimuli) occurring inside and outside the body •
4. Integration •
5. To process and interpret sensory input and decide if action is needed •
6. Motor output • A response to integrated stimuli
7. • The response activates muscles or glands 14.1 Overview of the nervous system T

Question 239

reverse.prompt

14.3 The Limbic System and Higher Mental Functions

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the limbic system.

Explain how the limbic system is involved in memory, language, and speech.

Summarize the types of memory associated with the limbic system.

Question 241

reversedprompt

1. 12 pairs attached to the brain
2. referred to by Roman numerals (Fig. 14.16).
3. Types:
   
   • sensory nerves—only sensory fibers;
   • some are motor nerves– only motor fibers;
   • others are mixed nerves– both sensory and motor fibers.
4. largely concerned with the head, neck, and facial regions of the body.
5. However, the vagus nerve (X) has branches not only to the pharynx and larynx but also to most of the internal organs.
   
   • It arises from the brain stem—specifically, the medulla oblongata, which communicates with the hypothalamus.
     
     • These two parts of the brain control the internal organs.
6. Overall function:
   
   • ​​receive sensory input from, and send motor outputs to, the head region.
   • The spinal nerves receive sensory input from, and send motor outputs to, the rest of the body.
   • Two important exceptions are the vagus nerve, X, which communicates with internal organs, and the spinal accessory nerve, XI, which controls neck and back muscles.

Answer 241

Cranial Nerves

Naming

Overall Function

Types

Figure 14.16

Review and understand upload Figure 14.16

Question 242

The CNS: Brain 4 major parts:

Answer 242

1. Cerebrum 2. Diencephalon 3. Cerebellum 4. Brainstem 14.2 The Central Nervous System 13 The CNS: Overview of the brain

Answer 244

14.5 Drug Therapy and Drug Abuse

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain the ways that drugs interact with the nervous system.

Classify drugs as to whether they have a depressant, stimulant, or psychoactive effect on the nervous system.

List the long-term effects of drug use on the body.

As you are reading these words, synapses throughout your brain are organizing, integrating, and cataloging the information you take in. Neurotransmitters at these synapses control the firing of countless action potentials, thus creating a network of neural circuits. It is amazing to realize that all thoughts, feelings, and actions of a human are dependent on neurotransmitters in the CNS and PNS. By modifying or controlling synaptic transmission, a wide variety of drugs with neurological activity, both legal pharmaceuticals and illegal drugs of abuse, can alter mood, emotional state, behavior, and personality.

Drug Mode of Action

As mentioned in Section 14.1, there are more than 100 known neurotransmitters. The most widely studied neurotransmitters to date are acetylcholine, norepinephrine, dopamine, serotonin, and gamma-aminobutyric acid (GABA). Acetylcholine is an essential CNS neurotransmitter for memory circuits in the limbic system. Norepinephrine is important to dreaming, waking, and mood. The neurotransmitter dopamine plays a central role in the brain’s regulation of mood. Dopamine is also the basal nuclei neurotransmitter that helps organize coordinated movements. Serotonin is involved in thermoregulation, sleeping, emotions, and perception. GABA is an abundant inhibitory neurotransmitter in the CNS.

Neuromodulators are naturally occurring molecules that block the release of a neurotransmitter or modify a neuron’s response to a neurotransmitter. Two well-known neuromodulators are substance P and endorphins. Substance P is a neuropeptide that is released by sensory neurons when pain is present. Endorphins block the release of substance P and serve as natural painkillers. Endorphins are produced by the brain during times of physical and/or emotional stress. They are associated with the “runner’s high” of joggers.

Both pharmaceuticals and illegal drugs have several basic modes of action:

They promote the action of a neurotransmitter, usually by increasing the amount of neurotransmitter at a synapse. Examples include drugs such as alprazolam (Xanax) and diazepam (Valium), which increase GABA. These medications are used for panic attacks and anxiety. Reduced levels of norepinephrine and serotonin are linked to depression. Drugs such as fluoxetine (Prozac), paroxetine (Paxil), and duloxetine (Cymbalta) allow norepinephrine and/or serotonin to accumulate at the synapse, which explains their effectiveness as antidepressants. Alzheimer disease causes a slow, progressive loss of memory (see Section 18.5). Drugs used for Alzheimer disease allow acetylcholine to accumulate at synapses in the limbic system.

They interfere with or decrease the action of a neurotransmitter. For instance, antipsychotic drugs used for the treatment of schizophrenia decrease the activity of dopamine. The caffeine in coffee, chocolate, and tea keeps us awake by interfering with the effects of inhibitory neurotransmitters in the brain.

They replace or mimic a neurotransmitter or neuromodulator. The opiates—namely, codeine, heroin, and morphine—bind to endorphin receptors and in this way reduce pain and produce a feeling of well-being.

Ongoing research into neurophysiology and neuropharmacology (the study of nervous system function and the way drugs work in the nervous system) continues to provide evidence that mental illnesses are caused by imbalances in neurotransmitters. These studies will undoubtedly improve treatments for mental illness, as well as provide insight into the problem of drug abuse.

Drug Abuse

Like mental illness, drug abuse is linked to neurotransmitter levels. As mentioned previously, the neurotransmitter dopamine is essential for mood regulation. Dopamine plays a central role in the working of the brain’s built-in reward circuit. The reward circuit is a collection of neurons that, under normal circumstances, promotes healthy, pleasurable activities, such as consuming food. It’s possible to abuse behaviors such as eating, spending, or gambling Page 300because the behaviors stimulate the reward circuit and make us feel good. Drug abusers take drugs that artificially affect the reward circuit to the point that they neglect their basic physical needs in favor of continued drug use.

Drug abuse is apparent when a person takes a drug at a dose level and under circumstances that increase the potential for a harmful effect. Drug abusers are apt to display a psychological and/or physical dependence on the drug. Psychological dependence is apparent when a person craves the drug, spends time seeking the drug, and takes it regularly. With physical dependence, formerly called “addiction,” the person has become tolerant to the drug. More is needed to get the same effect, and withdrawal symptoms occur when he or she stops taking the drug. This is true for not only teenagers and adults but also newborn babies of mothers who abuse and are addicted to drugs. Alcohol, drugs, and tobacco can all adversely affect the developing embryo, fetus, or newborn.

Alcohol

With the exception of caffeine, alcohol (ethanol) consumption is the most socially accepted form of drug use in the United States. According to a 2015 national survey, 26.9% of high school students reported drinking some alcohol (down from 37.4% in 2014), and 7% binge drank (five-plus drinks in one setting) during the 30 days preceding the survey. Among adults, 86.4% reported they had consumed alcohol during their lifetime, with 56% stating they had used alcohol in the past month.

Alcohol acts as a depressant on many parts of the brain (Table 14.2) by increasing the action of GABA, an inhibitory neurotransmitter. Depending on the amount consumed, the effects of alcohol on the brain can lead to a feeling of relaxation, lowered inhibitions, impaired concentration and coordination, slurred speech, and vomiting. If the blood level of alcohol becomes too high, coma or death can occur.

Table 14.2Drug Influence on the CNS

Table Summary: Table lists the names of different substances in column 1. Other information related to these substances appears in columns 2 and 3.

SubstanceEffectMode of Transmission

AlcoholDepressantDrink

NicotineStimulantSmoked or smokeless tobacco

CocaineStimulantSniffed/snorted, injected, or smoked

Methamphetamine/EcstasyStimulantSmoked or pill form

HeroinDepressantSniffed/snorted, injected, or smoked

Marijuana/K2PsychoactiveSmoked or consumed

Beginning in about 2005, several manufacturers began selling alcoholic energy drinks. With names like Four Loko, JOOSE, and Sparks, these drinks combine fairly high levels of alcohol with caffeine and other ingredients. Although interactions between drugs can be complex, the stimulant effects of caffeine can counteract some of the depressant effects of alcohol, so users feel able to drink more. Because caffeine does not reduce the intoxicating effects of alcohol, many state legislatures are banning these products, and in November 2010 the U.S. Food and Drug Administration warned several manufacturers that they would no longer be allowed to mix caffeine with alcohol in their products.

Nicotine

Although the numbers have been decreasing since 2011 according to the CDC, in 2015, 25.3% of high school students and 7.4% of middle school students reported using a tobacco product. When tobacco is smoked or chewed, nicotine is rapidly delivered throughout the body. It causes a release of epinephrine from the adrenal glands, increasing blood sugar and causing the initial feeling of stimulation. As blood sugar falls, depression and fatigue set in, causing the user to seek more nicotine. In the CNS, nicotine stimulates neurons to release dopamine, a neurotransmitter that promotes a temporary sense of pleasure, and reinforces dependence on the drug. About 70% of people who try smoking become addicted.

As mentioned in earlier chapters, smoking is strongly associated with serious diseases of the cardiovascular and respiratory systems. Once addicted, however, only 10–20% of smokers are able to quit. Most medical approaches to quitting smoking involve the administration of nicotine in safer forms, such as skin patches, gum, or a newly developed nicotine inhaler, so that withdrawal symptoms can be minimized while dependence is gradually reduced. Several antinicotine vaccines (such as NicVAX) are currently in development or in early clinical trials. These vaccines stimulate the production of antibodies that prevent nicotine from entering the brain.

Cocaine and Crack

Cocaine is an alkaloid derived from the shrub Erythroxylon coca. Approximately 35 million Americans have used cocaine by sniffing/snorting, injecting, or smoking. Cocaine is a powerful stimulant in the CNS that interferes with the reuptake of dopamine at synapses, increasing overall brain activity. The result is a rush of a sense of well-being that lasts from 5 to 30 minutes. However, long-term use of cocaine causes a loss of metabolic functions in the brain (Fig. 14.19).

Figure 14.19 Cocaine use. Brain activity before and after the use of cocaine.

(both photos): ©Science Source

“Crack” is the street name given to cocaine that is processed to a free-base form for smoking. The term crack refers to the crackling sound heard when the drug is smoked. Smoking allows high doses of the drug to reach the brain rapidly, providing an intense and immediate high, or “rush.” Approximately 8 million Americans use crack.

A cocaine binge is a period in which a user takes the drug at ever-higher doses. The user is hyperactive, with little desire for food or sleep, but has an increased sex drive. This is followed by a crash period, characterized by fatigue, depression, irritability, and a lack of interest in sex. In fact, men who use cocaine often become impotent.

Cocaine is highly addictive; with continued use, the brain makes less dopamine to compensate for a seemingly endless supply. The user experiences withdrawal symptoms and an intense craving for cocaine. Overdosing on cocaine can cause cardiac and/or respiratory arrest.Page 301

Methamphetamine and Ecstasy

Methamphetamine and ecstasy are considered club, or party, drugs. Methamphetamine (commonly called meth or crank) is a powerful CNS stimulant. Meth is often produced in makeshift home laboratories, usually starting with ephedrine or pseudoephedrine, common ingredients in many cold and asthma medicines. As a result, many states have passed laws making these medications more difficult to purchase. The number of toxic chemicals used to prepare the drug makes a former meth lab site hazardous to humans and to the environment. Over 9 million people in the United States have used methamphetamine at least once. It is available as a powder that can be snorted or as crystals (crystal meth or ice) that can be smoked.

The structure of methamphetamine is similar to that of dopamine, and the most immediate effect of taking meth is a rush of euphoria, energy, alertness, and elevated mood. However, this is typically followed by a state of agitation that, in some individuals, leads to violent behavior. Chronic use can result in what is called an amphetamine psychosis, characterized by paranoia, hallucinations, irritability, and aggressive, erratic behavior.

Ecstasy is the street name for MDMA (methylenedioxymethamphetamine), which is chemically similar to methamphetamine. Many users say that “X,” taken as a pill that looks like an aspirin or candy, increases their feelings of well-being and love for other people. However, it has many of the same side effects as other stimulants, plus it can interfere with temperature regulation, leading to hyperthermia, high blood pressure, and seizures. Although deaths from alcohol abuse are more common, ecstasy is identified as a cause of accidental death in young adults each year.

Drugs with sedative effects, known as date rape or predatory drugs, include flunitrazepam (Rohypnol, or roofies), gamma-hydroxybutyric acid (GHB), and ketamine (special K). Ketamine is actually a drug that veterinarians sometimes use to perform surgery on animals. Any of these drugs can be given to an unsuspecting person, who may fall into a dreamlike state in which he or she is unable to move and thus is vulnerable to sexual assault.

Heroin

Heroin is derived from the resin or sap of the opium poppy plant, which is widely grown in a region from Turkey to Southeast Asia and in parts of Latin America. Drugs derived from opium are called opiates, or more commonly, opioids. This class also includes morphine and codeine. After heroin is injected, snorted, or smoked, a feeling of euphoria, along with relief of any pain, occurs within a few minutes. It is estimated that 4 million Americans have used heroin sometime in their lives, and over 300,000 people use heroin annually.

As with other drugs of abuse, addiction is common. Heroin and opioids bind to receptors meant for the endorphins, naturally occurring neurotransmitters that kill pain and produce feelings of tranquility. With repeated use, the body’s production of endorphins decreases. Tolerance develops, so the user needs to take more of the drug just to prevent withdrawal symptoms (tremors, restlessness, cramps, vomiting), and the original euphoria is no longer felt. In the case of heroin, long-term users commonly acquire hepatitis, HIV/AIDS, and various bacterial infections due to the use of shared needles, and heavy users may experience convulsions and death by respiratory arrest.

Heroin addiction can be treated with synthetic opiate compounds, such as methadone or buprenorphine and naloxone (Suboxone), that decrease withdrawal symptoms and block heroin’s effects. However, methadone itself can be addictive, and methadone-related deaths are on the rise.

Marijuana and K2

Marijuana is the most commonly used illegal drug in the United States. Surveys vary, but in 2015, about 52% of young adults reported using marijuana in their lifetime, and 46% of the U.S. population had tried it at least once. It is derived from the dried flowering tops, leaves, and stems of the marijuana plant, Cannabis sativa, which contain and are covered by a resin that is rich in THC (tetrahydrocannabinol). The names cannabis and marijuana apply to either the plant or THC. Marijuana can be ingested, but usually it is smoked in a cigarette called a “joint.” Beginning with California in 1996, several states have legalized its use for medical purposes, such as in treating cancer, AIDS, and glaucoma. In 2012, Colorado became the first state to legalize recreational use. As of 2018, 8 states had joined Colorado in legalizing recreational use, and 22 additional states had authorized the use of marijuana for medicinal purposes. However, in 2005, the Supreme Court ruled that patients prescribed medical marijuana can still be prosecuted by federal agencies.

Page 302Researchers have found that THC binds to a receptor for anandamide, a naturally occurring neurotransmitter that is important for short-term memory processing, and perhaps for feelings of contentment. The occasional marijuana user experiences mild euphoria, along with alterations in vision and judgment. Heavy use can cause hallucinations, anxiety, depression, paranoia, and psychotic symptoms. Research is underway to identify the effects of long-term marijuana use on the brain, as well as on the effects of secondhand marijuana smoke on the respiratory system.

In recent years, awareness has been increasing about a synthetic compound called K2, or spice. Originally synthesized by an organic chemist at Clemson University, K2 is about ten times as potent as THC. The chemical is typically sprayed onto a mixture of other herbal products and smoked. However, because there is no regulation of how it is produced, the amount of K2 itself, or contaminants, can vary greatly. This may account for the several reports of serious medical problems and even deaths among K2 users.

CHECK YOUR PROGRESS 14.5

Contrast drug therapy and drug abuse.

Answer

Drug therapy is used to treat a disease or disorder. Drug abuse is using drugs without symptoms of disease or disorder.

List how the abuse of drugs, including alcohol and nicotine, affects the nervous system.

Answer

Alcohol and heroin are depressants; nicotine, cocaine, and methamphetamines are stimulants; marijuana produces euphoria.

Detail several modes of action of pharmaceutical and illegal drugs.

Answer

Alcohol increases the action of GABA and increases the release of endorphins in the hypothalamus. Nicotine stimulates dopamine release. Cocaine inhibits dopamine reuptake. Methamphetamine mimics the action of cocaine. Heroin is converted to morphine in the brain and binds to opioid receptors. Marijuana stimulates anandamide receptors.

CONNECTING THE CONCEPTS

For more on the long-term effects of drug use on the systems of the body, refer to the following discussions:

Section 5.7 explores the negative long-term effects of smoking on the cardiovascular system.

Section 11.4 provides information on how alcohol acts as a diuretic in the urinary system.

Section 20.2 examines the relationship between smoking and alcohol use and the increased risk of cancer.

CONCLUSION

The cause of multiple sclerosis (MS) is still unknown, but most researchers agree that there are most likely many contributing factors, including environmental influences, genetics, and a faulty immune system. Many individuals with MS are able to control their symptoms by using immunosuppressive medications, such as beta interferons. The fact that this treatment works suggests that, in many cases, MS is caused by the immune system incorrectly identifying the myelin sheaths as foreign material. The breakdown of the myelin can be detected using both MRI and SSEP tests (discussed in the chapter opener). However, environmental conditions are also suspected to cause MS. Studies have shown that the risk of contracting MS is influenced in part by where in the world you live, although the specific environmental factor or pollutant has not yet been identified. Genetics is also believed to play a role in some cases. But most researchers believe that a defect in a single gene is unlikely. Rather, it is more likely that a certain combination of genetic factors places an individual at a higher risk of contracting MS. Though there is no cure for MS, researchers have been very successful in developing disease-modifying drugs that reduce the symptoms and allow the individual to lead a normal life.

Question 245

• It includes cranial nerves (12 pairs),

spinal nerves (31 pairs), and

ganglia (neuronal cell bodies) outside the CNS. -

Spinal nerves conduct impulses to and from the spinal cord. - Cranial nerves conduct impulses to and from the brain. •

Answer 245

The peripheral nervous system (PNS)

The PNS is divided into 2 systems. - Somatic division - Autonomic division

Question 246

• • extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4).
• From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.
• • Structure of the Spinal Cord
• A cross-section of the spinal cord (Fig. 14.8a) shows a

1. central canal,
2. gray matter, and
3. white matter.

*

Answer 246

The Spinal Cord (Book)

Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

• Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.
• (a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

Question 247

reversedprompt

1. contains motor areas and sensory areas
2. association areas

• The primary motor area is in the

1. frontal lobe just anterior to (before) the central sulcus
   
   1. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section
   2. (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements.
      
      1. ​Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

Answer 247

Primary Motor and Sensory Area of the Cerebral Cortex

See and download Figure 14.11, Page 290

Question 248

reverse.prompt

What is the Limbic System and Higher Level Functions?

What areas may work with lower centers to produce learning and memory?

This is the ability to hold a thought in mind or recall a word from yesterday?

What are types of this

Page 294

Answer 248

• cortical areas may work with
• lower centers to produce learning and memory.
• Memory:
  
  • ​the ability to hold a thought in mind or
  • to recall events from the past, ranging from a word we learned only yesterday to an early emotional experience that has shaped our lives.
  • Types of Memory
    
    1. prefrontal area, active during short-term memory
       
       • seven-digit telephone number for a short time
    2. long term memory:
       
       • ​​memorized phone numbers; often associated w/ place or person associated with that number bc
       • mixture of
       1. semantic memory (numbers, words, etc.)
       2. episodic memory (persons, events, etc.).
       • stored in bits and pieces throughout the sensory association areas of the cerebral cortex.
       1. Visual perceptions: vision association area
       2. sounds: auditory association area
       3. hippocampus serves as a bridge
          
          • ​​between the sensory association areas (where memories are stored) and the prefrontal area (where memories are used).
          • prefrontal area communicates with the hippocampus when memories are stored and when these memories are brought to mind.
          • Some memories are emotionally charged, because the amygdala seems to be responsible for fear conditioning and associating danger with sensory stimuli received from various parts of the brain.
    3. Skill memory
    • independent of episodic memory.
    • performing motor activities
    • first learns a skill, more areas of the cerebral cortex are involved than after the skill is perfected
    • later automatic.
    • all the motor areas of the _cerebrum below the level of consciousness._
    • Long-Term Memory Storage and Retrieval
    • • Learning: when we retain and use past memories.
• *

Question 249

On her way to work, Sarah noticed that the colors of the traffic lights didn’t seem quite right; the red lights appeared to be more orange than red. At work, she realized she was having trouble reading her e-mail. By the end of the day, she had a splitting headache. She kept telling herself she was just working too hard. But even as she tried to remain calm, deep down she had a bad feeling. Within a few weeks, she was almost completely blind in one eye and the sensations in her feet felt muffled, as if they were wrapped in gauze. Her doctor referred her to a neurologist, who immediately ordered a magnetic resonance imaging (MRI) scan of her brain and a series of somatosensory evoked potential (SSEP) tests to examine how her nervous system was processing electrical impulses.

The results indicated that Sarah had multiple sclerosis (MS), which is an inflammatory disease. This disease affects the myelin sheaths, which wrap parts of some nerve cells like insulation around an electrical cord. As these sheaths deteriorate, the nerves no longer conduct impulses normally. For unknown reasons, multiple sclerosis often attacks the optic nerves first before proceeding to other areas of the brain. Sarah’s doctors were able to treat her MS symptoms using high doses of immunosuppressive medications. Unfortunately, there is no cure for MS, but most patients can control the symptoms with daily injections of medication.

As you read through the chapter, think about the following questions:

Why would a deterioration of the myelin sheaths cause a nerve cell to function incorrectly?

How would an MRI and SSEP test indicate there was a problem with Sarah’s neurological functions?

Why are many individuals who contract MS eventually confined to a wheelchair?

Question 251

Reverse

Expanding on neurons • Neuron structure (Ch. 4 review) • Cell body – main cell where nucleus and most organelles reside • Dendrites – many short extensions that carry impulses to a cell body • Axon (nerve fiber) – single, long extension that carries impulses away from the cell body

Question 253

Reverse

reasoning

critical thinking

formulating appropriate behaviors

Answer 253

prefrontal cortex

Question 254

• infection of the meninges and may be caused by either bacterial or viral pathogens.
• Meningitis

Answer 254

This is an infection that may be caused by either bacterial or viral pathogens and takes place in the protective covering of the brain.

Answer 255

Chapter Review

SUMMARIZE

14.1Overview of the Nervous System

The nervous system

Is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

Has three functions: (1) reception of input, (2) integration of data, and (3) generation of motor output.

Nervous Tissue

Nervous tissue contains the following two types of cells: neurons and neuroglia:

Neurons transmit nerve signals using action potentials.

Neuroglia nourish and support neurons.

Anatomy of a Neuron

A neuron is composed of dendrites, a cell body, and an axon. The axons of neurons may be clustered into nerves. There are three types of neurons:

Sensory neurons take nerve signals from sensory receptors to the CNS.

Interneurons occur within the CNS.

Motor neurons take nerve signals from the CNS to effectors (muscles or glands).

Myelin Sheath

Long axons are covered by a myelin sheath, which is formed by the neuroglia cells. Gaps in the myelin sheath are called nodes of Ranvier.

Multiple sclerosis (MS) occurs when the myelin sheath breaks down, causing a short-circuiting of nerve signals.

Physiology of a Neuron

Nerve signals move information within the nervous system. The generation of a nerve signal is based on the polarity across the membrane of the neuron.

Resting potential: There is more Na+ outside the axon and more K+ inside the axon. The axon does not conduct a signal. The resting potential is maintained by active transport using the sodium–potassium pump.

Action potential: On receipt of a stimulus strong enough to overcome the threshold, a change in polarity across the axonal membrane as a nerve signal occurs: When Na+ gates open, Na+ moves to the inside Page 303of the axon, and a depolarization occurs. When K+ gates open, K+ moves to the outside of the axon, and a repolarization occurs.

Signal propagation: The presence of the myelin sheath speeds the movement of the nerve signal by saltatory conduction. After the action potential has passed, a refractory period occurs during which no additional action potentials may be processed.

The Synapse

At the end of each axon is an axon terminal, which borders the synapse between another neuron or target cell.

When a neurotransmitter is released into a synaptic cleft, transmission of a nerve signal occurs.

Binding of the neurotransmitter to receptors in the receiving membrane causes excitation or inhibition.

Enzymes, such as acetylcholinesterase (AChE), assist in removing the neurotransmitter from the synaptic cleft.

Neurotransmitters

Neurotransmitters, such as acetylcholine, norepinephrine, and serotonin, are used to convey signals across the synapses.

Integration is the summing of excitatory and inhibitory signals.

14.2The Central Nervous System

The CNS receives and integrates sensory input and formulates motor output. The CNS consists of the spinal cord and brain. The CNS is protected by the meninges, which are filled with cerebrospinal fluid. The same fluid fills the four ventricles of the brain. In the CNS, gray matter contains cell bodies and nonmyelinated fibers. White matter contains myelinated axons organized as tracts.

The Spinal Cord

The spinal cord is responsible for conduction of information to and from the brain and carries out reflex actions.

The Brain

The cerebrum: The cerebrum has two cerebral hemispheres connected by the corpus callosum.

Sensation, reasoning, learning and memory, and language and speech take place in the cerebrum.

The cerebral cortex of each cerebral hemisphere has four lobes: frontal, parietal, occipital, and temporal.

The primary motor area in the frontal lobe sends out motor commands to lower brain centers, which pass them on to motor neurons.

The primary somatosensory area in the parietal lobe receives sensory information from lower brain centers in communication with sensory neurons.

Association areas are located in all the lobes. The prefrontal area in the frontal lobe is involved in reasoning and planning of actions.

Wernicke’s area and Broca’s area are two processing centers involved in speech.

Basal nuclei: The basal nuclei integrate commands to the muscles to coordinate movement. Parkinson disease is associated with the degradation of neurons in this area.

The diencephalon: The diencephalon contains both the hypothalamus and the thalamus. The hypothalamus controls homeostasis. The thalamus sends sensory input to the cerebrum.

The cerebellum: The cerebellum coordinates skeletal muscle contractions.

The brain stem: The brain stem includes the midbrain, the pons, and the medulla oblongata.

The medulla oblongata and pons have centers for breathing and the heartbeat.

The midbrain serves as a relay station between the cerebrum and spinal cord or cerebellum.

The reticular formation is part of the reticular activating system (RAS), which transfers sensory signals to higher processing centers in the brain.

14.3The Limbic System and Higher Mental Functions

The limbic system, located deep in the brain, is involved in determining emotions and higher mental functions, such as learning.

The amygdala determines when a situation deserves the emotion we call “fear.”

The hippocampus is particularly involved in storing and retrieving memories.

A memory may be processed as either short-term memory or long-term memory. Long-term memory may be classified as either semantic memory or episodic memory. Skill memory is involved with processes such as riding a bike.

14.4The Peripheral Nervous System

The PNS contains only nerves and ganglia (sing., ganglion).

Cranial nerves take impulses to and from the brain.

Spinal nerves take impulses to and from the spinal cord.

The PNS is divided into the somatic system and the autonomic system.

The Somatic System

The somatic system serves the skin, skeletal muscles, and tendons.

Some actions are due to reflexes, which are automatic and involuntary.

Other actions are voluntary and originate in the cerebral cortex.

The Autonomic System

The autonomic system is further divided into the sympathetic division and the parasympathetic division.

Sympathetic division: responses that occur during times of stress

Parasympathetic division: responses that occur during times of relaxation

Actions in these divisions are involuntary and automatic.

These divisions innervate internal organs.

Two neurons and one ganglion are used for each impulse.Page 304

14.5Drug Therapy and Drug Abuse

Neurotransmitters, such as acetylcholine, norepinephrine, dopamine, and serotonin, play an important role in moving signals within the nervous system.

Neuromodulators block the release of a neurotransmitter.

Neurological drugs promote, prevent, or mimic the action of a particular neurotransmitter.

Drugs, such as alcohol, nicotine, and marijuana, may have depressant, stimulant, or psychoactive effects.

Dependency occurs when the body compensates for the presence of neurological drugs.

ASSESS

TESTING YOURSELF

Choose the best answer for each question.

14.1Overview of the Nervous System

Which of the following neuron parts receive(s) signals from sensory receptors of other neurons?

cell body

axon

dendrites

Both a and c are correct.

The neuroglia cells that form myelin sheaths in the CNS are called

oligodendrocytes.

ganglionic cells.

Schwann cells.

astrocytes.

microglia.

Which of these correctly describes the distribution of ions on either side of an axon when it is not conducting a nerve signal?

more sodium ions (Na+) outside and more potassium ions (K+) inside

more K+ outside and more Na+ inside

charged protein outside and Na+ and K+ inside

Na+ and K+ outside and water only inside

chloride ions (Cl−) outside and K+ and Na+ inside

When the action potential begins, sodium gates open, allowing Na+ to cross the membrane. This causes the charge on the inside of the neuron to become

more negative.

more positive.

neutral.

None of these are correct.

Repolarization of an axon during an action potential is produced by

inward diffusion of Na+.

outward diffusion of K+.

inward active transport of Na+.

active extrusion of K+.

Transmission of the nerve signal across a synapse is accomplished by the

movement of Na+ and K+.

release of a neurotransmitter by a dendrite.

release of a neurotransmitter by an axon.

release of a neurotransmitter by a cell body.

All of these are correct.

14.2The Central Nervous System

Which of the following cerebral areas is not correctly matched with its function?

occipital lobe—vision

parietal lobe—somatosensory area

temporal lobe—primary motor area

frontal lobe—Broca’s motor speech area

Which of the following brain regions is not correctly described?

The medulla oblongata regulates heartbeat, breathing, and blood pressure.

The cerebellum coordinates voluntary muscle movements.

The thalamus secretes melatonin, which regulates daily body rhythms.

The midbrain acts as a reflex center for visual, auditory, and tactile responses.

This part of the brain forms the link between the nervous system and the endocrine system.

corpus callosum

reticular formation

amygdala

hypothalamus

14.3The Limbic System and Higher Mental Functions

The regulation of the information that is to be relayed to memory is the function of the

reticular formation.

hippocampus.

hypothalamus.

cerebellum.

pons.

Memories are stored in the sensory association areas of the

cerebral cortex.

spinal cord.

brain stem.

hypothalamus.

14.4The Peripheral Nervous System

Label this diagram.

Page 305Which of the following is correct regarding the autonomic nervous system?

The action of its two divisions tends to have opposite effects on its target organs.

It is divided into sympathetic and parasympathetic divisions.

It is involved in reflect responses.

Major responsibilities are regulation of cardiac muscle, smooth muscles, organs, and glands.

All of these are correct.

The sympathetic division of the autonomic system does not cause

the liver to release glycogen.

dilation of bronchioles.

the gastrointestinal tract to digest food.

an increase in the heart rate.

14.5Drug Therapy and Drug Abuse

This neurotransmitter plays an important role in sleeping, emotions, and perception.

dopamine

acetylcholine

GABA

seratonin

Which of the following is a depressant of the CNS?

cocaine

methamphetamine

ecstasy

alcohol

ENGAGE

THINKING CRITICALLY

Demyelinating disorders, such as multiple sclerosis (discussed in the chapter opener), are the subject of numerous research projects. Many investigations focus on the cells that create myelin: the Schwann cells of the PNS and oligodendrocytes in the CNS. Other studies focus on immune system cells that attack this myelin sheath. The goal of this research is to determine how to restore lost myelin, which might help (or possibly cure) people living with MS and other demyelinating diseases. Investigations into the role played by the sheath in nerve regeneration may offer hope to victims of spinal cord injury.

Why are impulses transmitted more quickly down a myelinated axon than down an unmyelinated axon?

A buildup of very-long-chain saturated fatty acids is believed to be the cause of myelin loss in adrenoleukodystrophy. This rare disease is a demyelinating disorder like MS. It is the subject of the film Lorenzo’s Oil. This real-life drama focuses on Lorenzo Odone, whose parents successfully developed a diet that helped their son.

From your study of chemistry in Chapter 2, a fatty acid is a part of what type of molecule?

What distinguishes a saturated fatty acid from an unsaturated fatty acid?

From your study of nutrition in Chapter 9, what types of foods contain saturated fatty acids?

Why would you expect the motor skills of a child to improve as myelination continued during early childhood development?

Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC

ANSWER KEY

Testing Yourself

Click here for the answers to the Testing Yourself questions.

Answer

Testing Yourself: 1. c; 2. a; 3. a; 4. b; 5. b; 6. c; 7. c; 8. c; 9. d; 10. b; 11. a; 12. a. central canal; b. gray matter; c. white matter; d. cell body of interneuron; e. cell body of sensory neuron; 13. e; 14. c; 15. d; 16. d

Thinking Critically

Click here for the answers to the Thinking Critically questions.

Answer

Thinking Critically: 1. Myelin enables the signal to jump from node to node quickly, because the depolarization process occurs only at the node of Ranvier. 2a. Triglycerides, phospholipids. 2b. Unsaturated fatty acids (usually liquid at room temperature) are characterized by one or more double bonds between carbons, whereas saturated fatty acids (usually solid at room temperature) have all single bonds. 2c. Animal fat, butter, fatty cuts of meat. 3. Myelination enables signals to travel through axons more quickly, which helps coordinate motor skills.

Answer 257

Master controlling and communicating system of the body • Sensory input – gathering information • To monitor changes (stimuli) occurring inside and outside the body • Integration • To process and interpret sensory input and decide if action is needed • Motor output • A response to integrated stimuli • The response activates muscles or glands 14.1 Overview of the nervous system T

Question 258

CNS 14.2 Lecture Notes

This is called the CEO of the brain and controls working memory.

Answer 258

• Prefrontal Cortex
• “CEO of the brain”
• Where you control and plan your actions
• Working memory
• Organization
• Modulate your mood
• Conscience
• Personality • Not fully developed until at least 25 years of age– maybe even later! (You can blame your bad decisions on this if you are younger than this– ha!– or flip it around: drugs, alcohol, excessive videogaming, etc. can really have a permanent negative impact on this developing brain area even if you are of ‘legal age’…)

Question 259

Reverse

SUMMARIZE

14.1Overview of the Nervous System

The nervous system

Is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

Has three functions: (1) reception of input, (2) integration of data, and (3) generation of motor output.

Nervous Tissue

Nervous tissue contains the following two types of cells: neurons and neuroglia:

Neurons transmit nerve signals using action potentials.

Neuroglia nourish and support neurons.

Anatomy of a Neuron

A neuron is composed of dendrites, a cell body, and an axon. The axons of neurons may be clustered into nerves. There are three types of neurons:

Sensory neurons take nerve signals from sensory receptors to the CNS.

Interneurons occur within the CNS.

Motor neurons take nerve signals from the CNS to effectors (muscles or glands).

Myelin Sheath

Long axons are covered by a myelin sheath, which is formed by the neuroglia cells. Gaps in the myelin sheath are called nodes of Ranvier.

Multiple sclerosis (MS) occurs when the myelin sheath breaks down, causing a short-circuiting of nerve signals.

Physiology of a Neuron

Nerve signals move information within the nervous system. The generation of a nerve signal is based on the polarity across the membrane of the neuron.

Resting potential: There is more Na+ outside the axon and more K+ inside the axon. The axon does not conduct a signal. The resting potential is maintained by active transport using the sodium–potassium pump.

Action potential: On receipt of a stimulus strong enough to overcome the threshold, a change in polarity across the axonal membrane as a nerve signal occurs: When Na+ gates open, Na+ moves to the inside Page 303of the axon, and a depolarization occurs. When K+ gates open, K+ moves to the outside of the axon, and a repolarization occurs.

Signal propagation: The presence of the myelin sheath speeds the movement of the nerve signal by saltatory conduction. After the action potential has passed, a refractory period occurs during which no additional action potentials may be processed.

The Synapse

At the end of each axon is an axon terminal, which borders the synapse between another neuron or target cell.

When a neurotransmitter is released into a synaptic cleft, transmission of a nerve signal occurs.

Binding of the neurotransmitter to receptors in the receiving membrane causes excitation or inhibition.

Enzymes, such as acetylcholinesterase (AChE), assist in removing the neurotransmitter from the synaptic cleft.

Neurotransmitters

Neurotransmitters, such as acetylcholine, norepinephrine, and serotonin, are used to convey signals across the synapses.

Integration is the summing of excitatory and inhibitory signals.

14.2The Central Nervous System

The CNS receives and integrates sensory input and formulates motor output. The CNS consists of the spinal cord and brain. The CNS is protected by the meninges, which are filled with cerebrospinal fluid. The same fluid fills the four ventricles of the brain. In the CNS, gray matter contains cell bodies and nonmyelinated fibers. White matter contains myelinated axons organized as tracts.

The Spinal Cord

The spinal cord is responsible for conduction of information to and from the brain and carries out reflex actions.

The Brain

The cerebrum: The cerebrum has two cerebral hemispheres connected by the corpus callosum.

Sensation, reasoning, learning and memory, and language and speech take place in the cerebrum.

The cerebral cortex of each cerebral hemisphere has four lobes: frontal, parietal, occipital, and temporal.

The primary motor area in the frontal lobe sends out motor commands to lower brain centers, which pass them on to motor neurons.

The primary somatosensory area in the parietal lobe receives sensory information from lower brain centers in communication with sensory neurons.

Association areas are located in all the lobes. The prefrontal area in the frontal lobe is involved in reasoning and planning of actions.

Wernicke’s area and Broca’s area are two processing centers involved in speech.

Basal nuclei: The basal nuclei integrate commands to the muscles to coordinate movement. Parkinson disease is associated with the degradation of neurons in this area.

The diencephalon: The diencephalon contains both the hypothalamus and the thalamus. The hypothalamus controls homeostasis. The thalamus sends sensory input to the cerebrum.

The cerebellum: The cerebellum coordinates skeletal muscle contractions.

The brain stem: The brain stem includes the midbrain, the pons, and the medulla oblongata.

The medulla oblongata and pons have centers for breathing and the heartbeat.

The midbrain serves as a relay station between the cerebrum and spinal cord or cerebellum.

The reticular formation is part of the reticular activating system (RAS), which transfers sensory signals to higher processing centers in the brain.

14.3The Limbic System and Higher Mental Functions

The limbic system, located deep in the brain, is involved in determining emotions and higher mental functions, such as learning.

The amygdala determines when a situation deserves the emotion we call “fear.”

The hippocampus is particularly involved in storing and retrieving memories.

A memory may be processed as either short-term memory or long-term memory. Long-term memory may be classified as either semantic memory or episodic memory. Skill memory is involved with processes such as riding a bike.

14.4The Peripheral Nervous System

The PNS contains only nerves and ganglia (sing., ganglion).

Cranial nerves take impulses to and from the brain.

Spinal nerves take impulses to and from the spinal cord.

The PNS is divided into the somatic system and the autonomic system.

The Somatic System

The somatic system serves the skin, skeletal muscles, and tendons.

Some actions are due to reflexes, which are automatic and involuntary.

Other actions are voluntary and originate in the cerebral cortex.

The Autonomic System

The autonomic system is further divided into the sympathetic division and the parasympathetic division.

Sympathetic division: responses that occur during times of stress

Parasympathetic division: responses that occur during times of relaxation

Actions in these divisions are involuntary and automatic.

These divisions innervate internal organs.

Two neurons and one ganglion are used for each impulse.Page 304

14.5Drug Therapy and Drug Abuse

Neurotransmitters, such as acetylcholine, norepinephrine, dopamine, and serotonin, play an important role in moving signals within the nervous system.

Neuromodulators block the release of a neurotransmitter.

Neurological drugs promote, prevent, or mimic the action of a particular neurotransmitter.

Drugs, such as alcohol, nicotine, and marijuana, may have depressant, stimulant, or psychoactive effects.

Dependency occurs when the body compensates for the presence of neurological drugs.

ASSESS

TESTING YOURSELF

Choose the best answer for each question.

14.1Overview of the Nervous System

Which of the following neuron parts receive(s) signals from sensory receptors of other neurons?

cell body

axon

dendrites

Both a and c are correct.

The neuroglia cells that form myelin sheaths in the CNS are called

oligodendrocytes.

ganglionic cells.

Schwann cells.

astrocytes.

microglia.

Which of these correctly describes the distribution of ions on either side of an axon when it is not conducting a nerve signal?

more sodium ions (Na+) outside and more potassium ions (K+) inside

more K+ outside and more Na+ inside

charged protein outside and Na+ and K+ inside

Na+ and K+ outside and water only inside

chloride ions (Cl−) outside and K+ and Na+ inside

When the action potential begins, sodium gates open, allowing Na+ to cross the membrane. This causes the charge on the inside of the neuron to become

more negative.

more positive.

neutral.

None of these are correct.

Repolarization of an axon during an action potential is produced by

inward diffusion of Na+.

outward diffusion of K+.

inward active transport of Na+.

active extrusion of K+.

Transmission of the nerve signal across a synapse is accomplished by the

movement of Na+ and K+.

release of a neurotransmitter by a dendrite.

release of a neurotransmitter by an axon.

release of a neurotransmitter by a cell body.

All of these are correct.

14.2The Central Nervous System

Which of the following cerebral areas is not correctly matched with its function?

occipital lobe—vision

parietal lobe—somatosensory area

temporal lobe—primary motor area

frontal lobe—Broca’s motor speech area

Which of the following brain regions is not correctly described?

The medulla oblongata regulates heartbeat, breathing, and blood pressure.

The cerebellum coordinates voluntary muscle movements.

The thalamus secretes melatonin, which regulates daily body rhythms.

The midbrain acts as a reflex center for visual, auditory, and tactile responses.

This part of the brain forms the link between the nervous system and the endocrine system.

corpus callosum

reticular formation

amygdala

hypothalamus

14.3The Limbic System and Higher Mental Functions

The regulation of the information that is to be relayed to memory is the function of the

reticular formation.

hippocampus.

hypothalamus.

cerebellum.

pons.

Memories are stored in the sensory association areas of the

cerebral cortex.

spinal cord.

brain stem.

hypothalamus.

14.4The Peripheral Nervous System

Label this diagram.

Page 305Which of the following is correct regarding the autonomic nervous system?

The action of its two divisions tends to have opposite effects on its target organs.

It is divided into sympathetic and parasympathetic divisions.

It is involved in reflect responses.

Major responsibilities are regulation of cardiac muscle, smooth muscles, organs, and glands.

All of these are correct.

The sympathetic division of the autonomic system does not cause

the liver to release glycogen.

dilation of bronchioles.

the gastrointestinal tract to digest food.

an increase in the heart rate.

14.5Drug Therapy and Drug Abuse

This neurotransmitter plays an important role in sleeping, emotions, and perception.

dopamine

acetylcholine

GABA

seratonin

Which of the following is a depressant of the CNS?

cocaine

methamphetamine

ecstasy

alcohol

ENGAGE

THINKING CRITICALLY

Demyelinating disorders, such as multiple sclerosis (discussed in the chapter opener), are the subject of numerous research projects. Many investigations focus on the cells that create myelin: the Schwann cells of the PNS and oligodendrocytes in the CNS. Other studies focus on immune system cells that attack this myelin sheath. The goal of this research is to determine how to restore lost myelin, which might help (or possibly cure) people living with MS and other demyelinating diseases. Investigations into the role played by the sheath in nerve regeneration may offer hope to victims of spinal cord injury.

Why are impulses transmitted more quickly down a myelinated axon than down an unmyelinated axon?

A buildup of very-long-chain saturated fatty acids is believed to be the cause of myelin loss in adrenoleukodystrophy. This rare disease is a demyelinating disorder like MS. It is the subject of the film Lorenzo’s Oil. This real-life drama focuses on Lorenzo Odone, whose parents successfully developed a diet that helped their son.

From your study of chemistry in Chapter 2, a fatty acid is a part of what type of molecule?

What distinguishes a saturated fatty acid from an unsaturated fatty acid?

From your study of nutrition in Chapter 9, what types of foods contain saturated fatty acids?

Why would you expect the motor skills of a child to improve as myelination continued during early childhood development?

Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC

ANSWER KEY

Testing Yourself

Click here for the answers to the Testing Yourself questions.

Answer

Testing Yourself: 1. c; 2. a; 3. a; 4. b; 5. b; 6. c; 7. c; 8. c; 9. d; 10. b; 11. a; 12. a. central canal; b. gray matter; c. white matter; d. cell body of interneuron; e. cell body of sensory neuron; 13. e; 14. c; 15. d; 16. d

Thinking Critically

Click here for the answers to the Thinking Critically questions.

Answer

Thinking Critically: 1. Myelin enables the signal to jump from node to node quickly, because the depolarization process occurs only at the node of Ranvier. 2a. Triglycerides, phospholipids. 2b. Unsaturated fatty acids (usually liquid at room temperature) are characterized by one or more double bonds between carbons, whereas saturated fatty acids (usually solid at room temperature) have all single bonds. 2c. Animal fat, butter, fatty cuts of meat. 3. Myelination enables signals to travel through axons more quickly, which helps coordinate motor skills.

Question 260

Reverse

The CNS: Brain 4 major parts: 1. Cerebrum 2. Diencephalon 3. Cerebellum 4. Brainstem 14.2 The Central Nervous System 13 The CNS: Overview of the brain

Question 261

• The unique ability of humans to speak is partially dependent on two processing centers
• found only in the left cerebral cortex.

1. Wernicke’s area is located in the posterior part of the left temporal lobe.
   * Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. ​
2. Broca’s area is located in the left frontal lobe.

• Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.​
• just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10).

Answer 261

Processing Centers

Speaking

and the left cerebral cortex

Question 262

Reverse

• Resting potential (RP): when the axon is not conducting a nerve impulse (when the axon is “at rest”)

1. More positive ions outside than inside thee membrane
2. Negative charge of -70 mV inside the axon
3. More Na+ outside than inside
4. More K+ inside than outside

• • The nerve impulse: action potential
• • Action potential – rapid change in the axon membrane; a nerve impulse– threshold is -55mV
  
  • Sodium gates open letting Na+ in
  • Depolarization occurs (-70mV to threshold-55mV)
  • Interior of axon loses negative charge (+35mV)
  • Potassium gates open letting K+ out
  • Repolarization occurs
  • Interior of axon regains negative charge (-70mV)
  • Wave of depolarization/repolarization travels down the axon.
  • Resting potential is restored by moving potassium inside and sodium outside

Answer 262

Nerve Impulse

14.1 Overview of the Nervous System

Slides

*** vital short answer slide from lecture** make sure to understand and add to study sheet

Question 263

Reverse

• It extends from the base of the brain and
• along the length of the vertebral canal formed by the vertebrae.
• provide communication between the brain and most of the body.
• It is the integrating center for reflex arcs.
• Gray matter in the center is in a butterfly shape.
• White matter surrounds the gray matter.

Answer 263

The CNS: Spinal Cord

Question 264

Reverse

These are categories of effects on drug abusers.

This defined physical dependence.

Answer 264

1. • Drug abusers tend to show a physiological and psychological effect. •
2. Once a person is physically dependent, they usually need more of the drug for the same effect because their body has become tolerant, where they are used to the presence of the drug and work at this level to maintain homeostasis.

Answer 265

SCIENCE IN YOUR LIFE

What is amnesia?

Amnesia results from disruption of the memory pathways and can be temporary or permanent. In anterograde amnesia, injury to the limbic system separates long-term memories of events that occurred prior to the injury from events that occur in the here and now. An affected person might carry on a conversation about past events (memories of a long-ago birthday) but be unable to recall a breakfast menu from that morning. In retrograde amnesia, a blow to the head or similar injury abolishes all memories for a variable time before the injury. For example, a head injury occurring during a car accident may abolish all memories from hours to days prior to the accident.

Question 266

14.2 The Central Nervous System 25 The reticular formation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RAS radiates to cerebral cortex. thalamus reticular formation ascending sensory tracts (touch, pain, temperature)

Question 267

These are the central nervous system, and this is what is primarily done.

Answer 267

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Answer 268

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 269

This part of the brain is the largest and this is what it’s function is

Answer 269

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

1. largest portion of the brain in mammals, including humans.
2. last center to receive sensory input and carry out integration before commanding voluntary motor responses.
3. It communicates with and coordinates the activities of the other parts of the brain.

Question 270

Reverse

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

Answer 270

Brain Ventricles

Question 271

Reverse

Lecture slides

Answer 271

1. Function:
2. Support and brace neurons (microfilaments) –Processes form barrier between capillaries and neurons, block out harmful substances in blood –
3. Guide migration of young neurons
4. Aids in synapse formation for learning and memory

Question 272

reversedprompt

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Answer 272

List the functions of the spinal cord.

Question 273

reverse.prompt

• emerge either side of the spinal cord
• 31 pairs
• The roots physically separate the axons of sensory neurons from the axons of motor neurons: forming an arrangement resembling a letter Y.
• The posterior root contains sensory fibers that direct sensory receptor information inward (toward the spinal cord).
• The cell body of a sensory neuron is in a posterior-root ganglion (also termed a dorsal-root ganglion).
• The anterior (also termed ventral) root of a spinal nerve contains motor fibers that conduct impulses outward (away from the cord) to the effectors.
• Observe in Figure 14.8
• that the anterior and posterior roots join to form a spinal nerve.
• All spinal nerves are called mixed nerves, because they contain both sensory and motor fibers.
• Each spinal nerve serves the particular region of the body in which it is located.
  
  • For example, the intercostal muscles of the rib cage are innervated by thoracic nerves.

Answer 273

Spinal Nerves

(see Fig. 14.8).

Question 274

Check Yourself

Answer 274

CHECK YOUR PROGRESS 14.1

Describe the three types of neurons, and list the three main parts of a neuron.

Answer

Sensory neurons take nerve signals from a sensory receptor to the CNS. Interneurons lie entirely within the CNS and communicate with other neurons. Motor neurons move nerve impulses away from the CNS to an effector. The parts are cell body, dendrites, and axon.

Describe how a nerve impulse is propagated.

Answer

An exchange of Na+ and K+ ions generates an action potential that moves along the length of an axon. An action potential in one location stimulates the production of an action potential in an adjacent part of the axon membrane. If the nerve is myelinated, the action potential moves more quickly, “jumping” from one node of Ranvier to the next.

Summarize how a nerve impulse is transmitted from one neuron to the next.

Answer

An action potential arrives at the axon terminal and calcium enters the terminal. Synaptic vesicles enclosing the neurotransmitter fuse with the sending neuron’s membrane. Neurotransmitters are released, travel across the synapse, and bind to receptors on the receiving neuron membrane. Sodium diffuses into the receiving neuron, and an action potential is created.

CONNECTING THE CONCEPTS

For more information on neurons and the nervous system, refer to the following discussions:

Section 4.4 explores how stem cells may be used to regenerate nervous tissue.

Figure 13.7 illustrates the role of the synapse in the neuromuscular junction.

Section 15.1 explains how the peripheral nervous system sends information to and from the central nervous system.

Question 275

• integration occurs and
• where memories are stored.
• Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time.
• Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs.
• The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Answer 275

Association Areas

Question 276

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

Answer 276

The central nervous system • The CNS consists of the brain and spinal cord. • Both are protected by • Scalp and skin • Bones – skull and vertebral column • Meninges – 3 protective membranes that wrap around CNS • Cerebral spinal fluid (CSF) – space between meninges is filled with this fluid that cushions and protects the CNS • Blood brain barrier (BBB)

Meninges • Dura mater • Double-layered external covering • Periosteum – dense connective tissue attached to surface of the skull • Meningeal layer – outer covering of the brain • Folds inward in several areas

Meninges • Arachnoid mater • Middle layer • Web-like • Pia mater • Internal layer • Clings to the surface of the brain • Many blood vessels http://droualb.faculty.mjc.edu/Lecture%20Notes/Unit%205/Meninges_peeled_away

Question 277

Primary Somatosensory Areas

Answer 277

Primary somatosensory area – for sensory information from skeletal muscle and skin •

Answer 278

Function: –Support and brace neurons (microfilaments) –Processes form barrier between capillaries and neurons, block out harmful substances in blood –Guide migration of young neurons –Aids in synapse formation for learning and memory

Answer 279

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Question 281

Drug Mode of Action

As mentioned in Section 14.1, there are more than 100 known neurotransmitters. The most widely studied neurotransmitters to date are acetylcholine, norepinephrine, dopamine, serotonin, and gamma-aminobutyric acid (GABA). Acetylcholine is an essential CNS neurotransmitter for memory circuits in the limbic system. Norepinephrine is important to dreaming, waking, and mood. The neurotransmitter dopamine plays a central role in the brain’s regulation of mood. Dopamine is also the basal nuclei neurotransmitter that helps organize coordinated movements. Serotonin is involved in thermoregulation, sleeping, emotions, and perception. GABA is an abundant inhibitory neurotransmitter in the CNS.

Question 282

Alcohol

Answer 282

Alcohol

With the exception of caffeine, alcohol (ethanol) consumption is the most socially accepted form of drug use in the United States. According to a 2015 national survey, 26.9% of high school students reported drinking some alcohol (down from 37.4% in 2014), and 7% binge drank (five-plus drinks in one setting) during the 30 days preceding the survey. Among adults, 86.4% reported they had consumed alcohol during their lifetime, with 56% stating they had used alcohol in the past month.

Alcohol acts as a depressant on many parts of the brain (Table 14.2) by increasing the action of GABA, an inhibitory neurotransmitter. Depending on the amount consumed, the effects of alcohol on the brain can lead to a feeling of relaxation, lowered inhibitions, impaired concentration and coordination, slurred speech, and vomiting. If the blood level of alcohol becomes too high, coma or death can occur.

Question 284

Cerebrospinal fluid abnormalities in children and adults

Answer 284

1. infant, the brain enlarges due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”).
2. If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Question 286

Neuron structure

Lecture slides

Answer 286

• (Ch. 4 review)
• Cell body – main cell where nucleus and most organelles reside
• Dendrites – many short extensions that carry impulses to a cell body
• Axon (nerve fiber) – single, long extension that carries impulses away from the cell body

Question 287

• This is a complex network of nuclei–masses of gray matter–and fibers that extends the length of the brain stem
• major component of the reticular activating system (RAS): receives sensory signals and sends them to higher centers.
  
  • Motor signals received by the RAS are sent to the spinal cord.
  • arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face.
  • The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on.
  • Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting.
  • To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep.
  • General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

Answer 287

What is The Reticular Formation

(Fig. 14.12)- See, download and study

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

Question 288

Reverse

Expanding on neurons • 3 types of neurons • Sensory – takes impulses from sensory receptor to CNS • Interneuron – receives information in the CNS and sends it to a motor neuron • Motor – takes impulses from the CNS to an effector (i.e., gland or muscle fiber)

Question 289

reversedprompt

• amygdala hippocampus olfactory bulb olfactory tract hypothalamus corpus thalamus callosum Figure 14.12 The regions of the brain associated with the limbic system. 14.3 The Limbic System and Higher Mental Functions 28 • Learning – what happens when we recall and use past memories • Memory – ability to hold a thought or to recall past events • Short-term memory – retention of information for only a few minutes 14.3 The Limbic System and Higher Mental Functions Higher mental functions 29 Higher mental functions • Long-term memory – retention of i

Answer 289

Limbic System

Question 290

reverse.prompt

1. integrates our emotions (fear, joy, sadness)
2. with our higher mental functions (reason, memory).
3. Because of the limbic system, activities such as sexual behavior and eating seem pleasurable, and mental stress can cause high blood pressure.

Answer 290

Limbic System 14.3

Question 291

This is found in the spaces between meninges as well as the ventricles of the brain and in the central canal of the spinal cord. This is what it does.

Answer 291

1. cerebrospinal fluid,
2. which cushions and protects the CNS.
3. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing.
4. within the ventricles of the brain and in the central canal of the spinal cord.

Question 292

The Cerebellum

Answer 292

• under the occipital lobe of the cerebrum
• and is separated from the brain stem by the fourth ventricle.
• The cerebellum has two portions joined by a narrow median portion.
  
  • white matter.
    
    • treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

1. receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts.
2. It also receives motor output from the cerebral cortex about where these parts should be located.
3. After integrating this information: sends motor signals by way of the brain stem to the skeletal muscles.
   
   • maintains posture and balance.
   • ensures that all the muscles work together to produce smooth, coordinated, voluntary movements.
   • assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

Question 293

Reverse

1. The brain: Cerebrum – The cerebral cortex • Cerebral cortex – thin, outer layer of gray matter

Question 294

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

Question 295

Reverse

integrates our emotions with our higher mental functions

contains the amygdala and hippocampus

accounts for why eating and sexual behavior seem pleasurable

Answer 295

limbic system

Question 296

Reverse

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to

Answer 296

Processing centers

Question 297

Reverse

Resting Potential

Think of all the devices, such as your cell phone and laptop, that are battery-powered. Every battery is an energy source manufactured by separating positively charged ions across a membrane from negative ions. The battery’s potential energy can be used to perform work—for example, using your phone or lighting a flashlight. A resting neuron also has potential energy, much like a fully charged battery. This energy, called the resting potential, Page 283exists because the plasma membrane is polarized: Positively charged ions are stashed outside the cell, with negatively charged ions inside.

As Figure 14.4a shows, the outside of the cell is positive because positively charged sodium ions (Na+) gather around the outside of the plasma membrane. At rest, the neuron’s plasma membrane is permeable to potassium, but not to sodium. Thus, positively charged potassium ions (K+) contribute to the positive charge by diffusing out of the cell to join the sodium ions. The inside of the cell is negative in relation to the exterior of the cell because of the presence of large, negatively charged proteins and other molecules that remain inside the cell because of their size.

Figure 14.4 Generation of an action potential. a. Resting potential occurs when a neuron is not conducting a nerve impulse. During an action potential, (b) the stimulus causes the cell to reach its threshold. c. Depolarization is followed by (d) repolarization. e. A graph depicting the generation of an action potential.

Tutorial: Neuron Action Potentials

Like a battery, the neuron’s resting potential energy can be measured in volts. Whereas a D-size flashlight battery has 1.5 volts, a nerve cell typically has 0.070 volt, or 70 millivolts (mV), of stored energy (Fig. 14.4a). By convention, the voltage measurement is always a negative number. This is because scientists compare the inside of the cell—where negatively charged proteins and other large molecules are clustered—to the outside of the cell—where positively charged sodium and potassium ions are gathered.

Just like rechargeable batteries, neurons must maintain their resting potential to be able to work. To do so, neurons actively transport sodium ions out of the cell and return potassium ions to Page 284the cytoplasm. A protein carrier in the membrane, called the sodium–potassium pump, pumps sodium ions (Na+) out of the neuron and potassium ions (K+) into the neuron (see Section 3.3). This action effectively “recharges” the cell so that, like a fresh battery, it can perform work.

Answer 297

Resting Potential

Question 298

Reverse

Nerve impulse reaches the axon terminal. • Calcium ions enter the axon terminal and stimulate the synaptic vesicles to fuse with the presynaptic membrane; the axon terminal membrane of the first neuron. • Neurotransmitters are released and diffuse across the synapse, where they bind with the postsynaptic membrane; the dendrite/cell body membrane on the second neuron to inhibit or excite the neuron.

Answer 298

How does transmissions across a synapse occur?

Question 300

The synapse

Answer 300

The Synapse

Every axon branches into many fine endings, each tipped by a small swelling called an axon terminal. Each terminal lies very close to either the dendrite or the cell body of another neuron. This region of close proximity is called a synapse (Fig. 14.5). At a synapse, a small gap called the synaptic cleft separates the sending neuron from the receiving neuron. The nerve signal is unable to jump the cleft. Therefore, another means is needed to pass the nerve signal from the sending neuron to the receiving neuron.

Figure 14.5 Signal transmission at the synapse. Transmission across a synapse from one neuron to another occurs when a neurotransmitter is released, diffuses across a synaptic cleft, and binds to a receptor in the membrane of the receiving neuron.

Tutorial: Synaptic Cleft

Transmission across a synapse is carried out by molecules called neurotransmitters, stored in synaptic Page 285vesicles in the axon terminals. (See Section 3.4 for a review of vesicle function.) The events (Fig. 14.5) at a synapse are (1) nerve signals traveling along an axon to reach an axon terminal; (2) calcium ions entering the terminal and stimulating synaptic vesicles to merge with the sending membrane; and (3) neurotransmitter molecules releasing into the synaptic cleft and diffusing across the cleft to the receiving membrane; there, neurotransmitter molecules bind with specific receptor proteins.

Depending on the types of receptors, the response of the receiving neuron can be toward excitation or toward inhibition. In Figure 14.6, excitation occurs because the neurotransmitter, such as acetylcholine (ACh), has caused the sodium gate to open. Sodium ions diffuse into the receiving neuron. Inhibition would occur if a neurotransmitter caused potassium ions to exit the receiving neuron.

Figure 14.6 Integration of excitatory and inhibitory signals at the synapse. a. Inhibitory signals and excitatory signals are summed up in the dendrite and cell body of the postsynaptic neuron. Only if the combined signals cause the membrane potential to rise above threshold does an action potential occur. b. In this example, threshold was not reached.

(photo): (a): ©Science Source

Chemical Synapses

Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft. In some synapses, the receiving membrane contains enzymes that rapidly inactivate the neurotransmitter. For example, the enzyme acetylcholinesterase (AChE) breaks down the neurotransmitter acetylcholine. In other synapses, the sending membrane rapidly reabsorbs the neurotransmitter, possibly for repackaging in synaptic vesicles or for molecular breakdown.

The short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of receiving membranes. The receiving cell needs to be able to respond quickly to changing conditions. If the neurotransmitter were to linger in the cleft, the receiving cell would be unable to respond to a new signal from a sending cell.

Neural Transmission: Synapse

Question 301

reverse.prompt

1. a few cranial nerves (e.g., the vagus nerve), as well as fibers that arise from the sacral (bottom) portion of the spinal cord.
   
   • craniosacral portion of the autonomic system
   • In the parasympathetic division, the preganglionic fiber is long, and the postganglionic fiber is short because the ganglia lie near or within the organ.
2. housekeeper division, promotes all the internal responses we associate with a relaxed state.
   
   1. ​ For example, it causes the pupil of the eye to contract,
   2. promotes digestion of food, and
   3. slows heart rate.
   4. rest-and-digest system.
   5. The neurotransmitter used by the parasympathetic division is acetylcholine (ACh).

Answer 301

The parasympathetic division

These are a few of its nicknames

This is the neurotransmitter it uses

Question 302

1. As you are reading these words, synapses throughout your brain are organizing, integrating, and cataloging the information you take in.
2. Neurotransmitters at these synapses control the firing of countless action potentials, thus creating a network of neural circuits.
3. It is amazing to realize that all thoughts, feelings, and actions of a human are dependent on neurotransmitters in the CNS and PNS.
4. By modifying or controlling synaptic transmission, a wide variety of drugs with neurological activity, both legal pharmaceuticals and illegal drugs of abuse, can alter mood, emotional state, behavior, and personality.

Answer 302

Drugs

Question 303

The Peripheral Nervous System Structures

Answer 303

The peripheral nervous system (PNS) • It includes cranial nerves (12 pairs), spinal nerves (31 pairs), and ganglia (neuronal cell bodies) outside the CNS. - Spinal nerves conduct impulses to and from the spinal cord. - Cranial nerves conduct impulses to and from the brain. • The PNS is divided into 2 systems. - Somatic division - Autonomic division 14.4 The Peripheral Nervous System 2 The peripheral nervous system Figure 14.14 The structure of a nerve. 14.4 The Peripheral Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. LM PNS single nerve fiber bundle of nerve fibers blood vessel Spinal or Cranial Nerve artery and vein spinal nerves cranial nerves © PASIEKA/Science Photo Library/Science Source 3 14.4 The Peripheral Nervous System The peripheral nervous system Know these cranial nerves! Copyright © The McGraw-Hill Companies, Inc. Permission r

Question 305

Dura mater • Double-layered external covering • Periosteum – dense connective tissue attached to surface of the skull • Meningeal layer – outer covering of the brain • Folds inward in several areas Supplemental material not in boo

Question 306

The Spinal Cord has these functions:

These can result from severed spinal cords

Answer 306

1. communication between the brain and the peripheral nerves that leave the cord.

• When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).
• The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

1. The brain coordinates the voluntary control of our limbs.
2. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows).

• Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.
• Page 288*

1. Reflex Actions
   
   • The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17).
   • A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord.
   • Interneurons integrate the incoming data and relay signals to motor neurons.
   • A response to the stimulus occurs when motor axons cause skeletal muscles to contract.
   • Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.
   • Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

Question 308

reversedprompt

1. Transmission across a synapse is carried out by molecules called neurotransmitters, stored in synapticvesicles in the axon terminals. (See Section 3.4 for a review of vesicle function.)
2. The events (Fig. 14.5) at a synapse are:
   
   1. nerve signals traveling along an axon to reach an axon terminal;
   2. calcium ions entering the terminal and stimulating synaptic vesicles to merge with the sending membrane;
   3. neurotransmitter molecules releasing into the synaptic cleft and diffusing across the cleft to the receiving membrane;
   4. there, neurotransmitter molecules bind with specific receptor proteins.
   5. Depending on the types of receptors, the response of the receiving neuron can be toward excitation or toward inhibition.
      
      • In Figure 14.6, excitation occurs because the neurotransmitter, such as acetylcholine (ACh), has caused the sodium gate to open. Sodium ions diffuse into the receiving neuron.
      • Inhibition would occur if a neurotransmitter caused potassium ions to exit the receiving neuron.

Answer 308

See and Study Figure 14.5

Figure 14.5 Signal transmission at the synapse. Transmission across a synapse from one neuron to another occurs when a neurotransmitter is released, diffuses across a synaptic cleft, and binds to a receptor in the membrane of the receiving neuron.

Transmission across Synapse

Figure 14.6

Page 285

Question 309

Reverse

Small, ovoid cells with spiny processes, “spider-like cells” –Phagocytes from red bone marrow that monitor the health of neurons – Dispose of dead cells, invading microorganisms M

Answer 309

Microglia

Question 310

The PNS Autonomic Division

Answer 310

The PNS: Autonomic division • The autonomic system regulates the activity of involuntary muscles (cardiac and smooth) and glands.

Question 311

Sympathetic Division

Answer 311

Sympathetic Division

Most preganglionic fibers of the sympathetic division arise from the middle portion of the spinal cord. They terminate almost immediately in ganglia that lie near the cord.

The sympathetic division is especially important during emergency situations when you might be required to fight or take flight. It accelerates the heartbeat and dilates the bronchi—active muscles, after all, require a ready supply of glucose and oxygen. Page 297Sympathetic neurons inhibit the digestive organs, as well as the kidneys and urinary bladder; the activities of these organs—digestion, defecation, and urination—are not immediately necessary if you’re under attack. The neurotransmitter released by the postganglionic axon is primarily norepinephrine (NE). The structure of NE is like that of epinephrine (adrenaline), an adrenal medulla hormone that usually increases heart rate and contractility.

Question 312

Limbic System (14.2)

Function

Structurally

(Fig. 14.13) Page 293

The regions of the brain associated with the limbic system. In the limbic system (purple), structures deep within each cerebral hemisphere and surrounding the diencephalon join higher mental functions, such as reasoning, with more primitive feelings, such as fear and pleasure. Therefore, primitive feelings can influence our behavior, but reason can also keep them in check.

Answer 312

• evolutionary ancient group of linked structures deep within the cerebrum.
• It is a functional grouping rather than an anatomical one

1. blends primitive emotions w/
2. higher mental functions into a united whole.
   
   • sexual behavior and eating seem pleasurable
   • unpleasant sensations or emotions (pain, frustration, hatred, despair) are translated by the limbic system into a stress response.

• Structurally: 2 significant
  
  1. amygdala
  • ​ particular emotional overtones, creating sensation of fear.
  • use past knowledge fed to it by association areas to assess a current situation.
  • if necessary, trigger fight-or-flight reaction
    
    • So if you are out late at night and you turn to see someone in a ski mask following you, the amygdala may immediately cause you to start running.
    • The frontal cortex can override the limbic system, cause you to rethink the situation, and prevent you from acting out strong reactions

2. The hippocampus

• learning and memory
• information gateway: determines what information about the world is to be sent to memory and how this information is to be encoded and stored by other regions in the brain.
• Most likely, the hippocampus can communicate with the frontal cortex, because we know that memories are an important part of our decision-making processes.

Question 314

Reverse

• A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord.
• The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.
• Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.
• (a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections
• The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c).
• Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter.
• The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.
• The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly).
• Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Answer 314

Structure Spinal Cord

Question 315

Reverse

Neurotransmitter Molecules

More than 100 substances are known or suspected to be neurotransmitters. Some of the more common ones in humans are acetylcholine, norepinephrine, dopamine, serotonin, glutamate, and GABA (gamma aminobutyric acid). Neurotransmitters transmit signals between nerves. Nerve-muscle, nerve-organ, and nerve-gland synapses also communicate using neurotransmitters.

Acetylcholine (ACh) and norepinephrine are active in both the CNS and PNS. In the PNS, these neurotransmitters act at synapses called neuromuscular junctions. We will explore the structure of the neuromuscular junctions in Section 13.2.

In the PNS, ACh excites skeletal muscle but inhibits cardiac muscle. It has either an excitatory or inhibitory effect on smooth muscle or glands, depending on their location.

Norepinephrine generally excites smooth muscle. In the CNS, norepinephrine is important to dreaming, waking, and mood. Serotonin is involved in thermoregulation, sleeping, emotions, and perception. Many drugs that affect the nervous system act at the synapse. Some interfere with the actions of neurotransmitters, and other drugs prolong the effects of neurotransmitters (see Section 14.5).

Answer 315

Neurotransmitter Molecules

Question 316

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Question 317

Functions of Spinal Cord

Answer 317

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Question 318

reversedprompt

1. axons outside the brain and spinal cord can regenerate—but axons inside these organs cannot After injury, axons in the human central nervous system (CNS) degenerate, resulting in permanent loss of nervous function.
2. Interestingly, about 90% of the cells in the brain and the spinal cord are not even neurons. They are neuroglia cells.
3. In nerves outside the brain and spinal cord, the neuroglia cells are Schwann cells that help axons regenerate. The neuroglia cells in the CNS include microglial cells, oligodendrocytes, and astrocytes, and they inhibit axon regeneration
4. 5. The spinal cord contains its own stem cells. When the spinal cord is injured in experimental animals, these stem cells proliferate. But instead of becoming functional neurons, they become neuroglia cells. Researchers are trying to understand the process that triggers the stem cells to become neuroglia cells. In the future, this understanding would allow manipulation of stem cells into neurons.
5. In early experiments with neural stem cells in the laboratory, scientists at Johns Hopkins University caused embryonic stem (ES) cells to differentiate into spinal cord motor neurons, the type of nerve cell that causes muscles to contract. The motor neurons then produced axons. When grown in the same dish with muscle cells, the motor neurons formed neuromuscular junctions and even caused muscle contractions. The cells were then transplanted into the spinal cords of rats with spinal cord injuries. Some of the transplanted cells survived for longer than a month within the spinal cord. However, no improvement in symptoms was seen and no functional neuron connections were made.
6. In later experiments by the same research group, paralyzed rats were first treated with drugs and nerve growth factors to overcome inhibition from the central nervous system. These techniques significantly increased the success of the transplanted neurons. Amazingly, axons of transplanted neurons reached the muscles, formed neuromuscular junctions, and provided partial relief from the paralysis. Research is being done on the use of both the body’s own stem cells and laboratory-grown stem cells to repair damaged CNS neurons. Though many questions remain, the current results are promising.
7. Questions to Consider
8. What is the likely reason neurons cannot simply be transplanted from other areas of the body?
9. How might this research also help patients who suffer from neurodegenerative diseases, such as Parkinson disease?
10. Long axons tend to have a myelin sheath, but short axons do not. The gray matter of the CNS is gray because it contains no myelinated axons; the white matter of the CNS is white because it does. In the PNS, myelin gives nerve fibers their white, glistening appearance and serves as an excellent insulator. When the myelin breaks down, as happens in multiple sclerosis (MS) (see chapter opener), then it becomes more difficult for the neurons to transmit information. In effect, MS “short-circuits” the nervous system. The myelin sheath also plays an important role in nerve regeneration within the PNS. If an axon is accidentally severed, the myelin sheath remains and serves as a passageway for new fiber growth.

Answer 318

BIOLOGY TODAY Science

Nerve Regeneration and Stem Cells

(Fig. 14A)- See Figure 14A Regeneration of nerve cells. Outside the CNS, nerves regenerate because new neuroglia called Schwann cells form a pathway for axons to reach a muscle. In the CNS, comparable neuroglia called oligodendrocytes do not have this function.

Question 319

Reverse

The Central Nervous System

Slides

Answer 319

• Both the brain and spinal cord are made up of 2 types of nervous tissue:

1. Gray matter – contains cell bodies and nonmyelinated fibers
2. White matter – contains myelinated axons

Question 320

Central nervous system

Answer 320

The central nervous system (CNS) consists of the brain and spinal cord. The brain is completely surrounded and protected by the skull. It connects directly to the spinal cord, similarly protected by the vertebral column.

Question 321

1. outside the nervous system, contains the nerves
   
   1. cranial nerves when they arise from the brain and are termed
   2. spinal nerves when they arise from the spinal cord.
   3. In any case, all nerves carry signals to and from the CNS.
2. anatomy of a nerve:

• The cell body and the dendrites of neurons are in either the CNS or the ganglia.
• Ganglia (sing., ganglion) are collections of nerve cell bodies outside the CNS.
• The axons of neurons project from the CNS and form the spinal cord.
• In other words, nerves, whether cranial or spinal, are composed of axons, the long part of neurons.

1. 2. Humans have 12 pairs of cranial nerves attached to the brain. By convention, the pairs of cranial nerves are referred to by Roman numerals (Fig. 14.16). Some cranial nerves are sensory nerves—they contain only sensory fibers; some are motor nerves that contain only motor fibers; others are mixed nerves that contain both sensory and motor fibers. Cranial nerves are largely concerned with the head, neck, and facial regions of the body. However, the vagus nerve (X) has branches not only to the pharynx and larynx but also to most of the internal organs. It arises from the brain stem—specifically, the medulla oblongata, which communicates with the hypothalamus. These two parts of the brain control the internal organs.
2. Figure 14.16 The cranial nerves. Overall, cranial nerves receive sensory input from, and send motor outputs to, the head region. The spinal nerves receive sensory input from, and send motor outputs to, the rest of the body. Two important exceptions are the vagus nerve, X, which communicates with internal organs, and the spinal accessory nerve, XI, which controls neck and back muscles.
3. The spinal nerves of humans emerge from either side of the spinal cord (see Fig. 14.8). There are 31 pairs of spinal nerves. The roots of a spinal nerve physically separate the axons of sensory neurons from the axons of motor neurons, forming an arrangement resembling a letter Y. The posterior root of a spinal nerve contains sensory fibers that direct sensory receptor information inward (toward the spinal cord). The cell body of a sensory neuron is in a posterior-root ganglion (also termed a dorsal-root ganglion). The anterior (also termed ventral) root of a spinal nerve contains motor fibers that conduct impulses outward (away from the cord) to the effectors. Observe in Figure 14.8 that the anterior and posterior roots join to form a spinal nerve. All spinal nerves are called mixed nerves, because they contain both sensory and motor fibers. Each spinal nerve serves the particular region of the body in which it is located. For example, the intercostal muscles of the rib cage are innervated by thoracic nerves.
   5.

Answer 321

What is the peripheral nervous system (PNS)

Nerves!

Figure 14.15

Figure 14.15 The structure of a nerve.

The peripheral nervous system consists of the cranial nerves and the spinal nerves. A nerve is composed of bundles of axons separated from one another by connective tissue

Question 322

1. CNS, where sensory information is received and
2. motor control is initiated.

Answer 322

What are the spinal cord and the brain?

Question 323

Both pharmaceuticals and illegal drugs have several basic modes of action:

They promote the action of a neurotransmitter, usually by increasing the amount of neurotransmitter at a synapse. Examples include drugs such as alprazolam (Xanax) and diazepam (Valium), which increase GABA. These medications are used for panic attacks and anxiety. Reduced levels of norepinephrine and serotonin are linked to depression. Drugs such as fluoxetine (Prozac), paroxetine (Paxil), and duloxetine (Cymbalta) allow norepinephrine and/or serotonin to accumulate at the synapse, which explains their effectiveness as antidepressants. Alzheimer disease causes a slow, progressive loss of memory (see Section 18.5). Drugs used for Alzheimer disease allow acetylcholine to accumulate at synapses in the limbic system.

They interfere with or decrease the action of a neurotransmitter. For instance, antipsychotic drugs used for the treatment of schizophrenia decrease the activity of dopamine. The caffeine in coffee, chocolate, and tea keeps us awake by interfering with the effects of inhibitory neurotransmitters in the brain.

They replace or mimic a neurotransmitter or neuromodulator. The opiates—namely, codeine, heroin, and morphine—bind to endorphin receptors and in this way reduce pain and produce a feeling of well-being.

Ongoing research into neurophysiology and neuropharmacology (the study of nervous system function and the way drugs work in the nervous system) continues to provide evidence that mental illnesses are caused by imbalances in neurotransmitters. These studies will undoubtedly improve treatments for mental illness, as well as provide insight into the problem of drug abuse.

Question 324

Unit 4Integration and Coordination in Humans

CHAPTER 14

Nervous System

©Jozef Polc/123RF

CHAPTER OUTLINE

14.1Overview of the Nervous System

14.2The Central Nervous System

14.3The Limbic System and Higher Mental Functions

14.4The Peripheral Nervous System

14.5Drug Therapy and Drug Abuse

BEFORE YOU BEGIN

Before beginning this chapter, take a few moments to review the following discussions:

Section 2.1How does an ion differ from an atom of an element?

Section 3.3How does the sodium–potassium pump move ions across the plasma membrane?

Section 4.1What is the function of nervous tissue in the body?

Question 325

This is the primary function of the nervous system.

2 main components

see Figure 14.1. and download and study; Page 281

Figure 14.2 Organization of the nervous system. The red arrows are the pathways by which the CNS receives sensory information. The black arrows are the pathways by which the CNS communicates information with the PNS.

Figure 14.1 The central nervous system. The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of the nerves, which lie outside the CNS.

Answer 325

• reception and processing of sensory information from both the external and the internal environments.
• two major divisions. The major structures of the nervous system are shown in

1. central nervous system (CNS) consists of the brain and spinal cord. The brain is completely surrounded and protected by the skull. It connects directly to the spinal cord, similarly protected by the vertebral column.
2. The peripheral nervous system (PNS) consists of nerves. Nerves lie outside the CNS.

• The division between the CNS and the PNS is arbitrary.
• The two systems work together and are connected to each other (Fig. 14.2).

Nervous System Functions (3 specific):

1. receives sensory input: Sensory receptors in skin and other organs respond to external and internal stimuli by generating nerve signals that travel by way of the PNS to the CNS.
   * ​For example, if you smell baking cookies, olfactory (smell) receptors in the nose use the PNS to transmit that information to the CNS.
2. The CNS performs information processing and integration, summing up the input it receives from all over the body.

• reviews the information, stores the information as memories, and creates the appropriate motor responses.
  
  • The smell of those baking cookies evokes memories of their taste.

3. The CNS generates motor output; Nerve signals from the CNS go by way of the PNS to the muscles, glands, and organs, all in response to the cookies.

• Signals to the salivary glands make you salivate. Your stomach generates the acid and enzymes needed to digest the cookies—even before you’ve had a bite.
• The CNS also coordinates the movement of your arms and hands as you reach for the cookies.

Question 327

Brain, Diencephalon

Answer 327

• 2. The brain: Diencephalon • Includes the • Hypothalamus – helps maintain homeostasis (hunger, sleep, thirst, body temperature, and water balance) and controls pituitary gland • Thalamus – 2 masses of gray matter that receive all sensory input except smell; involved in memory and emotions • Pineal gland – secretes melatonin, the hormone that controls our daily rhythms

Question 328

I work closely as a link between the nervous and endochrine systems.

Answer 328

What is the hypothalamus?

Question 329

reversedprompt

• electrochemical changes that convey information within the nervous system.

Answer 329

Nerve Signals

Question 330

The parasympathetic division

These are a few of its nicknames

This is the neurotransmitter it uses

Answer 330

1. a few cranial nerves (e.g., the vagus nerve), as well as fibers that arise from the sacral (bottom) portion of the spinal cord.
   
   • craniosacral portion of the autonomic system
   • In the parasympathetic division, the preganglionic fiber is long, and the postganglionic fiber is short because the ganglia lie near or within the organ.
2. housekeeper division, promotes all the internal responses we associate with a relaxed state.
   
   1. ​ For example, it causes the pupil of the eye to contract,
   2. promotes digestion of food, and
   3. slows heart rate.
   4. rest-and-digest system.
   5. The neurotransmitter used by the parasympathetic division is acetylcholine (ACh).

Question 331

The Spinal Cord

Download, attach image and study it, 14.8 a

Answer 331

1. from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4).
   * From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord (Visuals)

1. A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter.
   
   1. Figure 14.8b shows how an individual vertebra protects the spinal cord.
   2. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.
2. Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.
3. (a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections
4. The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c).
5. Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.
6. The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.
   7.

Question 333

Reverse

A lipid covering on long axons that acts to increase the speed of nerve impulse conduction, insulation for both CNS and PNS, and regeneration in the PNS • Schwann cells – neuroglia that make up the myelin sheath in the PNS • Oligodendrocytes- neuroglia that make up the myelin sheath in the CNS • Nodes of Ranvier – gaps between myelination on the axons • Saltatory conduction – conduction of the nerve impulse from node to node

Question 334

Reverse

• This part of the brain is where the hypothalamus and thalamus are
• It is in this ventricle
• It is an integrating center for homeostasis

Answer 334

Diencephalon

Question 335

CNS, brainstem, midbrain, pons, medulla oblangata

Answer 335

4.2 The Central Nervous System 24 4. The brain: The brainstem • Includes • Midbrain – relay station between the cerebrum and spinal cord or cerebellum; reflex center • Pons – a bridge between cerebellum and the CNS; regulates breathing rate; reflex center for head movements • Medulla oblongata – contains reflex centers for regulating breathing, heartbeat, and blood pressure

Question 336

Learning Outcomes

Answer 336

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

Question 338

reverse.prompt

The CNS generates motor output. Nerve signals from the CNS go by way of the PNS to the muscles, glands, and organs, all in response to the cookies. Signals to the salivary glands make you salivate. Your stomach generates the acid and enzymes Page 281needed to digest the cookies—even before you’ve had a bite. The CNS also coordinates the movement of your arms and hands as you reach for the cookies.

Answer 338

3 functions of the nervous system

Question 339

The peripheral nervous system (PNS) consists of nerves. Nerves lie outside the CNS. The division between the CNS and the PNS is arbitrary. The two systems work together and are connected to each other (Fig. 14.2).

Answer 339

Peripheral Nervous System

Question 340

1. Alcohol
2. With the exception of caffeine, alcohol (ethanol) consumption is the most socially accepted form of drug use in the United States. According to a 2015 national survey, 26.9% of high school students reported drinking some alcohol (down from 37.4% in 2014), and 7% binge drank (five-plus drinks in one setting) during the 30 days preceding the survey. Among adults, 86.4% reported they had consumed alcohol during their lifetime, with 56% stating they had used alcohol in the past month.

• Alcohol acts as a depressant on many parts of the brain by increasing the action of GABA, an inhibitory neurotransmitter.
• Depending on the amount consumed, the effects of alcohol on the brain can lead to a feeling of relaxation, lowered inhibitions, impaired concentration and coordination, slurred speech, and vomiting.
• If the blood level of alcohol becomes too high, coma or death can occur.

Beginning in about 2005, several manufacturers began selling alcoholic energy drinks. With names like Four Loko, JOOSE, and Sparks, these drinks combine fairly high levels of alcohol with caffeine and other ingredients. Although interactions between drugs can be complex, the stimulant effects of caffeine can counteract some of the depressant effects of alcohol, so users feel able to drink more. Because caffeine does not reduce the intoxicating effects of alcohol, many state legislatures are banning these products, and in November 2010 the U.S. Food and Drug Administration warned several manufacturers that they would no longer be allowed to mix caffeine with alcohol in their products.

Nicotine

• Although the numbers have been decreasing since 2011 according to the CDC, in 2015, 25.3% of high school students and 7.4% of middle school students reported using a tobacco product. When tobacco is smoked or chewed, nicotine is rapidly delivered throughout the body. It causes a release of epinephrine from the
• adrenal glands, increasing blood sugar and causing the initial feeling of stimulation. As blood sugar falls, depression and fatigue set in, causing the user to seek more nicotine. In the CNS, nicotine stimulates neurons to release dopamine, a neurotransmitter that promotes a temporary sense of pleasure, and reinforces dependence on the drug. About 70% of people who try smoking become addicted.

As mentioned in earlier chapters, smoking is strongly associated with serious diseases of the cardiovascular and respiratory systems. Once addicted, however, only 10–20% of smokers are able to quit. Most medical approaches to quitting smoking involve the administration of nicotine in safer forms, such as skin patches, gum, or a newly developed nicotine inhaler, so that withdrawal symptoms can be minimized while dependence is gradually reduced. Several antinicotine vaccines (such as NicVAX) are currently in development or in early clinical trials. These vaccines stimulate the production of antibodies that prevent nicotine from entering the brain.

Cocaine and Crack

Cocaine is an alkaloid derived from the shrub Erythroxylon coca. Approximately 35 million Americans have used cocaine by sniffing/snorting, injecting, or smoking. Cocaine is a powerful stimulant in the CNS that interferes with the reuptake of dopamine at synapses, increasing overall brain activity. The result is a rush of a sense of well-being that lasts from 5 to 30 minutes. However, long-term use of cocaine causes a loss of metabolic functions in the brain (Fig. 14.19).

Figure 14.19 Cocaine use. Brain activity before and after the use of cocaine.

(both photos): ©Science Source

“Crack” is the street name given to cocaine that is processed to a free-base form for smoking. The term crack refers to the crackling sound heard when the drug is smoked. Smoking allows high doses of the drug to reach the brain rapidly, providing an intense and immediate high, or “rush.” Approximately 8 million Americans use crack.

A cocaine binge is a period in which a user takes the drug at ever-higher doses. The user is hyperactive, with little desire for food or sleep, but has an increased sex drive. This is followed by a crash period, characterized by fatigue, depression, irritability, and a lack of interest in sex. In fact, men who use cocaine often become impotent.

Cocaine is highly addictive; with continued use, the brain makes less dopamine to compensate for a seemingly endless supply. The user experiences withdrawal symptoms and an intense craving for cocaine. Overdosing on cocaine can cause cardiac and/or respiratory arrest.Page 301

Methamphetamine and Ecstasy

Methamphetamine and ecstasy are considered club, or party, drugs. Methamphetamine (commonly called meth or crank) is a powerful CNS stimulant. Meth is often produced in makeshift home laboratories, usually starting with ephedrine or pseudoephedrine, common ingredients in many cold and asthma medicines. As a result, many states have passed laws making these medications more difficult to purchase. The number of toxic chemicals used to prepare the drug makes a former meth lab site hazardous to humans and to the environment. Over 9 million people in the United States have used methamphetamine at least once. It is available as a powder that can be snorted or as crystals (crystal meth or ice) that can be smoked.

The structure of methamphetamine is similar to that of dopamine, and the most immediate effect of taking meth is a rush of euphoria, energy, alertness, and elevated mood. However, this is typically followed by a state of agitation that, in some individuals, leads to violent behavior. Chronic use can result in what is called an amphetamine psychosis, characterized by paranoia, hallucinations, irritability, and aggressive, erratic behavior.

Ecstasy is the street name for MDMA (methylenedioxymethamphetamine), which is chemically similar to methamphetamine. Many users say that “X,” taken as a pill that looks like an aspirin or candy, increases their feelings of well-being and love for other people. However, it has many of the same side effects as other stimulants, plus it can interfere with temperature regulation, leading to hyperthermia, high blood pressure, and seizures. Although deaths from alcohol abuse are more common, ecstasy is identified as a cause of accidental death in young adults each year.

Drugs with sedative effects, known as date rape or predatory drugs, include flunitrazepam (Rohypnol, or roofies), gamma-hydroxybutyric acid (GHB), and ketamine (special K). Ketamine is actually a drug that veterinarians sometimes use to perform surgery on animals. Any of these drugs can be given to an unsuspecting person, who may fall into a dreamlike state in which he or she is unable to move and thus is vulnerable to sexual assault.

Heroin

Heroin is derived from the resin or sap of the opium poppy plant, which is widely grown in a region from Turkey to Southeast Asia and in parts of Latin America. Drugs derived from opium are called opiates, or more commonly, opioids. This class also includes morphine and codeine. After heroin is injected, snorted, or smoked, a feeling of euphoria, along with relief of any pain, occurs within a few minutes. It is estimated that 4 million Americans have used heroin sometime in their lives, and over 300,000 people use heroin annually.

As with other drugs of abuse, addiction is common. Heroin and opioids bind to receptors meant for the endorphins, naturally occurring neurotransmitters that kill pain and produce feelings of tranquility. With repeated use, the body’s production of endorphins decreases. Tolerance develops, so the user needs to take more of the drug just to prevent withdrawal symptoms (tremors, restlessness, cramps, vomiting), and the original euphoria is no longer felt. In the case of heroin, long-term users commonly acquire hepatitis, HIV/AIDS, and various bacterial infections due to the use of shared needles, and heavy users may experience convulsions and death by respiratory arrest.

Heroin addiction can be treated with synthetic opiate compounds, such as methadone or buprenorphine and naloxone (Suboxone), that decrease withdrawal symptoms and block heroin’s effects. However, methadone itself can be addictive, and methadone-related deaths are on the rise.

Marijuana and K2

Marijuana is the most commonly used illegal drug in the United States. Surveys vary, but in 2015, about 52% of young adults reported using marijuana in their lifetime, and 46% of the U.S. population had tried it at least once. It is derived from the dried flowering tops, leaves, and stems of the marijuana plant, Cannabis sativa, which contain and are covered by a resin that is rich in THC (tetrahydrocannabinol). The names cannabis and marijuana apply to either the plant or THC. Marijuana can be ingested, but usually it is smoked in a cigarette called a “joint.” Beginning with California in 1996, several states have legalized its use for medical purposes, such as in treating cancer, AIDS, and glaucoma. In 2012, Colorado became the first state to legalize recreational use. As of 2018, 8 states had joined Colorado in legalizing recreational use, and 22 additional states had authorized the use of marijuana for medicinal purposes. However, in 2005, the Supreme Court ruled that patients prescribed medical marijuana can still be prosecuted by federal agencies.

Page 302Researchers have found that THC binds to a receptor for anandamide, a naturally occurring neurotransmitter that is important for short-term memory processing, and perhaps for feelings of contentment. The occasional marijuana user experiences mild euphoria, along with alterations in vision and judgment. Heavy use can cause hallucinations, anxiety, depression, paranoia, and psychotic symptoms. Research is underway to identify the effects of long-term marijuana use on the brain, as well as on the effects of secondhand marijuana smoke on the respiratory system.

In recent years, awareness has been increasing about a synthetic compound called K2, or spice. Originally synthesized by an organic chemist at Clemson University, K2 is about ten times as potent as THC. The chemical is typically sprayed onto a mixture of other herbal products and smoked. However, because there is no regulation of how it is produced, the amount of K2 itself, or contaminants, can vary greatly. This may account for the several reports of serious medical problems and even deaths among K2 users.

CHECK YOUR PROGRESS 14.5

Contrast drug therapy and drug abuse.

Answer

Drug therapy is used to treat a disease or disorder. Drug abuse is using drugs without symptoms of disease or disorder.

List how the abuse of drugs, including alcohol and nicotine, affects the nervous system.

Answer

Alcohol and heroin are depressants; nicotine, cocaine, and methamphetamines are stimulants; marijuana produces euphoria.

Detail several modes of action of pharmaceutical and illegal drugs.

Answer

Alcohol increases the action of GABA and increases the release of endorphins in the hypothalamus. Nicotine stimulates dopamine release. Cocaine inhibits dopamine reuptake. Methamphetamine mimics the action of cocaine. Heroin is converted to morphine in the brain and binds to opioid receptors. Marijuana stimulates anandamide receptors.

CONNECTING THE CONCEPTS

For more on the long-term effects of drug use on the systems of the body, refer to the following discussions:

Section 5.7 explores the negative long-term effects of smoking on the cardiovascular system.

Section 11.4 provides information on how alcohol acts as a diuretic in the urinary system.

Section 20.2 examines the relationship between smoking and alcohol use and the increased risk of cancer.

CONCLUSION

The cause of multiple sclerosis (MS) is still unknown, but most researchers agree that there are most likely many contributing factors, including environmental influences, genetics, and a faulty immune system. Many individuals with MS are able to control their symptoms by using immunosuppressive medications, such as beta interferons. The fact that this treatment works suggests that, in many cases, MS is caused by the immune system incorrectly identifying the myelin sheaths as foreign material. The breakdown of the myelin can be detected using both MRI and SSEP tests (discussed in the chapter opener). However, environmental conditions are also suspected to cause MS. Studies have shown that the risk of contracting MS is influenced in part by where in the world you live, although the specific environmental factor or pollutant has not yet been identified. Genetics is also believed to play a role in some cases. But most researchers believe that a defect in a single gene is unlikely. Rather, it is more likely that a certain combination of genetic factors places an individual at a higher risk of contracting MS. Though there is no cure for MS, researchers have been very successful in developing disease-modifying drugs that reduce the symptoms and allow the individual to lead a normal life.

Answer 340

(Table 14.2) Drugs - See this table

Table 14.2Drug Influence on the CNS

Table Summary: Table lists the names of different substances in column 1. Other information related to these substances appears in columns 2 and 3.

SubstanceEffectMode of Transmission

AlcoholDepressantDrink

NicotineStimulantSmoked or smokeless tobacco

CocaineStimulantSniffed/snorted, injected, or smoked

Methamphetamine/EcstasyStimulantSmoked or pill form

HeroinDepressantSniffed/snorted, injected, or smoked

Marijuana/K2PsychoactiveSmoked or consumed

Question 341

reverse.prompt

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 342

Which class of neurons takes nerve signals to the central nervous system?

Answer 342

Sensory

Question 343

Functions of the Spinal Cord

Answer 343

1. The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord.
2. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

Question 344

reverse.prompt

SCIENCE IN YOUR LIFE

What is amnesia?

Amnesia results from disruption of the memory pathways and can be temporary or permanent. In anterograde amnesia, injury to the limbic system separates long-term memories of events that occurred prior to the injury from events that occur in the here and now. An affected person might carry on a conversation about past events (memories of a long-ago birthday) but be unable to recall a breakfast menu from that morning. In retrograde amnesia, a blow to the head or similar injury abolishes all memories for a variable time before the injury. For example, a head injury occurring during a car accident may abolish all memories from hours to days prior to the accident.

Question 345

See and Study Figure 14.5

Figure 14.5 Signal transmission at the synapse. Transmission across a synapse from one neuron to another occurs when a neurotransmitter is released, diffuses across a synaptic cleft, and binds to a receptor in the membrane of the receiving neuron.

Transmission across Synapse

Figure 14.6

Page 285

Answer 345

1. Transmission across a synapse is carried out by molecules called neurotransmitters, stored in synapticvesicles in the axon terminals. (See Section 3.4 for a review of vesicle function.)
2. The events (Fig. 14.5) at a synapse are:
   
   1. nerve signals traveling along an axon to reach an axon terminal;
   2. calcium ions entering the terminal and stimulating synaptic vesicles to merge with the sending membrane;
   3. neurotransmitter molecules releasing into the synaptic cleft and diffusing across the cleft to the receiving membrane;
   4. there, neurotransmitter molecules bind with specific receptor proteins.
   5. Depending on the types of receptors, the response of the receiving neuron can be toward excitation or toward inhibition.
      
      • In Figure 14.6, excitation occurs because the neurotransmitter, such as acetylcholine (ACh), has caused the sodium gate to open. Sodium ions diffuse into the receiving neuron.
      • Inhibition would occur if a neurotransmitter caused potassium ions to exit the receiving neuron.

Question 346

reversedprompt

• reception and processing of sensory information from both the external and the internal environments.
• two major divisions. The major structures of the nervous system are shown in

1. central nervous system (CNS) consists of the brain and spinal cord. The brain is completely surrounded and protected by the skull. It connects directly to the spinal cord, similarly protected by the vertebral column.
2. The peripheral nervous system (PNS) consists of nerves. Nerves lie outside the CNS.

• The division between the CNS and the PNS is arbitrary.
• The two systems work together and are connected to each other (Fig. 14.2).

Nervous System Functions (3 specific):

1. receives sensory input: Sensory receptors in skin and other organs respond to external and internal stimuli by generating nerve signals that travel by way of the PNS to the CNS.
   * ​For example, if you smell baking cookies, olfactory (smell) receptors in the nose use the PNS to transmit that information to the CNS.
2. The CNS performs information processing and integration, summing up the input it receives from all over the body.

• reviews the information, stores the information as memories, and creates the appropriate motor responses.
  
  • The smell of those baking cookies evokes memories of their taste.

3. The CNS generates motor output; Nerve signals from the CNS go by way of the PNS to the muscles, glands, and organs, all in response to the cookies.

• Signals to the salivary glands make you salivate. Your stomach generates the acid and enzymes needed to digest the cookies—even before you’ve had a bite.
• The CNS also coordinates the movement of your arms and hands as you reach for the cookies.

Answer 346

This is the primary function of the nervous system.

2 main components

see Figure 14.1. and download and study; Page 281

Figure 14.2 Organization of the nervous system. The red arrows are the pathways by which the CNS receives sensory information. The black arrows are the pathways by which the CNS communicates information with the PNS.

Figure 14.1 The central nervous system. The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of the nerves, which lie outside the CNS.

Question 347

reversedprompt

4 ventricles

Answer 347

These are the interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7).

Question 348

Neurons- 3 types and functions

Answer 348

Expanding on neurons • 3 types of neurons • Sensory – takes impulses from sensory receptor to CNS •

nterneuron – receives information in the CNS and sends it to a motor neuron • Motor – takes impulses from the CNS to an effector (i.e., gland or muscle fiber)

Question 349

• for curing mental disorders: important understanding memory on the cellular level
• After synapses have been used intensively for a short time, they release more neurotransmitters than before.
• This phenomenon, called long-term potentiation, may be involved in memory storage.

Answer 349

Memory on a cellular level

Long Term Potentiation

Question 350

Reverse

The Reflex Arc

Figure 14.17 illustrates the path of a reflex that involves only the spinal cord. If your hand touches a sharp pin, sensory receptors in the skin generate nerve signals that move along sensory fibers through the posterior (dorsal) root ganglia toward the spinal cord. Sensory neurons that enter the cord posteriorly pass signals on to many interneurons. Some of these interneurons synapse with motor neurons whose short dendrites and cell bodies are in the spinal cord. Nerve signals travel along these motor fibers to an effector, which brings about a response to the stimulus. In this case, the effector is a muscle, which contracts so that you withdraw your hand from the pin. Various other reactions are also possible—you will most likely look at the pin, wince, and cry out in pain. This whole series of responses occurs because some of the interneurons involved carry nerve signals to the brain. The brain makes you aware of the stimulus and directs these other reactions to it. In other words, you don’t feel pain until the brain receives the information and interprets it.

Figure 14.17 A spinal reflex arc. A stimulus (e.g., a sharp pin) causes sensory receptors in the skin to generate nerve signals that travel in sensory axons to the spinal cord. Interneurons integrate data from sensory neurons and then relay signals to motor neurons, causing contraction of a skeletal muscle and movement of the hand away from the stimulus.

Answer 350

Reflex Arc

Question 352

Reverse

Association areas and Processing Centers

Wernick’s and Broca’s aphasias

Answer 352

Association areas – integration occurs here • Processing centers – perform higher level analytical functions including Wernicke’s and Broca’s areas, both involved in speech. Prefrontal area is also a processing center 14.2 The Central Nervous System 18 Wernicke’s and Broca’s aphasias • Aphasias are brain lesions/areas of damage • When Wernicke’s area is damaged, the individual is not able to process language and responds nonsensically (“word salad”) • When Broca’s area is damaged, the individual understands what is spoken to him/her, however is unable to have motor control for speech to verbally respond 1

Question 353

The Central Nervous System

Cerebrospinal Fluid -

Answer 353

4 Cerebrospinal Fluid (CSF) •

1. Similar to blood plasma composition, as it is formed from blood plasma •
2. Forms a watery cushion to protect the brain •
3. Circulated in subarachnoid space, ventricles, and central canal of the spinal cord

Question 354

• 14.2 The Central Nervous System 10 The CNS: Spinal cord • Reflex arc: •

Answer 354

Central Nervous System

Download photos and understand study them from 14.2

From slides

1. Stimulus causes sensory receptors to generate afferent signals in sensory axons to the spinal cord •
2. Interneurons in spinal cord integrate the information and relay the information to the efferent motor neurons
3. • Motor neuron axons cause skeletal and/or smooth muscles to contract
4. 5. 14.2 The Central Nervous System 11
5. 7. What does the spinal cord look like?
6. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. dorsal root dorsal root ventral root ventral root vertebra spinal cord white matter central canal gray matter gray matter white matter gray matter meninges ventral dorsal white matter central canal vertebra b. a. c. dorsal root ganglion spinal nerve dorsal root ganglion spinal nerve dorsal root branches dorsal root ganglion cut vertebrae d. Dorsal view of spinal cord and dorsal roots of spinal nerves. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a: © Karl E. Deckart/Phototake; d: © The McGraw-Hill Companies, Inc./Rebecca Gray, photographer and Don Kincaid, dissections Figure 14.7 The organization of white and gray matter in the spinal cord and the spinal nerves. 14.2 The Central Nervous S

Question 356

See, download, review and study the following diagrams:

Figure 14.3 The structure of sensory neurons, interneurons, and motor neurons.

Answer 356

a. A sensory neuron has a long axon covered by a myelin sheath that takes nerve impulses all the way from dendrites to the CNS. b. In the CNS, some interneurons, such as this one, have a short axon that is not covered by a myelin sheath. c. A motor neuron has a long axon covered by a myelin sheath that takes nerve impulses from the CNS to an effector.

Answer 357

Identify the lobes and major areas of the human brain.

Question 358

reverse.prompt

What are the hemispheres?

Answer 358

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

Question 359

reversedprompt

• under the occipital lobe of the cerebrum
• and is separated from the brain stem by the fourth ventricle.
• The cerebellum has two portions joined by a narrow median portion.
  
  • white matter.
    
    • treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

1. receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts.
2. It also receives motor output from the cerebral cortex about where these parts should be located.
3. After integrating this information: sends motor signals by way of the brain stem to the skeletal muscles.
   
   • maintains posture and balance.
   • ensures that all the muscles work together to produce smooth, coordinated, voluntary movements.
   • assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

Answer 359

The Cerebellum

Question 361

Reverse

Classified according to function, the three types of neurons are sensory neurons, interneurons, and motor neurons (Fig. 14.3). Their functions are best described relative to the CNS. A sensory neuron takes nerve signals from a sensory receptor to the CNS. Sensory receptors are special structures that detect changes in the environment. An interneuron lies entirely within the CNS. Interneurons can receive input from sensory neurons and from other interneurons in the CNS. Thereafter, they sum up all the information received from other neurons before they communicate with motor neurons. A motor neuron takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland). Effectors carry out our responses to environmental changes, whether these are external or internal.

Answer 361

Anatomy of a Neuron

Answer 362

Drug abuse: Marijuana • Marijuana – psychoactive drug derived from a hemp plant called Cannabis; legal medical use and legal recreational use in some states • It is most often smoked as a “joint.” • Occasional users experience mild euphoria, alterations to vision and judgment, as well as impaired motor coordination with slurred speech. • Heavy users may experience depression, anxiety, hallucinations, paranoia, and psychotic symptoms. • Long term use may lead to brain damage. • K2 (“Spice”) is a synthetic drug with higher potency than THC, the active chemical in marijuana

Question 363

1. The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts.
2. It also receives motor output from the cerebral cortex about where these parts should be located.
3. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles.
4. In this way, the cerebellum maintains posture and balance.
5. It also ensures that all the muscles work together to
6. produce smooth, coordinated, voluntary movements.
7. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

Answer 363

Function of Cerebellum

Question 364

Drugs and Drug Abuse

Answer 364

Drugs and drug abuse • Both legal pharmaceuticals and illegal drugs of abuse have certain basic modes of action that are similar. They: – promote the action of a neurotransmitter. – interfere with or decrease the action of a neurotransmitter. – replace or mimic a neurotransmitter or neuromodulator. 1

Question 365

reverse.prompt

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

Question 366

Reverse

sensory, motor, interneurons… interneurons

Answer 366

These are the three types of neurons, and the one that is in the nervous system alone is…

Question 367

The nerve impulse: Resting potential (RP) • Resting potential – when the axon is not conducting a nerve impulse (when the axon is “at rest”) • More positive ions outside than inside the membrane • Negative charge of -70 mV inside the axon • More Na+ outside than inside • More K+ inside than outside

Question 368

• of the cortex receive information from the other association areas and perform higher-level analytical functions.
  1. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

Answer 368

Processing Centers- Prefrontal Area

Answer 369

Figure 14.17 A spinal reflex arc. A stimulus (e.g., a sharp pin) causes sensory receptors in the skin to generate nerve signals that travel in sensory axons to the spinal cord. Interneurons integrate data from sensory neurons and then relay signals to motor neurons, causing contraction of a skeletal muscle and movement of the hand away from the stimulus.

SCIENCE IN YOUR LIFE

How does aspirin work?

Aspirin is made of a chemical called acetylsalicylic acid (ASA). Damaged tissue produces large amounts of a type of fatty acid called prostaglandin. Prostaglandin acts as a signal to the peripheral nervous system that tissue damage has occurred, which the brain interprets as pain. Prostaglandins are manufactured in the cell by an enzyme called COX (cyclooxygenase). ASA reduces the capabilities of this enzyme, thus lowering the amount of prostaglandin produced and the perception of pain.

The Autonomic System

The autonomic system is also in the PNS (see Fig. 14.2). The autonomic system regulates the activity of cardiac and smooth muscles, organs, and glands. The system is divided into the sympathetic and parasympathetic divisions (Fig. 14.18). Activation of these two systems generally causes opposite responses.

Figure 14.18 The two divisions of the autonomic nervous system. Sympathetic preganglionic fibers (left) arise from the thoracic and lumbar portions of the spinal cord; parasympathetic preganglionic fibers (right) arise from the cranial and sacral portions of the spinal cord. Each system innervates the same organs but has contrary effects.

Although their functions are different, the two divisions share some features: (1) They usually function in an involuntary manner; (2) they innervate all internal organs; and (3) they use two neurons and one ganglion for each impulse. The first neuron has a cell body within the CNS and a preganglionic fiber that enters the ganglion. The second neuron has a cell body within a ganglion and a postganglionic fiber that leaves the ganglion.

Reflex actions, such as those that regulate blood pressure and breathing rate, are especially important to the maintenance of homeostasis. These reflexes begin when the sensory neurons in contact with internal organs send messages to the CNS. They are completed by motor neurons within the autonomic system.

Sympathetic Division

Most preganglionic fibers of the sympathetic division arise from the middle portion of the spinal cord. They terminate almost immediately in ganglia that lie near the cord.

The sympathetic division is especially important during emergency situations when you might be required to fight or take flight. It accelerates the heartbeat and dilates the bronchi—active muscles, after all, require a ready supply of glucose and oxygen. Page 297Sympathetic neurons inhibit the digestive organs, as well as the kidneys and urinary bladder; the activities of these organs—digestion, defecation, and urination—are not immediately necessary if you’re under attack. The neurotransmitter released by the postganglionic axon is primarily norepinephrine (NE). The structure of NE is like that of epinephrine (adrenaline), an adrenal medulla hormone that usually increases heart rate and contractility.

Parasympathetic Division

The parasympathetic division includes a few cranial nerves (e.g., the vagus nerve), as well as fibers that arise from the sacral (bottom) portion of the spinal cord. Therefore, this division is often referred to as the craniosacral portion of the autonomic system. In the parasympathetic division, the preganglionic fiber is long, and the postganglionic fiber is short because the ganglia lie near or within the organ.

The parasympathetic division, sometimes called the housekeeper division, promotes all the internal responses we associate with a relaxed state. For example, it causes the pupil of the eye to contract, promotes digestion of food, and slows heart rate. It has been suggested that the parasympathetic system could be called the rest-and-digest system. The neurotransmitter used by the parasympathetic division is acetylcholine (ACh).

The Somatic Versus the Autonomic Systems

Recall that the PNS includes the somatic system and the autonomic system. Table 14.1 compares the features and functions of the somatic motor pathway with the motor pathways of the autonomic system.

Table 14.1Comparison of Somatic Motor and Autonomic Motor Pathways

Table Summary: Columns are for somatic motor pathway and autonomic motor pathways. Rows are for different points of comparison. Autonomic motor pathways are grouped into sympathetic and parasympathetic, as the other column-headers.

Autonomic Motor Pathways

Somatic Motor PathwaySympatheticParasympathetic

Type of controlVoluntary/involuntaryInvoluntaryInvoluntary

Number of neurons per messageOneTwo (preganglionic shorter than postganglionic)Two (preganglionic longer than postganglionic)

Location of motor fiberMost cranial nerves and all spinal nervesThoracolumbar spinal nervesCranial (e.g., vagus) and sacral spinal nerves

NeurotransmitterAcetylcholineNorepinephrineAcetylcholine

EffectorsSkeletal musclesSmooth and cardiac muscle, glands, and organsSmooth and cardiac muscle, glands, and organs

CHECK YOUR PROGRESS 14.4

Contrast cranial and spinal nerves.

Answer

The 12 pairs of cranial nerves receive sensory input from and send motor outputs primarily to the head region. The 31 pairs of spinal nerves receive sensory input from and send motor outputs to the rest of the body.

Detail the fastest way for you to react to a stimulus.

Answer

A reflex action is fastest when it involves just the reflex arc that passes only through the spinal cord, not the brain.

Predict what could happen to homeostasis if the autonomic nervous system failed.

Answer

Without the autonomic nervous system, activities of the cardiac muscles, smooth muscles, and glands would have to be regulated voluntarily. Maintaining homeostasis would be an overwhelming task.

CONNECTING THE CONCEPTS

For more on the interaction of the PNS with the other systems of the body, refer to the following discussions:

Section 5.3 explores how the divisions of the autonomic system regulate the heart rate and help maintain homeostasis.

Section 10.5 examines how signals between the brain and the diaphragm control the rate of breathing.

Section 15.1 provides an overview of the types of sensory inputs processed by the peripheral nervous system.

Question 370

Reverse

The Brain Stem

• The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a).
• The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses.
• The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS.
• In addition, the pons functions with the medulla oblongata to regulate breathing rate.
• Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.
• The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

Answer 370

Brain Stem

Question 371

Structure of the Spinal Cord

(Fig. 14.8a)

Figure 14.8b

Answer 371

A cross-section of the spinal cord shows a

1. central canal,
2. gray matter, and
3. white matter

Question 372

Reverse

sensory

Answer 372

This type of neuron takes nerve signals from a sensory receptor to the CNS.

Question 373

Reverse

Sympathetic Division

Most preganglionic fibers of the sympathetic division arise from the middle portion of the spinal cord. They terminate almost immediately in ganglia that lie near the cord.

The sympathetic division is especially important during emergency situations when you might be required to fight or take flight. It accelerates the heartbeat and dilates the bronchi—active muscles, after all, require a ready supply of glucose and oxygen. Page 297Sympathetic neurons inhibit the digestive organs, as well as the kidneys and urinary bladder; the activities of these organs—digestion, defecation, and urination—are not immediately necessary if you’re under attack. The neurotransmitter released by the postganglionic axon is primarily norepinephrine (NE). The structure of NE is like that of epinephrine (adrenaline), an adrenal medulla hormone that usually increases heart rate and contractility.

Answer 373

Sympathetic Division

Answer 374

Important concepts to focus on • What are the divisions of the nervous system? • What are the functions of the nervous system? • What are the three types of neurons? • What are neuroglia? • What is the structure of a neuron? • What is the myelin sheath? Saltatory conduction? Schwann cell? Node of Ranvier? • Explain the resting and action potential as they relate to a nerve impulse. • How does the nerve impulse traverse the synapse? • What are the 4 parts of the brain and their functions? • What structures protect the CNS? • What are the 2 parts of the peripheral nervous system? • Describe the actions of some drugs of abuse

Question 375

Reverse

What are oligodendrocytes?

Answer 375

1. branched cells that wrap CNS nerve fibers
2. Produce fatty insulating coverings (myelin sheath) around nerve fibers in the CNS – Can coil around as many as 60 different fibers at one time

Question 376

What is Central White Matter?

Where is it found?

What process enables the brain to grow in size and complexity

What occurs?

Answer 376

1. Much of the rest of the cerebrum
2. Myelination occurs and white matter develops as a child grows. ​

• Progressive myelination
  
  • enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area.
  • Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10.
  • The corpus callosum contains tracts that join the two cerebral hemispheres

Question 378

Reverse

Blood Brain Barrier • Includes the least permeable capillaries of the body • Excludes many potentially harmful substances • Useless against some substances • Fats and fat soluble molecules • Respiratory gases • Alcohol • Nicotine • Anesthesia Supplemental material not in book 14.2 The Central Nervous System 8 The central nervous system • Both the brain and spinal cord are made up of 2 types of nervous tissue: • Gray matter – contains cell bodies and nonmyelinated fibers • White matter – contains myelinated axons 14.2 The Central Nervous System 9 The CNS: Spinal cord • It extends from the base of the brain and along the length of the vertebral canal formed by the vertebrae. • The spinal cord functions to provide communication between the brain and most of the body. • It is the integrating center for reflex arcs. • Gray matter in the center is in a butterfly shape. • White matter surrounds the gray matter. 14.2 The Central Nervous System 10 The CNS: Spinal cord • Reflex arc: • Stimulus causes sensory receptors to generate afferent signals in sensory axons to the spinal cord • Interneurons in spinal cord integrate the information and relay the information to the efferent motor neurons • Motor neuron axons cause skeletal and/or smooth muscles to contract 14.2 The Central Nervous System 11 What does the spinal cord look like? Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. dorsal root dorsal root ventral root ventral root vertebra spinal cord white matter central canal gray matter gray matter white matter gray matter meninges ventral dorsal white matter central canal vertebra b. a. c. dorsal root ganglion spinal nerve dorsal root ganglion spinal nerve dorsal root branches dorsal root ganglion cut vertebrae d. Dorsal view of spinal cord and dorsal roots of spinal nerves. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a: © Karl E. Deckart/Phototake; d: © The McGraw-Hill Companies, Inc./Rebecca Gray, photographer and Don Kincaid, dissections Figure 14.7 The organization of white and gray matter in the spinal cord and the spinal nerves. 14.2 The Central Nervous S

Question 379

Reverse

Somatic and Parasympathetic Pathways

Answer 379

Table 14.1Comparison of Somatic Motor and Autonomic Motor Pathways

Table Summary: Columns are for somatic motor pathway and autonomic motor pathways. Rows are for different points of comparison. Autonomic motor pathways are grouped into sympathetic and parasympathetic, as the other column-headers.

Autonomic Motor Pathways

Somatic Motor PathwaySympatheticParasympathetic

Type of controlVoluntary/involuntaryInvoluntaryInvoluntary

Number of neurons per messageOneTwo (preganglionic shorter than postganglionic)Two (preganglionic longer than postganglionic)

Location of motor fiberMost cranial nerves and all spinal nervesThoracolumbar spinal nervesCranial (e.g., vagus) and sacral spinal nerves

NeurotransmitterAcetylcholineNorepinephrineAcetylcholine

EffectorsSkeletal musclesSmooth and cardiac muscle, glands, and organsSmooth and cardiac muscle, glands, and organs

Question 380

Synaptic Integration

Answer 380

1. Integration is the summation (adding up) of the inhibitory and excitatory signals received by a postsynaptic neuron. • This occurs because a neuron receives many signals at once.

Question 381

Biology Today Nerve Regeneration and Stem Cells

Answer 381

BIOLOGY TODAY Science

Nerve Regeneration and Stem Cells

In humans, axons outside the brain and spinal cord can regenerate—but axons inside these organs cannot (Fig. 14A). After injury, axons in the human central nervous system (CNS) degenerate, resulting in permanent loss of nervous function. Interestingly, about 90% of the cells in the brain and the spinal cord are not even neurons. They are neuroglia cells. In nerves outside the brain and spinal cord, the neuroglia cells are Schwann cells that help axons regenerate. The neuroglia cells in the CNS include microglial cells, oligodendrocytes, and astrocytes, and they inhibit axon regeneration.

Figure 14A Regeneration of nerve cells. Outside the CNS, nerves regenerate because new neuroglia called Schwann cells form a pathway for axons to reach a muscle. In the CNS, comparable neuroglia called oligodendrocytes do not have this function.

The spinal cord contains its own stem cells. When the spinal cord is injured in experimental animals, these stem cells proliferate. But instead of becoming functional neurons, they become neuroglia cells. Researchers are trying to understand the process that triggers the stem cells to become neuroglia cells. In the future, this understanding would allow manipulation of stem cells into neurons.

In early experiments with neural stem cells in the laboratory, scientists at Johns Hopkins University caused embryonic stem (ES) cells to differentiate into spinal cord motor neurons, the type of nerve cell that causes muscles to contract. The motor neurons then produced axons. When grown in the same dish with muscle cells, the motor neurons formed neuromuscular junctions and even caused muscle contractions. The cells were then transplanted into the spinal cords of rats with spinal cord injuries. Some of the transplanted cells survived for longer than a month within the spinal cord. However, no improvement in symptoms was seen and no functional neuron connections were made.

In later experiments by the same research group, paralyzed rats were first treated with drugs and nerve growth factors to overcome inhibition from the central nervous system. These techniques significantly increased the success of the transplanted neurons. Amazingly, axons of transplanted neurons reached the muscles, formed neuromuscular junctions, and provided partial relief from the paralysis. Research is being done on the use of both the body’s own stem cells and laboratory-grown stem cells to repair damaged CNS neurons. Though many questions remain, the current results are promising.

Questions to Consider

What is the likely reason neurons cannot simply be transplanted from other areas of the body?

How might this research also help patients who suffer from neurodegenerative diseases, such as Parkinson disease?

Long axons tend to have a myelin sheath, but short axons do not. The gray matter of the CNS is gray because it contains no myelinated axons; the white matter of the CNS is white because it does. In the PNS, myelin gives nerve fibers their white, glistening appearance and serves as an excellent insulator. When the myelin breaks down, as happens in multiple sclerosis (MS) (see chapter opener), then it becomes more difficult for the neurons to transmit information. In effect, MS “short-circuits” the nervous system. The myelin sheath also plays an important role in nerve regeneration within the PNS. If an axon is accidentally severed, the myelin sheath remains and serves as a passageway for new fiber growth.

Question 382

This neuron takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland). Effectors carry out our responses to environmental changes, whether these are external or internal.

Answer 382

What is a motor neuron?

Question 383

Reverse

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Answer 383

Functions of Spinal Cord

Question 384

reversedprompt

• This structure receives and integrates sensory input from the eyes, ears, joints, and muscles about the current position of the body
• • Functions

1. • Maintains posture •
2. Coordinates voluntary movement •
3. Allows learning of new motor skills (i.e., playing the piano or hitting a baseball) 1

Answer 384

Cerebellum, CNS,

• 14.2 The Central Nervous System 23 3.
• • Slides

Question 385

Cerebellum (cont)

Cerebral Hemispheres with the 4 _____________

Answer 385

1. cerebrum.
2. left and right cerebral hemispheres
3. (Fig. 14.9b).
4. longitudinal fissure divides hemispheres.
5. the 2 hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.
6. Page 289
7. The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10).

• The frontal lobe is the most anterior of the lobes (directly behind the forehead): movement and higher reasoning, as well as the smell sensation
• The parietal lobe is posterior to the frontal lobe: Somatic sensing
• The occipital lobe is posterior to the parietal lobe (at the rear of the head): Visual information is received and processed .
• The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear):

Figure 14.10 Centers in the frontal lobe control is carried out by parietal lobe neurons, and those of the temporal lobe in the occipital lobe.

Question 386

Reverse

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

Answer 386

basal nuclei

Question 387

Spinal Cord Structure

Intervertebral Foramina

Fibrocartilage intervertebral discs separating vertebrae

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

Answer 387

1. .shows how an individual vertebra protects the spinal cord.
2. The spinal nerves project from the cord through small openings called intervertebral foramina.
3. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Question 388

Reverse

Cerebellum

Answer 388

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

Question 389

reversedprompt

• An exchange of Na+ and K+ ions generates an action potential that moves along the length of an axon.
• An action potential in one location stimulates the production of an action potential in an adjacent part of the axon membrane.
• If the nerve is myelinated, the action potential moves more quickly, “jumping” from one node of Ranvier to the next.

Answer 389

Lecture notes Na + and K+ action potential generation

Question 390

• 1. receive information from the other association areas
  2. and perform higher-level analytical functions.
• The prefrontal area,
  
  • ​ frontal lobe, receives information from the other association areas and uses to reason and plan our actions.
• Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.
• The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex.
  
  1. Wernicke’s area is located in the posterior part of the left temporal lobe: helps us understand both the written and the spoken word and sends the information to Broca’s area.
  2. Broca’s area is located in the left frontal lobe.
  • anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth)
  • adds grammatical refinements
  • and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

1.

Answer 390

Processing Centers

in the Cortex

(see Fig. 14.10). - download and see

Question 393

Reverse

The peripheral nervous system (PNS) • It includes cranial nerves (12 pairs), spinal nerves (31 pairs), and ganglia (neuronal cell bodies) outside the CNS. - Spinal nerves conduct impulses to and from the spinal cord. - Cranial nerves conduct impulses to and from the brain. • The PNS is divided into 2 systems. - Somatic division - Autonomic division 14.4 The Peripheral Nervous System 2 The peripheral nervous system Figure 14.14 The structure of a nerve. 14.4 The Peripheral Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. LM PNS single nerve fiber bundle of nerve fibers blood vessel Spinal or Cranial Nerve artery and vein spinal nerves cranial nerves © PASIEKA/Science Photo Library/Science Source 3 14.4 The Peripheral Nervous System The peripheral nervous system Know these cranial nerves! Copyright © The McGraw-Hill Companies, Inc. Permission r

Question 394

Nerve Impulse

Synapse

Lecture slides

Answer 394

1. This is the junction between the sending neuron (presynaptic membrane) and the receiving neuron (postsynaptic membrane).
2. Transmission is accomplished chemically across a small gap between the two neurons (synaptic cleft) by a neurotransmitter
   
   • (e.g., ACh, dopamine, or serotonin).
   • Neurotransmitters are stored in synaptic vesicles in the axon terminals

Question 395

Lecture Notes

True or False

The prefontal cortex is also a processing center in the cerebrum?

Answer 395

True

Question 396

Review 14.4

Answer 396

Autonomic Motor Pathways

Somatic Motor PathwaySympatheticParasympathetic

Type of controlVoluntary/involuntaryInvoluntaryInvoluntary

Number of neurons per messageOneTwo (preganglionic shorter than postganglionic)Two (preganglionic longer than postganglionic)

Location of motor fiberMost cranial nerves and all spinal nervesThoracolumbar spinal nervesCranial (e.g., vagus) and sacral spinal nerves

NeurotransmitterAcetylcholineNorepinephrineAcetylcholine

EffectorsSkeletal musclesSmooth and cardiac muscle, glands, and organsSmooth and cardiac muscle, glands, and organs

CHECK YOUR PROGRESS 14.4

Contrast cranial and spinal nerves.

Answer

The 12 pairs of cranial nerves receive sensory input from and send motor outputs primarily to the head region. The 31 pairs of spinal nerves receive sensory input from and send motor outputs to the rest of the body.

Detail the fastest way for you to react to a stimulus.

Answer

A reflex action is fastest when it involves just the reflex arc that passes only through the spinal cord, not the brain.

Predict what could happen to homeostasis if the autonomic nervous system failed.

Answer

Without the autonomic nervous system, activities of the cardiac muscles, smooth muscles, and glands would have to be regulated voluntarily. Maintaining homeostasis would be an overwhelming task.

CONNECTING THE CONCEPTS

For more on the interaction of the PNS with the other systems of the body, refer to the following discussions:

Section 5.3 explores how the divisions of the autonomic system regulate the heart rate and help maintain homeostasis.

Section 10.5 examines how signals between the brain and the diaphragm control the rate of breathing.

Section 15.1 provides an overview of the types of sensory inputs processed by the peripheral nervous system.

Question 397

reverse.prompt

14.4 The Peripheral Nervous System

LEARNING OUTCOMES

Upon completion of this section you should be able to

Describe the series of events during a spinal reflex.

Distinguish between the somatic and autonomic divisions of the peripheral nervous system.

Distinguish between the sympathetic and parasympathetic divisions of the autonomic division.

Question 398

Somatic Versus Autonomic Systems

Answer 398

The Somatic Versus the Autonomic Systems

Recall that the PNS includes the somatic system and the autonomic system. Table 14.1 compares the features and functions of the somatic motor pathway with the motor pathways of the autonomic system.

Table 14.1Comparison of Somatic Motor and Autonomic Motor Pathways

Table Summary: Columns are for somatic motor pathway and autonomic motor pathways. Rows are for different points of comparison. Autonomic motor pathways are grouped into sympathetic and parasympathetic, as the other column-headers.

Question 399

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

Question 400

Reverse

14.5 Drug Therapy and Drug Abuse

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain the ways that drugs interact with the nervous system.

Classify drugs as to whether they have a depressant, stimulant, or psychoactive effect on the nervous system.

List the long-term effects of drug use on the body.

As you are reading these words, synapses throughout your brain are organizing, integrating, and cataloging the information you take in. Neurotransmitters at these synapses control the firing of countless action potentials, thus creating a network of neural circuits. It is amazing to realize that all thoughts, feelings, and actions of a human are dependent on neurotransmitters in the CNS and PNS. By modifying or controlling synaptic transmission, a wide variety of drugs with neurological activity, both legal pharmaceuticals and illegal drugs of abuse, can alter mood, emotional state, behavior, and personality.

Answer 400

14.5 Learning Outcomes

Answer 401

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 402

Synaptic Integration

Answer 402

Synaptic Integration

A single neuron has a cell body and may have many dendrites (Fig. 14.6a). All can have synapses with many other neurons. Therefore, a neuron is on the receiving end of many signals, Page 286which can either be excitatory or inhibitory. Recall that an excitatory neurotransmitter produces an excitatory signal by opening sodium gates at a synapse. This drives the neuron closer to its threshold (illustrated by the green line in Fig. 14.6b). If threshold is reached, an action potential is inevitable. On the other hand, an inhibitory neurotransmitter drives the neuron farther from an action potential (red line in Fig. 14.6b) by opening the gates for potassium.

Neurons integrate these incoming signals. Integration is the summing up of excitatory and inhibitory signals. If a neuron receives enough excitatory signals (either from different synapses or at a rapid rate from a single synapse) to outweigh the inhibitory ones, chances are the axon will transmit a signal. On the other hand, if a neuron receives more inhibitory than excitatory signals, summing these signals may prohibit the axon from reaching threshold and then depolarizing (the solid black line in Fig. 14.6b).

Question 403

• axon branches into many fine endings, each tipped by a small swelling called an axon terminal.
  
  • Each terminal lies very close to either the dendrite or the cell body of another neuron, area called synapse (Fig. 14.5). At a synapse, a small gap called the synaptic cleft
    
    1. separates the sending neuron from the receiving neuron. The nerve signal is unable to jump the cleft. Therefore:
    2. The nerve signal is unable to jump the cleft. Therefore, another means is needed to pass the nerve signal from the sending neuron to the receiving neuron.

Answer 403

The Synapse

Axon terminal

See, download study Figure 14.5

See and Study Figure 14.5 Signal transmission at the synapse. Transmission across a synapse from one neuron to another occurs when a neurotransmitter is released, diffuses across a synaptic cleft, and binds to a receptor in the membrane of the receiving neuron.

• Tutorial: Synaptic Cleft
• Transmission across a synapse is carried out by molecules called neurotransmitters, stored in synaptic Page 285vesicles in the axon terminals. (See Section 3.4 for a review of vesicle function.) The events (Fig. 14.5) at a synapse are (1) nerve signals traveling along an axon to reach an axon terminal; (2) calcium ions entering the terminal and stimulating synaptic vesicles to merge with the sending membrane; and (3) neurotransmitter molecules releasing into the synaptic cleft and diffusing across the cleft to the receiving membrane; there, neurotransmitter molecules bind with specific receptor proteins.

Figure 14.6 Integration of excitatory and inhibitory signals at the synapse. a. Inhibitory signals and excitatory signals are summed up in the dendrite and cell body of the postsynaptic neuron. Only if the combined signals cause the membrane potential to rise above threshold does an action potential occur. b. In this example, threshold was not reached.

Question 404

Spinal Cord

Answer 404

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Question 405

Reverse

• cardiac and smooth muscles, organs, and glands
  *

Answer 405

The Autonomic Nervous System

The autonomic system is also in the PNS (see Fig. 14.2)

The system is divided into the sympathetic and parasympathetic divisions (Fig. 14.18). Activation of these two systems generally causes opposite responses.

Figure 14.18 The two divisions of the autonomic nervous system. Sympathetic preganglionic fibers (left) arise from the thoracic and lumbar portions of the spinal cord; parasympathetic preganglionic fibers (right) arise from the cranial and sacral portions of the spinal cord. Each system innervates the same organs but has contrary effects.

Question 406

reverse.prompt

1. 18 Wernicke’s and Broca’s aphasias
   
   1. Aphasias : brain lesions/areas of damage
   2. When Wernicke’s area is damaged, the individual is not able to process language and responds nonsensically (“word salad”) •
   3. When Broca’s area is damaged, the individual understands what is spoken to him/her, however is unable to have motor control for speech to verbally respond

Answer 406

14.2 The Central Nervous System

Lecture notes

Wernicke’s and Broca’s aphasias

Which one produces word salad?

(Diseases and Disorders)

Question 407

Reverse

The PNS has divisions:

• the somatic system nerves serve:

1. the skin
2. skeletal muscles
3. tendons

• nerves function to: take sensory information from external sensory receptors to the CNS; Motor commands leaving the CNS travel to skeletal muscles via somatic motor nerves and the autonomic system.
• Not all somatic motor actions are voluntary. Some are automatic. Automatic responses to a stimulus in the somatic system are called reflexes. A reflex occurs quickly, without your even having to think about it. For example, a reflex may cause you to blink your eyes in response to a flash of light, without your willing it. We will study the path of a reflex, because it allows us to study

Answer 407

PNS: Somatic Sensory System

(see Fig. 14.2).

Page 296

Answer 408

14.2 The Central Nervous System 24 4. The brain: The brainstem • Includes • Midbrain – relay station between the cerebrum and spinal cord or cerebellum; reflex center • Pons – a bridge between cerebellum and the CNS; regulates breathing rate; reflex center for head movements • Medulla oblongata – contains reflex centers for regulating breathing, heartbeat, and blood pressure • Reticular formation – major component of the reticular activating system (RAS) that regulates alertness (if it is damaged, you would be in a coma)

Question 409

List what limbic system structures are involved in the fight-or-flight reaction, learning, and long-term memory.

Answer 409

Amygdala—fight-or-flight; hippocampus—learning and memory. The hippocampus acts as a bridge between the sensory association areas of the cerebral cortex where memories are stored long term and the prefrontal areas of the cortex where memories are used.

Question 410

PNS Somatic Division

14.4 The Peripheral Nervous System 5 The PNS: Somatic division

Answer 410

1. serves the

• skin,
• skeletal muscles and
• tendons. •

1. Automatic responses are called reflexes. •
2. Reflexes consist of
   
   • sensory receptor
   • sensory neuron
   • interneuron 
   • motor neuron 
   • effector organ (we talked about this in 14.1)

Question 411

Reflex Arc

Answer 411

The Reflex Arc

Figure 14.17 illustrates the path of a reflex that involves only the spinal cord. If your hand touches a sharp pin, sensory receptors in the skin generate nerve signals that move along sensory fibers through the posterior (dorsal) root ganglia toward the spinal cord. Sensory neurons that enter the cord posteriorly pass signals on to many interneurons. Some of these interneurons synapse with motor neurons whose short dendrites and cell bodies are in the spinal cord. Nerve signals travel along these motor fibers to an effector, which brings about a response to the stimulus. In this case, the effector is a muscle, which contracts so that you withdraw your hand from the pin. Various other reactions are also possible—you will most likely look at the pin, wince, and cry out in pain. This whole series of responses occurs because some of the interneurons involved carry nerve signals to the brain. The brain makes you aware of the stimulus and directs these other reactions to it. In other words, you don’t feel pain until the brain receives the information and interprets it.

Figure 14.17 A spinal reflex arc. A stimulus (e.g., a sharp pin) causes sensory receptors in the skin to generate nerve signals that travel in sensory axons to the spinal cord. Interneurons integrate data from sensory neurons and then relay signals to motor neurons, causing contraction of a skeletal muscle and movement of the hand away from the stimulus.

Question 412

Neuroaglia

Answer 412

• CNS – Astrocytes – Microglia – Ependymal cells – Oligodendrocytes • PNS – Schwann cells – Satellite cells 1

Question 413

Reverse

Classified according to function, the three types of neurons are sensory neurons, interneurons, and motor neurons (Fig. 14.3). Their functions are best described relative to the CNS. A sensory neuron takes nerve signals from a sensory receptor to the CNS. Sensory receptors are special structures that detect changes in the environment. An interneuron lies entirely within the CNS. Interneurons can receive input from sensory neurons and from other interneurons in the CNS. Thereafter, they sum up all the information received from other neurons before they communicate with motor neurons. A motor neuron takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland). Effectors carry out our responses to environmental changes, whether these are external or internal.

Figure 14.3 The structure of sensory neurons, interneurons, and motor neurons. a. A sensory neuron has a long axon covered by a myelin sheath that takes nerve impulses all the way from dendrites to the CNS. b. In the CNS, some interneurons, such as this one, have a short axon that is not covered by a myelin sheath. c. A motor neuron has a long axon covered by a myelin sheath that takes nerve impulses from the CNS to an effector.

(photos) (a): ©McGraw-Hill Education/Dr. Dennis Emery, Dept. of Zoology and Genetics, Iowa State University, photographer; (b): ©David M. Phillips/Science Source

Neurons vary in appearance, but all of them have three distinct structures: a cell body, dendrites, and an axon. The cell body contains the nucleus, as well as other organelles. Dendrites are short extensions that receive signals from sensory receptors or other neurons. Incoming signals from dendrites can result in nerve signals that are then conducted by an axon. The axon is the portion of a neuron that conducts nerve impulses. An axon can be quite long. Individual axons are termed nerve fibers, and collectively they form a nerve.

In sensory neurons, a very long axon carries nerve signals from the dendrites associated with a sensory receptor to the CNS, and this axon is interrupted by the cell body. In interneurons and motor neurons, on the other hand, multiple dendrites take signals to the cell body, and then an axon conducts nerve signals away from the cell body.

Myelin Sheath

Many axons are covered by a protective myelin sheath. The myelin sheath develops when Schwann cells (PNS) or oligodendrocytes (CNS) wrap their membranes around an axon many times. Each neuroglia cell covers only a portion of an axon, so the myelin Page 282sheath is interrupted. The gaps where there is no myelin sheath are called nodes of Ranvier (Fig. 14.3). Later in this section, we will see how the myelin sheath plays an important role in the rate at which signals move through the neuron.

Answer 413

Neurons, and their anatomy

Question 414

Time to download all photographs, review slides, do PLQ again and review adaptive learning along with flash cards and making study guide for Chapter Nervous System!

Question 415

Drug abuse: Marijuana • Marijuana – psychoactive drug derived from a hemp plant called Cannabis; legal medical use and legal recreational use in some states • It is most often smoked as a “joint.” • Occasional users experience mild euphoria, alterations to vision and judgment, as well as impaired motor coordination with slurred speech. • Heavy users may experience depression, anxiety, hallucinations, paranoia, and psychotic symptoms. • Long term use may lead to brain damage. • K2 (“Spice”) is a synthetic drug with higher potency than THC, the active chemical in marijuana

Question 417

Reverse

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Answer 417

Science in your life? Strokes

Question 418

reversedprompt

The Cerebral Cortex, gray matter, myelinated.

Answer 418

1. a thin, highly convoluted outer layer of _____________ that covers the cerebral hemispheres.
2. _______________consists of neurons whose axons are not_________________
3. over 1 billion cell bodies
4. **accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness**
   5.

Question 419

reversedprompt

• Reticular formation – major component of the reticular activating system (RAS) that regulates alertness (if it is damaged, you would be in a coma) 14.2 The Central Nervous System 25 The reticular formation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RAS radiates to cerebral cortex. thalamus reticular formation ascending sensory tracts (touch, pain, temperature) Figure 14.11 The reticular formation of the brain.

Answer 419

Reticular Formation

Question 420

Reverse

Autonomic Motor Pathways

Somatic Motor PathwaySympatheticParasympathetic

Type of controlVoluntary/involuntaryInvoluntaryInvoluntary

Number of neurons per messageOneTwo (preganglionic shorter than postganglionic)Two (preganglionic longer than postganglionic)

Location of motor fiberMost cranial nerves and all spinal nervesThoracolumbar spinal nervesCranial (e.g., vagus) and sacral spinal nerves

NeurotransmitterAcetylcholineNorepinephrineAcetylcholine

EffectorsSkeletal musclesSmooth and cardiac muscle, glands, and organsSmooth and cardiac muscle, glands, and organs

CHECK YOUR PROGRESS 14.4

Contrast cranial and spinal nerves.

Answer

The 12 pairs of cranial nerves receive sensory input from and send motor outputs primarily to the head region. The 31 pairs of spinal nerves receive sensory input from and send motor outputs to the rest of the body.

Detail the fastest way for you to react to a stimulus.

Answer

A reflex action is fastest when it involves just the reflex arc that passes only through the spinal cord, not the brain.

Predict what could happen to homeostasis if the autonomic nervous system failed.

Answer

Without the autonomic nervous system, activities of the cardiac muscles, smooth muscles, and glands would have to be regulated voluntarily. Maintaining homeostasis would be an overwhelming task.

CONNECTING THE CONCEPTS

For more on the interaction of the PNS with the other systems of the body, refer to the following discussions:

Section 5.3 explores how the divisions of the autonomic system regulate the heart rate and help maintain homeostasis.

Section 10.5 examines how signals between the brain and the diaphragm control the rate of breathing.

Section 15.1 provides an overview of the types of sensory inputs processed by the peripheral nervous system.

Answer 420

Review 14.4

Question 421

Reverse

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Answer 421

What’s the Cerebral Cortex?

Question 422

Limbic System 14.2 The Central Nervous System 26 The limbic system

Class slides

Answer 422

• • • It joins primitive emotions (i.e., fear, pleasure) with higher functions, such as reasoning. •
• It can cause strong emotional reactions to situations but conscious thought can override and direct our behavior. •
• Includes • Amygdala – imparts emotional overtones •
• Hippocampus – important to learning and memory

Question 423

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Question 424

reversedprompt

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

1. Descending motor tracts (from the primary motor area)
2. and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla.
3. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over.
4. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere.
5. _**Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side._

Answer 424

Stroke

Question 425

Reticular Formation

Answer 425

• Reticular formation – major component of the reticular activating system (RAS) that regulates alertness (if it is damaged, you would be in a coma) 14.2 The Central Nervous System 25 The reticular formation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RAS radiates to cerebral cortex. thalamus reticular formation ascending sensory tracts (touch, pain, temperature) Figure 14.11 The reticular formation of the brain.

Question 427

14.2 The Central Nervous System

Lecture notes

Wernicke’s and Broca’s aphasias

Which one produces word salad?

(Diseases and Disorders)

Answer 427

1. 18 Wernicke’s and Broca’s aphasias
   
   1. Aphasias : brain lesions/areas of damage
   2. When Wernicke’s area is damaged, the individual is not able to process language and responds nonsensically (“word salad”) •
   3. When Broca’s area is damaged, the individual understands what is spoken to him/her, however is unable to have motor control for speech to verbally respond

Question 428

14.5 Drug Therapy and Drug Abuse

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain the ways that drugs interact with the nervous system.

Classify drugs as to whether they have a depressant, stimulant, or psychoactive effect on the nervous system.

Question 429

Reverse

1. under the occipital lobe of the cerebrum and is
2. separated from the brain stem by the fourth ventricle.
3. This structure has two portions joined by a narrow median portion.
   
   • Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae.
   • Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

Answer 429

Structure and Location of Cerebellum

Question 430

reversedprompt

1. In myelinated fibers, an action potential at one node of Ranvier causes an action potential at the next, jumping over the entire myelin-coated portion of the axon: This type of conduction is translated from (saltatio is a Latin word that means “to jump”) and is much faster. In thick, myelinated fibers, the rate of transmission is more than 100 m/s.
2. Regardless of whether an axon is myelinated or not, its action potentials are self-propagating. Each action potential generates another, along the entire length of the axon.
3. Like the action potential itself, conduction of an action potential is an all-or-none event—either an axon conducts its action potential or it does not. The intensity of a message is determined by how many action potentials are generated within a given time. An axon can conduct a volley of action potentials very quickly, because only a small number of ions are exchanged with each action potential. Once the action potential is complete, the ions are rapidly restored to their proper place through the action of the sodium–potassium pump
4. Neural Transmission: Action Potential Propagation

• As soon as the action potential has passed by each successive portion of an axon, that portion undergoes a short refractory period, during which it is unable to conduct an action potential.
  
  • This ensures the one-way direction of a signal from the cell body down the length of the axon to the axon terminal.

***all functions of the nervous system, from our deepest emotions to our highest reasoning abilities, are dependent on the conduction of nerve signals**

Propagation of an Action Potential

1. If an axon is unmyelinated, an action potential at one locale stimulates an adjacent part of the axon membrane to produce an action potential. Conduction along the entire axon in this fashion can be rather slow—approximately 1 meter/second (1 m/s) in thin axons—because each section of the axon must be stimulated.

Answer 430

Action Potential Propagation; This is Saltatory conduction

Neural Transmission

This is the name of the period during which an axon is unable to conduct an action potential, thereby ensuring :

Question 431

Reverse

These 4 frontal lobes are found here which is defined as (14.2)

Answer 431

• Cerebrum –
• largest portion of the brain •

4 lobes:

1. Frontal lobe: primary motor area and conscious thought
2. Temporal lobe: primary auditory, smell, and speech area
3. Parietal lobe: primary somatosensory and taste area
4. Occipital lobe: primary visual area
5. 2 The Central Nervous System 16 1. The brain: Cerebrum – The cerebral hemispheres

Question 432

These are the interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7).

Answer 432

4 ventricles

Question 433

reverse.prompt

1. The pineal gland
2. diencephalon
3. melatonin
   
   • *

Answer 433

This is a hormone I secrete, which is located in the diencephalon.

People love me for secreting it… for insomnia and maybe even for regulating the onset of puberty.

Question 434

Action Potential and the Sodium-Potassium Pump

Answer 434

How the Sodium–Potassium Pump Works

Action Potential

The resting potential energy of the neuron can be used to perform the work of the neuron: conduction of nerve signals. The process of conduction is termed an action potential, and it occurs in the axons of neurons. A stimulus activates the neuron and begins the action potential. For example, a stimulus for pain neurons in the skin would be the prick of a sharp pin. However, the stimulus must be strong enough to cause the cell to reach threshold, the voltage that will result in an action potential. In Figure 14.4b, the threshold voltage is around −55 mV. An action potential is an all-or-nothing event. Once threshold is reached, the action potential happens automatically and completely. On the other hand, if the threshold voltage is never reached, the action potential does not occur. Increasing the strength of a stimulus (such as pressing harder with the pin) does not change the strength of an action potential. However, it may cause more action potentials to occur in a given period. As a result, the person may perceive that pain has increased.

Answer 435

The myelin sheath • A lipid covering on long axons that acts to increase the speed of nerve impulse conduction, insulation for both CNS and PNS, and regeneration in the PNS • Schwann cells – neuroglia that make up the myelin sheath in the PNS • Oligodendrocytes- neuroglia that make up the myelin sheath in the CNS • Nodes of Ranvier – gaps between myelination on the axons • Saltatory conduction – conduction of the nerve impulse from node to node

Question 436

Reverse

A lipid covering on long axons that acts to
increase the speed of nerve impulse conduction,
insulation for both CNS and PNS, and
regeneration in the PNS
• Schwann cells – neuroglia that make up the
myelin sheath in the PNS
• Oligodendrocytes- neuroglia that make up the
myelin sheath in the CNS
• Nodes of Ranvier – gaps between myelination
on the axons
• Saltatory conduction – conduction of the nerve
impulse from node to node

Answer 436

Myelin Sheath

Question 437

Neurons are best classified according to:

Answer 437

role in CNS

Question 438

Reverse

• Primary motor area – voluntary control of skeletal muscle • P

Answer 438

Primary Motor Area

Question 439

Reverse

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 440

Reverse

Nerve Impulse

Sodium Gates Open

Protein channels specific for sodium ions are located in the plasma membrane of the axon. When an action potential begins in response to a threshold stimulus, these protein channels open and sodium ions rush into the cell. Adding positively charged sodium ions causes the inside of the axon to become positive compared to the outside (Fig. 14.4c). This change is called depolarization, because the charge (polarity) inside the axon changes from negative to positive.

Potassium Gates Open

Almost immediately after depolarization, the channels for sodium close and a separate set of potassium protein channels opens. Potassium flows rapidly from the cell. As positively charged potassium ions exit the cell, the inside of the cell becomes negative again because of the presence of large, negatively charged ions trapped inside the cell. This change in polarity is called repolarization, because the inside of the axon resumes a negative charge as potassium exits the axon (Fig. 14.4d). Finally, the sodium–potassium pump completes the action potential. Potassium ions are returned to the inside of the cell and sodium ions to the outside, and resting potential is restored.

Neural Transmission: Resting Membrane Potential and Propagation

Graph of an Action Potential

To visualize such rapid fluctuations in voltage across the axonal membrane, researchers generally find it useful to plot the voltage changes over time (Fig. 14.4e). During depolarization, the voltage increases from −70 mV to −55 mV to between +30 and +35 mV as sodium ions move to the inside of the axon. In repolarization, the opposite change occurs when potassium ions leave the axon. The entire process is very rapid, requiring only 3 to 4 milliseconds (ms) to complete.

Question 441

• lies entirely within the CNS.
• receive input from sensory neurons and from other interneurons in the CNS.
• sum up all the information received from other neurons before they communicate with motor neurons.

Answer 441

What is an interneuron?

Question 442

prefrontal cortex

Answer 442

reasoning

critical thinking

formulating appropriate behaviors

Question 443

Reversed prompt

Reflex Actions, Homeostasis, CNS

(see Fig. 14.17)

Page 288

Answer 443

The spinal cord is the center for

• thousands of reflex arcs .
  
  1. stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord.
  2. Interneurons integrate the incoming data and relay signals to motor neurons.
  3. A response to the stimulus occurs when motor axons cause skeletal muscles to contract.
  4. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands.
  5. Each interneuron in the spinal cord synapses with numerous other neurons.
  6. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs.

1. For example, when blood pressure falls,
2. internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord
3. and then up an ascending tract
4. to a cardiovascular center in the brain.
5. Thereafter, nerve signals pass down a descending tract to the spinal cord.
6. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

Question 444

Reverse

terminal

Answer 444

The small swelling at the end of an axon, which lies close to the dendrite or cell body of another neuron, is the axon

Question 445

The PNS: Somatic division • The somatic system serves the skin, skeletal muscles and tendons. • Automatic responses are called reflexes. • Reflexes consist of sensory receptorsensory neuron interneuron  motor neuron effector organ (we talked about this in 14.1)

Answer 446

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 449

3 functions of the nervous system

Answer 449

The CNS generates motor output. Nerve signals from the CNS go by way of the PNS to the muscles, glands, and organs, all in response to the cookies. Signals to the salivary glands make you salivate. Your stomach generates the acid and enzymes Page 281needed to digest the cookies—even before you’ve had a bite. The CNS also coordinates the movement of your arms and hands as you reach for the cookies.

Question 451

i

• Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter.
• integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited.
• Integration ensures that movements are coordinated and smooth.
• is believed to be caused by degeneration of specific neurons in the basal nuclei.

Answer 451

What are basal nuclei?

What is Parkinson disease (see Section 18.5)

Question 452

Nerve Signals

Answer 452

• electrochemical changes that convey information within the nervous system.

Question 453

reversedprompt

What is The Reticular Formation

(Fig. 14.12)- See, download and study

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

Answer 453

• This is a complex network of nuclei–masses of gray matter–and fibers that extends the length of the brain stem
• major component of the reticular activating system (RAS): receives sensory signals and sends them to higher centers.
  
  • Motor signals received by the RAS are sent to the spinal cord.
  • arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face.
  • The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on.
  • Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting.
  • To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep.
  • General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

Question 454

Neurotransmitters

These are what some of them do:

This one is essential for memory circuits in the limbic system

This one is important to dreaming, waking and mood

This one is also the basal nuclei neurotransmitter

This one is involved in thermoregulation

This is an abudant inhibitory neurotransmitter in CNS:

Answer 454

1. As mentioned in Section 14.1, there are more than 100 known neurotransmitters.
2. The most widely studied neurotransmitters to date are acetylcholine, norepinephrine, dopamine, serotonin, and gamma-aminobutyric acid (GABA).

• Acetylcholine is an essential CNS neurotransmitter for memory circuits in the limbic system.
• Norepinephrine is important to dreaming, waking, and mood.
• The neurotransmitter dopamine plays a central role in the brain’s regulation of mood. Dopamine is also the basal nuclei neurotransmitter that helps organize coordinated movements.
• Serotonin is involved in thermoregulation, sleeping, emotions, and perception.
• GABA is an abundant inhibitory neurotransmitter in the CNS

Question 455

The small swelling at the end of an axon, which lies close to the dendrite or cell body of another neuron, is the axon

Answer 455

terminal

Question 456

Reverse

Central Nervous System

Answer 456

14.2 The Central Nervous System 10 The CNS: Spinal cord • Reflex arc: • Stimulus causes sensory receptors to generate afferent signals in sensory axons to the spinal cord • Interneurons in spinal cord integrate the information and relay the information to the efferent motor neurons • Motor neuron axons cause skeletal and/or smooth muscles to contract 14.2 The Central Nervous System 11 What does the spinal cord look like? Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. dorsal root dorsal root ventral root ventral root vertebra spinal cord white matter central canal gray matter gray matter white matter gray matter meninges ventral dorsal white matter central canal vertebra b. a. c. dorsal root ganglion spinal nerve dorsal root ganglion spinal nerve dorsal root branches dorsal root ganglion cut vertebrae d. Dorsal view of spinal cord and dorsal roots of spinal nerves. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a: © Karl E. Deckart/Phototake; d: © The McGraw-Hill Companies, Inc./Rebecca Gray, photographer and Don Kincaid, dissections Figure 14.7 The organization of white and gray matter in the spinal cord and the spinal nerves. 14.2 The Central Nervous S

Question 457

What happens to excess cerebrospinal fluid?

Answer 457

1. Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur.

Question 458

reverse.prompt

Neuromodulators are naturally occurring molecules that block the release of a neurotransmitter or modify a neuron’s response to a neurotransmitter. Two well-known neuromodulators are substance P and endorphins. Substance P is a neuropeptide that is released by sensory neurons when pain is present. Endorphins block the release of substance P and serve as natural painkillers. Endorphins are produced by the brain during times of physical and/or emotional stress. They are associated with the “runner’s high” of joggers.

Both pharmaceuticals and illegal drugs have several basic modes of action:

They promote the action of a neurotransmitter, usually by increasing the amount of neurotransmitter at a synapse. Examples include drugs such as alprazolam (Xanax) and diazepam (Valium), which increase GABA. These medications are used for panic attacks and anxiety. Reduced levels of norepinephrine and serotonin are linked to depression. Drugs such as fluoxetine (Prozac), paroxetine (Paxil), and duloxetine (Cymbalta) allow norepinephrine and/or serotonin to accumulate at the synapse, which explains their effectiveness as antidepressants. Alzheimer disease causes a slow, progressive loss of memory (see Section 18.5). Drugs used for Alzheimer disease allow acetylcholine to accumulate at synapses in the limbic system.

They interfere with or decrease the action of a neurotransmitter. For instance, antipsychotic drugs used for the treatment of schizophrenia decrease the activity of dopamine. The caffeine in coffee, chocolate, and tea keeps us awake by interfering with the effects of inhibitory neurotransmitters in the brain.

They replace or mimic a neurotransmitter or neuromodulator. The opiates—namely, codeine, heroin, and morphine—bind to endorphin receptors and in this way reduce pain and produce a feeling of well-being.

Ongoing research into neurophysiology and neuropharmacology (the study of nervous system function and the way drugs work in the nervous system) continues to provide evidence that mental illnesses are caused by imbalances in neurotransmitters. These studies will undoubtedly improve treatments for mental illness, as well as provide insight into the problem of drug abuse.

Answer 458

Neuromodulators

Answer 459

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

Question 460

CNS

Autonomic Nervous System

Sympathic Nervous System and Parasympathetic Nervous System

Answer 460

1. In common:

• (1) They usually function in an involuntary manner;
• (2) they innervate all internal organs; and
• (3) they use two neurons and one ganglion for each impulse.
  
  • The first neuron has a cell body within the CNS and a preganglionic fiber that enters the ganglion.
  • The second neuron has a cell body within a ganglion and a postganglionic fiber that leaves the ganglion.

1. Reflex actions: such as those that

• regulate blood pressure
• and breathing rate,
• are especially important to the maintenance of homeostasis.

1. These reflexes begin when the sensory neurons in contact with internal organs send messages to the CNS.
2. They are completed by motor neurons within the autonomic system.

Question 461

Reverse

1. unmyelinated axon: then an action potential at one locale stimulates an adjacent part of the axon membrane to produce an action potential. Conduction along entire action can be slow—approximately 1 meter/second (1 m/s) in thin axons—because each section of the axon must be stimulated.

Answer 461

Propogation of an Action Potential

Question 462

Spinal Nerves

(see Fig. 14.8).

Answer 462

• emerge either side of the spinal cord
• 31 pairs
• The roots physically separate the axons of sensory neurons from the axons of motor neurons: forming an arrangement resembling a letter Y.
• The posterior root contains sensory fibers that direct sensory receptor information inward (toward the spinal cord).
• The cell body of a sensory neuron is in a posterior-root ganglion (also termed a dorsal-root ganglion).
• The anterior (also termed ventral) root of a spinal nerve contains motor fibers that conduct impulses outward (away from the cord) to the effectors.
• Observe in Figure 14.8
• that the anterior and posterior roots join to form a spinal nerve.
• All spinal nerves are called mixed nerves, because they contain both sensory and motor fibers.
• Each spinal nerve serves the particular region of the body in which it is located.
  
  • For example, the intercostal muscles of the rib cage are innervated by thoracic nerves.

Question 463

reverse.prompt

The Somatic SystemThe PNS has divisions: the somatic system and the autonomic system.

Question 464

reverse.prompt

Brain Ventricles

Answer 464

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

Question 465

Reverse

CHECK YOUR PROGRESS 14.1

Describe the three types of neurons, and list the three main parts of a neuron.

Answer

Sensory neurons take nerve signals from a sensory receptor to the CNS. Interneurons lie entirely within the CNS and communicate with other neurons. Motor neurons move nerve impulses away from the CNS to an effector. The parts are cell body, dendrites, and axon.

Describe how a nerve impulse is propagated.

Answer

An exchange of Na+ and K+ ions generates an action potential that moves along the length of an axon. An action potential in one location stimulates the production of an action potential in an adjacent part of the axon membrane. If the nerve is myelinated, the action potential moves more quickly, “jumping” from one node of Ranvier to the next.

Summarize how a nerve impulse is transmitted from one neuron to the next.

Answer

An action potential arrives at the axon terminal and calcium enters the terminal. Synaptic vesicles enclosing the neurotransmitter fuse with the sending neuron’s membrane. Neurotransmitters are released, travel across the synapse, and bind to receptors on the receiving neuron membrane. Sodium diffuses into the receiving neuron, and an action potential is created.

CONNECTING THE CONCEPTS

For more information on neurons and the nervous system, refer to the following discussions:

Section 4.4 explores how stem cells may be used to regenerate nervous tissue.

Figure 13.7 illustrates the role of the synapse in the neuromuscular junction.

Section 15.1 explains how the peripheral nervous system sends information to and from the central nervous system.

Answer 465

Check Yourself

Question 466

primary somatosensory area

see and download: (Fig. 14.11b), Page 291

Answer 466

1. posterior to the central sulcus in the parietal lobe.
2. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented
3. . Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation.
4. Once again, the face and hands require the largest proportion of the sensory cortex.
5. Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10).
   
   1. The primary taste area in the parietal lobe (pink) accounts for taste sensations.
   2. Visual information is received by the primary visual cortex (blue) in the occipital lobe.
   3. The primary auditory area in the temporal lobe (green) accepts information from our ears.
   4. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Question 467

Similar to blood plasma composition, as it is formed from blood plasma • Forms a watery cushion to protect the brain • Circulated in subarachnoid space, ventricles, and central canal of the spinal cord

Question 468

reversedprompt

The structures of neurons

These are the three parts:

The common parts

See download, review and study:

Page 282, (Fig. 14.3)

Answer 468

• vary in appearance, but all of them have
• three distinct structures:

1. a cell body: contains the nucleus, as well as other organelles.
2. dendrites: short extensions that receive signals from sensory receptors or other neurons. Incoming signals from dendrites can result in nerve signals that are then conducted by an axon. T.
3. axon: portion of a neuron that conducts nerve impulses; quite long. Individual axons are termed nerve fibers, and collectively they form a nerve

Myelin Sheath

• plays an important role in the rate at which signals move through the neuron.
• Many axons covered by this protective sheath
• develops when Schwann cells (PNS) or oligodendrocytes (CNS) wrap their membranes around an axon many times.
• Each neuroglia cell covers only a portion of an axon, so the myelin sheath is interrupted.
  
  • The gaps where there is no myelin sheath are called nodes of Ranvier. Later in this section, we will see how the myelin sheath

Question 469

reversedprompt

• The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated.
• Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx).
• Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing
• . Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Answer 469

Spinal cord

Question 470

Drug Abuse

Page 300

Answer 470

• Like mental illness, drug abuse is linked to neurotransmitter levels.
• dopamine – mood regulation.
  
  • Dopamine plays a central role in the working of the brain’s built-in reward circuit.
  • The reward circuit is a collection of neurons that, under normal circumstances, promotes healthy, pleasurable activities, such as consuming food.
  • behaviors stimulate the reward circuit and make us feel good.
  • Drug abusers take drugs that artificially affect the reward circuit to the point that they neglect their basic physical needs in favor of continued drug use.

1. Drug abuse is apparent when a person takes a drug at a dose level and under circumstances that increase the potential for a harmful effect.
2. Drug abusers are apt to display a psychological and/or physical dependence on the drug.
3. Psychological dependence is apparent when a person craves the drug, spends time seeking the drug, and takes it regularly.
4. With physical dependence, formerly called “addiction,” the person has become tolerant to the drug.
5. More is needed to get the same effect, and withdrawal symptoms occur when he or she stops taking the drug. This is true for not only teenagers and adults but also newborn babies of mothers who abuse and are addicted to drugs. Alcohol, drugs, and tobacco can all adversely affect the developing embryo, fetus, or newborn.

Question 471

A lipid covering on long axons that acts to increase the speed of nerve impulse conduction, insulation for both CNS and PNS, and regeneration in the PNS • Schwann cells – neuroglia that make up the myelin sheath in the PNS • Oligodendrocytes- neuroglia that make up the myelin sheath in the CNS • Nodes of Ranvier – gaps between myelination on the axons • Saltatory conduction – conduction of the nerve impulse from node to node

Question 472

Summarize how a nerve impulse is transmitted from one neuron to the next.

Answer 472

1. An action potential arrives at the axon terminal and
2. calcium enters the terminal.
3. Synaptic vesicles enclosing the neurotransmitter fuse with the sending neuron’s membrane.
4. Neurotransmitters are released, travel across the synapse, and bind to receptors on the receiving neuron membrane.
5. Sodium diffuses into the receiving neuron, and an action potential is created.

Question 473

The PNS: Somatic division • The somatic system serves the skin, skeletal muscles and tendons. • Automatic responses are called reflexes. • Reflexes consist of sensory receptorsensory neuron interneuron  motor neuron effector organ (we talked about this in 14.1)

Question 474

Reverse

Drug Abuse

Like mental illness, drug abuse is linked to neurotransmitter levels. As mentioned previously, the neurotransmitter dopamine is essential for mood regulation. Dopamine plays a central role in the working of the brain’s built-in reward circuit. The reward circuit is a collection of neurons that, under normal circumstances, promotes healthy, pleasurable activities, such as consuming food. It’s possible to abuse behaviors such as eating, spending, or gambling Page 300because the behaviors stimulate the reward circuit and make us feel good. Drug abusers take drugs that artificially affect the reward circuit to the point that they neglect their basic physical needs in favor of continued drug use.

Drug abuse is apparent when a person takes a drug at a dose level and under circumstances that increase the potential for a harmful effect. Drug abusers are apt to display a psychological and/or physical dependence on the drug. Psychological dependence is apparent when a person craves the drug, spends time seeking the drug, and takes it regularly. With physical dependence, formerly called “addiction,” the person has become tolerant to the drug. More is needed to get the same effect, and withdrawal symptoms occur when he or she stops taking the drug. This is true for not only teenagers and adults but also newborn babies of mothers who abuse and are addicted to drugs. Alcohol, drugs, and tobacco can all adversely affect the developing embryo, fetus, or newborn.

Answer 474

Drug Abuse

Question 475

reverse.prompt

What is the hypothalamus?

Answer 475

1. I regulate :

• hunger,
• sleep,
• thirst,
• body temperature,
• and water balance.

Question 476

Answer 476

Reticular Formation

Question 477

Review slides 14.2:

The brain: Cerebrum – The cerebral cortex Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. salivation vocalization mastication longitudinal fissure facial expression swallowing thumb, fingers, and hand forearm arm trunk pelvis thigh leg foot and toes lips upper face

Question 478

Oliogodendrocytes

Lectures

Answer 478

1. branched cells that wrap CNS nerve fibers
2. Produce fatty insulating coverings (myelin sheath) around nerve fibers in the CNS – Can coil around as many as 60 different fibers at one time

Answer 479

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 480

14.5 Drug Therapy and Drug Abuse

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain the ways that drugs interact with the nervous system.

Classify drugs as to whether they have a depressant, stimulant, or psychoactive effect on the nervous system.

List the long-term effects of drug use on the body.

As you are reading these words, synapses throughout your brain are organizing, integrating, and cataloging the information you take in. Neurotransmitters at these synapses control the firing of countless action potentials, thus creating a network of neural circuits. It is amazing to realize that all thoughts, feelings, and actions of a human are dependent on neurotransmitters in the CNS and PNS. By modifying or controlling synaptic transmission, a wide variety of drugs with neurological activity, both legal pharmaceuticals and illegal drugs of abuse, can alter mood, emotional state, behavior, and personality.

Drug Mode of Action

As mentioned in Section 14.1, there are more than 100 known neurotransmitters. The most widely studied neurotransmitters to date are acetylcholine, norepinephrine, dopamine, serotonin, and gamma-aminobutyric acid (GABA). Acetylcholine is an essential CNS neurotransmitter for memory circuits in the limbic system. Norepinephrine is important to dreaming, waking, and mood. The neurotransmitter dopamine plays a central role in the brain’s regulation of mood. Dopamine is also the basal nuclei neurotransmitter that helps organize coordinated movements. Serotonin is involved in thermoregulation, sleeping, emotions, and perception. GABA is an abundant inhibitory neurotransmitter in the CNS.

Neuromodulators are naturally occurring molecules that block the release of a neurotransmitter or modify a neuron’s response to a neurotransmitter. Two well-known neuromodulators are substance P and endorphins. Substance P is a neuropeptide that is released by sensory neurons when pain is present. Endorphins block the release of substance P and serve as natural painkillers. Endorphins are produced by the brain during times of physical and/or emotional stress. They are associated with the “runner’s high” of joggers.

Both pharmaceuticals and illegal drugs have several basic modes of action:

They promote the action of a neurotransmitter, usually by increasing the amount of neurotransmitter at a synapse. Examples include drugs such as alprazolam (Xanax) and diazepam (Valium), which increase GABA. These medications are used for panic attacks and anxiety. Reduced levels of norepinephrine and serotonin are linked to depression. Drugs such as fluoxetine (Prozac), paroxetine (Paxil), and duloxetine (Cymbalta) allow norepinephrine and/or serotonin to accumulate at the synapse, which explains their effectiveness as antidepressants. Alzheimer disease causes a slow, progressive loss of memory (see Section 18.5). Drugs used for Alzheimer disease allow acetylcholine to accumulate at synapses in the limbic system.

They interfere with or decrease the action of a neurotransmitter. For instance, antipsychotic drugs used for the treatment of schizophrenia decrease the activity of dopamine. The caffeine in coffee, chocolate, and tea keeps us awake by interfering with the effects of inhibitory neurotransmitters in the brain.

They replace or mimic a neurotransmitter or neuromodulator. The opiates—namely, codeine, heroin, and morphine—bind to endorphin receptors and in this way reduce pain and produce a feeling of well-being.

Ongoing research into neurophysiology and neuropharmacology (the study of nervous system function and the way drugs work in the nervous system) continues to provide evidence that mental illnesses are caused by imbalances in neurotransmitters. These studies will undoubtedly improve treatments for mental illness, as well as provide insight into the problem of drug abuse.

Drug Abuse

Like mental illness, drug abuse is linked to neurotransmitter levels. As mentioned previously, the neurotransmitter dopamine is essential for mood regulation. Dopamine plays a central role in the working of the brain’s built-in reward circuit. The reward circuit is a collection of neurons that, under normal circumstances, promotes healthy, pleasurable activities, such as consuming food. It’s possible to abuse behaviors such as eating, spending, or gambling Page 300because the behaviors stimulate the reward circuit and make us feel good. Drug abusers take drugs that artificially affect the reward circuit to the point that they neglect their basic physical needs in favor of continued drug use.

Drug abuse is apparent when a person takes a drug at a dose level and under circumstances that increase the potential for a harmful effect. Drug abusers are apt to display a psychological and/or physical dependence on the drug. Psychological dependence is apparent when a person craves the drug, spends time seeking the drug, and takes it regularly. With physical dependence, formerly called “addiction,” the person has become tolerant to the drug. More is needed to get the same effect, and withdrawal symptoms occur when he or she stops taking the drug. This is true for not only teenagers and adults but also newborn babies of mothers who abuse and are addicted to drugs. Alcohol, drugs, and tobacco can all adversely affect the developing embryo, fetus, or newborn.

Alcohol

With the exception of caffeine, alcohol (ethanol) consumption is the most socially accepted form of drug use in the United States. According to a 2015 national survey, 26.9% of high school students reported drinking some alcohol (down from 37.4% in 2014), and 7% binge drank (five-plus drinks in one setting) during the 30 days preceding the survey. Among adults, 86.4% reported they had consumed alcohol during their lifetime, with 56% stating they had used alcohol in the past month.

Alcohol acts as a depressant on many parts of the brain (Table 14.2) by increasing the action of GABA, an inhibitory neurotransmitter. Depending on the amount consumed, the effects of alcohol on the brain can lead to a feeling of relaxation, lowered inhibitions, impaired concentration and coordination, slurred speech, and vomiting. If the blood level of alcohol becomes too high, coma or death can occur.

Table 14.2Drug Influence on the CNS

Table Summary: Table lists the names of different substances in column 1. Other information related to these substances appears in columns 2 and 3.

SubstanceEffectMode of Transmission

AlcoholDepressantDrink

NicotineStimulantSmoked or smokeless tobacco

CocaineStimulantSniffed/snorted, injected, or smoked

Methamphetamine/EcstasyStimulantSmoked or pill form

HeroinDepressantSniffed/snorted, injected, or smoked

Marijuana/K2PsychoactiveSmoked or consumed

Beginning in about 2005, several manufacturers began selling alcoholic energy drinks. With names like Four Loko, JOOSE, and Sparks, these drinks combine fairly high levels of alcohol with caffeine and other ingredients. Although interactions between drugs can be complex, the stimulant effects of caffeine can counteract some of the depressant effects of alcohol, so users feel able to drink more. Because caffeine does not reduce the intoxicating effects of alcohol, many state legislatures are banning these products, and in November 2010 the U.S. Food and Drug Administration warned several manufacturers that they would no longer be allowed to mix caffeine with alcohol in their products.

Nicotine

Although the numbers have been decreasing since 2011 according to the CDC, in 2015, 25.3% of high school students and 7.4% of middle school students reported using a tobacco product. When tobacco is smoked or chewed, nicotine is rapidly delivered throughout the body. It causes a release of epinephrine from the adrenal glands, increasing blood sugar and causing the initial feeling of stimulation. As blood sugar falls, depression and fatigue set in, causing the user to seek more nicotine. In the CNS, nicotine stimulates neurons to release dopamine, a neurotransmitter that promotes a temporary sense of pleasure, and reinforces dependence on the drug. About 70% of people who try smoking become addicted.

As mentioned in earlier chapters, smoking is strongly associated with serious diseases of the cardiovascular and respiratory systems. Once addicted, however, only 10–20% of smokers are able to quit. Most medical approaches to quitting smoking involve the administration of nicotine in safer forms, such as skin patches, gum, or a newly developed nicotine inhaler, so that withdrawal symptoms can be minimized while dependence is gradually reduced. Several antinicotine vaccines (such as NicVAX) are currently in development or in early clinical trials. These vaccines stimulate the production of antibodies that prevent nicotine from entering the brain.

Cocaine and Crack

Cocaine is an alkaloid derived from the shrub Erythroxylon coca. Approximately 35 million Americans have used cocaine by sniffing/snorting, injecting, or smoking. Cocaine is a powerful stimulant in the CNS that interferes with the reuptake of dopamine at synapses, increasing overall brain activity. The result is a rush of a sense of well-being that lasts from 5 to 30 minutes. However, long-term use of cocaine causes a loss of metabolic functions in the brain (Fig. 14.19).

Figure 14.19 Cocaine use. Brain activity before and after the use of cocaine.

(both photos): ©Science Source

“Crack” is the street name given to cocaine that is processed to a free-base form for smoking. The term crack refers to the crackling sound heard when the drug is smoked. Smoking allows high doses of the drug to reach the brain rapidly, providing an intense and immediate high, or “rush.” Approximately 8 million Americans use crack.

A cocaine binge is a period in which a user takes the drug at ever-higher doses. The user is hyperactive, with little desire for food or sleep, but has an increased sex drive. This is followed by a crash period, characterized by fatigue, depression, irritability, and a lack of interest in sex. In fact, men who use cocaine often become impotent.

Cocaine is highly addictive; with continued use, the brain makes less dopamine to compensate for a seemingly endless supply. The user experiences withdrawal symptoms and an intense craving for cocaine. Overdosing on cocaine can cause cardiac and/or respiratory arrest.Page 301

Methamphetamine and Ecstasy

Methamphetamine and ecstasy are considered club, or party, drugs. Methamphetamine (commonly called meth or crank) is a powerful CNS stimulant. Meth is often produced in makeshift home laboratories, usually starting with ephedrine or pseudoephedrine, common ingredients in many cold and asthma medicines. As a result, many states have passed laws making these medications more difficult to purchase. The number of toxic chemicals used to prepare the drug makes a former meth lab site hazardous to humans and to the environment. Over 9 million people in the United States have used methamphetamine at least once. It is available as a powder that can be snorted or as crystals (crystal meth or ice) that can be smoked.

The structure of methamphetamine is similar to that of dopamine, and the most immediate effect of taking meth is a rush of euphoria, energy, alertness, and elevated mood. However, this is typically followed by a state of agitation that, in some individuals, leads to violent behavior. Chronic use can result in what is called an amphetamine psychosis, characterized by paranoia, hallucinations, irritability, and aggressive, erratic behavior.

Ecstasy is the street name for MDMA (methylenedioxymethamphetamine), which is chemically similar to methamphetamine. Many users say that “X,” taken as a pill that looks like an aspirin or candy, increases their feelings of well-being and love for other people. However, it has many of the same side effects as other stimulants, plus it can interfere with temperature regulation, leading to hyperthermia, high blood pressure, and seizures. Although deaths from alcohol abuse are more common, ecstasy is identified as a cause of accidental death in young adults each year.

Drugs with sedative effects, known as date rape or predatory drugs, include flunitrazepam (Rohypnol, or roofies), gamma-hydroxybutyric acid (GHB), and ketamine (special K). Ketamine is actually a drug that veterinarians sometimes use to perform surgery on animals. Any of these drugs can be given to an unsuspecting person, who may fall into a dreamlike state in which he or she is unable to move and thus is vulnerable to sexual assault.

Heroin

Heroin is derived from the resin or sap of the opium poppy plant, which is widely grown in a region from Turkey to Southeast Asia and in parts of Latin America. Drugs derived from opium are called opiates, or more commonly, opioids. This class also includes morphine and codeine. After heroin is injected, snorted, or smoked, a feeling of euphoria, along with relief of any pain, occurs within a few minutes. It is estimated that 4 million Americans have used heroin sometime in their lives, and over 300,000 people use heroin annually.

As with other drugs of abuse, addiction is common. Heroin and opioids bind to receptors meant for the endorphins, naturally occurring neurotransmitters that kill pain and produce feelings of tranquility. With repeated use, the body’s production of endorphins decreases. Tolerance develops, so the user needs to take more of the drug just to prevent withdrawal symptoms (tremors, restlessness, cramps, vomiting), and the original euphoria is no longer felt. In the case of heroin, long-term users commonly acquire hepatitis, HIV/AIDS, and various bacterial infections due to the use of shared needles, and heavy users may experience convulsions and death by respiratory arrest.

Heroin addiction can be treated with synthetic opiate compounds, such as methadone or buprenorphine and naloxone (Suboxone), that decrease withdrawal symptoms and block heroin’s effects. However, methadone itself can be addictive, and methadone-related deaths are on the rise.

Marijuana and K2

Marijuana is the most commonly used illegal drug in the United States. Surveys vary, but in 2015, about 52% of young adults reported using marijuana in their lifetime, and 46% of the U.S. population had tried it at least once. It is derived from the dried flowering tops, leaves, and stems of the marijuana plant, Cannabis sativa, which contain and are covered by a resin that is rich in THC (tetrahydrocannabinol). The names cannabis and marijuana apply to either the plant or THC. Marijuana can be ingested, but usually it is smoked in a cigarette called a “joint.” Beginning with California in 1996, several states have legalized its use for medical purposes, such as in treating cancer, AIDS, and glaucoma. In 2012, Colorado became the first state to legalize recreational use. As of 2018, 8 states had joined Colorado in legalizing recreational use, and 22 additional states had authorized the use of marijuana for medicinal purposes. However, in 2005, the Supreme Court ruled that patients prescribed medical marijuana can still be prosecuted by federal agencies.

Page 302Researchers have found that THC binds to a receptor for anandamide, a naturally occurring neurotransmitter that is important for short-term memory processing, and perhaps for feelings of contentment. The occasional marijuana user experiences mild euphoria, along with alterations in vision and judgment. Heavy use can cause hallucinations, anxiety, depression, paranoia, and psychotic symptoms. Research is underway to identify the effects of long-term marijuana use on the brain, as well as on the effects of secondhand marijuana smoke on the respiratory system.

In recent years, awareness has been increasing about a synthetic compound called K2, or spice. Originally synthesized by an organic chemist at Clemson University, K2 is about ten times as potent as THC. The chemical is typically sprayed onto a mixture of other herbal products and smoked. However, because there is no regulation of how it is produced, the amount of K2 itself, or contaminants, can vary greatly. This may account for the several reports of serious medical problems and even deaths among K2 users.

CHECK YOUR PROGRESS 14.5

Contrast drug therapy and drug abuse.

Answer

Drug therapy is used to treat a disease or disorder. Drug abuse is using drugs without symptoms of disease or disorder.

List how the abuse of drugs, including alcohol and nicotine, affects the nervous system.

Answer

Alcohol and heroin are depressants; nicotine, cocaine, and methamphetamines are stimulants; marijuana produces euphoria.

Detail several modes of action of pharmaceutical and illegal drugs.

Answer

Alcohol increases the action of GABA and increases the release of endorphins in the hypothalamus. Nicotine stimulates dopamine release. Cocaine inhibits dopamine reuptake. Methamphetamine mimics the action of cocaine. Heroin is converted to morphine in the brain and binds to opioid receptors. Marijuana stimulates anandamide receptors.

CONNECTING THE CONCEPTS

For more on the long-term effects of drug use on the systems of the body, refer to the following discussions:

Section 5.7 explores the negative long-term effects of smoking on the cardiovascular system.

Section 11.4 provides information on how alcohol acts as a diuretic in the urinary system.

Section 20.2 examines the relationship between smoking and alcohol use and the increased risk of cancer.

CONCLUSION

The cause of multiple sclerosis (MS) is still unknown, but most researchers agree that there are most likely many contributing factors, including environmental influences, genetics, and a faulty immune system. Many individuals with MS are able to control their symptoms by using immunosuppressive medications, such as beta interferons. The fact that this treatment works suggests that, in many cases, MS is caused by the immune system incorrectly identifying the myelin sheaths as foreign material. The breakdown of the myelin can be detected using both MRI and SSEP tests (discussed in the chapter opener). However, environmental conditions are also suspected to cause MS. Studies have shown that the risk of contracting MS is influenced in part by where in the world you live, although the specific environmental factor or pollutant has not yet been identified. Genetics is also believed to play a role in some cases. But most researchers believe that a defect in a single gene is unlikely. Rather, it is more likely that a certain combination of genetic factors places an individual at a higher risk of contracting MS. Though there is no cure for MS, researchers have been very successful in developing disease-modifying drugs that reduce the symptoms and allow the individual to lead a normal life.

Question 481

• 2 divisions – Central nervous system (CNS): –Brain and spinal cord –Peripheral nervous system (PNS): Nerves and ganglia (collections of cell bodies)

Question 482

Neuromodulators

Answer 482

Neuromodulators are naturally occurring molecules that block the release of a neurotransmitter or modify a neuron’s response to a neurotransmitter. Two well-known neuromodulators are substance P and endorphins. Substance P is a neuropeptide that is released by sensory neurons when pain is present. Endorphins block the release of substance P and serve as natural painkillers. Endorphins are produced by the brain during times of physical and/or emotional stress. They are associated with the “runner’s high” of joggers.

Both pharmaceuticals and illegal drugs have several basic modes of action:

They promote the action of a neurotransmitter, usually by increasing the amount of neurotransmitter at a synapse. Examples include drugs such as alprazolam (Xanax) and diazepam (Valium), which increase GABA. These medications are used for panic attacks and anxiety. Reduced levels of norepinephrine and serotonin are linked to depression. Drugs such as fluoxetine (Prozac), paroxetine (Paxil), and duloxetine (Cymbalta) allow norepinephrine and/or serotonin to accumulate at the synapse, which explains their effectiveness as antidepressants. Alzheimer disease causes a slow, progressive loss of memory (see Section 18.5). Drugs used for Alzheimer disease allow acetylcholine to accumulate at synapses in the limbic system.

They interfere with or decrease the action of a neurotransmitter. For instance, antipsychotic drugs used for the treatment of schizophrenia decrease the activity of dopamine. The caffeine in coffee, chocolate, and tea keeps us awake by interfering with the effects of inhibitory neurotransmitters in the brain.

They replace or mimic a neurotransmitter or neuromodulator. The opiates—namely, codeine, heroin, and morphine—bind to endorphin receptors and in this way reduce pain and produce a feeling of well-being.

Ongoing research into neurophysiology and neuropharmacology (the study of nervous system function and the way drugs work in the nervous system) continues to provide evidence that mental illnesses are caused by imbalances in neurotransmitters. These studies will undoubtedly improve treatments for mental illness, as well as provide insight into the problem of drug abuse.

Question 483

Association Areas are where

1)

2)

These are the association centers:

Answer 483

1. integration occurs
2. memories are stored.
3. premotor area.
   
   • ​​Anterior to the primary motor area
   • organizes motor functions for skilled motor activities,
     
     • such as walking and talking at the same time.
       
       • Next, the primary motor area sends signals to the cerebellum, which integrates them.
       • A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs.
4. The visual association area in the occipital lobe

• processes and analyzes sensory information from the skin and muscles.
• just posterior to the primary somatosensory area,​​​​
  
  • associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before.

1. The auditory association area in the temporal lobe performs the same functions with regard to sounds

Question 485

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Answer 485

The central nervous system • The CNS consists of the brain and spinal cord. • Both are protected by • Scalp and skin • Bones – skull and vertebral column • Meninges – 3 protective membranes that wrap around CNS • Cerebral spinal fluid (CSF) – space between meninges is filled with this fluid that cushions and protects the CNS • Blood brain barrier (BBB)

Meninges • Dura mater • Double-layered external covering • Periosteum – dense connective tissue attached to surface of the skull • Meningeal layer – outer covering of the brain • Folds inward in several areas

Meninges • Arachnoid mater • Middle layer • Web-like • Pia mater • Internal layer • Clings to the surface of the brain • Many blood vessels http://droualb.faculty.mjc.edu/Lecture%20Notes/Unit%205/Meninges_peeled_away

Question 486

Figure 14.11 The reticular formation of the brain. 14.2 The Central Nervous System 26 The limbic system • It joins primitive emotions (i.e., fear, pleasure) with higher functions, such as reasoning. • It can cause strong emotional reactions to situations but conscious thought can override and direct our behavior. • Includes • Amygdala – imparts emotional overtones • Hippocampus – important to learning and memory 14.3 The Limbic System and Higher Mental Functions 27 The limbic system Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. amygdala hippocampus olfactory bulb olfactory tract hypothalamus corpus thalamus callosum Figure 14.12 The regions of the brain associated with the limbic system. 14.3 The Limbic System and Higher Mental Functions 28 • Learning – what happens when we recall and use past memories • Memory – ability to hold a thought or to recall past events • Short-term memory – retention of information for only a few minutes 14.3 The Limbic System and Higher Mental Functions Higher mental functions 29 Higher mental functions • Long-term memory – retention of i

Question 487

Reverse

These are the primary Motor and Sensory Areas of the Cortex

Answer 487

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

Question 488

Wernicke’s area and Broca’s area in the left hemisphere are related to speech, comprehension, and writing. The right hemisphere is associated with more nonverbal and creative functions.

Answer 488

Describe the relationship between the left and right sides of the brain and language and speech.

Question 489

Reverse

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

Page 288

Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 490

reversedprompt

An action potential arrives at the axon terminal and calcium enters the terminal. Synaptic vesicles enclosing the neurotransmitter fuse with the sending neuron’s membrane. Neurotransmitters are released, travel across the synapse, and bind to receptors on the receiving neuron membrane. Sodium diffuses into the receiving neuron, and an action potential is created.

Answer 490

Summarize how a nerve impulse is transmitted from one neuron to the next.

Question 491

The peripheral nervous system (PNS) • It includes cranial nerves (12 pairs), spinal nerves (31 pairs), and ganglia (neuronal cell bodies) outside the CNS. - Spinal nerves conduct impulses to and from the spinal cord. - Cranial nerves conduct impulses to and from the brain. • The PNS is divided into 2 systems. - Somatic division - Autonomic division

Question 492

Reverse

Alcohol

With the exception of caffeine, alcohol (ethanol) consumption is the most socially accepted form of drug use in the United States. According to a 2015 national survey, 26.9% of high school students reported drinking some alcohol (down from 37.4% in 2014), and 7% binge drank (five-plus drinks in one setting) during the 30 days preceding the survey. Among adults, 86.4% reported they had consumed alcohol during their lifetime, with 56% stating they had used alcohol in the past month.

Alcohol acts as a depressant on many parts of the brain (Table 14.2) by increasing the action of GABA, an inhibitory neurotransmitter. Depending on the amount consumed, the effects of alcohol on the brain can lead to a feeling of relaxation, lowered inhibitions, impaired concentration and coordination, slurred speech, and vomiting. If the blood level of alcohol becomes too high, coma or death can occur.

Answer 492

Alcohol

Question 493

reversedprompt

The nervous tissue composing the central nervous system.

Answer 493

1. gray matter and white matter.
2. Gray matter contains cell bodies and short, nonmyelinated axons.
3. White matter contains myelinated axons that run together in bundles called tracts.

Answer 495

4.2 The Central Nervous System 19 Prefrontal Cortex • “CEO of the brain” • Where you control and plan your actions • Working memory • Organization • Modulate your mood • Conscience • Personality • Not fully developed until at least 25 years of age– maybe even later! (You can blame your bad decisions on this if you are younger than this– ha!– or flip it around: drugs, alcohol, excessive videogaming, etc. can really have a permanent negative impact on this developing brain area even if you are of ‘legal age’…) 14.2 The Central Nervous System 20 1. The brain: Cerebrum – The cerebral cortex Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. salivation vocalization mastication longitudinal fissure facial expression swallowing thumb, fingers, and hand forearm arm trunk pelvis thigh leg foot and toes lips upper face

Question 496

The Autonomic Nervous System (PNS)

(see Fig. 14.2).

(Fig. 14.18).

Figure 14.18 The two divisions of the autonomic nervous system. Sympathetic preganglionic fibers (left) arise from the thoracic and lumbar portions of the spinal cord; parasympathetic preganglionic fibers (right) arise from the cranial and sacral portions of the spinal cord. Each system innervates the same organs but has contrary effects.

Answer 496

• regulates the activity of
• cardiac and smooth muscles
• organs, and
• glands.
• • System:

1. sympathetic
2. parasympathetic divisions

• Activation generally causes opposite responses in 2.
• Shared features despite different functions:
  
  1. usually function involuntarily
  2. innervate all internal organs
  3. they use two neurons and one ganglion for each impulse.
     
     1. The first neuron has a cell body within the CNS and a preganglionic fiber that enters the ganglion.
     2. The second neuron has a cell body within a ganglion and a postganglionic fiber that leaves the ganglion.
• Reflex actions, such as those that regulate blood pressure and breathing rate, are especially important to the maintenance of homeostasis. These reflexes begin when the sensory neurons in contact with internal organs send messages to the CNS. They are completed by motor neurons within the autonomic system.

Question 497

This is action potential of a neuron.

Another word for action potential is:

List the steps in the process of action potential:

See, Download and Study Figure 14.4b

Answer 497

• The resting potential energy of the neuron can be used to perform the work of the neuron: conduction of nerve signals.
• The process of conduction is termed an action potential, and it occurs in the axons of neurons:

1. A stimulus activates the neuron and begins the action potential.
   
   • For example, a stimulus for pain neurons in the skin would be the prick of a sharp pin.
   • However, the stimulus must be strong enough to cause the cell to reach threshold, the voltage that will result in an action potential. In Figure 14.4b, the threshold voltage is around −55 mV.

• An action potential is an all-or-nothing event.
  
  • ​Once threshold is reached, the action potential happens automatically and completely. On the other hand, if the threshold voltage is never reached, the action potential does not occur. Increasing the strength of a stimulus (such as pressing harder with the pin) does not change the strength of an action potential. However, it may cause more action potentials to occur in a given period. As a result, the person may perceive that pain has increased.

Question 500

Central White Matter

Answer 500

• white matter. Myelination occurs and white matter develops as a child grows.
• Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech.
• Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area.
• Tracts take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Answer 501

The Somatic System

The PNS has divisions: the somatic system and the autonomic system. The nerves in the somatic system serve the skin, skeletal muscles, and tendons (see Fig. 14.2). The somatic system sensory nerves take sensory information from external sensory receptors to the CNS. Motor commands leaving the CNS travel to skeletal muscles via somatic motor nerves.

Not all somatic motor actions are voluntary. Some are automatic. Automatic responses to a stimulus in the somatic system are called reflexes. A reflex occurs quickly, without your even having to think about it. For example, a reflex may cause you to blink your eyes in response to a flash of light, without your willing it. We will study the path of a reflex, because it allows us to study in detail the path of nerve signals to and from the CNS.Page 296

The Reflex Arc

Figure 14.17 illustrates the path of a reflex that involves only the spinal cord. If your hand touches a sharp pin, sensory receptors in the skin generate nerve signals that move along sensory fibers through the posterior (dorsal) root ganglia toward the spinal cord. Sensory neurons that enter the cord posteriorly pass signals on to many interneurons. Some of these interneurons synapse with motor neurons whose short dendrites and cell bodies are in the spinal cord. Nerve signals travel along these motor fibers to an effector, which brings about a response to the stimulus. In this case, the effector is a muscle, which contracts so that you withdraw your hand from the pin. Various other reactions are also possible—you will most likely look at the pin, wince, and cry out in pain. This whole series of responses occurs because some of the interneurons involved carry nerve signals to the brain. The brain makes you aware of the stimulus and directs these other reactions to it. In other words, you don’t feel pain until the brain receives the information and interprets it.

Answer 502

14.3 The Limbic System and Higher Mental Functions

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the limbic system.

Explain how the limbic system is involved in memory, language, and speech.

Summarize the types of memory associated with the limbic system.

Question 503

reversedprompt

1. four ventricles.
2. A lateral ventricle is found on each side of the brain.
3. They join at the third ventricle.
4. The third ventricle connects with the fourth ventricle superiorly;
5. the central canal of the spinal cord joins the fourth ventricle inferiorly.
6. All structures are filled with cerebrospinal fluid.
7. 8. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

Answer 503

The brain has 4 of these.

This is where they are found

This is where they join - this number ventricle

the third and fourth ventricle join ____________

The _______________ joins the 4th ventricle ________________

All are filled with ________________________.

See/Download image

Question 504

reverse.prompt

What is the hypothalamus?

Answer 504

I work closely as a link between the nervous and endochrine systems.

Question 505

Processing centers

Answer 505

• cortex receive information from the other association areas and perform higher-level analytical functions.
• The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities.
• Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.
• The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex.
• Wernicke’s area is located in the posterior part of the left temporal lobe.
• Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10).
• Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area.
• Broca’s area adds grammatical refinements and directs the primary motor area to

Question 506

reversedprompt

1. protected by bone
   
   • The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull.
2. Also, both the spinal cord and the brain are wrapped in protective membranes known as meninges (sing., meninx).

Answer 506

The spinal cord and brain are protected by ______________, with the spinal cord being surrounded by _______________, and the brain enclosed by ____________.

They are both also wrapped in _________________.

Question 507

Similarly, the spinal cord creates reflex arcs for the internal organs.

• For example, when blood pressure falls:

1. internal receptors in the carotid arteries and aorta generate nerve signals
2. that pass through sensory fibers to the cord
3. and then up an ascending tract to a
4. cardiovascular center in the brain.
5. Thereafter, nerve signals pass down a descending tract to the spinal cord.
6. Motor signals then cause blood vessels to constrict,
7. so that the blood pressure rises.

Know this process

Answer 507

Spinal Cord Reflex Arcs for Internal Organs

What are the steps of regulating blood pressure?

Question 508

Action Potential Propagation; This is Saltatory conduction

Neural Transmission

This is the name of the period during which an axon is unable to conduct an action potential, thereby ensuring :

Answer 508

1. In myelinated fibers, an action potential at one node of Ranvier causes an action potential at the next, jumping over the entire myelin-coated portion of the axon: This type of conduction is translated from (saltatio is a Latin word that means “to jump”) and is much faster. In thick, myelinated fibers, the rate of transmission is more than 100 m/s.
2. Regardless of whether an axon is myelinated or not, its action potentials are self-propagating. Each action potential generates another, along the entire length of the axon.
3. Like the action potential itself, conduction of an action potential is an all-or-none event—either an axon conducts its action potential or it does not. The intensity of a message is determined by how many action potentials are generated within a given time. An axon can conduct a volley of action potentials very quickly, because only a small number of ions are exchanged with each action potential. Once the action potential is complete, the ions are rapidly restored to their proper place through the action of the sodium–potassium pump
4. Neural Transmission: Action Potential Propagation

• As soon as the action potential has passed by each successive portion of an axon, that portion undergoes a short refractory period, during which it is unable to conduct an action potential.
  
  • This ensures the one-way direction of a signal from the cell body down the length of the axon to the axon terminal.

***all functions of the nervous system, from our deepest emotions to our highest reasoning abilities, are dependent on the conduction of nerve signals**

Propagation of an Action Potential

1. If an axon is unmyelinated, an action potential at one locale stimulates an adjacent part of the axon membrane to produce an action potential. Conduction along the entire axon in this fashion can be rather slow—approximately 1 meter/second (1 m/s) in thin axons—because each section of the axon must be stimulated.

Question 509

reverse.prompt

What is the peripheral nervous system (PNS)

Nerves!

Figure 14.15

Figure 14.15 The structure of a nerve.

The peripheral nervous system consists of the cranial nerves and the spinal nerves. A nerve is composed of bundles of axons separated from one another by connective tissue

Answer 509

1. outside the nervous system, contains the nerves
   
   1. cranial nerves when they arise from the brain and are termed
   2. spinal nerves when they arise from the spinal cord.
   3. In any case, all nerves carry signals to and from the CNS.
2. anatomy of a nerve:

• The cell body and the dendrites of neurons are in either the CNS or the ganglia.
• Ganglia (sing., ganglion) are collections of nerve cell bodies outside the CNS.
• The axons of neurons project from the CNS and form the spinal cord.
• In other words, nerves, whether cranial or spinal, are composed of axons, the long part of neurons.

1. 2. Humans have 12 pairs of cranial nerves attached to the brain. By convention, the pairs of cranial nerves are referred to by Roman numerals (Fig. 14.16). Some cranial nerves are sensory nerves—they contain only sensory fibers; some are motor nerves that contain only motor fibers; others are mixed nerves that contain both sensory and motor fibers. Cranial nerves are largely concerned with the head, neck, and facial regions of the body. However, the vagus nerve (X) has branches not only to the pharynx and larynx but also to most of the internal organs. It arises from the brain stem—specifically, the medulla oblongata, which communicates with the hypothalamus. These two parts of the brain control the internal organs.
2. Figure 14.16 The cranial nerves. Overall, cranial nerves receive sensory input from, and send motor outputs to, the head region. The spinal nerves receive sensory input from, and send motor outputs to, the rest of the body. Two important exceptions are the vagus nerve, X, which communicates with internal organs, and the spinal accessory nerve, XI, which controls neck and back muscles.
3. The spinal nerves of humans emerge from either side of the spinal cord (see Fig. 14.8). There are 31 pairs of spinal nerves. The roots of a spinal nerve physically separate the axons of sensory neurons from the axons of motor neurons, forming an arrangement resembling a letter Y. The posterior root of a spinal nerve contains sensory fibers that direct sensory receptor information inward (toward the spinal cord). The cell body of a sensory neuron is in a posterior-root ganglion (also termed a dorsal-root ganglion). The anterior (also termed ventral) root of a spinal nerve contains motor fibers that conduct impulses outward (away from the cord) to the effectors. Observe in Figure 14.8 that the anterior and posterior roots join to form a spinal nerve. All spinal nerves are called mixed nerves, because they contain both sensory and motor fibers. Each spinal nerve serves the particular region of the body in which it is located. For example, the intercostal muscles of the rib cage are innervated by thoracic nerves.
   5.

Answer 510

Functional Classification of the Peripheral Nervous System  Sensory (afferent) division  Nerve fibers that carry information to the central nervous system  Motor (efferent) division  Nerve fibers that carry impulses away from the central nervous system  Somatic nervous system = voluntary (skeletal muscles)  Autonomic nervous system = involuntary (cardiac and smooth muscles, glands)

Question 511

Reverse

The Somatic Versus the Autonomic Systems

Recall that the PNS includes the somatic system and the autonomic system. Table 14.1 compares the features and functions of the somatic motor pathway with the motor pathways of the autonomic system.

Table 14.1Comparison of Somatic Motor and Autonomic Motor Pathways

Table Summary: Columns are for somatic motor pathway and autonomic motor pathways. Rows are for different points of comparison. Autonomic motor pathways are grouped into sympathetic and parasympathetic, as the other column-headers.

Answer 511

Somatic Versus Autonomic Systems

Question 512

Synaptic Integration

Page 286

(illustrated by the green line in Fig. 14.6b)

What is integration?

What are excitatory and inhibitory signals?

Answer 512

1. A single neuron has a cell body and may have many dendrites (Fig. 14.6a). All can have synapses with many other neurons. Therefore, a neuron is on the receiving end of many signals,which can either be excitatory or inhibitory.
2. Recall that an excitatory neurotransmitter produces an excitatory signal by opening sodium gates at a synapse. This drives the neuron closer to its threshold. If threshold is reached, an action potential is inevitable. On the other hand, an inhibitory neurotransmitter drives the neuron farther from an action potential (red line in Fig. 14.6b) by opening the gates for potassium.

Neurons integrate these incoming signals. Integration is the summing up of excitatory and inhibitory signals. If a neuron receives enough excitatory signals (either from different synapses or at a rapid rate from a single synapse) to outweigh the inhibitory ones, chances are the axon will transmit a signal. On the other hand, if a neuron receives more inhibitory than excitatory signals, summing these signals may prohibit the axon from reaching threshold and then depolarizing (the solid black line in Fig. 14.6b).

Question 513

Identify the structures of the brain and provide a function for each.

Question 514

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Question 515

Reverse

5
The nerve impulse: action potential
• Action potential – rapid change in the axon membrane; a nerve impulse– threshold is -55mV
• Sodium gates open letting Na+ in
• Depolarization occurs (-70mV to threshold-55mV)
• Interior of axon loses negative charge (+35mV)
• Potassium gates open letting K+ out
• Repolarization occurs
• Interior of axon regains negative charge (-70mV)
• Wave of depolarization/repolarization travels down the axon.
• Resting potential is restored by moving potassium inside and sodium outside
14.1 Overview of the nervous system

Answer 515

Nerve Impulse Action Potential

Question 516

The nerve impulse: action potential • Action potential – rapid change in the axon membrane; a nerve impulse– threshold is -55mV • Sodium gates open letting Na+ in • Depolarization occurs (-70mV to threshold55mV) • Interior of axon loses negative charge (+35mV) • Potassium gates open letting K+ out • Repolarization occurs • Interior of axon regains negative charge (-70mV)

Question 517

• cortical areas may work with
• lower centers to produce learning and memory.
• Memory:
  
  • ​the ability to hold a thought in mind or
  • to recall events from the past, ranging from a word we learned only yesterday to an early emotional experience that has shaped our lives.
  • Types of Memory
    
    1. prefrontal area, active during short-term memory
       
       • seven-digit telephone number for a short time
    2. long term memory:
       
       • ​​memorized phone numbers; often associated w/ place or person associated with that number bc
       • mixture of
       1. semantic memory (numbers, words, etc.)
       2. episodic memory (persons, events, etc.).
       • stored in bits and pieces throughout the sensory association areas of the cerebral cortex.
       1. Visual perceptions: vision association area
       2. sounds: auditory association area
       3. hippocampus serves as a bridge
          
          • ​​between the sensory association areas (where memories are stored) and the prefrontal area (where memories are used).
          • prefrontal area communicates with the hippocampus when memories are stored and when these memories are brought to mind.
          • Some memories are emotionally charged, because the amygdala seems to be responsible for fear conditioning and associating danger with sensory stimuli received from various parts of the brain.
    3. Skill memory
    • independent of episodic memory.
    • performing motor activities
    • first learns a skill, more areas of the cerebral cortex are involved than after the skill is perfected
    • later automatic.
    • all the motor areas of the _cerebrum below the level of consciousness._
    • Long-Term Memory Storage and Retrieval
    • • Learning: when we retain and use past memories.
• *

Answer 517

What is the Limbic System and Higher Level Functions?

What areas may work with lower centers to produce learning and memory?

This is the ability to hold a thought in mind or recall a word from yesterday?

What are types of this

Page 294

Question 518

The cerebral cortex • Cerebral cortex – thin, outer layer of gray matter • Primary motor area – voluntary control of skeletal muscle • Primary somatosensory area – for sensory information from skeletal muscle and skin • Association areas – integration occurs here • Processing centers – perform higher level analytical functions including Wernicke’s and Broca’s areas, both involved in speech. Prefrontal area is also a processing center 14.2 The Central Nervous System 18 Wernicke’s and

Answer 518

The Central Nervous System

1. The brain: Cerebrum –

Answer 519

The PNS: Somatic division • The somatic system serves the skin, skeletal muscles and tendons. • Automatic responses are called reflexes. • Reflexes consist of sensory receptorsensory neuron interneuron  motor neuron effector organ (we talked about this in 14.1)

Question 521

reversedprompt

1. In interneurons and motor neurons, multiple dendrites take signals to the cell body, and then an axon conducts nerve signals away from the cell body.
2. ​In sensory neurons, a very long axon carries nerve signals from the dendrites associated with a sensory receptor to the CNS, and this axon is interrupted by the cell body.

Answer 521

These are the differences amongst neuron types insofar as structures

Question 522

limbic system

Answer 522

integrates our emotions with our higher mental functions

contains the amygdala and hippocampus

accounts for why eating and sexual behavior seem pleasurable

Answer 523

Both pharmaceuticals and illegal drugs have several basic modes of action:

They promote the action of a neurotransmitter, usually by increasing the amount of neurotransmitter at a synapse. Examples include drugs such as alprazolam (Xanax) and diazepam (Valium), which increase GABA. These medications are used for panic attacks and anxiety. Reduced levels of norepinephrine and serotonin are linked to depression. Drugs such as fluoxetine (Prozac), paroxetine (Paxil), and duloxetine (Cymbalta) allow norepinephrine and/or serotonin to accumulate at the synapse, which explains their effectiveness as antidepressants. Alzheimer disease causes a slow, progressive loss of memory (see Section 18.5). Drugs used for Alzheimer disease allow acetylcholine to accumulate at synapses in the limbic system.

They interfere with or decrease the action of a neurotransmitter. For instance, antipsychotic drugs used for the treatment of schizophrenia decrease the activity of dopamine. The caffeine in coffee, chocolate, and tea keeps us awake by interfering with the effects of inhibitory neurotransmitters in the brain.

They replace or mimic a neurotransmitter or neuromodulator. The opiates—namely, codeine, heroin, and morphine—bind to endorphin receptors and in this way reduce pain and produce a feeling of well-being.

Ongoing research into neurophysiology and neuropharmacology (the study of nervous system function and the way drugs work in the nervous system) continues to provide evidence that mental illnesses are caused by imbalances in neurotransmitters. These studies will undoubtedly improve treatments for mental illness, as well as provide insight into the problem of drug abuse.

Question 524

Neurons, and their anatomy

Answer 524

Classified according to function, the three types of neurons are sensory neurons, interneurons, and motor neurons (Fig. 14.3). Their functions are best described relative to the CNS. A sensory neuron takes nerve signals from a sensory receptor to the CNS. Sensory receptors are special structures that detect changes in the environment. An interneuron lies entirely within the CNS. Interneurons can receive input from sensory neurons and from other interneurons in the CNS. Thereafter, they sum up all the information received from other neurons before they communicate with motor neurons. A motor neuron takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland). Effectors carry out our responses to environmental changes, whether these are external or internal.

Figure 14.3 The structure of sensory neurons, interneurons, and motor neurons. a. A sensory neuron has a long axon covered by a myelin sheath that takes nerve impulses all the way from dendrites to the CNS. b. In the CNS, some interneurons, such as this one, have a short axon that is not covered by a myelin sheath. c. A motor neuron has a long axon covered by a myelin sheath that takes nerve impulses from the CNS to an effector.

(photos) (a): ©McGraw-Hill Education/Dr. Dennis Emery, Dept. of Zoology and Genetics, Iowa State University, photographer; (b): ©David M. Phillips/Science Source

Neurons vary in appearance, but all of them have three distinct structures: a cell body, dendrites, and an axon. The cell body contains the nucleus, as well as other organelles. Dendrites are short extensions that receive signals from sensory receptors or other neurons. Incoming signals from dendrites can result in nerve signals that are then conducted by an axon. The axon is the portion of a neuron that conducts nerve impulses. An axon can be quite long. Individual axons are termed nerve fibers, and collectively they form a nerve.

In sensory neurons, a very long axon carries nerve signals from the dendrites associated with a sensory receptor to the CNS, and this axon is interrupted by the cell body. In interneurons and motor neurons, on the other hand, multiple dendrites take signals to the cell body, and then an axon conducts nerve signals away from the cell body.

Myelin Sheath

Many axons are covered by a protective myelin sheath. The myelin sheath develops when Schwann cells (PNS) or oligodendrocytes (CNS) wrap their membranes around an axon many times. Each neuroglia cell covers only a portion of an axon, so the myelin Page 282sheath is interrupted. The gaps where there is no myelin sheath are called nodes of Ranvier (Fig. 14.3). Later in this section, we will see how the myelin sheath plays an important role in the rate at which signals move through the neuron.

Question 525

Reverse

SCIENCE IN YOUR LIFE

How does aspirin work?

Aspirin is made of a chemical called acetylsalicylic acid (ASA). Damaged tissue produces large amounts of a type of fatty acid called prostaglandin. Prostaglandin acts as a signal to the peripheral nervous system that tissue damage has occurred, which the brain interprets as pain. Prostaglandins are manufactured in the cell by an enzyme called COX (cyclooxygenase). ASA reduces the capabilities of this enzyme, thus lowering the amount of prostaglandin produced and the perception of pain.

Answer 525

Science in your life: aspirin

Question 526

Neurotransmitter Molecules

Answer 526

1. More than 100 substances are known or suspected to be neurotransmitters.
2. Common: acetylcholine, norepinephrine, dopamine, serotonin, glutamate, and GABA (gamma aminobutyric acid).
3. transmit signals between nerves; Nerve-muscle, nerve-organ, and nerve-gland synapses also communicate using neurotransmitters.
4. Acetylcholine (ACh) and norepinephrine are active in both the CNS and PNS.
   
   • In the PNS, these neurotransmitters act at synapses called neuromuscular junctions. We will explore the structure of the neuromuscular junctions in Section 13.2.
   1. In the PNS, ACh excites skeletal muscle but inhibits cardiac muscle. It has either an excitatory or inhibitory effect on smooth muscle or glands, depending on their location.

• Norepinephrine generally excites smooth muscle.
  
  • In the CNS, norepinephrine is important to dreaming, waking, and mood.
• Serotonin is involved in thermoregulation, sleeping, emotions, and perception.
• Many drugs that affect the nervous system act at the synapse. Some interfere with the actions of neurotransmitters, and other drugs prolong the effects of neurotransmitters (see Section 14.5).

1.

Answer 527

14.3 Check Yourself

Answer 528

14.5 Drug Therapy and Drug Abuse

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain the ways that drugs interact with the nervous system.

Classify drugs as to whether they have a depressant, stimulant, or psychoactive effect on the nervous system.

Question 529

Ependymal Cells

Answer 529

Range in shape from squamous to columnar, many are ciliated • They line the ventricles of the brain and central canal of the spinal cord to form a permeable barrier between cerebrospinal fluid (CSF) and tissue fluid bathing cells of CNS, as well as from blood • Functionally, ependymal cells produce, possibly monitor, and assist in the circulation of cerebrospinal fluid – Help circulate CNS with their cilia

Question 531

Nerve Impulse

Answer 531

Nerve Impulse

Sodium Gates Open

• Protein channels specific for sodium ions are located in the plasma membrane of the axon.
• When an action potential begins in response to a threshold stimulus, these protein channels open and sodium ions rush into the cell.
• Adding positively charged sodium ions causes the inside of the axon to become positive compared to the outside (Fig. 14.4c). T
• his change is called depolarization, because the charge (polarity) inside the axon changes from negative to positive.
• Potassium Gates Open
• Almost immediately after depolarization, the channels for sodium close and a separate set of potassium protein channels opens.
• Potassium flows rapidly from the cell. As positively charged potassium ions exit the cell, the inside of the cell becomes negative again because of the presence of large, negatively charged ions trapped inside the cell. This change in polarity is called repolarization, because the inside of the axon resumes a negative charge as potassium exits the axon (Fig. 14.4d). Finally, the sodium–potassium pump completes the action potential. Potassium ions are returned to the inside of the cell and sodium ions to the outside, and resting potential is restored.
• Neural Transmission: Resting Membrane Potential and Propagation
• Graph of an Action Potential
• To visualize such rapid fluctuations in voltage across the axonal membrane, researchers generally find it useful to plot the voltage changes over time (Fig. 14.4e). During depolarization, the voltage increases from −70 mV to −55 mV to between +30 and +35 mV as sodium ions move to the inside of the axon. In repolarization, the opposite change occurs when potassium ions leave the axon. The entire process is very rapid, requiring only 3 to 4 milliseconds (ms) to complete.

Question 532

• vary in appearance, but all of them have
• three distinct structures:

1. a cell body: contains the nucleus, as well as other organelles.
2. dendrites: short extensions that receive signals from sensory receptors or other neurons. Incoming signals from dendrites can result in nerve signals that are then conducted by an axon. T.
3. axon: portion of a neuron that conducts nerve impulses; quite long. Individual axons are termed nerve fibers, and collectively they form a nerve

Myelin Sheath

• plays an important role in the rate at which signals move through the neuron.
• Many axons covered by this protective sheath
• develops when Schwann cells (PNS) or oligodendrocytes (CNS) wrap their membranes around an axon many times.
• Each neuroglia cell covers only a portion of an axon, so the myelin sheath is interrupted.
  
  • The gaps where there is no myelin sheath are called nodes of Ranvier. Later in this section, we will see how the myelin sheath

Answer 532

The structures of neurons

These are the three parts:

The common parts

See download, review and study:

Page 282, (Fig. 14.3)

Question 533

Action potential animation

Answer 533

difference in electron potential inside and outside of axon of neuron

across membrane about 40 …, inside the axon is negative- resting potential- no impulse -

structure of membrane- cell membranes channel proteins- neurons- 2 channels Na + or K+ channel- resting - Sodium is greater, outside than inside, and K is greater inside than outside

3 sodium out for every 2 K into the cell

when nerve impulse or action potential reaches center of membrane- stiumuls causes membraine to depolarize-

action potential- all or none event- if depolarize certain level- threshold- action potential occurs

gates open first, potential begins, sodium float into axon- positive charge- once moved- membrane potental- -70 to +35, threshold- overcomes, all or nothing event- depolarization- sudden rush of sodium axons into-

negative to positive,

K- channels open- float down to oustide of axon- concentration gradient- also + = repolarization- resumes negative charge as potassium exits- travels down axon- one membrane at a time- depolarization- stiumulus to neighboring - sections of membrane

refractory period- sodium gates- unpoen- action potential can’t move backwards and always moves in same direction

• K and Na pump- restores previous ion distribution Na outside and K inside- each small segment depolarization and repolarization takes just a few miliseconds, ready to transmit another action potential

Question 534

reversedprompt

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Answer 534

Summarize the major regions of the brain and describe the general function of each.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.

Question 535

3 functions of the nervous system

Answer 535

The CNS performs information processing and integration, summing up the input it receives from all over the body. The CNS reviews the information, stores the information as memories, and creates the appropriate motor responses. The smell of those baking cookies evokes memories of their taste.

Question 536

reversedprompt

What are the spinal cord and the brain?

Answer 536

1. CNS, where sensory information is received and
2. motor control is initiated.

Question 537

Reverse

This is the human brain

Answer 537

has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

Question 538

reversedprompt

Check out the pic

14.2 The Central Nervous System 22 2. The brain:

Slides!

Answer 538

• Diencephalon
• skull
• meninges
• pituitary gland
• fourth ventricle
• cord
• Cerebrum
• Diencephalon
• Cerebellum
• hypothalamus
• midbrain
• pons
• Brain stem
• a. Parts of brain
• b. Cerebral hemispheres
• lateral ventricle
• third ventricle
• pineal gland
• corpus callosum
• thalamus (surrounds the third ventricle)
• medulla oblongata
• Study!
• • Figure 14.8 The human brain.

Question 539

reversedprompt

Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur.

Answer 539

What happens to excess cerebrospinal fluid?

Question 540

3 types, support cells, neuroglia- types of neuroglia

structure of a neuron

speed up conduction- neuron to another- myelin sheath- saltatory conduction

what creates myelin sheath-

communication down a portion- axon- sped up through myelenation, action potentials occur at nodes of ranvier-

resting and action potentials - nerve impulse- neuron communicates with others

how nerve impulse traverses synapse

synapse- where two neurons come together

four parts of brain and functions

central nervous system- protection

pns

actions- drugs of abuse

Answer 540

Important for Nervous SyStem

Question 541

Reverse

The resting potential energy of the neuron can be used to perform the work of the neuron: conduction of nerve signals. The process of conduction is termed an action potential, and it occurs in the axons of neurons.

1. A stimulus activates the neuron and must be strong enough to reach threshold
   
   • all-or-nothing event.
   • Once threshold: is reached, happens automatically and completely.
   • On the other hand, if the threshold voltage is never reached, the action potential does not occur.
     
     • Increasing the strength of a stimulus (such as pressing harder with the pin) does not change the strength of an action potential. However, it
     • may cause more action potentials to occur in a given period. As a result, the person may perceive that pain has increased.
       the voltage that will result in an action potential.

Answer 541

All or Nothing Action Potential

In Figure 14.4b, the threshold voltage is around −55 mV.

Question 542

reversedprompt

Action Potential - Polarization

Answer 542

resting neuron- charge difference between inside and outside, maintained by K and Na pumps- while other channels- sodium – can’t get back in- exterior net positive, net negative interior- resting membrane potential- nerve impulse begins when stiumulus disturbs on dendrite- Na ions float into cell- charge reduced, change is enough, will cause nearby Na channels to open- depolarize-d local region- positively charged on inside and negative on the outside- neighboring channels open- depolarization on membrane- action potential- changes occur - to restore resting membrane potential, Na closes, K opens- allows K to float out repolarization membrane

Question 543

Reverse

The Synapse

Every axon branches into many fine endings, each tipped by a small swelling called an axon terminal. Each terminal lies very close to either the dendrite or the cell body of another neuron. This region of close proximity is called a synapse (Fig. 14.5). At a synapse, a small gap called the synaptic cleft separates the sending neuron from the receiving neuron. The nerve signal is unable to jump the cleft. Therefore, another means is needed to pass the nerve signal from the sending neuron to the receiving neuron.

Figure 14.5 Signal transmission at the synapse. Transmission across a synapse from one neuron to another occurs when a neurotransmitter is released, diffuses across a synaptic cleft, and binds to a receptor in the membrane of the receiving neuron.

Tutorial: Synaptic Cleft

Transmission across a synapse is carried out by molecules called neurotransmitters, stored in synaptic Page 285vesicles in the axon terminals. (See Section 3.4 for a review of vesicle function.) The events (Fig. 14.5) at a synapse are (1) nerve signals traveling along an axon to reach an axon terminal; (2) calcium ions entering the terminal and stimulating synaptic vesicles to merge with the sending membrane; and (3) neurotransmitter molecules releasing into the synaptic cleft and diffusing across the cleft to the receiving membrane; there, neurotransmitter molecules bind with specific receptor proteins.

Depending on the types of receptors, the response of the receiving neuron can be toward excitation or toward inhibition. In Figure 14.6, excitation occurs because the neurotransmitter, such as acetylcholine (ACh), has caused the sodium gate to open. Sodium ions diffuse into the receiving neuron. Inhibition would occur if a neurotransmitter caused potassium ions to exit the receiving neuron.

Figure 14.6 Integration of excitatory and inhibitory signals at the synapse. a. Inhibitory signals and excitatory signals are summed up in the dendrite and cell body of the postsynaptic neuron. Only if the combined signals cause the membrane potential to rise above threshold does an action potential occur. b. In this example, threshold was not reached.

(photo): (a): ©Science Source

Chemical Synapses

Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft. In some synapses, the receiving membrane contains enzymes that rapidly inactivate the neurotransmitter. For example, the enzyme acetylcholinesterase (AChE) breaks down the neurotransmitter acetylcholine. In other synapses, the sending membrane rapidly reabsorbs the neurotransmitter, possibly for repackaging in synaptic vesicles or for molecular breakdown.

The short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of receiving membranes. The receiving cell needs to be able to respond quickly to changing conditions. If the neurotransmitter were to linger in the cleft, the receiving cell would be unable to respond to a new signal from a sending cell.

Neural Transmission: Synapse

Answer 543

The synapse

Question 544

What are the tracts crossing in the ___________________________ that contain the midbrain, _________________, and ___________________.

See and download diagram 14.9,

(see Fig. 14.9a).

Page 292

What do ascending and descending tracts do?

What are the functions of each part of the brain stem?

Answer 544

• The tracts cross containing:
  1. midbrain:
• relay station for tracts between the cerebrum and the spinal cord or cerebellum.
• reflex centers for visual, auditory, and tactile responses.

1. pons:

• (“bridge” in Latin)
• contains bundles of axons traveling
• between the cerebellum and the rest of the CNS.
• functions with the medulla oblongata to regulate breathing rate.
• Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

3. medulla oblongata

• reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure).
• It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing.
• superior to the spinal cord
• • groups of axons that travel together;
    
    • Between brain and higher level brain centers
      
      1. Ascending tracts convey sensory information.
      2. Motor information is transmitted on descending tracts.

Question 545

Reverse

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

Answer 545

These are the 14.2 learning goals.

Question 546

1. The peripheral nervous system (PNS), which lies outside the central nervous system, contains the nerves.
2. Nerves are designated as cranial nerves when they arise from the brain and are termed spinal nerves when they arise from the spinal cord. In any case, all nerves carry signals to and from the CNS. So right now, your eyes are sending messages by way of a cranial nerve to the brain, allowing you to read this text. When you’re finished, your brain will direct the muscles in your fingers, by way of the spinal cord and a spinal nerve, to proceed to the next chapter.
3. Figure 14.15 illustrates the anatomy of a nerve. The cell body and the dendrites of neurons are in either the CNS or the ganglia. Ganglia (sing., ganglion) are collections of nerve cell bodies outside the CNS. The axons of neurons project from the CNS and form the spinal cord. In other words, nerves, whether cranial or spinal, are composed of axons, the long part of neurons.

Question 547

reversedprompt

1. cerebrum.
2. left and right cerebral hemispheres
3. (Fig. 14.9b).
4. longitudinal fissure divides hemispheres.
5. the 2 hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.
6. Page 289
7. The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10).

• The frontal lobe is the most anterior of the lobes (directly behind the forehead): movement and higher reasoning, as well as the smell sensation
• The parietal lobe is posterior to the frontal lobe: Somatic sensing
• The occipital lobe is posterior to the parietal lobe (at the rear of the head): Visual information is received and processed .
• The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear):

Figure 14.10 Centers in the frontal lobe control is carried out by parietal lobe neurons, and those of the temporal lobe in the occipital lobe.

Answer 547

Cerebellum (cont)

Cerebral Hemispheres with the 4 _____________

Question 548

The synapse • The synapse is the junction between the sending neuron (presynaptic membrane) and the receiving neuron (postsynaptic membrane). • Transmission is accomplished chemically across a small gap between the two neurons (synaptic cleft) by a neurotransmitter (e.g., ACh, dopamine, or serotonin). • Neurotransmitters are stored in synaptic vesicles in the axon terminals.

Question 549

Reversed prompt

The central nervous system • The CNS consists of the brain and spinal cord. • Both are protected by • Scalp and skin • Bones – skull and vertebral column • Meninges – 3 protective membranes that wrap around CNS • Cerebral spinal fluid (CSF) – space between meninges is filled with this fluid that cushions and protects the CNS • Blood brain barrier (BBB)

Meninges • Dura mater • Double-layered external covering • Periosteum – dense connective tissue attached to surface of the skull • Meningeal layer – outer covering of the brain • Folds inward in several areas

Meninges • Arachnoid mater • Middle layer • Web-like • Pia mater • Internal layer • Clings to the surface of the brain • Many blood vessels http://droualb.faculty.mjc.edu/Lecture%20Notes/Unit%205/Meninges_peeled_away

Answer 549

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

Question 550

Nervous tissue

Answer 550

contains two types of cells: neurons and neuroglia (sometimes referred to as glial cells). Neurons are the cells that transmit nerve impulses between parts of the nervous system; neuroglia support and nourish neurons.

Neuroglia (see Section 4.4) greatly outnumber neurons in the brain. There are several types of neuroglia in the CNS, each with specific functions. Microglia are phagocytic cells that help remove bacteria and debris, whereas astrocytes provide metabolic and structural support directly to the neurons. The myelin sheath is formed from the membranes of tightly spiraled neuroglia. In the PNS, Schwann cells perform this function, leaving gaps called nodes of Ranvier. In the CNS, neuroglia cells called oligodendrocytes form the myelin sheath. We will focus our attention on the anatomy and physiology of neurons.

Question 551

reversedprompt

• Nerve Impulse
• Sodium Gates Open

See download and study 14.4c

(Fig. 14.4d).

1. Neural Transmission: Resting Membrane Potential and Propagation
2. Graph of an Action Potential
3. To visualize such rapid fluctuations in voltage across the axonal membrane, researchers generally find it useful to plot the voltage changes over time (Fig. 14.4e). During depolarization, the voltage increases from −70 mV to −55 mV to between +30 and +35 mV as sodium ions move to the inside of the axon. In repolarization, the opposite change occurs when potassium ions leave the axon. The entire process is very rapid, requiring only 3 to 4 milliseconds (ms) to complete.

***Proccess, know steps- short answers** study slides

Answer 551

1. Protein channels specific for sodium ions are located in the plasma membrane of the axon. When an action potential begins in response to a threshold stimulus, these protein channels open and sodium ions rush into the cell.
2. Adding positively charged sodium ions causes the inside of the axon to become positive compared to the outside (Fig. 14.4c). This change is called depolarization, because the charge (polarity) inside the axon changes from negative to positive.
3. Almost immediately after depolarization, the channels for sodium close and a separate set of potassium protein channels opens. Potassium flows rapidly from the cell. As positively charged potassium ions exit the cell, the inside of the cell becomes negative again because of the presence of large, negatively charged ions trapped inside the cell. This change in polarity is called repolarization, because the inside of the axon resumes a negative charge as potassium exits the axon
4. Finally, the sodium–potassium pump completes the action potential. Potassium ions are returned to the inside of the cell and sodium ions to the outside, and resting potential is restored.

Question 552

Association areas – integration occurs here • Processing centers – perform higher level analytical functions including Wernicke’s and Broca’s areas, both involved in speech. Prefrontal area is also a processing center 14.2 The Central Nervous System 18 Wernicke’s and Broca’s aphasias • Aphasias are brain lesions/areas of damage • When Wernicke’s area is damaged, the individual is not able to process language and responds nonsensically (“word salad”) • When Broca’s area is damaged, the individual understands what is spoken to him/her, however is unable to have motor control for speech to verbally respond 1

Answer 552

Association areas and Processing Centers

Wernick’s and Broca’s aphasias

Question 554

14.5 Drug Therapy and Drug Abuse 10 Drugs and drug abuse • Most drug abusers take drugs that affect the neurotransmitter dopamine and thus artificially affect this reward circuit to the point that they ignore basic physical needs in favor of the drug. • Drug abusers tend to show a physiological and psychological effect. • Once a person is physically dependent, they usually need more of the drug for the same effect because their body has become tolerant, where they are used to the presence of the drug and work at this level to maintain homeostasis. 14.5 Drug Therapy and Drug Abuse 11 Drug abuse: Alcohol • Alcohol – a depressant directly absorbed from the stomach and small intestine • Alcohol is the most socially accepted form of drug use. • About 80% of college-aged people drink. • Alcohol denatures proteins and causes damage to tissues such as the brain and liver; chronic consumption can damage the frontal lobe. • High blood alcohol levels can lead to poor judgment, loss of coordination, or even coma and death. 14.5 Drug Therapy and Drug Abuse 12 Drug abuse: Nicotine • Nicotine – stimulant derived from tobacco plant • Nicotine stimulates neurons to release dopamine that reinforces dependence on the drug. • It adversely affects a developing embryo or fetus. • Smoking increases heart rate and blood pressure. • Nicotine causes psychological and physiological dependency. 14.5 Drug Therapy and Drug Abuse 13 Drug abuse: Cocaine • Cocaine – stimulant derived from a shrub • Cocaine causes a rush sensation that lasts from 5-30 minutes. • A cocaine binge occurs when a user takes the drug at ever-higher doses, resulting in hyperactivity, little desire for food and sleep, and an increased sex drive. • There is extreme physical dependence with this drug. • “Crack” is the street name for cocaine that is processed to a free-base form for smoking. 14.5 Drug Therapy and Drug Abuse 14 Drug abuse: Methamphetamine • Powder form is called ‘speed’ and crystal form is called ‘meth’ or ‘ice.’ • It is a stimulant that reverses the effects of fatigue and is a mood elevator. • High agitation is common after the rush and can lead to violent behavior. • Methamphetamine causes psychological dependency and hallucinations. • “Ecstasy” is the street name for a drug that has the same effects as meth without the hallucinations. 14.5 Drug Therapy and Drug Abuse 15 Drug abuse: Heroin • Heroin – depressant from the sap of the opium poppy plant • It leads to a feeling of euphoria and no pain because it is delivered to the brain and converted into morphine. • Side effects are nausea, vomiting, and depression of the respiratory and circulatory systems. • Heroin use can lead to HIV, hepatitis, and other infections due to shared needles. • Extreme dependency is common. 14.5 Drug Therapy and Drug Abuse 16 Drug abuse: Marijuana • Marijuana – psychoactive drug derived from a hemp plant called Cannabis; legal medical use and legal recreational use in some states • It is most often smoked as a “joint.” • Occasional users experience mild euphoria, alterations to vision and judgment, as well as impaired motor coordination with slurred speech. • Heavy users may experience depression, anxiety, hallucinations, paranoia, and psychotic symptoms. • Long term use may lead to brain damage. • K2 (“Spice”) is a synthetic drug with higher potency than THC, the active chemical in marijuana.

Question 555

Reverse

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Answer 555

These are the central nervous system, and this is what is primarily done.

Answer 556

Drug Mode of Action

As mentioned in Section 14.1, there are more than 100 known neurotransmitters. The most widely studied neurotransmitters to date are acetylcholine, norepinephrine, dopamine, serotonin, and gamma-aminobutyric acid (GABA). Acetylcholine is an essential CNS neurotransmitter for memory circuits in the limbic system. Norepinephrine is important to dreaming, waking, and mood. The neurotransmitter dopamine plays a central role in the brain’s regulation of mood. Dopamine is also the basal nuclei neurotransmitter that helps organize coordinated movements. Serotonin is involved in thermoregulation, sleeping, emotions, and perception. GABA is an abundant inhibitory neurotransmitter in the CNS.

Question 557

14.5 Learning Outcomes

Answer 557

14.5 Drug Therapy and Drug Abuse

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain the ways that drugs interact with the nervous system.

Classify drugs as to whether they have a depressant, stimulant, or psychoactive effect on the nervous system.

List the long-term effects of drug use on the body.

As you are reading these words, synapses throughout your brain are organizing, integrating, and cataloging the information you take in. Neurotransmitters at these synapses control the firing of countless action potentials, thus creating a network of neural circuits. It is amazing to realize that all thoughts, feelings, and actions of a human are dependent on neurotransmitters in the CNS and PNS. By modifying or controlling synaptic transmission, a wide variety of drugs with neurological activity, both legal pharmaceuticals and illegal drugs of abuse, can alter mood, emotional state, behavior, and personality.

Question 558

These are the three types of neurons, and the one that is in the nervous system alone is…

Answer 558

sensory, motor, interneurons… interneurons

Question 559

This type of neuron takes nerve signals from a sensory receptor to the CNS.

Answer 559

sensory

Question 560

Reversed prompt

The central nervous system • The CNS consists of the brain and spinal cord. • Both are protected by • Scalp and skin • Bones – skull and vertebral column • Meninges – 3 protective membranes that wrap around CNS • Cerebral spinal fluid (CSF) – space between meninges is filled with this fluid that cushions and protects the CNS • Blood brain barrier (BBB)

Meninges • Dura mater • Double-layered external covering • Periosteum – dense connective tissue attached to surface of the skull • Meningeal layer – outer covering of the brain • Folds inward in several areas

Meninges • Arachnoid mater • Middle layer • Web-like • Pia mater • Internal layer • Clings to the surface of the brain • Many blood vessels http://droualb.faculty.mjc.edu/Lecture%20Notes/Unit%205/Meninges_peeled_away

Answer 560

14.2 The Central Nervous System

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the spinal cord and provide a function for each.

Identify the structures of the brain and provide a function for each.

Identify the lobes and major areas of the human brain.

Distinguish between the functions of the primary motor and the primary somatosensory areas of the brain.

The spinal cord and the brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae, and the brain is enclosed by the skull. Also, both the spinal cord and the brain are wrapped in protective Page 287membranes known as meninges (sing., meninx). Meningitis is an infection of the meninges and may be caused by either bacterial or viral pathogens. The spaces between the meninges are filled with cerebrospinal fluid, which cushions and protects the CNS. In a spinal tap (lumbar puncture), a small amount of this fluid is withdrawn from around the spinal cord for laboratory testing. Cerebrospinal fluid is also contained within the ventricles of the brain and in the central canal of the spinal cord. The brain has four ventricles, interconnecting chambers that produce and serve as a reservoir for cerebrospinal fluid (Fig. 14.7). Normally, any excess cerebrospinal fluid drains away into the cardiovascular system. However, blockages can occur. In an infant, the brain can enlarge due to cerebrospinal fluid accumulation, resulting in a condition called hydrocephalus (“water on the brain”). If cerebrospinal fluid collects in an adult, the brain cannot enlarge. Instead, it is pushed against the skull. Such situations cause severe brain damage and can be fatal unless quickly corrected.

Figure 14.7 The ventricles of the brain. The brain has four ventricles. A lateral ventricle is found on each side of the brain. They join at the third ventricle. The third ventricle connects with the fourth ventricle superiorly; the central canal of the spinal cord joins the fourth ventricle inferiorly. All structures are filled with cerebrospinal fluid. a. Lateral view of ventricles seen through a transparent brain. b. Anterior view of ventricles seen through a transparent brain.

The CNS is composed of two types of nervous tissue—gray matter and white matter. Gray matter contains cell bodies and short, nonmyelinated axons. White matter contains myelinated axons that run together in bundles called tracts.

The Spinal Cord

The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum (see Fig. 12.4). From the foramen magnum, the spinal cord proceeds inferiorly in the vertebral canal.

Structure of the Spinal Cord

A cross-section of the spinal cord (Fig. 14.8a) shows a central canal, gray matter, and white matter. Figure 14.8b shows how an individual vertebra protects the spinal cord. The spinal nerves project from the cord through small openings called intervertebral foramina. Fibrocartilage intervertebral discs separate the vertebrae. If a disc ruptures or herniates, it may compress a spinal nerve, resulting in pain and a loss of mobility.

Figure 14.8 The organization of white and gray matter in the spinal cord and the spinal nerves. a. Cross-section of the spinal cord, showing arrangements of white and gray matter. b. Spinal nerves originating from the spinal cord. c. The spinal cord is protected by vertebrae. d. Spinal nerves emerging from the cord.

(a): ©Scientifica/Corbis Documentary/Getty Images; (d): ©McGraw-Hill Education/Rebecca Gray, photographer & Don Kincaid, dissections

The central canal of the spinal cord contains cerebrospinal fluid, as do the meninges that protect the spinal cord. The gray matter is centrally located and shaped like the letter H (Fig. 14.8a–c). Portions of sensory neurons and motor neurons are found in gray matter, as are interneurons that communicate with these two types of neurons. The dorsal root of a spinal nerve contains sensory fibers entering the gray matter. The ventral root of a spinal nerve contains motor fibers exiting the gray matter. The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal (Fig. 14.8c, d), forming a mixed nerve. Spinal nerves are a part of the PNS.

The white matter of the spinal cord occurs in areas around the gray matter. The white matter contains ascending tracts taking information to the brain (primarily located posteriorly) and descending tracts taking information from the brain (primarily located anteriorly). Many tracts cross just after they enter and exit the brain, so the left side of the brain controls the right side of the body. Likewise, the right side of the brain controls the left side of the body.

Functions of the Spinal Cord

The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand, sensory receptors generate nerve signals that pass through sensory fibers to the spinal cord and up ascending tracts to the brain (see Fig. 14.2b, red arrows).

The gate control theory of pain proposes that the tracts in the spinal cord have “gates” and that these gates control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain. Pain messages may also be blocked by other inputs, such as those received from touch receptors or endorphins.

The brain coordinates the voluntary control of our limbs. Motor signals originating in the brain pass down descending tracts to the spinal cord and out to our muscles by way of motor fibers (see Fig. 14.2b, black arrows). Therefore, if the spinal cord is severed, we suffer a loss of sensation and a loss of voluntary control—paralysis. If the cut occurs in the thoracic region, the lower body and legs are paralyzed, a condition known as paraplegia. If the injury is in the neck region, all four limbs are usually affected, a condition called quadriplegia.

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Reflex Actions

The spinal cord is the center for thousands of reflex arcs (see Fig. 14.17). A stimulus causes sensory receptors to generate signals that travel in sensory axons to the spinal cord. Interneurons integrate the incoming data and relay signals to motor neurons. A response to the stimulus occurs when motor axons cause skeletal muscles to contract. Motor neurons in a reflex arc may also affect smooth muscle, organs, or glands. Each interneuron in the spinal cord synapses with numerous other neurons. From there, interneurons send signals to other interneurons and motor neurons.

Similarly, the spinal cord creates reflex arcs for the internal organs. For example, when blood pressure falls, internal receptors in the carotid arteries and aorta generate nerve signals that pass through sensory fibers to the cord and then up an ascending tract to a cardiovascular center in the brain. Thereafter, nerve signals pass down a descending tract to the spinal cord. Motor signals then cause blood vessels to constrict, so that the blood pressure rises.

The Brain

The human brain has been called the last great frontier of biology. The goal of modern neuroscience is to understand the structure and function of the brain’s various parts so well that it will be possible to prevent or correct the thousands of mental disorders that rob humans of a normal life. This section gives only a glimpse of what is known about the brain and the modern avenues of research.

We discuss the parts of the brain with reference to the cerebrum, the diencephalon, the cerebellum, and the brain stem. The brain’s four ventricles (see Fig. 14.7) are called, in turn, the two lateral ventricles, the third ventricle, and the fourth ventricle. It may be helpful for you to associate the cerebrum with the two lateral ventricles, the diencephalon with the third ventricle, and the brain stem and cerebellum with the fourth ventricle (Fig. 14.9a).

Figure 14.9 The human brain. a. The cerebrum is the largest part of the brain in humans. b. Viewed from above, the cerebrum has left and right cerebral hemispheres. The hemispheres are connected by the corpus callosum.

(b): ©McGraw-Hill Education; photo and dissection by Christine Eckel

The Cerebrum

The cerebrum is the largest portion of the brain in mammals, including humans. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain.

Cerebral Hemispheres

Just as the human body has two halves, so does the cerebrum. These halves are called the left and right cerebral hemispheres (Fig. 14.9b). A deep groove called the longitudinal fissure divides the left and right cerebral hemispheres. The two cerebral hemispheres communicate via the corpus callosum, an extensive bridge of nerve tracts.

Page 289The characteristic appearance of the cerebrum is the result of thick folds, called gyri (sing., gyrus) separated by shallow grooves called sulci (sing., sulcus). The sulci divide each hemisphere into lobes (Fig. 14.10). The frontal lobe is the most anterior of the lobes (directly behind the forehead). The parietal lobe is posterior to the frontal lobe. The occipital lobe is posterior to the parietal lobe (at the rear of the head). The temporal lobe lies inferior to the frontal and parietal lobes (at the temple and the ear).

Figure 14.10 The lobes of the cerebral hemispheres. Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Centers in the frontal lobe control movement and higher reasoning, as well as the smell sensation. Somatic sensing is carried out by parietal lobe neurons, and those of the temporal lobe receive sound information. Visual information is received and processed in the occipital lobe.

The Cerebral Cortex

The cerebral cortex is a thin, highly convoluted outer layer of gray matter that covers the cerebral hemispheres. Recall that gray matter consists of neurons whose axons are not myelinated. The cerebral cortex contains over 1 billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes we associate with consciousness.

Primary Motor and Sensory Areas of the Cortex

The cerebral cortex contains motor areas and sensory areas, as well as association areas. The primary motor area is in the frontal lobe just anterior to (before) the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section (Fig. 14.11a). In this illustration, notice that large areas of the cerebral cortex are devoted to controlling structures that carry out very fine, precise movements. Thus, the muscles that control facial movements—swallowing, salivation, expression—take up an especially large portion of the primary motor area. Likewise, hand movements require tremendous accuracy. Together, these two structures command nearly two-thirds of the primary motor area.Page 290

Figure 14.11 The primary motor and primary somatosensory areas of the brain. a. The primary motor area (blue) is located in the frontal lobe, adjacent to (b) the primary somatosensory area in the parietal lobe. The primary taste area is colored pink. The size of each body region shown indicates the relative amount of cortex devoted to control of that body region.

SCIENCE IN YOUR LIFE

Why does a stroke on the right side of the brain cause weakness or paralysis on the left side of the body?

Descending motor tracts (from the primary motor area) and ascending sensory tracts (from the primary somatosensory area) cross over in the spinal cord and medulla. Motor neurons in the right cerebral hemisphere control the left side of the body and vice versa because of crossing-over. Likewise, sensation from the left half of the body travels to the right cerebral hemisphere. Destruction of brain tissue by a stroke interferes with outgoing motor signals to the opposite side of the body, as well as incoming sensory information from that side.

The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented (Fig. 14.11b). Like the primary motor cortex, large areas of the primary sensory cortex are dedicated to those body areas with acute sensation. Once again, the face and hands require the largest proportion of the sensory cortex.

Page 291Reception areas for the other primary sensations—taste, vision, hearing, and smell—are located in other areas of the cerebral cortex (see Fig. 14.10). The primary taste area in the parietal lobe (pink) accounts for taste sensations. Visual information is received by the primary visual cortex (blue) in the occipital lobe. The primary auditory area in the temporal lobe (green) accepts information from our ears. Smell sensations travel to the primary olfactory area (yellow) found on the deep surface of the frontal lobe.

Association Areas

Association areas are places where integration occurs and where memories are stored. Anterior to the primary motor area is a premotor area. The premotor area organizes motor functions for skilled motor activities, such as walking and talking at the same time. Next, the primary motor area sends signals to the cerebellum, which integrates them. A momentary lack of oxygen during birth can damage the motor areas of the cerebral cortex, resulting in cerebral palsy, a condition characterized by a spastic weakness of the arms and legs. The somatosensory association area, located just posterior to the primary somatosensory area, processes and analyzes sensory information from the skin and muscles. The visual association area in the occipital lobe associates new visual information with stored visual memories. It might “decide,” for example, if we have seen a face, scene, or symbol before. The auditory association area in the temporal lobe performs the same functions with regard to sounds.

Processing Centers

Processing centers of the cortex receive information from the other association areas and perform higher-level analytical functions. The prefrontal area, an association area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities. Reasoning, critical thinking, and formulating appropriate behaviors are possible because of integration carried out in the prefrontal area.

The unique ability of humans to speak is partially dependent on two processing centers found only in the left cerebral cortex. Wernicke’s area is located in the posterior part of the left temporal lobe. Broca’s area is located in the left frontal lobe. Broca’s area is just anterior to the portion of the primary motor area for speech musculature (lips, tongue, larynx, and so forth) (see Fig. 14.10). Wernicke’s area helps us understand both the written and the spoken word and sends the information to Broca’s area. Broca’s area adds grammatical refinements and directs the primary motor area to stimulate the appropriate muscles for speaking and writing.

Central White Matter

Much of the rest of the cerebrum is composed of white matter. Myelination occurs and white matter develops as a child grows. Progressive myelination enables the brain to grow in size and complexity. For example, as neurons become myelinated within tracts designed for language development, children become more capable of speech. Descending tracts from the primary motor area communicate with lower brain centers, and ascending tracts from lower brain centers send sensory information up to the primary somatosensory area. Tracts within the cerebrum also take information among the different sensory, motor, and association areas pictured in Figure 14.10. The corpus callosum contains tracts that join the two cerebral hemispheres.

Basal Nuclei

Though the majority of each cerebral hemisphere is composed of tracts, there are masses of gray matter deep within the white matter. These basal nuclei integrate motor commands to ensure that the proper muscle groups are stimulated or inhibited. Integration ensures that movements are coordinated and smooth. Parkinson disease (see Section 18.5) is believed to be caused by degeneration of specific neurons in the basal nuclei.

The Diencephalon

The hypothalamus and thalamus are in the diencephalon, a region that encircles the third ventricle. The hypothalamus forms the floor of the third ventricle. The hypothalamus is an integrating center that helps maintain homeostasis. It regulates hunger, sleep, thirst, body temperature, and water balance. The hypothalamus controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.

The thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle. The thalamus is on the receiving end for all sensory input except the sense of smell. Visual, auditory, and somatosensory information arrives at the thalamus via the cranial nerves and tracts from the spinal cord. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum. The thalamus is involved in arousal of the cerebrum, and it participates in higher mental functions, such as memory and emotions.

The pineal gland, which secretes the hormone melatonin, is located in the diencephalon. Presently, there is much popular interest in the role of melatonin in our daily rhythms. Some researchers believe it can help alleviate jet lag or insomnia. Scientists are also interested in the possibility that the hormone may regulate the onset of puberty.

The Cerebellum

The cerebellum lies under the occipital lobe of the cerebrum and is separated from the brain stem by the fourth ventricle. The cerebellum has two portions joined by a narrow median portion. Each portion is primarily composed of white matter. In a longitudinal section, the white matter has a treelike pattern called arbor vitae. Overlying the white matter is a thin layer of gray matter that forms a series of complex folds.

The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts. It also receives motor output from the cerebral cortex about where these parts should be located. After integrating this information, the cerebellum sends motor signals by way of the brain stem to the skeletal muscles. In this way, the cerebellum maintains posture and balance. It also ensures that all the muscles work together to produce smooth, coordinated, voluntary movements. The cerebellum assists in the learning of new motor skills, such as playing the piano or hitting a baseball.

The Brain Stem

The tracts cross in the brain stem, which contains the midbrain, the pons, and the medulla oblongata (see Fig. 14.9a). The midbrain acts as a relay station for tracts passing between the cerebrum and Page 292the spinal cord or cerebellum. It also has reflex centers for visual, auditory, and tactile responses. The pons (“bridge” in Latin) contains bundles of axons traveling between the cerebellum and the rest of the CNS. In addition, the pons functions with the medulla oblongata to regulate breathing rate. Reflex centers in the pons coordinate head movements in response to visual and auditory stimuli.

The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction (blood pressure). It also contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing. The medulla oblongata lies just superior to the spinal cord, and it contains tracts that ascend or descend between the spinal cord and higher brain centers. Recall that tracts are groups of axons that travel together. Ascending tracts convey sensory information. Motor information is transmitted on descending tracts.

The Reticular Formation

The reticular formation is a complex network of nuclei, which are masses of gray matter, and fibers that extends the length of the brain stem (Fig. 14.12). The reticular formation is a major component of the reticular activating system (RAS). The RAS receives sensory signals and sends them to higher centers. Motor signals received by the RAS are sent to the spinal cord.

Figure 14.12 The reticular formation of the brain. The reticular formation receives and sends on motor and sensory information to various parts of the CNS. One portion, the reticular activating system (RAS) (see arrows), arouses the cerebrum and, in this way, controls alertness versus sleep.

The RAS arouses the cerebrum via the thalamus and causes a person to be alert. If you want to awaken the RAS, surprise it with sudden stimuli, such as an alarm clock ringing, bright lights, smelling salts, or cold water splashed on your face. The RAS can filter out unnecessary sensory stimuli, explaining why you can study with the TV on. Similarly, the RAS allows you to take a test without noticing the sounds of the people around you—unless the sounds are particularly distracting. To inactivate the RAS, remove visual or auditory stimuli, allowing yourself to become drowsy and drop off to sleep. General anesthetics function by artificially suppressing the RAS. A severe injury to the RAS can cause a person to become comatose, from which recovery may be impossible.

CHECK YOUR PROGRESS 14.2

List the functions of the spinal cord.

Answer

Provides a means of communication between the brain and the peripheral nerves, and is the center for reflex actions.

Summarize the major regions of the brain and describe the general function of each.

Answer

Cerebrum—largest part of the brain, integrates sensory inputs and coordinates the activities of the other parts of the brain; diencephalon—contains the hypothalamus and thalamus, maintains homeostasis, receives sensory input; cerebellum—sends out motor impulses by way of the brain stem to the skeletal muscles, produces smooth, coordinated voluntary movements; brain stem—contains the midbrain, pons, and medulla oblongata, acts as a relay station, and the medulla has reflex centers.

Relate how the RAS aids in homeostasis.

Answer

The RAS regulates a person’s alertness, relays sensory signals to higher centers, and filters out unnecessary stimuli, which are important functions in being properly responsive to one’s environment.

CONNECTING THE CONCEPTS

For more information on the central nervous system, refer to the following discussions:

Section 10.5 examines how the central nervous system controls breathing.

Section 18.5 explores how aging and diseases such as Alzheimer disease influence the brain.

Sections 23.3 and 23.4 outline how the size of the brain has changed over the course of human evolution.