Lecture Exam #3 Flashcards
Compare and contrast the general composition, function, and distribution of gray and white
matter throughout the central nervous system.
In general, gray matter within the brain and spinal cord consists primarily of dendrites and cell bodies that serve as processing, or “decision-making,” areas, whereas white matter is composed of myelinated axons that relay nerve signals to and from the gray matter.
Both the brain and the spinal cord contain white and gray matter. In both organs, the white matter contains the myelinated axons, whereas the gray matter contains the cell bodies and nonmyelinated processes of neurons. Yet, the location of the white and gray matters differ in the brain and the spinal cord.
The gray matter is the outermost structure in both the cerebrum and cerebellum. Having the gray matter on the outside assures a large surface area for the cell bodies of the cortical neurons, which send and receive information via the white matter underneath.
In the spinal cord, the gray matter is on the inside, surrounded by the white matter. The spinal cord has thirty-one spinal nerves arising at every vertebral level. Hence, in the spinal cord, the white matter on the outside carries axon tracts to and from the brain but also allows easy exit at every vertebral level. This leaves the cell bodies of interneurons and motor neurons to be on the inner core of the spinal cord.
Explain the three functions of cerebrospinal fluid
Protect brain and spinal cord from trauma.
Supply nutrients to nervous system tissue.
Remove waste products from cerebral metabolism.
Describe the components that form the blood-brain barrier & how it protects the brain.
The perivascular feet of
the astrocytes and the tight endothelial junctions and thickened basement membrane of the capillaries work together to prevent harmful materials in the blood from reaching the brain. The blood brain barrier regulates the movement of most substances, but lipid-soluble substances can pass through the barrier freely.
The BBB is formed of specialized capillaries surrounded by astrocytes. Capillaries are typically composed of an endothelial lining resting on a basement membrane. Capillaries forming the BBB exhibit three significant structural differences from other capillaries. (1) The endothelial cells contain tight junctions, which prevent the passage of materials between cells. Thus, most substances are forced through the endothelial cells and their movement is controlled by membrane transport processes . (2) The capillary wall is made more substantial by a thickened basement membrane that further restricts the passage of substances from the blood into the brain. (3) The capillaries forming the BBB are wrapped in the perivascular feet of astrocytes, which form the outermost portion of the BBB.
explain the general functions of the left and right cerebral hemispheres
- Right hemi = language comprehension and visuospatial tasks (artistic tasks) and control the left side of the body
- Left hemi = The left hemisphere function is to control the right side of the body and is the educational and rational side of the brain. The left hemispheres functions are language, logic, science, written, communications, numbers. Skills, and reasoning.
Identify the role of the corpus callosum
The largest of these white matter tracts, the corpus callosum connects the hemispheres. The corpus callosum provides the main method of communication between these hemispheres.
Locate and list the general functions of the motor cortical regions & their association areas
Primary motor cortex- located at precentral gyrus of frontal lobe; controls skeletal muscles
Association- Premotor cortex, located in front of precentral gyrus, deals with coordination
Certain motor functions have been mapped to specific areas of the frontal lobe, including the motor speech area and the frontal eye field. The motor speech area (also known as the Broca area) is located in most individuals within the inferolateral portion of the left frontal lobe. This region is responsible for regulating the breathing and controlling the muscular movements necessary for vocalization. The frontal eye field is within the frontal lobe immediately superior to the motor speech area. This cortical area controls and regulates the eye movements needed for reading and coordinating binocular vision. Some investigators include the frontal eye fields within the premotor area.
The primary motor cortical regions are connected to adjacent association areas that coordinate discrete skeletal muscle movement. The premotor cortex also is called the somatic motor association area, and it is located within the frontal lobe immediately anterior to the precentral gyrus. It is primarily responsible for coordinating learned, skilled motor activities, such as moving the eyes in a coordinated fashion when reading a book or playing the guitar.
Compare and contrast the sensory cortical regions and their association areas
Each primary cortical region typically has an association area. The general function of association areas is to receive input from the primary region and integrate the current sensory input with previous experiences and memories.
The primary somatosensory cortex is housed within the postcentral gyrus of the parietal lobes. Neurons within this cortex receive general somatic sensory information from receptors of the skin regarding touch, pressure, pain, and temperature, as well as sensory input from proprioceptors from the joints and muscles regarding the conscious interpretation of body position. We typically are conscious of the sensations received by this cortex. A sensory homunculus may be traced on the postcentral gyrus surface, similar to a motor homunculus. Thus, the lips, fingers, and genital region occupy larger portions of the homunculus, whereas the trunk of the body has proportionately fewer receptors, so its associated homunculus region is smaller.
The somatosensory association area is located within the parietal lobe and lies immediately posterior to the primary somatosensory cortex. It integrates sensory information and interprets sensations to determine the texture, temperature, pressure, and shape of objects. The somatosensory association area allows us to identify known objects without seeing them.
The primary visual cortex is located within the occipital lobe, where it receives and processes incoming visual information. The visual association area is located within the occipital lobe and it surrounds the primary visual area. The primary auditory cortex is located within the temporal lobe, where it receives and processes auditory information. The auditory association area is located within the temporal lobe, posteroinferior to the primary auditory cortex. Within this association area, the cortical neurons interpret the characteristics of sound and store memories of sounds heard in the past.
The primary olfactory cortex is also located within the temporal lobe and provides conscious awareness of smells. Finally, the primary gustatory cortex is within the insula and is involved in processing taste information.
Explain the functions of the prefrontal cortex
A functional brain region acts as a multi-association area between lobes for integrating information from individual association areas. One functional brain region is the prefrontal cortex, located in the most anterior (rostral) portions of the frontal lobes. The prefrontal cortex is associated with many higher intellectual functions such as complex thought, judgment, expression of personality, planning future behaviors, and decision making.
Describe the main functions of the Wernicke area
Another functional brain region is the Wernicke area, which is typically located only within the left hemisphere. The Wernicke area is involved in recognizing, understanding, and comprehending spoken or written language.
Identify and describe the three main tracts of the central white matter.
Association tracts connect different regions of the cerebral cortex within the same hemisphere.
Commissural tracts extend between the cerebral hemispheres through axonal bridges called commissures. The prominent commissural tracts that link the left and right cerebral hemispheres include the large, C-shaped corpus callosum and the smaller anterior and posterior commissure
Projection tracts link the cerebral cor- tex to both the inferior brain regions and the spinal cord.
Define cerebral lateralization & contrast the general functions of the left and right hemispheres in most individuals.
The hemispheres also differ with respect to some of their functions. Each hemisphere tends to be specialized for certain tasks, a phenomenon called cerebral lateralization or hemispheric lateralization. Higher-order centers in both hemispheres tend to have different but complementary functions.
In most people, the left hemisphere is the categorical hemisphere. It usually contains the Wernicke area and the motor speech area. It is specialized for language abilities and is important in performing sequential and analytical reasoning tasks, such as those required in science and mathematics. The term categorical hemisphere reflects this hemisphere’s function in categorization and identification.
The right hemisphere is called the representational hemisphere, because it is concerned with visuospatial relationships and analyses. It is the seat of imagination and insight, musical and artistic skill, perception of patterns and spatial relationships, and comparison of sights, sounds, smells, and tastes.
Describe the primary function of the cerebral nuclei
The cerebral nuclei are paired, irregular masses of gray matter buried deep within the central white matter in the basal (deepest) region of the cerebral hemispheres inferior to the floor of the lateral ventricle. In general, the cerebral nuclei primarily help regulate motor output initiated by the cerebral cortex, to help inhibit unwanted movements. Diseases that affect the cerebral nuclei often are associated with jerky, involuntary movements.
Describe components composing the epithalamus and explain their functions.
The epithalamus partially forms the posterior roof of the diencephalon and covers the third ventricle. The posterior portion of the epithalamus houses the pineal gland and the habenular nuclei.
The pineal gland, or pineal body, is an endocrine gland. It secretes the hormone melatonin, which appears to help regulate day-night cycles known as the body’s circadian rhythm.
The habenular nuclei relay signals from the limbic system to the midbrain and are involved in visceral and emotional responses to odors.
Explain the functions of the thalamus
The thalamus is the principal and final relay point for incoming sensory information that is processed and then projected to the appropriate lobe of the cerebral cortex. Only a relatively small portion of the sensory information that arrives at the thalamus is forwarded to the cerebrum because the thalamus acts as an information filter. For example, the thalamus is responsible for filtering out the sounds and sights in a crowded cafeteria when you are trying to study. The thalamus also “clues in” the cerebrum about where this sensory information came from. For example, the thalamus lets the cerebrum know that sensory information it receives came from the eye, indicating that the information is visual.
Describe the functions of the hypothalamus.
Master control of the autonomic nervous system
Master control of the endocrine system
Regulation of body temperature
Control of food intake
Control of water intake
Regulation of sleep-wake (circadian) rhythms
Control of emotional behavior
Describe the major components of the midbrain and explain their functions
The midbrain and hindbrain together form the brainstem. The midbrain connects the forebrain to the hindbrain. The parts of the midbrain are:
- Tectum: It is involved in reflex actions in response to the auditory and visual stimuli.
- Cerebral aqueduct: It links the third and the fourth ventricle and is involved in the continuity of the cerebrospinal fluid. This area controls all the major movements of the eye.
- Tegmentum: It is involved in giving our memory its sharpness. It is also involved in homeostasis and reflex actions. The ventral tegmental area is the largest dopamine-producing area of the brain.The tegmentum contains the pigmented red nuclei and the reticular formation. The reddish color of these nuclei is due to both blood vessel density and iron pigmentation in the neuronal cell bodies. The tegmentum integrates information from the cerebrum and cerebellum and issues involuntary motor commands to the erector spinae muscles of the back to help maintain posture while standing, bending at the waist, or walking.
- Cerebral peduncles: It is mainly involved in motor-planning, learning, addiction, and other activities.
5.The substantia nigra consists of bilaterally symmetric nuclei within the midbrain. Its name derives from its almost black appearance due to melanin pigmentation. The substantia nigra houses clusters of neurons that produce the neurotransmitter dopamine, which affects brain processes to control movement, emotional response, and ability to experience pleasure and pain.
Describe the major structures in the pons and explain their functions.
The pons is a bulging region on the anterior part of the brainstem. Sensory and motor tracts are located within the pons and extend through it to connect to the brain and spinal cord. Additionally, the middle cerebellar peduncles are transverse axons that connect the pons to the cerebellum.
The pons houses autonomic nuclei in the pontine respiratory center . This vital center, along with the medullary respiratory center within the medulla oblongata, regulates the skeletal muscles of breathing. The primary function of the pontine respiratory center is to regulate a smooth transition between breathing in and breathing out.
The superior olivary nuclei are located in the inferior portion of the pons. Each nucleus receives auditory input and is involved in the pathway for sound localization.
The pons also houses sensory and motor cranial nerve nuclei for the trigeminal (CN V), abducens (CN VI), and facial (CN VII) cranial nerves. Some of the nuclei for the vestibulocochlear cranial nerve (CN VIII) also are located there.
Describe the major structures in the medulla oblongata and explain their functions.
It is the most inferior part of the brainstem and is continuous with the spinal cord inferiorly. All communication between the brain and spinal cord involves tracts that ascend or descend through the medulla oblongata
The anterior surface exhibits two longitudinal ridges called the pyramids, which house the motor projection tracts called the corticospinal (pyramidal) tracts that extend through the medulla oblongata. In the anterior region of the medulla, most of the axons of the pyramidal tracts cross to the opposite side of the brain at a point called the decussation of the pyramids. As a result of the crossover, each cerebral hemisphere controls the voluntary movements of the opposite side of the body. Immediately lateral to each pyramid is a distinct bulge, called the olive, which contains a large fold of gray matter called the inferior olivary nucleus. The inferior olivary nuclei relay ascending sensory nerve signals, especially proprioceptive information, to the cerebellum. Additionally, paired inferior cerebellar peduncles are tracts that connect the medulla oblongata to the cerebellum.
Explain the functions of the cerebellum
The cerebellum is the second largest part of the brain. It coordinates fine control over skeletal muscle actions and stores memories of movement patterns, such as the playing of scales on a piano. The cerebellum has several additional functions. Skeletal muscle activity is adjusted to maintain equilibrium and posture. It also receives proprioceptive (sensory) information from the muscles and joints and uses this information to regulate the body’s position.
Explain how the cerebellum and cerebral regions control and modify motor programming to produce somatic motor movement.
Voluntary movements
The primary motor cortex and the basal nuclei in the forebrain send impulses through the nuclei of the pons to the cerebellum.
Assessment of voluntary movements
Proprioceptors in skeletal muscles and joints report degree of movement to the cerebellum.
Integration and analysis
The cerebellum compares the planned movements (motor signals) against the results of the actual movements (sensory signals).
Corrective feedback
The cerebellum sends impulses through the thalamus to the primary motor cortex and to motor nuclei in the brainstem.
Describe the main functions of the limbic system
The limbic system is composed of multiple cerebral and diencephalic structures that collectively process and experience emotions. Thus, the limbic system is sometimes referred to as the emotional brain.
Describe the function of the reticular formation
Projecting vertically through the core of the midbrain, pons, and medulla is a loosely organized mass of gray matter called the reticular formation. The reticular formation extends slightly into the diencephalon and the spinal cord as well. This func- tional brain system has both motor and sensory components.
The motor component of the reticular formation communicates with the spinal cord and is responsible for regulating muscle tone. This motor component also assists in autonomic motor functions, such as respiration, blood pressure, and heart rate, by working with the autonomic centers in the medulla and pons.
The sensory component of the reticular formation is responsible for alerting the cerebrum to incoming sensory information. This sensory component is called the reticular activating system (RAS), and it contains sensory axons that project to the cerebral cortex. The RAS processes visual, auditory, and touch stimuli and uses this infor- mation to keep us in a state of mental alertness. Additionally, the RAS arouses us from sleep.
Consciousness includes an awareness of sensation, voluntary control of motor activities, and the activities necessary for higher mental processing.
Describe how an electroencephalogram examines brain activity
An electroencephalogram (EEG) is a diagnostic test where electrodes are attached to the head to record the electrical activity of the brain. This procedure is performed to investigate sleep disorders and lesions, and to determine if an individual is in a coma or a persistent vegetative state. EEGs also may evaluate a seizure, which is an event of abnormal electrical activity in the brain. Epilepsy is the condition where a person experiences repeated seizures over time.
An EEG measures and plots four types of brain waves (i.e., alpha, beta, theta, and delta). The distribution and frequency of these waves vary, depending upon whether the person is a child or an adult and if the individual is in a deep sleep, having a seizure, or experiencing a pathologic state of consciousness. For example, alpha and beta waves are typically seen in an awake or alert state, whereas theta and delta waves are more common during sleep. The presence of theta and delta waves in an awake adult is suggestive of a brain abnormality. Each electrode attached to a person’s head will register a brain wave over that region of the head, so a patient’s EEG printout will show multiple brain waves over a period of time.
Describe the main characteristics of sleep, including comparing non-REM & REM sleep
Both types are distinguished by their EEG patterns and the absence or presence of rapid eye movements, respectively. In addition, it is during REM sleep that we have our most memorable dreams. We spend about 75% of our total sleep time in non-REM sleep, and the remaining 25% in REM sleep.
Non-REM sleep may be further subdivided into four stages. The EEG has helped scientists detect these four stages. We cycle through these non-REM stages and REM sleep multiple times throughout a normal-length sleep cycle. The different stages of non-REM sleep differ in the types of brain waves present (e.g., alpha, beta, theta, and delta) and the ease at which one may be awakened. After about 90 minutes of non-REM sleep, the first incidence of REM sleep occurs and typically lasts about 10 minutes. The body then cycles back into non-REM sleep and then a longer period of REM sleep.
Compare and contrast short-term and long-term memory and describe the parts of the brain involved with each
Short-term memory (STM) is generally characterized by limited capacity (approximately seven small segments of information) and brief duration (typically lasting less than 1 minute unless the information is rehearsed). Once information is placed into long-term memory (LTM), it may exist for limitless periods of time. So, for example, if over the weekend you practice retrieving the information from lecture and/or work with a study partner to explain a lecture concept, you likely will store the information as LTM within the cerebral lobes.
Conversion from STM to LTM is called encoding, or memory consolidation. Encoding requires the proper functioning of two components of the limbic system: the hippocampus and the amygdaloid body. The hippocampus is required for the formation of STM, whereas LTM is stored primarily in the corresponding association areas of the cerebral cortex. For example, voluntary motor activity memory is stored in the premotor cortex, whereas memory of sounds is stored in the auditory association area.
Because STM and LTM involve different anatomic structures, loss of the ability to form STM does not affect the maintenance or accessibility of LTM.
Explain the interactions of the prefrontal cortex and the limbic system inexpression of
emotions
Expression of our emotions is interpreted by our limbic system but ultimately is controlled by the prefrontal cortex. Irrespective of how we feel, this cortical region decides the appropriate way to show our feelings. Researchers have learned that many important aspects of emotion also depend upon an intact, functional amygdaloid body and hippocampus (components of the limbic system). If specific regions of either of these structures are damaged or artificially stimulated, we exhibit either deadened or exaggerated expressions of aggression, affection, anger, fear, love, pain, pleasure, or sexuality, as well as anomalies in learning and memory.
Cranial nerves I
CN I Olfactory Nerve
Sensory nerve for olfaction (smell)
CN II Optic Nerve
Sensory nerve for vision
CN III Oculomotor Nerve
Motor nerve that controls muscles that move eye, lift eyelid, change pupil diameter, change lens shape
CN IV Trochlear Nerve
Motor nerve that controls superior oblique eye muscle
CN V Trigeminal Nerve
Mixed nerve that receives somatic sensation from face; controls muscles involved in chewing
CN VI Abducens Nerve
Motor nerve that controls lateral rectus muscle that abducts eye
Cranial nerves II
CN VII Facial Nerve
Mixed nerve that controls muscles of facial expression and conducts taste sensations from tongue
CN VIII Vestibulocochlear Nerve
Sensory nerve involved in hearing and equilibrium
CN IX Glossopharyngeal Nerve
Mixed nerve that receives taste and touch from tongue; motor control of pharynx muscle
CN X Vagus Nerve
Mixed nerve that controls muscles in pharynx and larynx; conducts sensation from many viscera; major source of parasympathetic output
CN XI Accessory Nerve
Motor nerve that controls muscles of neck, pharynx
CN XII Hypoglossal Nerve
Motor nerve that controls tongue muscles
Describe the two primary functions of the spinal cord and spinal nerves.
Their first function is to provide an essential structural and functional link between the brain and the torso and limbs of the body. Sensory input is relayed from the torso and limbs to the brain, and motor output is relayed from the brain to the torso and limbs. These vital inputs and outputs are relayed along neuron pathways that are within the spinal cord and spinal nerves. Notice that both sensory input and motor output are relayed along the pathways within the spinal cord and spinal nerves.
The second important function of the spinal cord and spinal nerves is their role in spinal reflexes. These involve nervous system responses that do not require the involvement of the brain, but instead have the spinal cord as the integration center. Spinal reflexes initiate our quickest reactions to a stimulus. It is through spinal reflexes that the spinal cord exhibits some functional independence from the brain.
Trace sensory input to the spinal cord and motor output from the spinal cord
sensory input is relayed from receptors within torso and limbs to the brain
motor output is relayed from brain to effectors, or the muscles and glands of the torso and limbs.
This information is relayed along various neural pathways
List the four anatomic divisions of the white matter and describe their general composition
White matter is (1) primarily composed of myelinated axons and (2) functioning to relay nerve signals. The white matter of the spinal cord is external to the gray matter and on each side of the cord is partitioned into three distinct anatomic structural regions based upon their location within the spinal cord. Each of these regions is called a funiculus. Each posterior funiculus is white matter that lies between the posterior gray horns on the posterior side of the cord and the posterior median sulcus. The lateral funiculus is the white matter on each lateral side of the spinal cord. The anterior funiculus is composed of white matter that occupies the space on each anterior side of the cord between anterior gray horns and the anterior median fissure; the anterior funiculi are interconnected by the white commissure. The axons within each funiculus are organized into smaller structural units (or bundles of myelinated axons) called fasciculi.
Define a tract and differentiate between sensory and motor tracts.
White matter on each side of the cord can also be referred to as tracts, which have common functions. Individual tracts are either (1) sensory (or ascending) tracts, which conduct nerve signals from the spinal cord to the brain, or (2) motor (or descending) tracts, which conduct nerve signals from the brain to the spinal cord.
Differentiate between sensory pathways and motor pathways
Sensory pathways include the sensory neurons that relay sensory input to the brain. Sensory pathways are also called ascending pathways because the nerve signals are relayed from the sensory receptors superiorly to the brain. Motor pathways include the series of motor neurons that relay motor output from the brain. Motor pathways are also called descending pathways because the nerve signals are relayed from the brain inferiorly to the body’s muscles and glands.
List and explain the features common to all pathways
Most conduction pathways—whether sensory or motor—share several general characteristics:
Paired tracts. All pathways are composed of paired tracts. Thus, a pathway on one side of the CNS has a matching tract on the other side of the CNS.
Composed of two or more neurons. Most pathways are composed of a series of two or three neurons that form the pathway.
Common location of neuron cell bodies. Neuron cell bodies are located in one of three general places: the posterior root ganglion, the gray horns within the spinal cord, or nuclei within the brain along the pathway.
Common location of axons. The axons of the different neurons extend through spinal nerves, the spinal cord (as named tracts or fasciculi), and the brain.
Decussation. Most pathways include neurons that cross over, or decussate, from one side of the body to the other side at some point along the pathway—within either the spinal cord or the brain. This means that the left side of the brain receives sensory input from or initiates motor output to the right side of the body, whereas the right side of the brain receives sensory input from or initiates motor output to the left side of the body. The term contralateral is used to indicate the relationship to the opposite side. Over 90% of all neurons within pathways decussate.
Limited ipsilateral pathway. Pathways may have some neurons (about 10%) that remain on the same side of the body. The term ipsilateral is used to indicate the relationship to the same side.
Define a general sense receptor and describe its subcategories
General sense receptors are sensory receptors located throughout the body. General sense receptors are subdivided into two categories: somatic sensory (or somatosensory) receptors and visceral sensory receptors. Somatic sensory (or somatosensory) receptors are tactile receptors or proprioceptors. Tactile receptors are housed within both the skin and mucous membranes that line body cavities. These sensory receptors monitor characteristics of an object (e.g., texture). Proprioceptors are located within joints, muscles, and tendons to detect stretch and pressure relative to position and movement of the skeleton and skeletal muscles. Visceral sensory receptors are located in the walls of the viscera (internal organs) and blood vessels. They detect changes to an organ or a blood vessel (e.g., stretch).
List and describe the three type of neurons used in a sensory pathway
Sensory pathways use a series of two or three neurons to transmit nerve signals from the sensory receptors to the brain, which are the primary neuron, secondary neuron, and tertiary neuron.
∙ The primary neuron is the first neuron in the chain of neurons. The primary neuron extends from the sensory receptor to the CNS (brain or spinal cord), where it synapses with a secondary neuron.
∙ The secondary neuron is an interneuron that extends from the primary neuron to either the tertiary neuron or the cerebellum.
∙ The tertiary neuron is also an interneuron. It extends from the secondary neuron to the cerebrum (specifically, the primary somatosensory cortex of the parietal lobe). Pathways that lead to the cerebellum do not have a tertiary neuron.
List the three major somatosensory pathways and type of receptors in each
The anterolateral pathway (or spinothalamic pathway) uses a chain of three neurons to communicate with the brain about a specific stimulus. This pathway originates at tactile somatosensory receptors within both the skin and mucous membranes. This sensory input is providing information to the brain (specifically, the cerebral cortex) about crude touch and pressure as well as pain and temperature.
The posterior funiculus–medial lemniscal pathway uses a chain of three sensory neurons to communicate with the brain about a specific stimulus. This pathway originates at either of the two types of somatosensory receptors: (1) tactile receptors housed within both the skin and mucous membranes or (2) proprioceptors within joints, muscles, and tendons. This sensory input is providing information to the brain (specifically, the cerebral cortex) about discriminative touch, precise pressure, and vibration sensations from the tactile receptors of the skin and with conscious perception of the skeleton and skeletal muscles from proprioceptors.
The spinocerebellar pathway uses a chain of only two neurons to communicate with the brain about a specific stimulus. This pathway originates at proprioceptors within joints, muscles, and tendons at different locations in the body. This sensory input is providing information to the brain (specifically, the cerebellum) related to subconscious postural input, which helps in maintaining balance
Distinguish between an upper motor neuron and a lower motor neuron, based upon function and cell body location
An upper motor neuron is the first neuron in a chain of neurons. The cell body of the upper motor neuron is housed within the cerebral cortex, cerebral nuclei, or a specific nucleus within the brainstem. Axons of the upper motor neuron synapse either directly upon lower motor neurons (in direct pathways) or upon interneurons that ultimately synapse upon lower motor neurons (in indirect pathways). The upper motor neurons either excite or inhibit the activity of lower motor neurons.
∙ The lower motor neuron is the last neuron in the chain of neurons. The cell body of a lower motor neuron is housed within the anterior horn of the spinal cord. Axons of the lower motor neurons exit the spinal cord through the anterior root and project to and innervate a specific skeletal muscle. The lower motor neuron always excites the skeletal muscle fibers to contract.
Compare the functions of the direct and indirect motor pathways
DIRECT PATHWAY houses Corticobulbar tracts: Voluntary movement of cranial and facial muscles
Lateral corticospinal tracts: Voluntary movement of appendicular muscles
Anterior corticospinal tracts: Voluntary movement of axial muscles
INDIRECT PATHWAY houses
Lateral Pathway:
Rubrospinal tract: Regulates and controls precise discrete movements and tone in flexor muscles of the limbs
Medial Pathway:
Reticulospinal tract:
Controls reflexive movements related to posture and maintaining balance
Tectospinal tract: Regulates reflexive positional changes of the upper limbs, eyes, head, and neck due to visual and auditory stimuli
Vestibulospinal tract: Regulates reflexive muscular activity that helps maintain balance during sitting, standing, and walking
Describe the properties of a reflex
Reflexes are rapid, preprogrammed, involuntary responses of muscles or glands to a stimulus.
All reflexes have similar properties:
∙ A stimulus is required to initiate a reflex.
∙ A rapid response requires that few neurons are involved and synaptic delay is minimal.
∙ A preprogrammed response occurs the same way every time.
∙ An involuntary response requires no conscious intent or preawareness of the reflex activity. Thus, reflexes are usually not suppressed.
Explain the general function of a reflex
A reflex is a survival mechanism; it allows us to quickly respond to a stimulus that may be detrimental to our well-being without having to wait for the brain to process the information. Awareness of the stimulus occurs after the reflex action has been completed, in time to correct or avoid a potentially dangerous situation. (This is possible because sensory input has reached the cerebral cortex.)
List the structures involved in a reflex arc and explain the main steps in a reflex
1 A stimulus activates a sensory receptor. A sensory receptor (dendritic endings of a sensory neuron or specialized receptor cells) responds to external and internal stimuli, such as temperature, pressure, or tactile changes. Proprioceptors are sensory receptors found in muscles and tendons, and a stimulus to a proprioceptor (such as the tapping of tendon) may initiate a reflex as well.
2 The sensory neuron transmits a nerve signal to the CNS. A sensory neuron transmits a nerve signal from the receptor to the spinal cord (or brain).
3 Information from the nerve signal is processed in the integration center by interneurons.
4 The motor neuron transmits a nerve signal from the CNS to an effector. A motor neuron transmits a nerve signal from the CNS to a peripheral effector organ—a gland or a muscle.
5 The effector responds to the nerve signal from the motor neuron. An effector is a muscle or a gland that responds to the nerve signal from the motor neuron. This response is intended to counteract or remove the original stimulus.
Explain the five ways a reflex may be classified
Spinal reflex or cranial reflex. A reflex may be identified by the specific area of the central nervous system (integration center) that serves as the processing site. Spinal reflexes involve the spinal cord, whereas cranial reflexes involve the brain.
∙ Somatic reflex or visceral reflex. This classification criterion is determined by the type of effector that is stimulated by the motor neurons involved in the reflex. Somatic reflexes involve skeletal muscle as the effector. Visceral (or autonomic) reflexes involve cardiac muscle, smooth muscle, or a gland as the effector.
∙ Monosynaptic reflex or polysynaptic reflex. A reflex may also be classified by the number of neurons participating in the reflex. A monosynaptic reflex has only a sensory neuron and a motor neuron. The axon of the sensory neuron synapses directly on the motor neuron, whose axon projects to the effector. Thus, there is only one synapse between neurons.
Ipsilateral reflex or contralateral reflex. The reflex may also be classified based upon whether it involves only one side of the body. An ipsilateral reflex is a reflex in which both the receptor and effector organs are on the same side of the spinal cord. A contralateral reflex is a reflex that involves an effector on the opposite side of the body from the receptor that detected the stimulus. Note that this terminology is only applicable to reflexes that involve the limbs.
Innate reflex or acquired reflex. The reflex may be classified based upon whether you are born with it.
Name and describe four common spinal reflexes
A stretch reflex is a simple monosynaptic reflex. A stretching force detected by a muscle spindle results in the contraction of that muscle. Conversely, antagonistic muscle contraction is dampened, in a process called reciprocal inhibition.
A Golgi tendon reflex is a polysynaptic reflex. A contraction force detected by a Golgi tendon organ (within the tendon of the muscle) results in relaxation of that muscle. Conversely, antagonistic muscles are stimulated to contract, a process called reciprocal activation.
A withdrawal reflex is a polysynaptic reflex that is initiated by a painful stimulus. The crossed-extensor reflex occurs in response to the withdrawal reflex, by stimulating the extensor muscles in the opposite limb and thereby ensuring the opposite limb supports the body’s weight.
Differentiate between the SNS and the ANS
Similarities: Sensory & Motor components
Differences: SNS: Detect stimuli and transmission of nerve signals from the special senses, skin, & proprioreceptors to the CNS.
Ex. Taste dinner, see mountains, or smell a babies skin.
ANS: functions below the conscious level. Detect stimuli associated with blood vessels & internal organs. Initiate and transmit nerve signals from the CNS to cardiac, smooth muscle, and glands.
Ex. Monitor carbon dioxide concentration in the blood
Compare and contrast lower motor neurons in the SNS and ANS
Somatic Nervous System
Number of neurons in pathway
One neuron from CNS: Somatic motor neuron axon extends from CNS to effector
Axon properties
Myelinated and thicker in diameter; fast nerve signal propagation
Neurotransmitter released
Acetylcholine (ACh)
Response of effector
Excitation only
Ganglia associated with motor neurons: None
Autonomic Nervous Systems
Number of neurons:
Two neurons from CNS: Preganglionic neuron has preganglionic axon that projects to ganglionic neuron; ganglionic neuron has postganglionic axon that projects to effector
Axon properties
Preganglionic axons are myelinated and small in diameter
Postganglionic axons are unmyelinated and smaller in diameter; both have relatively slow nerve signal propagation
Neurotransmitter released
Preganglionic axons release ACh
Postganglionic axons release either ACh or norepinephrine (NE)
Response of effector
Either excitation or inhibition
Ganglia associated
Parasympathetic division: terminal ganglia, intramural ganglia Sympathetic division: sympathetic trunk ganglia, prevertebral ganglia
Describe how the two-neuron chain in the ANS facilitates communication and control
The autonomic nervous system employs a chain of two lower motor neurons, a preganglionic neuron and a ganglionic neuron. The dendrites and cell body of a preganglionic neuron are housed within the CNS (brain or spinal cord). The preganglionic axon synapses with a ganglionic neuron within an autonomic ganglion. The postganglionic axon extends to an effector organ, which includes cardiac muscle, smooth muscle, and glands.
Describe the CNS hierarchy that controls the autonomic nervous system
Autonomic function is regulated by three CNS regions: the hypothalamus, brainstem, and spinal cord. These CNS regions may be influenced by the cerebrum, thalamus, and limbic system.
Hypothalamus: Integration and command center for autonomic functions; involved in emotions
Brainstem: Contains major ANS reflex centers
Spinal Cord: Contains ANS reflex centers for defecation and urination
