Chapter 3: Nervous System Organization Flashcards
hemorrhagic stroke
A severe stroke that results from a burst vessel bleeding into the brain.
Tissue plasminogen activator (t-PA)
A drug for treating ischemic stroke that breaks up clots and allows the return of normal blood flow to the affected region if administered within 3 hours.
ischemic stroke
An ischemic stroke occurs when the blood supply to part of the brain is interrupted or reduced, preventing brain tissue from getting oxygen and nutrients. Brain cells begin to die in minutes.
nuclei
A spherical structure in the soma of a cell; contains DNA and is essential to cell function; also, a cluster of cells that can be identified histologically and has specific functions in mediating behavior.
- Neurons are organized in either layers or in clusters called nuclei
fiber pathways, or tracts
Tracts (fiber pathways) are large collections of axons projecting toward or away from a nucleus or layer in the (central nervous system) CNS i.e., a large collection of axons coursing together within the CNS.
- Within layers and nuclei, cells that are close together make the most of their connections with one another, but they also make long-distance connections, forming distinctive fiber pathways, or tracts.
Neuron
A neuron, also known as a nerve cell, is a specialized type of cell that transmits information throughout the body.
- an essential part of the nervous system
- responsible for transmitting information in the form of electrical and chemical signals between different parts of the body, including the brain and the muscles.
Glial Cell
Glial cells, also known as neuroglia or glia, are non-neuronal cells that provide support and protection for neurons in the nervous system. There are several different types of glial cells, including:
- astrocytes,
- oligodendrocytes
- microglia
Astrocyte
Astrocytes are the most abundant type of glial cell and are found throughout the central nervous system (CNS). They provide structural support for neurons, help to regulate the chemical environment around neurons, and provide nutrients and support to developing neurons.
Oligodendrocytes
Oligodendrocytes are found in the CNS and are responsible for producing the myelin sheath, a protective layer that surrounds the axons of neurons and helps to insulate them. This helps to increase the speed of neural transmission.
Microglia
Microglia are the immune cells of the CNS and are responsible for protecting the brain and spinal cord from infection and injury. They also play a role in clearing away dead or damaged cells.
Neuroplasticity/brain plasticity/neural plasticity
Refers to the ability of the brain to reorganize and modify its connections in response to changes in the environment or experiences. This process allows the brain to adapt and learn new things throughout life.
- occurs at all stages of life, but is especially pronounced during childhood and adolescence
- plays a key role in learning, memory, and the ability to adapt to new situations.
Inferior
away from the head, below, toward the feet; lower (example, the foot is part of the inferior extremity).
caudal
Toward the back of the brain or the bottom of the spinal cord
Anterior
In front of; toward the face (example, the kneecap is located on the anterior side of the leg)..
Ventral
Toward the bottom of the brain or the front of the spinal cord
Posterior
Behind; toward the back (example, the shoulder blades are located on the posterior side of the body).
Superior
Above; toward the head
Dorsal
Toward the top of the brain or the back of the spinal cord
Medial
toward the midline of the body (example, the middle toe is located at the medial side of the foot).
Lateral
away from the midline of the body, toward the edge (example, the little toe is located at the lateral side of the foot).
Proximal
toward or nearest the trunk or the point of origin of a part (example, the proximal end of the femur joins with the pelvic bone).
Distal
away from or farthest from the trunk or the point or origin of a part (example, the hand is located at the distal end of the forearm).
Rostral
Toward the front of the brain or the top of the spinal cord
A coronal section/cut reveals
A vertical plane running from side to side; divides the body or any of its parts into anterior and posterior portions; you should be able to see both lobes/right and left hemispheres - sliding the brain front to back
A horizontal/transverse section/cut reveals
A horizontal plane; divides the body or any of its parts into upper and lower parts; a dorsal view, looking down on the brain from above; should see two hemispheres
A sagittal section/cut reveals
A vertical plane running from front to back; divides the body or any of its parts into right and left sides; revealing a medial view, from the side; only seeing one hemisphere at a time
- mid sagittal slice includes (slice directly in the middle): corpus callosum - very white structure due to white matter, you should see regions of brain stem
ipsilateral
Residing in the same side of the body as the point of reference; structure on the same side
contralateral
Residing in the side of the body opposite the reference point; structures on the opposite side.
- the motor system is contralateral; left hemisphere controls right side of body
- contralesional
bilateral
Applying to both sides of the body; structures that lie in both hemispheres are bilateral
Proximal
Close to some point.
Distal
Distant from some point.
Afferent
Conducting toward a central nervous system area.
- sensory signals
Efferent
Conducting away from the central nervous system area and toward a muscle or gland.
- motor signals
Precentral Gyrus
It is the gyrus lying in front of the central sulcus. Also called M1 or primary motor cortex, or “somatomotor strip,” or “the motor homunculus” or “area pyramidalis.”
The precentral gyrus, also known as the primary motor cortex, is a region of the brain that is located in the frontal lobe. It is responsible for controlling voluntary movement and is an important part of the motor system.
The precentral gyrus is organized somatotopically, meaning that different areas of the gyrus control different parts of the body. The part of the precentral gyrus that is closest to the midline of the brain controls the muscles of the face and mouth, while the part of the gyrus that is further from the midline controls the muscles of the arms and legs.
In addition to controlling movement, the precentral gyrus also plays a role in the planning and execution of movements. It receives input from other brain areas, including the somatosensory cortex and the basal ganglia, and it sends output to the spinal cord, where it activates the muscles needed to produce movement.
Pyramidal (corticospinal) tract
The pyramidal (corticospinal) tract is a group of nerve fibers that carries information from the brain to the muscles of the body. It is an important part of the motor system and is responsible for controlling voluntary movement.
The pyramidal tract begins in the motor cortex, specifically the precentral gyrus (also known as the primary motor cortex), which is located in the frontal lobe of the brain. From there, the fibers of the pyramidal tract descend through the brainstem and the spinal cord, eventually terminating in the muscles of the body. The fibers of the pyramidal tract are arranged in a pyramid-like shape as they travel through the brainstem, which is why it is also known as the pyramidal tract.
The pyramidal tract is also known as the corticospinal tract because it carries information from the cortex of the brain (the corticospinal fibers) to the muscles of the body. It is called the pyramidal tract because the nerve fibers are arranged in a pyramid-like shape as they travel through the brainstem.
The pyramidal tract is an essential part of the motor system, and it is responsible for transmitting information from the brain to the muscles, enabling the voluntary control of movement.
Precentral gyrus: sources of input
Input into the precentral gyrus comes from a number of different sources, including other brain areas and sensory receptors in the body.
- somatosensory cortex
- basal ganglia
- cerebellum
One important source of input to the precentral gyrus is the somatosensory cortex, which is located in the parietal lobe of the brain. The somatosensory cortex processes information about touch, temperature, and other sensations from the skin and muscles, and it sends this information to the precentral gyrus to help guide movement.
The precentral gyrus also receives input from the basal ganglia, which are a group of structures located deep within the brain that play a role in the planning and execution of movement. In addition, the precentral gyrus receives input from the cerebellum, which is a structure located at the base of the brain that is involved in the coordination and regulation of movement.
Overall, the precentral gyrus receives input from a variety of sources in the brain and the body, which it uses to coordinate and control voluntary movement.
Precentral gyrus: output
Output from the precentral gyrus is transmitted through a group of nerve fibers called the pyramidal (corticospinal) tract, which carries information from the brain to the muscles of the body.
Somatic (body) Nervous System (SNS)
The SNS is made up of the sensory neurons, which transmit information from the body’s senses (such as touch, sight, and hearing) to the brain and spinal cord, and the motor neurons, which transmit information from the brain and spinal cord to the muscles and organs of the body. The SNS is also responsible for controlling the body’s reflexes, which are automatic responses to stimuli that do not require conscious thought.
The SNS is distinct from the autonomic (involuntary) nervous system (ANS), which is responsible for controlling the body’s involuntary functions, such as heart rate, digestion, and respiration.
The Somatic Nervous System (SNS) is comprised of:
Spinal & cranial nerves.
Spinal and cranial nerves interact with one another within the SNS to transmit sensory information from the body to the brain and to control the body’s voluntary movements. Sensory information is transmitted from the body to the brain through the spinal and cranial nerves, and motor commands are transmitted from the brain to the muscles and organs of the body through the same nerves.
- motor neurons
- cranial nerves (don’t need to know numbers or functions)
Overall, spinal and cranial nerves work together within the SNS to transmit sensory information and control the body’s voluntary movements.
Spinal Nerves
Spinal nerves are a type of nerve that arises from the spinal cord and innervates (supplies nerves to) the muscles and organs of the body. There are 31 pairs of spinal nerves in the body, and each one is responsible for innervating a specific part of the body.
Cranial Nerves
A set of 12 pairs of nerves that convey sensory and motor signals to and from the head.
Cranial nerves are a type of nerve that arises from the brain and innervates the muscles and organs of the head and neck. There are 12 pairs of cranial nerves in the body, and each one is responsible for innervating a specific part of the head and neck.
- (don’t need to know numbers or functions)
The peripheral nervous system (PNS) is comprised of:
The somatic (body) nervous system (SNS): divided in cranial and spinal nerves
The autonomic (automatic) nervous system (ANS): is divided into parasympathetic and sympathetic. - HYPOTHALAMUS, HORMONAL GLANDS, PITUAITY GLAND
parasympathetic (calming) nerves
Calming nerves of the autonomic nervous system that enable the body to “rest and digest.” Compare sympathetic nerves.
- the parasympathetic system connects with parasympathetic ganglia near the target organs
The parasympathetic system calms the body—for example, by slowing the heartbeat and stimulating digestion to allow us to “rest and digest” after exertion and during quiet times.
sympathetic (arousing) nerves
Arousing nerves of the autonomic nervous system that enable the body to “fight or flee” or engage in vigorous activity. Compare parasympathetic nerves.
- The sympathetic system is composed of a chain of ganglia that run parallel to the spinal cord, connecting the spinal cord to body organs.
The sympathetic system arouses the body for action—for example, by stimulating the heart to beat faster and inhibiting digestion when we exert ourselves during exercise or times of stress, bodily changes associated with the “fight-or-flight” response
The brain and spinal cord are supported and protected from injury and infection in four ways:
The brain is enclosed in a thick bone, the skull, and the spinal cord is encased in a series of interlocking bony vertebrae.
- the inner surface of the skull is not completely smooth (front is jagged); having skull is better than not having
central nervous system (CNS)
The central nervous system (CNS) is the part of the nervous system that consists of the brain and the spinal cord (both encased in bone). It is the control center of the body, and it is responsible for receiving and processing information from the body’s senses, issuing commands to the muscles and organs of the body, and coordinating the body’s movements and functions.
peripheral nervous system (PNS)
The peripheral nervous system (PNS) is the part of the nervous system that consists of all the nerves that lie outside the central nervous system (CNS), which includes the brain and the spinal cord. The PNS is responsible for transmitting information between the CNS and the rest of the body, including the muscles, organs, and sensory receptors.
The PNS is divided into two main divisions: the somatic (body) nervous system (SNS) and the autonomic (involuntary) nervous system (ANS). The SNS is responsible for transmitting sensory information from the body to the brain and for controlling the body’s voluntary movements. The ANS is responsible for controlling the body’s involuntary functions, such as heart rate, digestion, and respiration.
The brain and spinal cord are supported and protected from injury and infection in four ways:
- The brain is enclosed in a thick bone, the skull, and the spinal cord is encased in a series of interlocking bony vertebrae.
- Within the bony case enclosing the CNS is a triple-layered set of membranes, the meninges.
- The brain and spinal cord are cushioned from shock and sudden pressure changes by the cerebrospinal fluid (CSF), which circulates through the brain’s four ventricles, the spinal column, and within the subarachnoid space in the brain’s meninges.
- The blood–brain barrier protects the brain and spinal cord by limiting the movement of chemicals from the rest of the body into the CNS and by protecting it from toxic substances and infection.
meninges
Three layers of protective tissue — dura mater, arachnoid, and pia mater — that encase the brain and spinal cord.
Outer meninges
The outer dura mater (Latin for “hard mother”) is a tough double layer of tissue enclosing the brain in a kind of loose sack.
Middle meninges
The middle arachnoid membrane (from the Greek, meaning “resembling a spider’s web”) is a very thin sheet of delicate tissue that follows the brain’s contours.
Inner meninges
The inner pia mater (Latin for “soft mother”) is a moderately tough tissue that clings to the brain’s surface.
hydrocephalus (“water brain”)
A buildup of pressure in the brain and, in infants, swelling of the head, caused by blockage in the flow of cerebrospinal fluid; can result in intellectual disabilities.
Blood-brain barrier
Tight junctions between capillary cells block entry of an array of substances, including toxins, into the brain. Glial cells called astroglia stimulate the cells of capillaries—minute blood vessels—to form tight junctions with one another, thus preventing many blood-borne substances from crossing from the capillaries into the CNS tissues.
anterior cerebral artery (ACA)
A vessel originating from the carotid artery that irrigates the medial and dorsal parts of the cortex, including the orbitofrontal and dorsolateral frontal regions, anterior cingulate cortex, corpus callosum, and striatum.
middle cerebral artery (MCA)
An artery that runs along the length of the lateral (Sylvian) fissure to irrigate the lateral surface of the cortex, including the ventral part of the frontal lobe, most of the parietal lobe, and the temporal lobe.
posterior cerebral artery (PCA)
A vessel that irrigates the ventral and posterior surfaces of the cortex, including the occipital lobe and hippocampal formation.
neural stem cell (a germinal cell)
A self-renewing, multipotential cell that gives rise to any of the different types of neurons and glia in the nervous system.
Process of Brain-Cell Origin/Renewal
Through a four-step process, as indicated by the colors in the figure, brain cells
(1) begin as multipotential stem cells, which
(2) become progenitor cells, which
(3) become blasts, which
(4) finally develop into specialized neurons and glia.
Adult stem cells line the brain’s subventricular zone, which surrounds the ventricles, and are also located in some brain areas, in the spinal cord, and in the retina of the eye.
progenitor cells
A precursor cell derived from a stem cell that migrates and produces a neuron or glial cell. Also called precursor cell, which migrate and act as precursor cells, giving rise to nondividing primitive nervous system cell types called blasts.
Three basic types of neurons from different parts of the nervous system:
- Sensory neurons
- Interneurons
- Motor neurons
What is the difference between sensory receptors and sensory neurons?
Sensory neurons are responsible for transmitting sensory information from the body to the brain, while sensory receptors are specialized cells that detect stimuli and send messages about these stimuli to the brain through sensory neuron.
For example, the eyes contain sensory receptors called photoreceptors that are sensitive to light, and the ears contain sensory receptors called hair cells that are sensitive to sound. When a stimulus activates a sensory receptor, it sends a message through the sensory neuron to the brain, which processes the information and generates a response.
Sensory Receptor
A cell that transduces sensory information into nervous system activity. Sensory receptors are specialized cells that are responsible for detecting stimuli, such as light, sound, and touch, and for sending messages about these stimuli to the brain through sensory neurons. Sensory receptors are located throughout the body and are specialized to detect different types of stimuli.
Bipolar Neuron
Bipolar neurons are characterized by their bipolar shape, with one process extending from each end of the cell body. They play an important role in the functioning of the nervous system by transmitting information from one type of neuron to another—neurons with processes at both poles, characteristic especially of retinal cells.
Somatosensory Neuron
A neuron that projects from the body’s sensory receptors into the spinal cord; the dendrite and axon are connected, which speeds information conduction because messages do not have to pass through the cell body.
Interneuron
Any neuron lying between a sensory neuron and a motor neuron.
Motor Neuron / “Motor Unit”
Charles Scott Sherrington’s term for the unit formed by motor neurons and the muscle fibre to which their axon terminations are connected. The motor unit refers to the functional unit formed by a motor neuron and the muscle fibres that it innervates. The axon of a motor neuron extends from the cell body and terminates at a muscle fibre, forming a synapse (a specialized junction) between the two. When the motor neuron is activated, it sends a signal across the synapse to the muscle fibre, causing it to contract.
A motor neuron is a type of neuron that is responsible for transmitting information from the central nervous system (CNS) to muscles and organs. Motor neurons are part of the peripheral nervous system (PNS) and are responsible for transmitting information from the CNS to the muscles and organs of the body.
Motor neurons have a cell body and two processes: axons and dendrites. The axon is a long, single process that extends from the cell body and transmits information to muscles and organs. The dendrite is a short, branching process responsible for receiving information from other neurons.
Types of Glial Cells
- Ependymal cell
- Astrocyte
- Microglial cell
- Oligodendroglial cell
- Schwann cell
Ependymal cells
Glial cells that make and secrete cerebrospinal fluid and form the lining of the ventricles - line the brain’s ventricles and make CSF.
Astroglia (singular astrocyte, star-shaped glia)
A star-shaped glial cell that provides structural support to neurons in the central nervous system and transports substances between neurons and blood vessels. Play a role in the blood–brain barrier, and provide structural support and nutrition to neurons.
- Star-shaped, symmetrical; nutritive and support function
Microglia (tiny glia)
Glial cells that originate in the blood aid in cell repair, and scavenge debris in the nervous system - fight infection and remove debris.
- Small, mesodermally derived; defensive function
Oligodendroglia (singular oligodendrocyte, glia with few branches)
Glial cells in the central nervous system myelinate axons - insulate neurons in the CNS
- Asymmetrical; forms insulating myelin around axons in brain and spinal cord
Schwann cells
Glial cells in the peripheral nervous system that myelinate sensory and motor axons. insulate sensory and motor neurons in the PNS. This insulation is called myelin.
- Asymmetrical; wraps around peripheral nerves to form insulating myelin
myelin
A lipid substance that forms an insulating sheath around certain nerve fibers; formed by oligodendroglia in the central nervous system and by Schwann cells in the peripheral nervous system.
Gray Matter
Any brain area composed predominantly of cell bodies and capillaries.
White Matter
Areas of the nervous system rich in fat-sheathed neural axons that form the connections between brain cells. The axons are myelinated (insulated) by oligodendrocytes and Schwann cells that are composed of the same fatty substance (lipid) that gives milk its white appearance. As a result, areas of the brain that consist of axon pathways appear white
Reticular matter (from the Latin rete, meaning “net”)
contains a mixture of cell bodies and axons from which it acquires its mottled gray and white, or netlike, appearance.
- A mixture of nuclei and fibers that runs through the center of the brainstem, extending from the spinal cord to the thalamus; associated with sleep–wake behavior and behavioral arousal. Also called the reticular activating system.
- the function of the reticular formation is to control sleeping and waking and to maintain “general arousal,” or “consciousness.” As a result, the reticular formation came to be known as the reticular activating system. Damage to this area can result in permanent unconsciousness.
- The mid-regions of the brainstem are referred to as reticular matter.
ganglia (singular, ganglion)
A collection of nerve cells that function somewhat like a brain. Well-defined groups of cell bodies in the CNS form either layers or nuclei (clusters). Within the PNS, such clusters are called ganglia.
Steps in Brain Development
The development of the brain goes through several stages as an embryo grows and develops into a fetus. During these early stages of development, the brain is divided into three primary regions: the forebrain, midbrain, and hindbrain. These three regions are collectively known as the “three-chambered brain.”
- Here, the spinal cord is considered part of the hindbrain.
- The adult brain of a fish, an amphibian, or a reptile is roughly equivalent to this three-part brain.
As development continues, the three-chambered brain becomes more complex and is divided into five primary regions: the forebrain, midbrain, hindbrain, and two additional regions called the cerebral hemispheres. This five-chambered brain structure is more similar to the adult brain and is present in the fetus by the end of the first trimester of pregnancy.
ventricles (“bladders”)
A cavity of the brain that contains cerebrospinal fluid. Four prominent pockets of the hollow region are called ventricles (“bladders”), numbered first through fourth. The “lateral ventricles” (first and second) form C-shaped lakes underlying the cerebral cortex, whereas the third and fourth ventricles extend as a channel into the brainstem and spinal cord.
Thalamus (relay center)
- sensory information will route to the thalamus first; nuclei in the thalamus will process information and decide where the information should be sent.
- A group of nuclei in the diencephalon that integrates information from all sensory systems and projects it into the appropriate cortical regions.
Inferior Colliculus (ears - “lower hills”)
Nuclei of the midbrain tectum that receive auditory projections and mediate orientation to auditory stimuli.
- receive projections from the auditory receptors of the ear
Superior Colliculus (eyes - “upper hills”)
Bilateral nuclei of the midbrain tectum that receive projections from the retina of the eye and mediate visually related behavior.
- a reflexive response to visual stimulation
Limbic System
Disparate forebrain structures lying between the neocortex and the brainstem that form a functional system controlling affective and motivated behaviors and certain forms of memory; includes cingulate (limbic) cortex, amygdala, hippocampus, and hypothalamus, among other structures. Also called reptilian brain; formerly, limbic lobe.
- plays a role in self-regulatory behaviors including emotion, personal memories, spatial behavior, and social behavior.
Includes:
- Amygdala: Emotion & species-typical behaviours
- Hippocampus: Memory & spatial navigation
- Septum: Emotion & species-typical behavior
- Cingulate Cortex (cingulate gyrus): Emotion, cognition, executive function, motor control, response inhibition
Amygdala (“almond”)
An almond-shaped collection of nuclei in the base of the temporal lobe; the part of the limbic system that participates in emotional and species-typical behaviors.
- nuclei in the base of the temporal lobe that participate in emotion
Cingulate (“girdle”) Cortex
A strip of three- to four-layered limbic cortex that lies just above the corpus callosum along the medial walls of the cerebral hemispheres.
- is involved in sexual behavior, among other social interactions, and decision making, or executive function.
- above the corpus callosum
- cortical tissue
Layering in the Neocortex
- 6 layers in total; different functions/cells are organized differently
- layers 5 & 6: sending output to other regions of the brain (efferent); some tracts may go to the thalamus
- layers 1, 2, &3: have integrative functions
- layer 4: sensory input (afferent) - receiving input/sensory information
- layers may differ depending on the region in the brain
- motor cortex needs sensory information/feedback; therefore still needs to have layer 4
- myelination differs within the layers
Fissure
A cleft is called a fissure if it extends deeply enough into the brain to indent the ventricles, as do the longitudinal and lateral fissures; it is called a sulcus (plural, sulci) if it is shallower.
- A cleft in the cortex deep enough to indent ventricles
- increases the surface area
Figure 3.24C illustrates the main sulci and fissures on the lateral surface of the cortex, and Figure 3.24D locates some of the main sulci and fissures on the medial surface.
Sulcus (sulci)
A shallow cleft in the cortex
- The complexity of sulci increased throughout the evolution
- increases the surface area
Figure 3.24C illustrates the main sulci and fissures on the lateral surface of the cortex, and Figure 3.24D locates some of the main sulci and fissures on the medial surface.
Gyrus (gyri)
Ridge in cortex
- primary motor cortex
The major gyri on the outer surface of the neocortex are shown in Figure 3.24A, and those on the medial surface are shown in Figure 3.24B. Note that the cingulate gyrus, part of the limbic system located just above the corpus callosum, spans the inner surface of all four neocortical lobes.
Organization of the Cortex
Regions within each of the four lobes are designated as primary, secondary, or tertiary.
- Frontal lobe - Motor functions - CEO of te brain
- Parietal lobe - Body senses
- Temporal lobe - Auditory functions
- Occipital lobe - Visual functions - is the only one that focuses on one aspect of sensory input (vision)
Four types of axon projections
- Long connections between one lobe & another
- Shorter connections between one part of lobe & another
- Interhemispheric connections
- Commissures (largest is the corpur callosum)
- Homotopic points
*Connections through the thalamus
Projection Map
A map of the cortex made by tracing axons from the sensory systems into the brain and from the neocortex to the motor systems of the brainstem and spinal cord.
- shows locations on the cortex that process various types of sensory information and those that produce movement. Because of these specialized regions, each cortical lobe is associated with a specific sense or with movement: vision in the occipital lobe, audition in the temporal, body senses in the parietal, and motor functions in the frontal. This arrangement makes the posterior cortex (parietal, temporal, and occipital lobes) largely sensory and the anterior cortex (frontal lobe) largely motor.
Image: Projection Map - Primary areas receive input from the sensory systems or project to spinal motor systems. Secondary areas interpret sensory input or organize movement. Black arrows indicate information flows from primary to secondary sensory areas and from secondary motor areas to primary motor areas. Information also flows from secondary to higher-order association, or tertiary, areas and among association areas of the four cortical lobes.
Broadman Areas
Brains cellular organization was different depending on the area in the brain; later on additions were made (names) that revelaed the functions of these areas
17 - V1
18 - V2
4 - Primary
Spinal Cord
The spinal cord lies inside the bony spinal column, a series of vertebrae categorized into five anatomical regions from top to tail: cervical (C), thoracic (T), lumbar (L), sacral (S), and coccygeal.
dermatome (“skin cut”)
A body segment corresponding to a segment of the spinal cord.
There is a segmental organization within the human body. Each body segment forms a ring, or dermatome (“skin cut”), that encircles the spinal column.
Mammalian limbs evolved perpendicular to the spinal cord to accommodate their four-legged stance, but humans’ upright posture, on two legs, distorts the dermatomes in the arms and legs.
Spinal Nerve Connections
Cross section of the spinal cord, anterior view, illustrates a sensory neuron bringing in information in the posterior root and a motor neuron that commands muscle movements in the anterior root.
- Collateral branches of sensory fibers allow sensory information to influence movement at the level of the spinal cord. Collaterals cross to the cord’s far side to influence motor neurons on that side and may extend to adjacent spinal segments to influence adjacent body parts.
- Inner regions of the spinal cord consist of neuronal cell bodies (gray matter); outer regions consist of tracts (white matter) traveling to and from the brain.
posterior/dorsal root
A nerve composed of fibers carrying sensory information that enters each segment of the posterior spinal cord.
- Afferent Arrives: Afferent neurons carry information from sensory receptors of the skin and other organs to the central nervous system (i.e., brain and spinal cord)
- Fibers entering the posterior root bring sensory information from sensory receptors.
- cutting the dorsal roots caused loss of sensation
anterior/ventral root
A nerve composed of fibers carrying motor information from the anterior part of the human spinal cord.
- Efferent Exits: efferent neurons carry motor information away from the central nervous system to the muscles and glands of the body.
- Fibers leaving the anterior root carry motor information to the muscles.
- cutting the ventral roots caused loss of movement
The Brainstem
The brainstem begins where the spinal cord enters the skull and extends upward into the lower areas of the forebrain.
- main regions: the midbrain and the hindbrain.
- cranial-nerve nuclei are located in the brainstem and send their axons to the head muscles.
- mediate a variety of regulatory functions.
-bundles of sensory nerve fibers from the spinal cord pass through posterior regions of the brainstem on their way to the forebrain, and motor fibers from the forebrain pass through anterior regions of the brainstem on their way to the spinal cord.
Cerebellum
A major structure of the hindbrain that is specialized for learning and coordinating skilled movements. In large-brained animals, may also participate in coordinating other mental processes.
- The cerebellum plays a role in motor coordination and motor learning and integrates motor functions with mental processes. Damage to it results in equilibrium problems, postural defects, and impairments of skilled motor activity.
- It protrudes above the brainstem core, and its surface is gathered into narrow folds, or folia, like the gyri and sulci of the cortex but smaller
- In primates, it contains about four times more neurons than the cerebral cortex and about 80% of all the brain’s cells
Colliculi
Behaviors mediated by the colliculi include locating objects in surrounding space and orienting to those objects, be they visual or auditory. These structures also mediate simple pattern recognition of visual stimuli or sound stimuli.
Thalamic Projections
- One group of thalamic nuclei relays information from sensory systems to their appropriate targets. For example, the lateral geniculate body (LGB) receives visual projections; the medial geniculate body (MGB) receives auditory projections; and the ventrolateral posterior nuclei (VLP) receive touch, pressure, pain, and temperature projections from the body. In turn, these areas project to the visual, auditory, and somatosensory regions of the cortex.
- Some thalamic nuclei relay information between cortical areas. For example, visual areas of the cortex interconnect with other brain regions through the pulvinar nucleus (P).
- Some thalamic nuclei relay information between the cortex and a number of brainstem regions.
(A) Black arrows indicate the sources of input and output from major thalamic nuclei: anterior nucleus, A; dorsomedial nucleus, DM; ventral anterior nucleus, VA; ventrolateral nucleus, VL; lateral posterior nucleus, LP; ventrolateral posterior nucleus, VLP; pulvinar, P; lateral geniculate body, LGB; and medial geniculate body, MGB. (B) The cortical areas to which the major thalamic nuclei diagrammed in part (A) project.
Basal Ganglia (“lower knots,” referring to “knots below the cortex”)
Subcortical forebrain nuclei (caudate nucleus, putamen, globus pallidus) that connect to the thalamus and midbrain and coordinate voluntary movements of the limbs and body.
- include the putamen (“shell”), the globus pallidus (“pale globe”), and the caudate nucleus (“tailed nucleus”).
- The basal ganglia perform three main functions: they (1) connect sensory regions of the cortex to motor regions of the cortex, (2) regulate movement so that it is fluid, and (3) are involved in associative learning, learning by which one stimulus or event is associated with another, as is necessary to coordinate sensory and motor skills
- People with disorders of the basal ganglia can have difficulty performing such stimulus–response actions. Many of our bad habits acquired through associative learning involve the basal ganglia, including addictions to drugs, gambling, or food.
Frontal Lobe
The frontal lobes are bounded posteriorly by the central sulcus, inferiorly by the lateral fissure, and medially by the cingulate sulcus—the limbic cortex
Primary Areas
Neocortical regions that receive projections from the major sensory systems or send projections to the muscles.p
- Primary areas also extend down into the cortical gyri and fissures. Much of the auditory zone, for example, is located within the lateral fissure. The motor cortex sends projections to brainstem and spinal-cord motor systems.
Secondary Areas
A cortical region that receives inputs from the primary areas and is thought to participate in more complex sensory and perceptual or motor functions. Also called secondary projection area.
located adjacent to primary areas and are interconnected with them. Secondary areas are involved in elaborating information received from primary areas or, in the case of the primary motor area, sending commands to it. In vision, for example, different secondary areas are involved in visual aspects that include color, movement, and form.
Tertiary Areas / Association Areas
Cortical regions that receive projections from secondary areas or send projections to them; encompasses all cortex not specialized for sensory or motor function and mediates complex activities such as language, planning, memory, and attention. Also known as association cortex.
Cortical Function
The organization depicted in the Figure 3.25 projection map allows for a simple representation of cortical function. Sensory information enters the primary areas and then is passed to the secondary areas, each of which has a specific sensory-related function (e.g., color, form, and motion for vision, and music, words, and location for audition). The tertiary area in the posterior neocortex receives projections from the secondary areas and forms more complex associations, including ideas or concepts through which we represent the world. This information is then passed on to frontal tertiary areas, where it can be formulated into plans of action that may then be performed by the frontal cortex secondary and primary areas, respectively.
Cellular Organization in the Cortex
Neurons in the neocortex are arranged in six layers. Regional differences among the six layers include the shapes, sizes, and connections of cells. The layers’ functions relate to information input and output.
Outer layers I, II, and III receive input
mainly from other cortical areas and are well developed in the secondary and tertiary areas of the cortex to perform their integrative functions.
Axons in layer IV receive input from
sensory systems and other cortical areas. This layer features large numbers of stellate neurons, small, densely packed cells in the primary areas of vision, body senses, audition, and taste–olfaction that receive large projections from their respective sensory organs. Cortical areas rich in layer IV neurons are also called granular cortex, referring to their grainy appearance.
Inner cortical layers V and VI send
axons to other brain areas. Both layers and the pyramidal neurons that compose them are particularly large and distinctive in the motor cortex, which sends projections to the spinal cord. (Large size is typical of cells that send information over long distances.)
Brodmann’s map
A map of the cerebral cortex devised by Korbinian Brodmann circa 1905 and based on cytoarchitectonic structure with anatomical areas identified by number; conforms remarkably closely to functional areas identified by the results of later lesion and recording studies.
- the numbers of Brodmann’s map have no special meaning.
Cortical Connections
There are four types of axon projections that interconnect neocortical regions:
- Long connections between one lobe and another (Figure 3.28A)
- Relatively short connections between one part of a lobe and another (Figure 3.28B)
- Interhemispheric connections (commissures) between one hemisphere and the other (Figure 3.28C)
- Connections through the thalamus
Most interhemispheric connections link homotopic points in the two hemispheres—that is, points that correspond to each other in the brain’s mirror-image structure.
