INTRODUCTION TO THE NERVOUS SYSTEM
The nervous system is the major controlling, regulatory, and communicating system in the body. It is the center of all mental activity, including thought, learning, and memory. Together with the endocrine system, the nervous system is responsible for regulating and maintaining homeostasis. Through its receptors, the nervous system keeps us in touch with our environment, both external and internal.
Like other systems in the body, the nervous system is composed of organs, principally the brain, spinal cord, nerves, and ganglia. These, in turn, consist of various tissues, including nerve, blood, and connective tissues. Together these carry out the complex activities of the nervous system.
FUNCTIONS OF THE NERVOUS SYSTEM  
ORGANIZATION OF THE NERVOUS SYSTEM
There is really only one nervous system in the body, although terminology seems to indicate otherwise. Although each subdivision of the system is also called a “nervous system,” all of these smaller systems belong to the single, highly integrated nervous system. Each subdivision has structural and functional characteristics that distinguish it from the others. The nervous system as a whole is divided into two subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS) (Figure 9-1).
Central Nervous System
The brain and spinal cord are the organs of the CNS. Because they are so vitally important, the brain and spinal cord, located in the dorsal body cavity, are encased in bone for protection. The brain is in the cranial vault, and the spinal cord is in the vertebral canal of the vertebral column. Although considered to be two separate organs, the brain and spinal cord are continuous at the foramen magnum.
Peripheral Nervous System
The organs of the PNS are the nerves and ganglia. Nerves are bundles of nerve fibers, much as muscles are bundles of muscle fibers. Cranial nerves (12 pairs) and spinal nerves (31 pairs) extend from the CNS to peripheral organs, such as muscles and glands. Ganglia are collections, or small knots, of nerve cell bodies outside the CNS.
The PNS is further subdivided into an afferent (sensory) division and an efferent (motor) division. The afferent or sensory division transmits impulses from peripheral organs to the CNS. The efferent or motor division transmits impulses from the CNS out to the peripheral organs to cause an effect or action.
Finally, the efferent or motor division is again subdivided into the somatic nervous system and the autonomic nervous system (ANS). The somatic nervous system, also called the somatomotor or somatic efferent nervous system, supplies motor impulses to the skeletal muscles. Because these nerves permit conscious control of the skeletal muscles, the somatic nervous system is sometimes called the voluntary nervous system. The ANS, also called the visceral efferent nervous system, supplies motor impulses to cardiac muscle, smooth muscle, and glandular epithelium. It is further subdivided into sympathetic and parasympathetic divisions. Because the ANS regulates involuntary or automatic functions, it is sometimes called the involuntary nervous system.
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Amyotrophic lateral sclerosis (a-my-oh-TROF-ick LAT-er-al sclair-OH-sis) A neurologic disease caused by degeneration of motor neurons of the spinal cord, medulla, and cortex; marked by progressive muscular weakness and atrophy with spasticity and exaggerated reflexes; mental capabilities are not impaired; also called Lou Gehrig disease or motor neuron disease
Bell palsy (BELL PAUL-zee) Neuropathy of the seventh cranial nerve (facial) that causes paralysis of the muscles on one side of the face with sagging of the mouth on the affected side of the face
Cerebral concussion (seh-REE-brull kon-KUSH-un) Loss of consciousness as the result of a blow to the head; usually clears within 24 hours; no evidence of permanent structural damage to the brain tissue
Cerebral contusion (seh-REE-brull kon-TOO-shun) Bruising of brain tissue as a result of direct trauma to the head; neurologic problems persist longer than 24 hours
Cerebral palsy (seh-REE-brull PAWL-zee) Partial paralysis and lack of muscular coordination caused by damage to the cerebrum during fetal life, birth, or infancy Cerebrovascular accident (CVA) (seh-ree-broh-VAS-kyoo-lar AK-sih-dent) Most common brain disorder; may be caused by decreased blood supply to the brain or rupture of a blood vessel in the brain; commonly called a stroke Multiple sclerosis (MS) (MULL-tih-pull skler-OH-sis) A disorder in which there is progressive destruction of the myelin sheaths of central nervous system neurons, interfering with their ability to transmit impulses; characterized by progressive loss of function interspersed with periods of remission; cause is unknown and there is no satisfactory treatment Reye syndrome (RS) (RYE SIN-drohm) Brain dysfunction that occurs primarily in children and teenagers and is characterized by edema of the brain that leads to disorientation, lethargy, and personality changes and may progress to a coma; seems to
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occur after chickenpox and influenza, and taking aspirin is a risk factor
Shingles (SHING-gulls) Viral disease affecting peripheral nerves; characterized by blisters and pain spread over the skin in a bandlike pattern that follows the affected nerves; caused by the same herpes virus that causes chickenpox Tic douloureux (TICK doo-loo-ROO) A painful disorder of the fifth cranial nerve (trigeminal) that is characterized by sudden,
intense, sharp pain in the face and forehead on the affected side; also known as trigeminal neuralgia Transient ischemic attack (TIA) (TRANS-ee-ent iss-KEE-mik ah-TACK) An episode of temporary cerebral dysfunction caused by impaired blood flow to the brain; the onset is sudden, and the attack is of short duration and leaves no long-lasting neurologic impairment; common causes are blood clots and atherosclerosis ■
Although the nervous system is complex, there are only two main types of cells in nerve tissue. The actual nerve cell is the neuron. It is the “conducting” cell that transmits impulses. It is the structural unit of the nervous system. The other type of cell is the neuroglia or glial cell. The word neuroglia means “nerve glue.” These cells are nonconductive and provide a support system for the neurons. They are a special type of “connective tissue” for the nervous system.
Neurons, or nerve cells, carry out the functions of the nervous system by conducting nerve impulses. They are highly specialized and amitotic. This means that if a neuron is destroyed, it cannot be replaced because neurons do not undergo mitosis.
Each neuron has three basic parts:
Figure 9-2 illustrates a typical neuron. The main part of the neuron is the cell body or soma. In many ways the cell body is similar to other types of cells. It has a nucleus with at least one nucleolus and contains many of the typical cytoplasmic organelles. It lacks centrioles, however. Because centrioles function in cell division, the fact that neurons lack these organelles is consistent with the amitotic nature of the cell.
Dendrites and axons are cytoplasmic extensions, or processes, that project from the cell body. They are sometimes referred to as fibers. Dendrites are usually, but not always, short and branching, which increases their surface area to receive signals from other neurons. The number of dendrites on a neuron varies. They are called afferent processes because they transmit impulses to the neuron cell body. Only one axon projects from each cell body. It is usually elongated, and because it carries impulses away from the cell body, it is called an efferent process.
An axon may have infrequent branches called axon collaterals. Axons and axon collaterals terminate in many short branches or telodendria (tell-oh-DEN-dree-ah). The distal ends of the telodendria are slightly enlarged to form synaptic bulbs. Many axons are surrounded by a segmented, white, fatty substance called myelin (MY-eh-lin) or the myelin sheath. Myelinated fibers make up the white matter in the CNS, whereas cell bodies and unmyelinated fibers make up the gray matter. The unmyelinated regions between the myelin segments are called the nodes of Ranvier (nodes of ron-vee-AY). In the PNS the myelin is produced by Schwann cells. The cytoplasm, nucleus, and outer cell membrane of the Schwann cell form a tight covering around the myelin and around the axon itself at the nodes of Ranvier. This covering is the neurilemma (noo-rih-LEM-mah), which plays an important role in the regeneration of nerve fibers. In the CNS, oligodendrocytes (ah-lee-go-DEN-droh-sites) produce myelin, but there is no neurilemma, which is why fibers within the CNS do not regenerate. The structure of an axon and its coverings is illustrated in Figure 9-2.
Functionally, neurons are classified as afferent, efferent, or interneurons (association neurons) according to the direction in which they transmit impulses relative to the CNS (Table 9-1). Afferent, or sensory, neurons carry impulses from peripheral sense receptors to the CNS. They usually have long dendrites and relatively short axons. Efferent, or motor, neurons transmit impulses from the CNS to effector organs, such as muscles and glands. Efferent neurons usually have short dendrites and long axons. Interneurons, or association neurons, are located entirely within the CNS, where they form the connecting link between the afferent and efferent neurons. They have short dendrites and may have either a short or a long axon.
Neuroglia cells do not conduct nerve impulses; instead, they support, nourish, and protect the neurons. They are far more numerous than neurons and, unlike neurons, are capable of mitosis.
The functional characteristics of neurons are excitability and conductivity. Excitability is the ability to respond to a stimulus; conductivity is the ability to transmit an impulse from one point to another. All the functions associated with the nervous system, including thought, learning, and memory, are based on these two characteristics. These functional characteristics are the result of structural features of the cell membrane.
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Brain tumors: Because neurons are not capable of mitosis, primary malignant tumors of the brain are tumors of the glial cells rather than of the neurons themselves. These tumors, called gliomas, have extensive roots, making them extremely difficult to remove.
Blood-brain barrier: Neuroglia, particularly astrocytes, form a wall around the outside of the blood vessels in the nervous system. This astrocyte wall plus the blood vessel wall form the blood- brain barrier. Water, oxygen, carbon dioxide, alcohol, and a few other substances are able to pass through this barrier and move between the blood and brain tissue. Other substances, such as toxins, pathogens, and certain drugs, cannot pass through this barrier. This is a protective mechanism to keep harmful substances out of the brain. It has clinical significance because drugs such as penicillin that may be used to treat disorders in other parts of the body have no effect on the brain because they do not cross the blood-brain barrier.
Anesthetics: Some anesthetics produce their effects by inhibiting the diffusion of sodium through the cell membrane and thus blocking the initiation and conduction of nerve impulses.
Meningitis: Meningitis is an acute inflammation of the pia mater and the arachnoid. It is most commonly caused by bacteria. However, viral infections, fungal infections, and tumors may also cause inflammation of the meninges. Depending on the primary cause, meningitis may be mild or it may progress to a severe and life-threatening condition.
Left and right brain: In most people (approximately 90%), the left cerebral hemisphere dominates for language and mathematic abilities. It is the reasoning and analytic side of the brain. The right cerebral hemisphere is involved with motor skills, intuition, emotion, art, and music appreciation. It is the poetic and creative side of the brain. These people are generally right-handed. In about 10% of the people, these sides are reversed. In some cases, neither hemisphere dominates. This may result in “confusion” and learning disabilities.
Parkinson disease: Parkinson disease is a condition in which the basal ganglia do not produce enough of the inhibitory transmitter dopamine. Without dopamine, there is an excess of excitatory signals that affect certain voluntary muscles, producing rigidity and tremors.
Emotions: The limbic system consists of scattered but interconnected regions of gray matter in the cerebral hemispheres and diencephalon. The limbic system is involved in memory and in emotions such as sadness, happiness, anger, and fear. It is our emotional brain.
Hydrocephalus: In hydrocephalus, an obstruction in the normal flow of cerebrospinal fluid (CSF) causes the fluid to accumulate in the ventricles. The obstruction may be a congenital defect or an acquired lesion such as a tumor. As the fluid accumulates, it causes the ventricles to enlarge and CSF pressure to increase. When this happens in an infant, before the cranial bones ossify, the cranium enlarges. In an older child or adult, the pressure damages the soft brain tissue.
Lumbar puncture: A lumbar puncture is the withdrawal of some CSF from the subarachnoid space in the lumbar region of the spinal cord. The extension of the meninges beyond the end of the cord makes it possible to do this without injury to the spinal cord. The needle is usually inserted just above or just below the fourth lumbar vertebra, and the spinal cord ends at the first lumbar vertebra. The CSF that is removed can be tested for abnormal characteristics that may indicate an injury or infection.
Carpal tunnel syndrome: Carpal tunnel syndrome is a common occupational injury to the hand and wrist that is associated with repetitive hand motions. It is also associated with several diseases, including arthritis, diabetes, and gout. Symptoms, which include tingling of the thumb and fingers, result from the compression of the median nerve because of inflammation and swelling of the tendons within the carpal tunnel. ■
|Table 9-1||Types of Neurons Classified According to Function|
|Type of Neuron||Structure||Function|
|Afferent (sensory)||Long dendrites and short axon; cell body located in ganglia in PNS; dendrites in PNS; axon extends into CNS||Transmits impulses from peripheral sense receptors to CNS|
|Efferent (motor)||Short dendrites and long axon; dendrites and cell body located within CNS; axons extend to PNS||Transmits impulses from CNS to effectors, such as muscles and glands in periphery|
|Association (interneurons)||Short dendrites; axon may be short or long; located entirely within CNS||Transmits impulses from afferent neurons to efferent neurons|
A resting membrane is the cell membrane of a nonconducting, or resting, neuron. The membrane is impermeable to the passive diffusion of sodium (Na+) and potassium (K+) ions. An active transport mechanism, the sodium-potassium pump, maintains a difference in concentration of these ions on the two sides of the membrane. Sodium ions are concentrated in the extracellular fluid, whereas the potassium ions are inside the cell. The intracellular fluid also contains proteins and other negatively charged ions. The result is a polarized membrane with more positive charges outside the cell and more negative charges inside the cell. This difference in charges on the two sides of the resting membrane is the resting membrane potential. Electrical measurements show the resting membrane potential to be about -70 millivolts (mV), which means that the inside of the membrane is 70 mV less positive (more negative) than the outside.
Stimulation of a Neuron
A stimulus is a physical, chemical, or electrical event that alters the permeability of the neuron cell membrane. This allows sodium ions to move inside the cell, then potassium ions move to the outside. This ionic movement briefly changes the polarization of the membrane.
This response to a stimulus—namely, depolarization, reverse polarization, and repolarization—is called the action potential. Electrical measurements show the action potential to peak at approximately +30 mV (Figure 9-3). At the conclusion of the action potential, the sodium-potassium pump actively transports sodium ions out of the cell and
potassium ions into the cell to completely restore resting conditions.
The minimum stimulus necessary to initiate an action potential is called a threshold stimulus or liminal stimulus. A weaker stimulus, called a subthreshold (subliminal) stimulus, does not cause sufficient depolarization to elicit an action potential.
Conduction along a Neuron
Once a threshold stimulus has been applied and an action potential generated, it must be conducted along the total length of the neuron either to an effector or to another neuron.
The threshold stimulus causes a localized area of reverse polarization on the membrane. In that one area, the membrane is negative on the outside and positive on the inside. The rest of the membrane is in the resting condition. When a given area reverses its polarity, the difference in potential between that area and the adjacent area creates a current flow that depolarizes the second point. When the second point reverses its polarity, current flow between the second point and the third point depolarizes the third point. This continues point by point, in domino fashion, along the entire length of the neuron, creating a propagated action potential, or nerve impulse.
The conduction described in the previous paragraph is representative of an unmyelinated axon. Because myelin is an insulating substance, it inhibits the flow of current from one point to another. In myelinated fibers, depolarization occurs only at the places where there is no myelin, at the nodes of Ranvier. The action potential “jumps” from node to node. This “jumping” is saltatory conduction, which is faster than conduction in unmyelinated fibers.
The period of time during which a point on the cell membrane is “recovering” from depolarization is called the refractory period. Although the membrane is permeable to sodium ions, it cannot respond to a second stimulus, no matter how strong the stimulus. This is the absolute refractory period. For a brief period after the absolute refractory period it takes a stronger than normal stimulus to reach threshold. This is the relative refractory period.
Nerve fibers obey the all-or-none principle. If a threshold stimulus is applied, an action potential is generated and propagated along the entire length of the neuron at maximum strength and speed for the existing conditions. A stronger stimulus does not increase the strength of the action potential or change the rate of conduction. A weaker stimulus is subthreshold and does not evoke an action potential. If a stimulus is threshold or greater, an impulse is conducted. If the stimulus is subthreshold, there is no conduction.
Conduction across a Synapse
A nerve impulse, or propagated action potential, travels along a nerve fiber until it reaches the end of the axon; then it must be transmitted to the next neuron. The region of communication between two neurons is called a synapse (SIN-aps). This is similar to the neuromuscular junction described in Chapter 8. A synapse has three parts (Figure 9-4):
The first neuron, the one preceding the synapse, is called thepresynaptic neuron; the second neuron, the one following the synapse, is called the postsynaptic neuron. Synaptic knobs are tiny bulges at the end of the telodendria on the presynaptic neuron. Small sacs within the synaptic knobs, called synaptic vesicles, contain chemicals known as neurotransmitters (noo-roh-TRANS-mitters).
When a nerve impulse reaches the synaptic knob, a series of reactions releases neurotransmitters into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and react with receptors on the postsynaptic cell membrane. This is synaptic transmission. To prevent prolonged reactions with the postsynaptic receptors, the transmitters are quickly inactivated by enzymes. One of the best known neurotransmitters is acetylcholine (ah-see-till-KOH- leen), which is inactivated by the enzyme cholinesterase (koh-lin-ES-ter-ase). Table 9-2 lists some of the common neurotransmitters.
In excitatory transmission, the neurotransmitter-receptor reaction on the postsynaptic membrane depolarizes the membrane and initiates an action potential. This is excitation or stimulation. Acetylcholine is typically an excitatory neurotransmitter. Some neurotransmitters result in inhibitory transmission. In this case, the reaction between the neurotransmitter and the receptor makes it more difficult to generate an action potential. This is inhibition. Gamma- aminobutyric acid (GABA) is an inhibitory neurotransmitter in the CNS.
The billions of neurons in the CNS are organized into functional groups called neuronal pools. The neuronal pools
Figure 9-4 Components of a synapse. The impulse travels from the presynaptic neuron to the postsynaptic neuron.
Table 9-2 Some Common Neurotransmitters
|Acetylcholine||CNS and PNS||Generally excitatory but inhibitory to some visceral effectors||Found in skeletal neuromuscular junctions and in many ANS synapses|
|Norepinephrine||CNS and PNS||May be excitatory or inhibitory depending on receptors||Found in visceral and cardiac muscle neuromuscular junctions; cocaine and amphetamines exaggerate effects|
|Epinephrine||CNS and PNS||May be excitatory or inhibitory depending on receptors||Found in pathways concerned with behavior and mood|
|Dopamine||CNS and PNS||Generally excitatory||Found in pathways that regulate emotional responses; decreased levels in Parkinson disease|
|Serotonin||CNS||Generally inhibitory||Found in pathways that regulate temperature, sensory perception, mood, onset of sleep|
|Gamma-aminobutyric acid (GABA)||CNS||Generally inhibitory||Inhibits excessive discharge of neurons|
|Endorphins and enkephalins||CNS||Generally inhibitory||Inhibit release of sensory pain neurotransmitters; opiates mimic effects of these peptides|
From Applegate E: The anatomy and physiology learning system, ed 4, St Louis, 2011, Saunders. ANS, Autonomic nervous system; CNS, central nervous system; PNS, peripheral nervous system.
receive information, process and integrate that information, and then transmit it to some other destination. Neuronal pools are arranged in pathways, or circuits, over which the nerve impulses are transmitted. The simplest pathway is the simple series circuit (Figure 9-5, A) in which a single neuron synapses with another neuron, which in turn synapses with another, and so on. Most pathways are more complex. In a divergence circuit (Figure 9-5, B), a single neuron synapses with multiple neurons within the pool. This permits the same information to diverge or go along different pathways at the same time. This type of pathway is important in muscle contraction when many muscle fibers, or even several muscles, must contract at the same time. Another type of pathway is the convergence circuit (Figure 9-5, C). In this case, several presynaptic neurons synapse with a single postsynaptic neuron. This accounts for the fact that many different stimuli may have the same ultimate effect. For example, thinking about food, smelling food, and seeing food all have the same effect—the flow of saliva.
The neuron is the structural unit of the nervous system; the reflex arc is the functional unit. The reflex arc is a type of conduction pathway. It is similar to a one-way street because it allows impulses to travel in only one direction. The simplest reflex arc consists of two neurons, but most have three or more neurons in the conduction pathway. Figure 9-6 illustrates a three-neuron reflex arc. There are five basic components in a reflex arc (Table 9-3):
A reflex is an automatic, involuntary response to some change, either inside or outside the body. Reflexes are important in maintaining homeostasis by making adjustments to
Figure 9-5 Neuronal pools. A, Simple series circuit: one neuron synapses with another. B, Divergence circuit: a single neuron synapses with multiple neurons. C, Convergence circuit: several neurons synapse with a single postsynaptic neuron.
|Table 9-3||Components of a Reflex Arc|
|Receptor||Site of stimulus action; receptor end of dendrite or special cell in receptor organ||Responds to some change in internal or external environment|
|Sensory neuron||Afferent neuron; cell body is in ganglion outside CNS; axon extends into CNS||Transmits nerve impulses from receptor to CNS|
|Integration||Always within CNS; in simplest reflexes, it consists of synapse||Processing center; region in CNS where incoming|
|center||between sensory and motor neurons; more commonly one or more interneurons are involved||sensory impulses generate appropriate outgoing motor impulses|
|Motor||Efferent neuron; dendrites and cell body are in CNS; axon extends||Transmits nerve impulses from integration center in|
|neuron||to periphery||CNS to effector organ|
|Effector||Muscle or gland outside CNS||Responds to impulses from motor neuron to produce an action, such as contraction or secretion|
From Applegate E: The anatomy and physiology learning system, ed 4, St Louis, 2011, Saunders. CNS, Central nervous system.
Figure 9-6 Components of a generalized reflex arc. Note the five components of a reflex arc.
heart rate, breathing rate, and blood pressure. Reflexes are also involved in coughing, sneezing, and reactions to painful stimuli. Everyone is familiar with the withdrawal reflex. When you step on a tack or touch a hot iron, you immediately, without conscious thought, withdraw the injured foot or hand from the source of the irritation. Clinicians frequently test an individual’s reflexes to determine if the nervous system is functioning properly.
CENTRAL NERVOUS SYSTEM
The CNS consists of the brain and spinal cord, which are located in the dorsal body cavity. These are vital to our well-being and are enclosed in bone for protection. The brain is surrounded by the cranium, and the spinal cord is protected by the vertebrae. The brain is continuous with the spinal cord at the foramen magnum in the occipital bone. In addition to bone, the CNS is surrounded by connective tissue membranes, called meninges, and by cerebrospinal fluid (CSF).
Three layers of meninges (men-IN-jeez) surround the brain and spinal cord (Figure 9-7). The outer layer, the dura mater (DOO-rah MAY-ter), is tough, white fibrous connective tissue. It is just inside the cranial bones and lines the vertebral canal. The dura mater contains channels, called dural sinuses, that collect venous blood to return it to the cardiovascular system.
The middle layer of meninges is the arachnoid (ah-RAK- noyd). The arachnoid, which resembles a cobweb in appearance, is a thin layer with numerous threadlike strands that attach it to the innermost layer. The space under the arachnoid, the subarachnoid space, is filled with CSF and contains blood vessels.
The pia mater (PEE-ah MAY-ter) is the innermost layer of meninges. This thin, delicate membrane is tightly bound to the surface of the brain and spinal cord and cannot be dissected away without damaging the surface. It closely follows all surface contours.
The brain is divided into the cerebrum, diencephalon, brain stem, and cerebellum.
The largest and most obvious portion of the brain is the cerebrum (seh-REE-brum), which is divided by a deep longitudinal fissure (FISH-ur) into two cerebral hemispheres. The two hemispheres are two separate entities but are connected by an arching band of white fibers, called the corpus callosum (KOR-pus kah-LOH-sum), that provides a communication pathway between the two halves. The surface of the cerebrum is marked by convolutions, or gyri (JYE- rye), separated by grooves, or sulci (SULL-see). The pia mater closely follows the convolutions and goes deep into the sulci, and then up and over the gyri.
Each cerebral hemisphere is divided into five lobes, as illustrated in Figure 9-8. Four of the lobes have the same name as the bone over them. The frontal lobe, under the frontal bone, is the most anterior portion of each hemisphere. The posterior boundary of the frontal lobe is the central sulcus. The parietal lobe is immediately posterior to the central sulcus, under the parietal bone. The occipital lobe, under the occipital bone, is the most posterior portion of the cerebral hemisphere. Laterally, the temporal lobe is inferior to the frontal and parietal lobes. The lateral sulcus (fissure) separates the temporal lobe from the two lobes that
are superior to it. A fifth lobe, the insula (IN-sull-ah) or island of Reil, lies deep within the lateral sulcus. It is covered by parts of the frontal, parietal, and temporal lobes.
The cerebral hemispheres consist of gray matter and white matter. A thin layer of gray matter, the cerebral cortex, forms the outermost portion of the cerebrum. Gray matter consists of neuron cell bodies and unmyelinated fibers. The white matter, which makes up the bulk of the cerebrum, is just beneath the cerebral cortex. White matter is myelinated nerve fibers that form communication pathways in the cerebrum.
The cerebral cortex is the neural basis of what makes us “human.” It is the center for sensory and motor functions. It is concerned with memory, language, reasoning, intelligence, personality, and all the other factors that we associate with human life. Even though the two cerebral hemispheres are nearly symmetric in structure, they are not always equal in function; instead, there are areas of specialization. However, there is considerable overlap in these regions and no area really works alone; all the areas are dependent on one another for mental “consciousness”—those abilities that involve higher mental processing, such as memory, reasoning, logic, and judgment.
It is possible to identify regions of the cerebral cortex that have specific functions. Sensory areas receive information from the various sense organs and receptors throughout the body. The primary sensory area, the somatosensory (soh- mat-oh-SEN-soar-ee) cortex, is located in the postcentral gyrus of the parietal lobe, immediately posterior to the central sulcus. This region receives sensory input from sensory receptors in the skin and skeletal muscles. The right
side of the somatosensory cortex receives input from the left side of the body and vice versa. Motor areas responsible for muscle contraction are located in the frontal lobe. The primary motor area, the somatomotor (soh-mat-oh-MOH- ter) cortex, is in the precentral gyrus, immediately anterior to the central sulcus. Neurons in this area allow us to consciously control our skeletal muscles. The right primary motor gyrus controls muscles on the left side of the body and vice versa. The primary motor cortex is also highly organized in a manner similar to the primary sensory cortex, with neurons in a specific region responsible for controlling movement in a specific part of the body.
Association areas of the cerebral cortex are involved in the process of recognition. They analyze and interpret sensory information, and based on previous experiences they integrate appropriate responses through the motor areas. Table 9-4 describes some of the specific functional areas of the cerebral cortex.
The basal ganglia are functionally related regions of gray matter that are scattered throughout the white matter of the cerebral hemispheres. These regions function as relay stations, or areas of synapse, in pathways going to and from the cortex. The major effects of the basal ganglia are to decrease muscle tone and inhibit muscular activity. Because of these effects, they play an important role in posture and coordinating motor movements. Also, nearly all the inhibitory neurotransmitter dopamine is produced in the basal ganglia.
The diencephalon (dye-en-SEF-ah-lon) is centrally located and is nearly surrounded by the cerebral hemispheres. Regions of the diencephalon are illustrated in Figure 9-9.
The thalamus (THAL-ah-mus), about 80% of the diencephalon, consists of two oval masses of gray matter that serve as relay stations for sensory impulses, except for the sense of smell, going to the cerebral cortex. The thalamus channels the impulses to the appropriate region of the cortex for discrimination, localization, and interpretation.
The hypothalamus (HYE-poh-thal-ah-mus) is a small region below the thalamus. It plays a key role in maintaining homeostasis because it regulates many visceral activities. The hypothalamus also serves as a link between the nervous and endocrine systems because it regulates secretion of hormones from the pituitary gland. A slender stalk, the infundibulum, extends from the floor of the hypothalamus to the pituitary gland and acts as a connector between the two structures. Functions of the hypothalamus include the following:
Regulates and integrates the ANS
Regulates emotional responses and behavior
Regulates body temperature
Regulates food intake
Regulates water balance and thirst
Regulates endocrine system activity
The epithalamus (ep-ih-THAL-ah-mus) is the most dorsal, or superior, portion of the diencephalon. The pineal (PIE-nee-al) gland, or body, extends from its posterior margin. This small gland is involved with the onset of puberty and rhythmic cycles in the body. It is similar to a biologic clock.
The brain stem is the region between the diencephalon and the spinal cord. It consists of three regions:
|Table 9-4 Functional Regions of the Cerebral Cortex|
|Primary sensory cortex (somatosensory cortex)||Postcentral gyrus in parietal lobe||Receives sensory input from receptors in skin and skeletal muscles||Functions in sensations of temperature, touch, pressure, pain|
|Primary visual cortex||Posterior region of occipital lobe||Receives sensory input from retina of eye||Perceives current visual image|
|Auditory cortex||Superior margin of temporal lobe, along lateral sulcus||Receives auditory impulses related to pitch, rhythm, and loudness from inner ear||Allows the hearing of “sounds”|
|Olfactory cortex||Medial aspect of temporal lobe||Receives input from olfactory (smell) receptors in nasal cavity||Permits perception of different odors|
|Gustatory cortex||Parietal lobe where it is overlapped by temporal lobe||Receives input from taste buds on tongue||Permits perception of different tastes|
|Primary motor cortex (somatomotor cortex)||Precentral gyrus in frontal lobe||Initiates efferent action potentials that control voluntary movements||Permits skeletal muscle contraction|
|Premotor cortex||Anterior to primary motor cortex in frontal lobe||Controls learned motor skills that involve skeletal muscles, either simultaneously or sequentially||Examples of learned motor skills are playing piano, typing, writing|
|Broca area (motor speech area)||Inferior portion of frontal lobe in one hemisphere, usually the left||Programs and coordinates muscular movements necessary to articulate words||Person with injury in this area is able to understand words but is unable to speak because of inability to coordinate muscles necessary to form words|
|Prefrontal cortex||Anterior portion of frontal lobes||Involved with thought, reasoning, intelligence, judgment, planning, conscience||This area is well developed only in humans|
|Gnostic area (general interpretation area)||Region where parietal, temporal, and occipital lobes meet; found in one hemisphere (usually the left)||Integrates sensory interpretations from adjacent association areas to form thoughts; then transmits signals for appropriate responses||Stores complex memory patterns; allows person to recognize words and arrange them appropriately to express thoughts or to read and understand written ideas|
From Applegate E: The anatomy and physiology learning system, ed 4, St Louis, 2011, Saunders.
Regions of the brain stem are illustrated in Figure 9-9.
The midbrain is the most superior portion of the brain stem, the region next to the diencephalon. It consists of bundles of myelinated fibers that contain the voluntary motor tracts descending from the cerebral cortex. The pons is the bulging middle portion of the brain stem. This region primarily consists of nerve fibers that form conduction tracts between the higher brain centers and the spinal cord. Four cranial nerves originate in the pons. It also contains the pneumotaxic (noo-moh-TACK-sik) and apneustic (ap-NOO- stick) areas, which help regulate breathing movements.
The medulla oblongata (meh-DULL-ah ahb-long-GAH- tah), or simply medulla, extends inferiorly from the pons. It is continuous with the spinal cord at the foramen magnum. All the ascending (sensory) and descending (motor) nerve fibers connecting the brain and spinal cord pass through the medulla. Most of the descending fibers cross over from one side to the other. In other words, fibers descending on the left side cross over to the right and vice versa. This is called decussation (dee-kuh-SAY-shun). Because the fibers decussate, or cross over, the brain controls motor functions on the opposite side of the body. The medulla contains three vital centers that control visceral activities. The cardiac center adjusts the heart rate and contraction strength to meet body needs. The vasomotor center regulates blood pressure by effecting changes in blood vessel diameter. The respiratory center acts with the centers in the pons to regulate the rate, rhythm, and depth of breathing. Other centers are involved in coughing, sneezing, swallowing, and vomiting.
The cerebellum (sair-eh-BELL-um), the second largest portion of the brain, is located below the occipital lobes of the cerebrum. It consists of two cerebellar hemispheres connected in the middle by a structure called the vermis.
Like the cerebrum, the cerebellum consists of white matter surrounded by a thin layer of gray matter, the cerebellar cortex. Because the surface convolutions are less prominent in the cerebellum than in the cerebrum, the cerebellum has proportionately less gray matter.
Bundles of myelinated nerve fibers form communication pathways between the cerebellum and other parts of the CNS.
The cerebellum functions as a motor area of the brain that mediates subconscious contractions of skeletal muscles necessary for coordination, posture, and balance. The cerebellum coordinates skeletal muscles to produce smooth muscle movement rather than jerky, trembling motion. When the cerebellum is damaged, movements such as running, walking, and writing become uncoordinated. Posture is dependent on muscle tone, which is mediated by the cerebellum. Impulses from the inner ear concerning position and equilibrium are directed to the cerebellum, which uses that information to maintain balance.
Ventricles and Cerebrospinal Fluid
A series of interconnected, fluid-filled cavities is found within the brain. These cavities are the ventricles of the brain, and the fluid is cerebrospinal (seh-ree-broh-SPY-null) fluid (CSF). The lateral ventricles in the cerebrum are the largest of these cavities. One lateral ventricle exists in each cerebral hemisphere. The CSF is a clear fluid that forms as a filtrate from the blood in specialized capillary networks, the choroid plexus (KOR-oyd PLEKS-us), within the ventricles of the brain. It circulates through the ventricles and the central canal of the spinal cord and surrounds the brain in the subarachnoid space. From the subarachnoid space, CSF carrying waste products is returned to the blood. In addition to providing support and protection for the CNS, the CSF helps to nourish the brain and maintain constant ionic conditions for the brain and spinal cord and provides a pathway for removal of waste products.
The spinal cord, illustrated in Figure 9-10, extends from the foramen magnum at the base of the skull to the level of the first lumbar vertebra, a distance of about 43 to 46 cm (approximately 17 to 18 inches). The cord is continuous with the medulla oblongata at the foramen magnum. Distally, it terminates in the conus medullaris (KOH-nus med- yoo-LAIR-is). Like the brain, the spinal cord is surrounded by bone, meninges, and CSF. The spinal dura is separated from the vertebral bones by an epidural space. The meninges extend beyond the end of the spinal cord, down to the upper part of the sacrum. From there, a fibrous cord of pia mater, the filum terminale (FYE-lum term-ih-NAL-ee), extends down to the coccyx, where it is anchored.
The spinal cord is divided into 31 segments, with each segment giving rise to a pair of spinal nerves. At the distal end of the cord, many spinal nerves extend beyond the conus medullaris to form a collection that resembles a horse’s tail. This is the cauda equina (KAW-dah ee-KWYNE-ah). There are two enlargements in the cord, one in the cervical region and one in the lumbar region. The cervical enlargement gives rise to the nerves that supply the upper extremity. Nerves from the lumbar enlargement supply the lower extremity.
In cross section, the spinal cord appears oval (Figure 9-11). Peripheral white matter surrounds a core of gray matter that resembles a butterfly or the letter H. The gray
matter contains the terminal portions of sensory neuron axons, entire interneurons, and the dendrites and cell bodies of motor neurons. The central connecting bar between the two large areas of gray matter is the gray commissure (KOM- ih-shur). This surrounds the central canal, which contains CSF. The white matter that surrounds the gray matter contains longitudinal bundles of myelinated nerve fibers, called nerve tracts.
The spinal cord has two main functions. It is a conduction pathway for impulses going to and from the brain, and it serves as a reflex center. The conduction pathways that carry sensory impulses from body parts to the brain are called ascending tracts. Pathways that carry motor impulses from the brain to muscles and glands are descending tracts.
In addition to serving as a conduction pathway, the spinal cord functions as a center for spinal reflexes. The reflex arc, described earlier in this chapter and illustrated in Figure 9-6, is the functional unit of the nervous system. Reflexes are responses to stimuli that do not require conscious thought, and consequently they occur more quickly than reactions that require thought processes. For example, with the withdrawal reflex, the reflex action withdraws the affected part before one is aware of the pain. Many reflexes are mediated in the spinal cord without going to the higher brain centers. Table 9-5 describes some clinically significant reflexes.
|Figure 9-11 Cross section of the spinal cord.|
|Table 9-5||Some Clinically Significant Reflexes|
|Patellar (knee-jerk reflex)||Stretch reflex; two-neuron path; reflex hammer strikes patellar tendon just below knee; receptors in quadriceps femoris muscle are stretched; reflex results in immediate “kick”||Reflex is blocked by damage to nerves involved and by damage to lumbar segments of spinal cord; also absent in people with chronic diabetes mellitus and neurosyphilis|
|Achilles tendon (ankle-jerk reflex)||Stretch reflex; two-neuron path; reflex hammer strikes Achilles tendon just above heel; gastrocnemius and soleus muscles contract to plantarflex foot||Weak or no reflex action indicates damage to nerves involved or to L5-S2 segments of spinal cord; also absent in chronic diabetes, neurosyphilis, and alcoholism|
|Abdominal||Stroking lateral abdominal wall produces reflex action that compresses abdominal wall and moves umbilicus toward stimulus||Absent in lesions of peripheral nerves, in lesions in thoracic segments of spinal cord, and in multiple sclerosis|
|Babinski||Lateral sole of foot is stroked from heel to toe; positive sign results in dorsiflexion of big toe and spreading of other toes; negative sign results in toes curling under with a light inversion of foot||Positive Babinski sign is normal in children younger than 18 months of age; negative sign is normal after 18 months of age; if motor tracts in spinal cord are damaged, positive Babinski sign reappears|
From Applegate E: The anatomy and physiology learning system, ed 4, St Louis, 2011, Saunders.
PERIPHERAL NERVOUS SYSTEM
The PNS consists of the nerves that branch out from the brain and spinal cord. These nerves form the communication network between the CNS and the remainder of the body. The PNS is further subdivided into the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of nerves that go to the skin and muscles and is involved in conscious activities. The ANS consists of nerves that connect the CNS to the visceral organs such as the heart, stomach, and intestines. It mediates unconscious activities.
Structure of a Nerve
A nerve contains bundles of nerve fibers, either axons or dendrites, surrounded by connective tissue. Sensory nerves contain only afferent fibers—long dendrites of sensory neurons. Motor nerves have only efferent fibers—long axons of motor neurons. Mixed nerves contain both types of fibers.
Twelve pairs of cranial nerves, illustrated in Figure 9-12, emerge from the inferior surface of the brain. All of these nerves except the vagus nerve innervate structures in the head, neck, and facial region. The vagus nerve, cranial nerve X, has numerous branches that supply the viscera in the body.
The cranial nerves are designated both by name and by Roman numerals, according to the order in which they appear on the inferior surface of the brain. Most of the nerves have both sensory and motor components. Three of the nerves (I, II, VIII) are associated with the special senses of smell, vision, hearing, and equilibrium and have only sensory fibers. Five other nerves (III, IV, VI, XI, XII) are primarily motor in function but do have some sensory fibers for proprioception. The remaining four nerves (V, VII, IX, X) consist of significant amounts of both sensory and motor fibers. Table 9-6 itemizes the cranial nerves.
Thirty-one pairs of spinal nerves emerge laterally from the spinal cord. Each pair of nerves corresponds to a segment of the cord, and the nerves are named accordingly. This means there are eight cervical nerves (C1 to C8), 12 thoracic nerves (T1 to T12), five lumbar nerves (L1 to L5), five sacral nerves (S1 to S5), and one coccygeal nerve (Co).
Each spinal nerve is connected to the spinal cord by a dorsal root and a ventral root. The dorsal root has only sensory fibers, and the ventral root has only motor fibers. The two roots join to form the spinal nerve just before the nerve leaves the vertebral column. Because all spinal nerves have both sensory and motor components, they are all mixed nerves.
Immediately after they leave the vertebral column, the spinal nerves divide into several branches that provide the nerve supply to the muscles and the skin of the body wall. In the thoracic region, the main portions of the nerves go
|Table 9-||-6 Summary of Cranial Nerves|
|I||Olfactory||Sensory||Sense of smell|
|III||Oculomotor||Primarily motor||Movement of eyes and eyelids|
|IV||Trochlear||Primarily motor||Movement of eyes|
|Ophthalmic branch||Sensory fibers from cornea, skin of nose, forehead, and scalp|
|Maxillary branch||Sensory fibers from cheek, nose, upper lip, and teeth|
|Mandibular branch||Sensory fibers from skin over mandible, lower lip, and teeth|
|Motor fibers to muscles of mastication|
|VI||Abducens||Primarily motor||Eye movement|
|VII||Facial||Mixed||Sensory fibers from taste receptors on anterior two thirds of tongue|
|Motor fibers to muscles of facial expression, lacrimal glands, and salivary glands|
|VIII||Vestibulocochlear||Sensory||Hearing and equilibrium|
|IX||Glossopharyngeal||Mixed||Sensory fibers from taste receptors on posterior one third of tongue|
|Motor fibers to muscles used in swallowing and to salivary glands|
|X||Vagus||Mixed||Sensory fibers from pharynx, larynx, esophagus, and visceral organs|
|Somatic motor fibers to muscles of pharynx and larynx|
|Autonomic motor fibers to heart, smooth muscles, and glands to alter gastric motility, heart rate, respiration, and blood pressure|
|XI||Accessory||Primarily motor||Contraction of trapezius and sternocleidomastoid muscles|
|XII||Hypoglossal||Primarily motor||Contraction of muscles of tongue|
From Applegate E: The anatomy and physiology learning system, ed 4, St Louis, 2011, Saunders.
directly to the thoracic wall, where they are called intercostal nerves. In other regions, the main portions of the nerves form complex networks called plexuses (see Figure 9-10). In the plexus the fibers are sorted and recombined so that the fibers associated with a particular body part are together even though they may originate from different regions of the cord. The cervicalplexus is located in the neck and sends nerves to the skin and muscles of the neck, shoulder, and diaphragm. The brachial plexus is deep to the clavicle and innervates the skin and muscles of the upper extremities. The lumbosacral plexus is in the lumbar region of the back. Nerves from this plexus go to the skin and muscles of the lower abdominal wall, the lower extremities, the buttocks, and the external genitalia.
Autonomic Nervous System
The ANS is a visceral efferent system, which means it sends motor impulses to the visceral organs. It functions automatically and continuously, without conscious effort, to innervate smooth muscle, cardiac muscle, and glands. It is concerned with heart rate, breathing rate, blood pressure, body temperature, and other visceral activities that work together to maintain homeostasis.
The ANS has two parts: the sympathetic division and the parasympathetic division (Table 9-7). Many visceral organs are supplied with fibers from both divisions (dual innervation). In this case, one stimulates and the other inhibits. This antagonistic functional relationship serves as a balance to help maintain homeostasis.
The sympathetic division, illustrated in Figure 9-13, is concerned primarily with preparing the body for stressful or emergency situations. Sometimes called the fight-or-flight system, it is an energy-expending system. It stimulates the responses that are necessary to meet the emergency and inhibits the visceral activities that can be delayed momentarily. For example, during an emergency, the sympathetic system increases breathing rate, heart rate, and blood flow to skeletal muscles. At the same time, it decreases activity in the digestive tract because that is not necessary to meet the emergency.
The sympathetic preganglionic fibers arise from the thoracic and lumbar regions of the spinal cord; thus the sympathetic division is sometimes called the thoracolumbar (thoar-ah-koh-LUM-bar) division.
The parasympathetic division is most active under ordinary, relaxed conditions (see Figure 9-13). It also brings the body’s systems back to a normal state after an emergency by slowing the heart rate and breathing rate, decreasing blood pressure, decreasing blood flow to skeletal muscles, and increasing digestive tract activity. Sometimes called the rest-and-repose system, it is an energy-conserving system.
The parasympathetic preganglionic fibers arise from the brain stem and sacral region of the spinal cord; thus the parasympathetic division is sometimes called the craniosacral (kray-nee-oh-SAY-kral) division.
|Table 9-7 Comparison of Sympathetic and Parasympathetic Actions on Selected Visceral Effectors|
|Visceral Effectors||Sympathetic Action||Parasympathetic Action|
|Pupil of eye||Dilates||Constricts|
|Lens of eye||Lens flattens for distance vision||Lens bulges for near vision|
|Sweat glands||Stimulates||No innervation|
|Arrector pili muscles of hair||Stimulates contraction; goose bumps||No innervation|
|Heart||Increases heart rate||Decreases heart rate|
|Digestive glands||Decreases secretion of digestive enzymes||Increases secretion of digestive enzymes|
|Digestive tract||Decreases peristalsis||Increases peristalsis|
|Digestive tract sphincters||Stimulates—closes sphincters||Inhibits—opens sphincters|
|Blood vessels to digestive organs||Constricts||No innervation|
|Blood vessels to skeletal muscles||Dilates||No innervation|
|Blood vessels to skin||Constricts||No innervation|
|Adrenal medulla||Stimulates secretion of epinephrine||No innervation|
|Liver||Increases release of glucose||No innervation|
|Urinary bladder||Relaxes bladder and closes sphincter||Contracts bladder and opens sphincter|
From Applegate E: The anatomy and physiology learning system, ed 4, St Louis, 2011, Saunders.
AGING OF THE NERVOUS SYSTEM
Aging of the nervous system is of major importance because changes in this system affect organs in other systems and can cause disturbances of many bodily functions. For example, changes in nerves decrease stimulation of skeletal muscle, which contributes to muscle atrophy with age. Because of its widespread consequences, aging of the nervous system is one of the most distressing aspects of growing old.
Like other cells, nerve cells are lost as a person ages, even in the absence of disease processes. Because neurons are amitotic, those that are lost are not replaced. Loss of neurons is largely responsible for the decrease in brain mass that occurs with aging. Fortunately, the brain has a large reserve supply of neurons, many more than are necessary to carry out its functions, so the decrease in neuron number alone is not devastating. The loss of neurons is not constant in all areas of the brain. For example, about 25% of the specialized cells in the cerebellum, which are responsible for coordinated movements, are lost during aging. This may affect balance and cause difficulty in coordinating fine movements. In other areas of the brain, the number of neurons remains essentially constant throughout life.
It is generally accepted that there is a decline in intelligence with aging, and this is thought to be associated with the loss of neurons. However, it is important to remember that there are wide variations in individuals regarding changes in intellect with age. Because a person is old does not mean that person is “dumb.” Many elderly people retain a keen intellect until death. Along with the decline in intelligence, there is a general decline in memory. Again, this varies from person to person. In general, short-term memory seems to be affected more than long-term memory. Intellect and memory appear to be retained better in people who remain mentally and physically active.
Another change observed in older people is a decrease in the rate of impulse conduction along an axon and across a synapse. A reduction in the amount of myelin around the axon probably accounts for the diminished conduction rate along the axon. Decreases in the quantity of neurotransmitter and in the number of receptor sites cause slower conduction across the synapses. These factors contribute to the slower reflexes and the longer time required to process information that are observed in many elderly people.