CHAPTER 2 Neuroanatomy · brain, and major motor or efferent tracts relay transmis-sions from the brain to smooth and skeletal muscles. Sensory information enters the CNS through

8

C H A P T E R

INTRODUCTION

The purpose of this chapter is to provide the student with areview of neuroanatomy. Basic structures within the nerv-ous system are described and their functions discussed. Thisinformation is important to physical therapists and physicaltherapist assistants who treat patients with neurologic dys-function because it assists clinicians with identifying clinicalsigns and symptoms. In addition, it allows the physical ther-apist assistant to develop an appreciation of the patient’sprognosis and potential functional outcome. It is, however,outside the scope of this text to provide a comprehensivediscussion of neuroanatomy. The reader is encouraged toreview the works of Cohen (1999), Curtis (1990), Farber(1982), FitzGerald (1996), Gilman and Newman (2003),Littell (1990), Lundy-Ekman (2002), and others for a morein-depth review of these concepts.

MAJOR COMPONENTS OF THE NERVOUS SYSTEM

The nervous system is divided into two parts, the centralnervous system (CNS) and the peripheral nervous system (PNS).The CNS is composed of the brain, the cerebellum, thebrain stem, and the spinal cord, whereas the PNS comprisesall the components outside the cranium and spinal cord.Physiologically, the PNS is divided into the somatic nervoussystem and the autonomic nervous system (ANS). Figure 2-1illustrates the major components of the CNS.

The nervous system is a highly organized communicationsystem that serves the body. Nerve cells within the nervous sys-tem receive, transmit, analyze, and communicate informationto other areas throughout the body. For example, sensationssuch as touch, proprioception, pain, and temperature are trans-mitted from the periphery as electrochemical impulses to theCNS through sensory tracts. Once information is processedwithin the brain, it is relayed as new electrochemical impulses

to peripheral structures through motor tracts. This transmissionprocess is responsible for an individual’s ability to interact withthe environment. Individuals are able to perceive sensory expe-riences, to initiate movement, and to perform cognitive tasks asa result of a functioning nervous system.

Types of Nerve Cells

The brain, brain stem, and spinal cord are composed of twobasic types of nerve cells called neurons and neuroglia. Threedifferent subtypes of neurons have been identified based ontheir function: (1) afferent neurons, (2) interneurons, and (3)efferent neurons. Afferent or sensory neurons are responsiblefor receiving sensory input from the periphery of the bodyand transporting it into the CNS. Interneurons connect neu-rons to other neurons. Their primary function is to organizeinformation received from many different sources for laterinterpretation. Efferent or motor neurons transmit informationto the extremities to signal muscles to produce movement.

Neuroglia are non-neuronal supporting cells that providecritical services for neurons. Three different types of neuroglia(astrocytes, oligodendrocytes, and microglia) have been iden-tified. Astrocytes are responsible for maintaining the capillaryendothelium and as such provide a vascular link to neurons.Additionally, astrocytes contribute to the metabolism of theCNS and regulate extracellular concentrations of neurotrans-mitters (Gilman and Newman, 2003). Oligodendrocytes wrapmyelin sheaths around axons in the white matter and pro-duce satellite cells in the gray matter that participate in ionexchange between neurons. Microglia cells are known as thephagocytes of the CNS. They engulf and digest pathogensand assist with nervous system repair after injury.

Neuron Structures

As depicted in Figure 2-2, a neuron consists of a cell body,dendrites, and an axon. The dendrite is responsible for

2OBJECTIVES After reading this chapter, the student will be able to

1. Differentiate between the central and peripheral nervous systems.

2. Identify significant structures within the nervous system.

3. Understand primary functions of structures within the nervous system.

4. Describe the vascular supply to the brain.

5. Discuss components of the cervical, brachial, and lumbosacral plexuses.

Neuroanatomy

receiving information and transferring it to the cell body,where it is processed. Dendrites bring impulses into the cellbody from other neurons. The number and arrangement ofdendrites present in a neuron vary. The cell body or soma iscomposed of a nucleus and a number of different cellularorganelles. The cell body is responsible for synthesizingproteins and supporting functional activities of the neuron,such as transmitting electrochemical impulses and repairingcells. Cell bodies that are grouped together in the CNSappear gray and thus are called gray matter. Groups of cellbodies with similar functions are assembled together toform nuclei. The axon is the message-sending component ofthe nerve cell. It extends from the cell body and is respon-sible for transmitting impulses from the cell body to targetcells that can include muscle cells, glands, or other neurons.

Synapses

The space between the axon of one neuron and the dendriteof the next neuron is called the synapse. Synapses are the

connections between neurons that allow different parts ofthe nervous system to communicate with and influenceeach other. An axon transports electrical impulses or chem-icals called neurotransmitters to and across synapses. Therelaying of information from one neuron to the next takesplace at the synapse.

Neurotransmitters

Neurotransmitters are chemicals that transmit informationacross the synapse. An in-depth discussion of neurotrans-mitters is beyond the scope of this text. We will, however,discuss some common neurotransmitters because of theirrelationship to CNS disease. Furthermore, many of thepharmacologic interventions available to patients withCNS pathology act by facilitating or inhibiting neuro-transmitter activity. Common neurotransmitters includeacetylcholine, glutamate, γ-aminobutyric acid (GABA),dopamine, and norepinephrine. “Acetylcholine is the neu-rotransmitter used by all neurons that synapse with muscle

Neuroanatomy ■ CHAPTER 2 9

Cerebrum

Brain stem and cerebellum

Spinal region

Peripheral region

Cerebralhemispheres

Diencephalon

Midbrain

Pons

Medulla

Dendrites

NucleusOligodendrocyte

Nodes of Ranvier

AxonMyelin sheath

Cell body

FIGURE 2-1. Regions of the nervous system. Regions are listedon the left, and subdivisions are listed on the right. (From Lundy-Ekman L. Neuroscience: Fundamentals for Rehabilitation, 2ndedition. Philadelphia, WB Saunders, 2002.)

FIGURE 2-2. Diagram of a neuron.

fibers (lower motor neurons)” (Lundy-Ekman, 2002).Acetylcholine also plays a role in regulating heart rate andother autonomic functions. Glutamate is an excitatoryneurotransmitter and facilitates neuronal change duringdevelopment. Glutamate is also thought to contribute toneuron destruction after an injury to the CNS. GABA isan inhibitory neurotransmitter and exerts its influenceover interneurons within the spinal cord. Dopamine influ-ences motor activity, motivation, and cognition.Norepinephrine is used by the ANS and produces the“fight-or-flight response” to stress (Lundy-Ekman, 2002).

Axons

Once information is processed, it is conducted to other neu-rons, muscle cells, or glands by the axon. Axons can bemyelinated or unmyelinated. Myelin is a lipid/protein thatencases and insulates the axon. The presence of a myelinsheath increases the speed of impulse conduction, thusallowing for increased responsiveness of the nervous system.The myelin sheath surrounding the neuron is not continu-ous; it contains interruptions or spaces within the myelincalled the nodes of Ranvier. Saltatory conduction is theprocess whereby electrical impulses are conducted along anaxon by jumping from one node to the next (Fig. 2-3). Thisprocess increases the velocity of nervous system impulseconduction. Unmyelinated axons send messages moreslowly than myelinated ones.

White Matter

Areas of the nervous system with a high concentration ofmyelin appear white because of the fat present within themyelin. Consequently, white matter is composed of axonsthat carry information away from cell bodies. White matteris found in the brain and spinal cord. Myelinated axons arebundled together within the CNS to form fiber tracts.

Gray Matter

Gray matter refers to areas that contain large numbers ofnerve cell bodies and dendrites. Collectively, these cell bod-

ies give the region its grayish coloration. Gray matter coversthe entire surface of the cerebrum and is called the cerebralcortex. The cortex is estimated to contain 14 billion neurons(Gilman and Newman, 2003). Gray matter is also presentdeep within the spinal cord and is discussed in more detaillater in this chapter.

Fibers and Pathways

Major sensory or afferent tracts carry information to thebrain, and major motor or efferent tracts relay transmis-sions from the brain to smooth and skeletal muscles.Sensory information enters the CNS through the spinalcord or by the cranial nerves as the senses of smell, sight,hearing, touch, taste, heat, cold, pressure, pain, and move-ment. Information travels in fiber tracts composed ofaxons that ascend in a particular path from the sensoryreceptor to the cortex for interpretation. Motor signalsdescend from the cortex to the spinal cord through effer-ent fiber tracts for muscle activation. Fiber tracts are desig-nated by their point of origin and by the area in whichthey terminate. Thus, the corticospinal tract, the primarymotor tract, originates in the cortex and terminates in thespinal cord. The lateral spinothalamic tract, a sensorytract, begins in the lateral white matter of the spinal cordand terminates in the thalamus. A more thorough discus-sion of motor and sensory tracts is presented later in thischapter.

Brain

The brain consists of the cerebrum, which is divided intotwo cerebral hemispheres (the right and the left), the cere-bellum, and the brain stem. The surface of the cerebrum orcerebral cortex is composed of depressions (sulci) and ridges(gyri). These convolutions increase the surface area of thecerebrum without requiring an increase in the size of thebrain. The outer surface of the cerebrum is composed ofgray matter and is estimated to be 1.3 to 4.5 mm thick,whereas the inner surface is composed of white matter fibertracts (Gilman and Newman, 2003). Therefore, informationis conveyed by the white matter and is processed and inte-grated within the gray matter.

Supportive and Protective Structures

The brain is protected by a number of different structuresand substances to minimize the possibility of injury. First,the brain is surrounded by a bony structure called the skullor cranium. The brain is also covered by three layers ofmembranes called meninges, which provide additional pro-tection. The outermost layer is the dura mater. The dura is athick, fibrous connective tissue membrane that adheres tothe cranium. The area between the dura mater and the skullis known as the epidural space. The next or middle layer is thearachnoid. The space between the dura and the arachnoid iscalled the subdural space. The third protective layer is the piamater. This is the innermost layer and adheres to the brain

10 SECTION 1 ■ FOUNDATIONS

Myelin sheath Axoplasm Node ofRanvier

1 2 3

FIGURE 2-3. Saltatory conduction along a myelinated axon.(Redrawn from Guyton AC, Hall JE. Textbook of MedicalPhysiology, 9th edition. Philadelphia, WB Saunders, 1996.)

itself. The pia mater also contains the cerebral circulation.The cranial meninges are continuous with the membranesthat cover and protect the spinal cord. Cerebrospinal fluidbathes the brain and circulates within the subarachnoidspace. Figure 2-4 shows the relationship of the skull with thecerebral meninges.

Lobes of the Cerebrum

The cerebrum is divided into four lobes—frontal, parietal,temporal, and occipital—each having unique functions, asshown in Figure 2-5, A. The hemispheres of the brain,although apparent mirror images of one another, have spe-cialized functions as well. This sidedness of brain functionis called hemispheric specialization or lateralization.

Frontal Lobe. The frontal lobe is frequently referred toas the primary motor cortex. The frontal lobe is responsiblefor voluntary control of complex motor activities. In addi-tion to its motor responsibilities, the frontal lobe alsoexhibits a strong influence over cognitive functions, includ-ing judgment, attention, awareness, abstract thinking,mood, and aggression. The principal motor region responsi-ble for speech (Broca’s area) is located within the frontallobe. In the left hemisphere, Broca’s area plans movementsof the mouth to produce speech. In the opposite hemi-sphere, this same area is responsible for nonverbal commu-nication, including gestures and adjustments of theindividual’s tone of voice.

Parietal Lobe. The parietal lobe is the primary sensorycortex. Incoming sensory information is processed andmeaning is provided to stimuli within this lobe. Perceptionis the process of attaching meaning to sensory information.Much of our perceptual learning requires a functioningparietal lobe. Specific body regions are assigned locationswithin the parietal lobe for this interpretation. This map-ping is known as the sensory homunculus (Fig. 2-5, B). Theparietal lobe also plays a role in short-term memory func-tions.

Temporal Lobe. The temporal lobe is the primary audi-tory cortex. Wernicke’s area of the temporal lobe allows anindividual to hear and comprehend spoken language. Visualperception, musical discrimination, and long-term memorycapabilities are all functions of the temporal lobe.

Occipital Lobe. The occipital lobe is the primary visualcortex providing for the organization, integration, and inter-pretation of visual information. The eyes take in visualinformation and then send it to the occipital cortex forinterpretation.

Association Cortex

Association areas are regions within the parietal, temporal,and occipital lobes that horizontally link different parts ofthe cortex. For example, the sensory association cortex inte-grates and interprets information from all the lobes receiv-ing sensory input and allows individuals to perceive andattach meaning to sensory experiences. Additional functionsof the association areas include personality, memory, intel-ligence (problem solving and comprehension of spatial rela-tionships), and the generation of emotions (Lundy-Ekman,2002). Figure 2-5, C, depicts association areas within thecerebral hemispheres.

Motor Areas of the Cerebral Cortex

The primary motor cortex, located in the frontal lobe, is pri-marily responsible for contralateral voluntary control ofupper extremity and facial movements. Thus, a greater pro-portion of the total surface area of this region is devoted toneurons that control these body parts. Other motor areasinclude the premotor area, which controls muscles of thetrunk and anticipatory postural adjustments, and the sup-plementary motor area, which controls initiation of move-ment, orientation of the eyes and head, and bilateral,sequential movements (Lundy-Ekman, 2002).

Hemispheric Specialization

The cerebrum can be further divided into the right and left cere-bral hemispheres. Gross anatomic differences have been demon-strated within the hemispheres. The hemisphere that isresponsible for language is considered the dominant hemi-sphere. Approximately 95 percent of the population, includingall right-handed individuals, are left hemisphere dominant.Even in individuals who are left-hand dominant, the lefthemisphere is the primary speech center in about 50 percent of


Pia mater

Subarachnoid space

Arachnoid

Dura mater

Cerebral hemisphere

FIGURE 2-4. A coronal section through the skull, meninges, andcerebral hemispheres. The section shows the midline structuresnear the top of the skull. The three layers of meninges are indi-cated. (From Lundy-Ekman L. Neuroscience: Fundamentals forRehabilitation, 2nd edition. Philadelphia, WB Saunders, 2002.)

these people (Geschwind and Levitsky, 1968; Gilman andNewman, 2003; Guyton, 1991; Lundy-Ekman, 2002). Table 2-1lists primary functions of both the left and right cerebral hemi-spheres.

Left Hemisphere Functions. The left hemisphere hasbeen described as the verbal or analytic side of the brain.The left hemisphere allows for the processing of informa-tion in a sequential, organized, logical, and linear manner.


A

B C

Cerebrum

Central sulcus

Parietal lobe

Occipital lobe

Cerebellum

Spinal cord

Medulla

Pons

Temporal lobe

Sylvianfissure

Frontal lobe

FIGURE 2-5. The brain. A, Left lateral view of the brain, showing the principal divisions of thebrain and the four major lobes of the cerebrum. B, Sensory homunculus. C, Primary and asso-ciation sensory and motor areas of the brain. (A, From Guyton AC. Basic Neuroscience:Anatomy and Physiology, 2nd edition. Philadelphia, WB Saunders, 1991; B and C, from CechD, Martin S. Functional Movement Development Across the Life Span, 2nd edition.Philadelphia, WB Saunders, 2002.)

The processing of information in a step-by-step or detailedfashion allows for thorough analysis. For the majority ofpeople, language is produced and processed in the lefthemisphere, specifically the frontal and temporal lobes. Theleft parietal lobe allows an individual to recognize wordsand to comprehend what has been read. In addition, math-ematical calculations are performed in the left parietal lobe.An individual is able to sequence and perform movementsand gestures as a result of a functioning left frontal lobe. Afinal behavior assigned to the left cerebral hemisphere is theexpression of positive emotions such as happiness and love.Common impairments seen in patients with left hemi-spheric injury include an inability to plan motor tasks(apraxia); difficulty in initiating, sequencing, and processinga task; difficulty in producing or comprehending speech;perseveration of speech or motor behaviors; and anxious-ness (O’Sullivan, 2001).

Right Hemisphere Functions. The right cerebral hemi-sphere is responsible for an individual’s nonverbal and artis-tic abilities. The right side of the brain allows individuals toprocess information in a complete or holistic fashion with-out specifically reviewing all the details. The individual isable to grasp or comprehend general concepts. Visual-per-ceptual functions including eye-hand coordination, spatialrelationships, and perception of one’s position in space arecarried out in the right hemisphere. The ability to commu-nicate nonverbally and to comprehend what is beingexpressed is also assigned to the right parietal lobe.Nonverbal skills including understanding facial gestures,recognizing visual-spatial relationships, and being aware ofbody image are processed in the right side of the brain.Other functions include mathematical reasoning and judg-ment, sustaining a movement or posture, and perceivingnegative emotions such as anger and unhappiness(O’Sullivan, 2001). Specific deficits that can be observed inpatients with right hemisphere damage include poor judg-

ment, unrealistic expectations, denial of disability ordeficits, disturbances in body image, irritability, andlethargy.

Hemispheric Connections

Even though the two hemispheres of the brain have discretefunctional capabilities, they perform many of the sameactions. Communication between the two hemispheres isconstant, so individuals can be analytic and yet still graspbroad general concepts. It is possible for the right hand toknow what the left hand is doing and vice versa. The corpuscallosum is a large group of axons that connect the right andleft cerebral hemispheres and allow communicationbetween the two cortices.

Deeper Brain Structures

Subcortical structures lie deep within the brain and includethe internal capsule, the diencephalon, and the basal gan-glia. These structures are briefly discussed because of theirfunctional significance to motor function.

Internal Capsule. All descending fibers leaving themotor areas of the frontal lobe travel through the internalcapsule, a deep structure within the cerebral hemisphere. Theinternal capsule is made up of axons that project from thecortex to the white matter fibers (subcortical structures)located below and from subcortical structures to the cerebralcortex. The capsule is shaped like a less-than sign (<), withan anterior and a posterior limb. The corticospinal tract trav-els in the posterior part of the capsule and allows informa-tion to be transmitted from the cortex to the brain stem andspinal cord. A lesion within this area can cause contralateralloss of voluntary movement and conscious somatosensation,which is the ability to perceive tactile and proprioceptiveinput. The internal capsule is pictured in Figure 2-6.

Diencephalon. The diencephalon is situated deepwithin the cerebrum and is composed of the thalamus and


TABLE 2-1 Behaviors Attributed to the Left and Right Brain Hemispheres

Behavior Left Hemisphere Right Hemisphere

Cognitive style Processing information in a sequential, linear Processing information in a simultaneous, holistic,manner or gestalt manner

Observing and analyzing details Grasping overall organization or patternPerception/cognition Processing and producing language Processing nonverbal stimuli (environmental

sounds, speech intonation, complex shapes, and designs)

Visual-spatial perceptionDrawing inferences, synthesizing information

Academic skills Reading: sound-symbol relationships, word Mathematical reasoning and judgmentrecognition, reading comprehension Alignment of numerals in calculations

Performing mathematical calculationsMotor Sequencing movements Sustaining a movement or posture

Performing movements and gestures to commandEmotions Expressing positive emotions Expressing negative emotions

Perceiving emotion

From O’Sullivan SB. Stroke. In O’Sullivan SB, Schmitz TJ (eds). Physical Rehabilitation Assessment and Treatment, 4th edition. Philadelphia, FA Davis,2001, p 536.

hypothalamus. The diencephalon is the area where themajor sensory tracts (dorsal columns and lateral spinothala-mic) and the visual and auditory pathways synapse. Thethalamus consists of a large collection of nuclei andsynapses. In this way, the thalamus serves as a central relaystation for sensory impulses traveling upward from otherparts of the body and brain to the cerebrum. It receives allsensory impulses except those associated with the sense ofsmell and channels them to appropriate regions of the cor-tex for interpretation. Moreover, the thalamus relays sensoryinformation to the appropriate association areas within thecortex. Motor information received from the basal gangliaand cerebellum is transmitted to the correct motor regionthrough the thalamus. Sensations of pain and peripheralnumbness can also be identified at the level of the thalamus.

Hypothalamus. The hypothalamus is a group of nucleithat lie at the base of the brain, underneath the thalamus.The hypothalamus regulates homeostasis, which is themaintenance of a balanced internal environment. Thisstructure is primarily involved in automatic functions,including the regulation of hunger, thirst, digestion, bodytemperature, blood pressure, sexual activity, and sleep-wakecycles. The hypothalamus is responsible for integrating thefunctions of both the endocrine system and the ANSthrough its regulation of the pituitary gland and its releaseof hormones.

Basal Ganglia. Another group of nuclei located at thebase of the cerebrum comprise the basal ganglia. The basalganglia form a subcortical structure made up of the caudate,putamen, globus pallidus, substantia nigra, and subthalamicnuclei. The globus pallidus and putamen form the lentiformnucleus, and the caudate and putamen are known as thestriatum. The nuclei of the basal ganglia influence themotor planning areas of the cerebral cortex through variousmotor circuits. Primary responsibilities of the basal gangliainclude the regulation of posture and muscle tone and thecontrol of volitional and automatic movement. In additionto their role in motor control, the caudate nucleus isinvolved in cognitive functions. The most common condi-tion that results from dysfunction within the basal gangliais Parkinson disease. Patients with Parkinson disease exhibitbradykinesia (slowness initiating movement), akinesia (diffi-culty in initiating movement), tremors, rigidity, and pos-tural instability. Death of the cells in the substantia nigra,which produces dopamine, has been identified as the causeof this disease.

Limbic System. The limbic system is a group of deepbrain structures in the diencephalon and cortex that includesparts of the thalamus and hypothalamus and a portion of thefrontal and temporal lobes. The hypothalamus controlsprimitive emotional reactions, including rage and fear. Thelimbic system guides the emotions that regulate behavior


Corona radiata

Corpus callosum

Internal capsule

Mamillary body Subthalamic nucleus Substantia nigra

Caudate nucleus

Thalamus

Putamen

Globus pallidus

Amygdala

A

White matter

Coronaradiata

Internalcapsule

Cerebralpeduncle

Optic nerve

Pons Olive CerebellumB Medulla Pyramid

R. oculomotornerve

L. trochlearnerve

Cerebral cortex

Superiorcerebellarpeduncle

FIGURE 2-6. The cerebrum. A, Diencephalon and cerebral hemispheres. B, A deep dissectionof the cerebrum showing the radiating nerve fibers, the corona radiata, that conduct signals inboth directions between the cerebral cortex and the lower portions of the central nervous sys-tem. (A, From Lundy-Ekman L. Neuroscience: Fundamentals for Rehabilitation, 2nd edition.Philadelphia, WB Saunders, 2002; B, from Guyton AC. Basic Neuroscience: Anatomy andPhysiology, 2nd edition. Philadelphia, WB Saunders, 1991.)

and is involved in learning and memory. More specifically,the limbic system appears to control memory, pain, pleasure,rage, affection, sexual interest, fear, and sorrow.

Cerebellum

The cerebellum controls balance and complex muscularmovements. It is located below the occipital lobe of thecerebrum and is posterior to the brain stem. It fills the pos-terior fossa of the cranium. Like the cerebrum, it also con-sists of two symmetric hemispheres. The cerebellum isresponsible for the integration, coordination, and executionof multijoint movements. The cerebellum regulates the ini-tiation, timing, sequencing, and force generation of musclecontractions. It sequences the order of muscle firing when agroup of muscles work together to perform a movementsuch as stepping or reaching. The cerebellum also assistswith balance and posture maintenance and has been identi-fied as a comparator of actual motor performance to thatwhich is anticipated. The cerebellum monitors and com-pares the movement requested, for instance, the step, withthe movement actually performed (Horak, 1991).

Brain Stem

The brain stem is located between the base of the cerebrumand the spinal cord and is divided into three sections (Fig. 2-7). Moving cephalocaudally, the three areas are themidbrain, pons, and medulla. Each of the different areas isresponsible for specific functions. The midbrain connects thediencephalon to the pons and acts as a relay station for tractspassing between the cerebrum and the spinal cord or cere-bellum. The midbrain also houses reflex centers for visual,auditory, and tactile responses. The pons contains bundles ofaxons that travel between the cerebellum and the rest of theCNS and functions with the medulla to regulate the breath-

ing rate. It also contains reflex centers that assist with orien-tation of the head in response to visual and auditory stimu-lation. Cranial nerve nuclei can also be found within thepons, specifically, cranial nerves V through VIII, which carrymotor and sensory information to and from the face. Themedulla is an extension of the spinal cord and contains thefiber tracts that run through the spinal cord. Motor and sen-sory nuclei for the neck and mouth region are located withinthe medulla, as well as the control centers for heart and res-piration rates. Reflex centers for vomiting, sneezing, andswallowing are also located within the medulla.

The reticular activating system is also situated within thebrain stem and extends vertically throughout its length. Thesystem maintains and adjusts an individual’s level ofarousal, including sleep-wake cycles. In addition, the reticu-lar activating system facilitates the voluntary and autonomicmotor responses necessary for certain self-regulating, home-ostatic functions and is involved in the modulation of mus-cle tone throughout the body.

Spinal Cord

The spinal cord has two primary functions: coordination ofmotor information and movement patterns and communi-cation of sensory information. Subconscious reflexes,including withdrawal and stretch reflexes, are integratedwithin the spinal cord. Additionally, the spinal cord pro-vides a means of communication between the brain and theperipheral nerves. The spinal cord is a direct continuation ofthe brain stem, specifically the medulla. The spinal cord ishoused within the vertebral column and extends approxi-mately to the level of the first lumbar vertebra. The spinalcord has two enlargements, one that extends from the thirdcervical segment to the second thoracic segment andanother that extends from the first lumbar to the third sacral


FIGURE 2-7. Midsagittal view of the brain. (Redrawn from Farber SD. Neurorehabilitation: AMultisensory Approach. Philadelphia, WB Saunders, 1982.)

segment. These enlargements accommodate the great num-ber of neurons needed to innervate the upper and lowerextremities located in these regions. At approximately theL1 level, the spinal cord becomes a cone-shaped structurecalled the conus medullaris. The conus medullaris is com-posed of sacral spinal segments. Below this level, the spinalcord becomes a mass of spinal nerve roots called the caudaequina. The cauda equina consists of the nerve roots forspinal nerves L2 through S5. Figure 2-8 depicts the spinalcord and its relation to the brain. A thin filament, the filumterminale, extends from the caudal end of the spinal cordand attaches to the coccyx. In addition to the bony protec-tion offered by the vertebrae, the spinal cord is also coveredby the same protective meningeal coverings as in the brain.

Internal Anatomy

The internal anatomy of the spinal cord can be visualized incross-sections and is viewed as two distinct areas. Figure 2-9, A,illustrates the internal anatomy of the spinal cord. Like thebrain, the spinal cord is composed of gray and white matter.The center of the spinal cord, the gray matter, is distin-guished by its H-shaped or butterfly-shaped pattern. The gray

matter contains cell bodies of motor and sensory neurons andsynapses. The upper portion is known as the dorsal or poste-rior horn and is responsible for transmitting sensory stimuli.The lower portion is referred to as the anterior or ventral horn(Fig. 2-9, B). It contains cell bodies of lower motor neurons,and its primary function is to transmit motor impulses. Thelateral horn is present at the T1 to L2 levels and contains cellbodies of preganglionic sympathetic neurons. It is responsiblefor processing autonomic information. The periphery of thespinal cord is composed of white matter. The white matter iscomposed of sensory (ascending) and motor (descending)fiber tracts. A tract is a group of nerve fibers that are similar inorigin, destination, and function. These fiber tracts carryimpulses to and from various areas within the nervous system.In addition, these fiber tracts cross over from one side of thebody to the other at various points within the spinal cord andbrain. Therefore, an injury to the right side of the spinal cordmay produce a loss of motor or sensory function on the con-tralateral side.

Major Afferent (Sensory) Tracts

Two primary ascending sensory tracts are present in thewhite matter of the spinal cord. The dorsal or posteriorcolumns carry information about position sense (proprio-ception), vibration, two-point discrimination, and deeptouch. Figure 2-10 shows the location of this tract. Thefibers of the dorsal columns cross in the brain stem. Painand temperature sensations are transmitted in the spinothal-amic tract located anterolaterally in the spinal cord (seeFig. 2-10). Fibers from this tract enter the spinal cord,synapse, and cross within three segments. Sensory informa-tion must be relayed to the thalamus. Touch informationhas to be processed by the cerebral cortex for discriminationto occur. Light touch and pressure sensations enter thespinal cord, synapse, and are carried in the dorsal and ven-tral columns.

Major Efferent (Motor) Tract

The corticospinal tract is the primary motor pathway andcontrols skilled movements of the extremities. This tractoriginates in the frontal lobe from the primary and premo-tor cortices and continues through interconnections andvarious synapses, finally to synapse on anterior horn cells inthe spinal cord. This tract also crosses from one side to theother in the brain stem. A common indicator of corti-cospinal tract damage is the Babinski sign. To test for thissign, the clinician takes a blunt object such as the back of apen and runs it along the lateral border of the patient’s foot(Fig. 2-11). The sign is present when the great toe extendsand the other toes splay. The presence of a Babinski signindicates that damage to the corticospinal tract hasoccurred.

Other Descending Tracts

Other descending motor pathways that affect muscle toneare the rubrospinal, lateral and medial vestibulospinal,


Frontal lobe

Frontal lobeMotor areaParietal lobe

Sensory areaOccipital lobe

Temporal lobeCerebellumMedulla

Cervicalsegment

Thoracicsegment

Lumbarsegment

Dural saccontainingcauda equinaand filumterminale

Sacralsegment

Conusmedullaris

THE BRAIN

THE SPINAL CORD

FIGURE 2-8. The principal anatomic parts of the nervous sys-tem. (From Guyton AC. Basic Neuroscience: Anatomy andPhysiology, 2nd edition. Philadelphia, WB Saunders, 1991.)

tectospinal, and medial and lateral reticulospinal tracts. Therubrospinal tract originates in the red nucleus of the mid-brain and terminates in the anterior horn, where it synapseswith lower motor neurons that primarily innervate theupper extremities. Fibers from this tract facilitate flexormotor neurons and inhibit extensor motor neurons.Proximal muscles are primarily affected, although the tractdoes exhibit some influence over more distal muscle groups.The rubrospinal tract has been said to assist in the correc-tion of movement errors. The lateral vestibulospinal tractassists in postural adjustments through facilitation of proxi-

mal extensor muscles. Regulation of muscle tone in the neckand upper back is a function of the medial vestibulospinaltract. The medial reticulospinal tract facilitates limb exten-sors, whereas the lateral reticulospinal tract facilitates flexorsand inhibits extensor muscle activity. The tectospinal tractprovides for orientation of the head toward a sound or amoving object.

Anterior Horn Cell

An anterior horn cell is a large neuron located in the gray mat-ter of the spinal cord. An anterior horn cell sends out axons


Lateral white column

Spinal dura mater

Spinal arachnoid

Subarachnoid space

Spinal pia mater

Ventral white column

Ventral gray hornLateral gray horn

Dorsal root filaments

Dorsal root

Ventral root

Ventral root filaments

Spinal nerve

Dorsal root ganglion

Dorsal grayhorn

Dorsal whitecolumns

ANTERIOR

POSTERIOR

A

B

Dorsal column

WHITE MATTER

Lateral column

Anterior column

Dorsal horn

GRAY MATTER

Lateral horn

Ventral horn

FIGURE 2-9. The spinal cord. A, Structures of the spinal cord and its connections with thespinal nerve by way of the dorsal and ventral spinal roots. Note also the coverings of the spinalcord, the meninges. B, Cross-section of the spinal cord. The central gray matter is divided intohorns and a commissure. The white matter is divided into columns. (A, From Guyton AC. BasicNeuroscience: Anatomy and Physiology, 2nd edition. Philadelphia, WB Saunders, 1991.)

through the ventral or anterior spinal root; these axonseventually become peripheral nerves and innervate musclefibers. Thus, activation of an anterior horn cell stimulatesskeletal muscle contraction. Alpha motor neurons are a typeof anterior horn cell that innervates skeletal muscle. Becauseof axonal branching, several muscle fibers can be innervatedby one neuron. A motor unit consists of an alpha motorneuron and the muscle fibers it innervates. Gamma motorneurons are also located within the anterior horn. Thesemotor neurons transmit impulses to the intrafusal fibers ofthe muscle spindle.

Muscle Spindle

The muscle spindle is the sensory organ found in skeletalmuscle and is composed of motor and sensory endings andmuscle fibers. These fibers respond to stretch and thereforeprovide feedback to the CNS regarding the muscle’s length.

The easiest way to conceptualize how the muscle spin-dle functions within the nervous system is to review thestretch reflex mechanism. Stretch or deep tendon reflexescan easily be facilitated in the biceps, triceps, quadriceps,and gastrocnemius muscles. If a sensory stimulus such as atap on the patellar tendon is applied to the muscle and itsspindle, the input will enter through the dorsal root of thespinal cord to synapse on the anterior horn cell (alphamotor neurons). Stimulation of the anterior horn cell elic-its a motor response, reflex contraction of the quadriceps(extension of the knee), as information is carried throughthe anterior root to the skeletal muscle. An important noteabout stretch or deep tendon reflexes is that their activa-tion and subsequent motor response can occur withouthigher cortical influence. The sensory input coming intothe spinal cord does not have to be transmitted to the


Fasciculus gracilis

Dorsal columns

Posterior fissure

Anterior median fissure

Fasciculus cuneatus

Posteriorspinocerebellar tract

Anterior spinocerebellartract ascending from

proprioceptors in muscle and tendons for position

sense

Anterior corticospinal tractReticulospinal tract (fibers scattered)

Lateralspinothalamic tractascending forpain and temperature

Lateralcorticospinal tractdescending to skeletalmuscle for voluntarymovement

Rubrospinal tractdescending forposture and muscle coordination

Vestibulospinal tract

Tectospinal tract

FIGURE 2-10. Cross-section of the spinal cord showing tracts. (From Gould BE.Pathophysiology for the Health-Related Professions. Philadelphia, WB Saunders, 1997.)

A

B

FIGURE 2-11. A, Stroking from the heel to the ball of the footalong the lateral sole, then across the ball of the foot, normallycauses the toes to flex. B, Babinski sign in response to the samestimulus. In corticospinal tract lesions, or in infants less than6 months old, the big toe extends, and the other toes fan out-ward. (From Lundy-Ekman L. Neuroscience: Fundamentals forRehabilitation, 2nd edition. Philadelphia, WB Saunders, 2002.)

cortex for interpretation. This has clinical implicationsbecause it means that a patient with a cervical spinal cordinjury can continue to exhibit lower extremity deep ten-don reflexes despite lower extremity paralysis.

Peripheral Nervous System

The peripheral nervous system (PNS) consists of the nervesleading to and from the CNS, including the cranial nervesexiting the brain stem and the spinal roots exiting the spinalcord, many of which combine to form peripheral nerves.These nerves connect the CNS functionally with the rest ofthe body through sensory and motor impulses. Figure 2-12provides a schematic representation of the PNS and its tran-sition to the CNS.

The PNS is divided into two primary components: thesomatic (body) nervous system and the ANS. The somaticor voluntary nervous system is concerned with reactions tooutside stimulation. This system is under conscious control

and is responsible for skeletal muscle contraction by way ofthe 31 pairs of spinal nerves. By contrast, the ANS is aninvoluntary system that innervates glands, smooth (visceral)muscle, and the myocardium. The primary function of theANS is to maintain homeostasis, an optimal internal envi-ronment. Specific functions include the regulation of diges-tion, circulation, and cardiac muscle contraction.

Somatic Nervous System

Within the PNS are 12 pairs of cranial nerves, 31 pairs ofspinal nerves, and the ganglia or cell bodies associated withthe cranial and spinal nerves. The cranial nerves are locatedin the brain stem and can be either sensory or motor nerves.Primary functions of the cranial nerves include eye move-ment, smell, sensation perceived by the face and tongue,and innervation of the sternocleidomastoid and trapeziusmuscles. See Table 2-2 for a more detailed list of cranialnerves and their major functions.


Spinal cord segment

Posterior horn

Anterior horn

Cell body

Posterior root

Anterior root

T1

CNS

Brain

Spinalcord

Primary sensory cell body

Dorsal root ganglion

Spinal nerve

Posterior primary ramus

Anterior primary ramus

Epineurium

Endoneurium

Sensoryneuron

Perineurium

Nerve bundle(fascicle)

Node ofRanvier

Axon

Skin MuscleBloodvessels

Myelinsheath

Painreceptors

Motorneuron

Motor end plate

Sympatheticchain ganglion

FIGURE 2-12. Schematic representation of the peripheral nervous system and the transitionto the central nervous system.

The spinal nerves consist of 8 cervical, 12 thoracic, 5lumbar, and 5 sacral nerves and 1 coccygeal nerve. Cervicalspinal nerves C1 through C7 exit above the correspondingvertebrae. Because there are only 7 cervical vertebrae, theC8 spinal nerve exits above the T1 vertebra. From that pointon, each succeeding spinal nerve exits below its respectivevertebra. Figure 2-13 shows the distribution and innervationof the peripheral nerves.

Spinal nerves, consisting of sensory (posterior or dorsalroot) and motor (anterior or ventral root) components, exitthe intervertebral foramen. The region of skin innervated bysensory afferent fibers from an individual spinal nerve iscalled a dermatome. Myotomes are a group of muscles inner-vated by a spinal nerve. Once through the foramen, thespinal nerve divides into two primary rami. This divisionrepresents the beginning of the PNS. The dorsal or posteriorrami innervate the paravertebral muscles, the posterioraspects of the vertebrae, and the overlying skin. The ventralor anterior primary rami innervate the intercostal muscles,the muscles and skin in the extremities, and the anterior andlateral trunk.

The 12 pairs of thoracic nerves do not join with othernerves and maintain their segmental relationship. However,the anterior primary rami of the other spinal nerves jointogether to form local networks known as the cervical,brachial, and lumbosacral plexuses (Guyton, 1991). Thereader is given only a brief description of these nerveplexuses, because a detailed description of these structures isbeyond the scope of this text.

Cervical Plexus. The cervical plexus is composed of theC1 through C4 spinal nerves. These nerves primarily inner-vate the deep muscles of the neck, the superficial anteriorneck muscles, the levator scapulae, and portions of thetrapezius and sternocleidomastoid. The phrenic nerve, oneof the specific nerves within the cervical plexus, is formedfrom branches of C3 through C5. This nerve innervates the

diaphragm, the primary muscle of respiration, and is theonly motor and main sensory nerve for this muscle(Guyton, 1991). Figure 2-14 identifies components of thecervical plexus.

Brachial Plexus. The anterior primary rami of C5through T1 form the brachial plexus. The plexus divides andcomes together several times, providing muscles with motorand sensory innervation from more than one spinal nerveroot level. The five primary nerves of the brachial plexus arethe musculocutaneous, axillary, radial, median, and ulnarnerves. Figure 2-15 depicts the constituency of the brachialplexus. These five peripheral nerves innervate the majority ofthe upper extremity musculature, with the exception of themedial pectoral nerve (C8), which innervates the pectoralismuscles; the subscapular nerve (C5 and C6), which inner-vates the subscapularis; and the thoracodorsal nerve (C7),which supplies the latissimus dorsi muscle (Guyton, 1991).

The musculocutaneous nerve innervates the forearm flex-ors. The elbow, wrist, and finger extensors are innervated bythe radial nerve. The median nerve supplies the forearmpronators and the wrist and finger flexors, and it allowsthumb abduction and opposition. The ulnar nerve assiststhe median nerve with wrist and finger flexion, abducts andadducts the fingers, and allows for opposition of the fifthfinger (Guyton, 1991).

Lumbosacral Plexus. Although some authors discuss thelumbar and sacral plexuses separately, they are discussed hereas one unit because together they innervate lower extremitymusculature. The anterior primary rami of L1 through S3form the lumbosacral plexus. This plexus innervates themuscles of the thigh, lower leg, and foot. This plexus doesnot undergo the same separation and reuniting as does thebrachial plexus. The lumbosacral plexus has eight roots,which eventually form six primary peripheral nerves: obtura-tor, femoral, superior gluteal, inferior gluteal, commonperoneal, and tibial. The sciatic nerve, which is frequently


TABLE 2-2 Cranial Nerves

Number Name Function Connection to Brain

I Olfactory Smell Inferior frontal lobeII Optic Vision DiencephalonIII Oculomotor Moves eye up, down, medially; Midbrain (anterior)

raises upper eyelid; constricts pupilIV Trochlear Moves eye medially and down Midbrain (posterior)V Trigeminal Facial sensation, chewing, sensation Pons (lateral)

from temporomandibular jointVI Abducens Abducts eye Between pons and medullaVII Facial Facial expression, closes eye, tears, Between pons and medulla

salivation, tasteVIII Vestibulocochlear Sensation of head position relative to Between pons and medulla

gravity and head movement; hearingIX Glossopharyngeal Swallowing, salivation, taste MedullaX Vagus Regulates viscera, swallowing, speech, taste MedullaXI Accessory Elevates shoulders, turns head Spinal cord and medullaXII Hypoglossal Moves tongue Medulla

From Lundy-Ekman L. Neuroscience: Fundamentals for Rehabilitation, 2nd edition. Philadelphia, WB Saunders, 2002, p 299.

discussed in physical therapy practice, is actually composedof the common peroneal and tibial nerves encased in asheath. This nerve innervates the hamstrings and causes hipextension and knee flexion. The sciatic nerve separates intoits components just above the knee (Guyton, 1991). Thelumbosacral plexus is shown in Figures 2-16 and 2-17.

Peripheral Nerves. Two major types of nerve fibers arecontained in peripheral nerves: motor (efferent) and sensory(afferent) fibers. Motor fibers have a large cell body withmultiple branched dendrites and a long axon. The cell bodyand the dendrites are located within the anterior horn of thespinal cord. The axon exits the anterior horn throughthe white matter and is located with other similar axons inthe anterior root, which is located outside the spinal cordin the intervertebral foramen. The axon then eventually

becomes part of a peripheral nerve and innervates a motorend plate in a muscle. The sensory neuron, on the otherhand, has a dendrite that originates in the skin, muscle ten-don, or Golgi tendon organ and travels all the way to its cellbody, which is located in the dorsal root ganglion within theintervertebral foramen (Fig. 2-18). Golgi tendon organs areencapsulated nerve endings found at the musculotendinousjunction. They are sensitive to tension within muscle ten-dons and transmit this information to the spinal cord. Theaxon travels through the dorsal (posterior) root of a spinalnerve and into the spinal cord through the dorsal horn. Theaxon may terminate at this point, or it may enter the whitematter fiber tracts and ascend to a different level in thespinal cord or brain stem. Thus, a sensory neuron sendsinformation from the periphery to the spinal cord.


C3C4

C5T1 T2

T3T4T5T6T7T8T9T10

T11

T12

L1

L2

L3

L4

L5

S1

C6

C8C7

C2

C3C4C5

C6

C7C8T1

T2T3T4T5

T7T9

T11

L1

L3

L5

T6T8

T10

T12

L2

L4

S1S2

S3S4S5

L1

L2

L3

L4

S1

L5

S2

L4

S2S3

Obturator

Common peroneal

Saphenous

Superficial peroneal

Deep peroneal

LumboinguinalMedianRadialUlnar

Ilioinguinal

Musculocutaneous

Axillary

Supraclavicular

Cervical cutaneousPosterior rami of cervical

Sural

PERIPHERAL NERVESDERMATOMES DERMATOMES

Intercostobrachial cutaneousLateral brachial cutaneousMedial brachial cutaneous

Anterior thoracic ramiPosterior brachial cutaneous

Lateral thoracic ramiPosterior thoracic rami

Medial antebrachial cutaneousPosterior lumbar rami

Posterior antebrachial cutaneous

Posterior sacral ramiLateral femoral cutaneousAnterior femoral cutaneous

Posterior femoral cutaneous

FIGURE 2-13. Dermatomes and cutaneous distribution of peripheral nerves. (From Lundy-Ekman L. Neuroscience: Fundamentals for Rehabilitation, 2nd edition. Philadelphia, WBSaunders, 2002.)


FIGURE 2-15. The brachial plexus and its branches. (From Guyton AC. Basic Neuroscience:Anatomy and Physiology, 2nd edition. Philadelphia, WB Saunders, 1991.)

FIGURE 2-14. The cervical plexus and its branches. (From Guyton AC. Basic Neuroscience:Anatomy and Physiology, 2nd edition. Philadelphia, WB Saunders, 1991.)

Autonomic Nervous System

Functions of the autonomic nervous system (ANS) include theregulation of circulation, respiration, digestion, metabolism,secretion, body temperature, and reproduction. Control cen-ters for the ANS are located in the hypothalamus and thebrain stem. The ANS is composed of motor neurons locatedwithin spinal nerves that innervate smooth muscle, cardiacmuscle, and glands, which are also called effectors or targetorgans. The ANS is divided into the sympathetic and para-sympathetic divisions. Both the sympathetic and parasympa-thetic divisions innervate internal organs, use a two-neuronpathway and one-ganglion impulse conduction, and functionautomatically. Autoregulation is achieved by integrating infor-mation from peripheral afferents with information fromreceptors within the CNS. The two-neuron pathway (pregan-glionic and postganglionic neurons) provides the connectionfrom the CNS to the autonomic effector organs. Cell bodiesof the preganglionic neurons are located within the brain orspinal cord. The myelinated axons exit the CNS and synapsewith collections of postganglionic cell bodies. Unmyelinatedaxons from the postganglionic neurons ultimately innervatethe effector organs (Farber, 1982).


FIGURE 2-16. The lumbar plexus and its branches, especially thefemoral nerve. (From Guyton AC. Basic Neuroscience: Anatomyand Physiology, 2nd edition. Philadelphia, WB Saunders, 1991.)

FIGURE 2-17. The sacral plexus and its branches, especially the sciatic nerve. (From Guyton AC.Basic Neuroscience: Anatomy and Physiology, 2nd edition. Philadelphia, WB Saunders, 1991.)

The sympathetic fibers of the ANS arise from the thoracicand lumbar portions of the spinal cord. Axons of pregan-glionic neurons terminate in either the sympathetic chain orthe prevertebral ganglia located in the abdomen. The sympa-thetic division of the ANS assists the individual in respondingto stressful situations and is often referred to as the fight-or-flight response. Sympathetic responses help the individual toprepare to cope with the perceived stimulus by maintaining anoptimal blood supply. Activation of the sympathetic systemstimulates smooth muscle in the blood vessels to contract,thereby causing vasoconstriction. Norepinephrine, alsoknown as noradrenaline, is the major neurotransmitter respon-sible for this action. Consequently, heart rate and blood pres-sure are increased as the body prepares for a fight or to flee adangerous situation. Blood flow to muscles is increased bybeing diverted from the gastrointestinal tract.

The parasympathetic division maintains vital bodilyfunctions or homeostasis. The parasympathetic divisionreceives its information from the brain stem, specificallycranial nerves III (oculomotor), VII (facial), IX (glossopha-ryngeal), and X (vagus), and from lower sacral segmentsof the spinal cord. The vagus nerve is a parasympatheticpreganglionic nerve. Motor fibers within the vagus nerveinnervate the myocardium and the smooth muscles of thelungs and digestive tract. Activation of the vagus nerve can

produce the following effects: bradycardia, decreased forceof cardiac muscle contraction, bronchoconstriction,increased mucus production, increased peristalsis, andincreased glandular secretions. Efferent activation of thesacral components results in emptying of the bowels andbladder and arousal of sexual organs. Acetylcholine is thechemical transmitter responsible for sending nervous systemimpulses to effector cells in the parasympathetic division.Acetylcholine is used for both divisions at the preganglionicsynapse and dilates arterioles. Thus, activation of theparasympathetic division produces vasodilation. When anindividual is calm, parasympathetic activity decreases heartrate and blood pressure and signals a return of normal gas-trointestinal activity. Figure 2-19 shows the influence of thesympathetic and parasympathetic divisions on effectororgans (Farber, 1982; Lundy-Ekman, 2002).

The CNS also exerts influence over the ANS. Theregions most closely associated with this control are thehypothalamus, which regulates functions such as digestion,and the medulla, which controls heart and respiration rates.


Dorsalroot

ganglion

Dura mater

Arachnoid

Pia mater

Spinalcord

Dorsalramus Ventral

ramus

Meninges

Ramicommunicantes

Ventralroot

Dorsalroot

Vertebrallamina

Vertebralbody

Spinalnerve

FIGURE 2-18. Spinal region. The spinal nerve is formed ofaxons from the dorsal and ventral roots. The bifurcation of thespinal nerve into dorsal and ventral rami marks the transitionfrom the spinal to the peripheral region. (From Lundy-Ekman L.Neuroscience: Fundamentals for Rehabilitation, 2nd edition.Philadelphia, WB Saunders, 2002.)

C-1

S-1

2345678

345

Parasympatheticfibers — CRANIAL NERVES III, VII, IX, X

Brain stem

Phrenic nerve to diaphragm —RESPIRATION

Sympathetic nervous system — • HEART• BLOOD VESSELS• TEMPERATURE

Intercostal muscles —RESPIRATION

Parasympathetic nerves• BOWEL• BLADDER• EXTERNAL

GENITALIA

LEGS

ARMS

2345

L-1

23456789101112

T-1

2

FIGURE 2-19. Functional areas of the spinal cord. (From GouldBE. Pathophysiology for the Health-Related Professions.Philadelphia, WB Saunders, 1997.)

Cerebral Circulation

A final area that must be reviewed when discussing thenervous system is the circulation to the brain. The cellswithin the brain completely depend on a continuous supplyof blood for glucose and oxygen. The neurons within thebrain are unable to carry out glycolysis and to store glyco-gen. It is therefore absolutely essential that these neuronsreceive a constant supply of blood. Knowledge of cere-brovascular anatomy is the basis for understanding the clini-cal manifestations, diagnosis, and management of patientswho have sustained cerebrovascular accidents and traumaticbrain injuries.

Anterior Circulation

All arteries to the brain arise from the aortic arch. The firstmajor arteries ascending anteriorly and laterally within theneck are the common carotid arteries. The carotid arteriesare responsible for supplying the bulk of the cerebrum withcirculation. The right and left common carotid arteriesbifurcate just behind the posterior angle of the jaw tobecome the external and internal carotids. The externalcarotid arteries supply the face, whereas the internal carotidsenter the cranium and supply the cerebral hemispheres,including the frontal lobe, the parietal lobe, and parts of thetemporal lobe. In addition, the internal carotid artery sup-plies the optic nerves and the retina of the eyes. At the baseof the brain, the internal carotid bifurcates into the rightand left anterior and middle cerebral arteries. The middlecerebral artery is the largest of the cerebral arteries and ismost often occluded. It is responsible for supplying the lat-eral surface of the brain with blood and also the deep por-tions of the frontal and parietal lobes. The anterior cerebralartery supplies the superior border of the frontal and pari-etal lobes. Both the middle cerebral artery and the anteriorcerebral artery make up what is called the anterior circula-tion to the brain. Figures 2-20 and 2-21 depict the cerebralcirculation.

Posterior Circulation

The posterior circulation is composed of the two vertebralarteries, which are branches of the subclavian. The vertebralarteries supply blood to the brain stem and cerebellum. Thevertebral arteries leave the base of the neck and ascend pos-teriorly to enter the skull through the foramen magnum.The two vertebral arteries then unite to form the basilarartery. The basilar artery supplies the brain stem and themedial portion of the temporal and occipital lobes with cir-culation. This artery also bifurcates to form the right andleft posterior cerebral arteries. The two posterior cerebralarteries supply blood to the occipital and temporal lobes.

The anterior and posterior communicating arteries,which are branches of the carotid and basilar arteries, areinterconnected at the base of the brain and form the circleof Willis. This connection of blood vessels provides a pro-tective mechanism to the structures within the brain.

Because of the circle of Willis, failure or occlusion of onecerebral artery does not critically decrease blood flow to thatregion. Consequently, the occlusion can be circumventedor bypassed to meet the nutritional and metabolic needs ofcerebral tissue.

REACTION TO INJURY

What happens when the CNS or the PNS is injured? TheCNS and the PNS are prone to different types of injury, andeach system reacts differently. Within the CNS, arteryobstruction of sufficient duration produces cell and tissuedeath within minutes. Neurons that die because they aredeprived of oxygen do not possess the capacity to regener-ate. Neurons in the vicinity of damage are also at risk ofinjury secondary to the release of glutamate, an excitatoryneurotransmitter. At normal levels, glutamate assists withCNS functions; however, at higher levels glutamate can betoxic to neurons and can promote neuronal death. The pres-ence of excessive glutamate also facilitates calcium release,which ultimately produces a cascade of events including theliberation of calcium-dependent digestive enzymes, cellularedema, cell injury, and death (Lundy-Ekman, 2002).

Changes within the neurons themselves are not evidentfor 12 to 24 hours. By 24 to 36 hours, the damaged area


Anterior cerebralartery

Internal carotid artery

Anteriorcommunicatingartery

Posteriorcommunicatingartery

Posterior cerebralartery

Basilar artery

FIGURE 2-20. From anterior to posterior, the arteries that formthe circle of Willis are the anterior communicating, two anteriorcerebral, two internal carotid, two posterior communicating, andtwo posterior cerebral arteries. (From Lundy-Ekman L. Neuro-science: Fundamentals for Rehabilitation, 2nd edition. Philadelphia,WB Saunders, 2002.)

becomes soft and edematous. Liquefaction and cavitationbegin, and the area of necrotic tissue is eventually convertedinto a cyst. In time, the infarct will eventually retract, andthe cystic cavity will by surrounded by a glial scar. The dam-aged neurons will not be replaced, and the original functionof the area will be lost (Branch, 1987).

Nearby undamaged axons demonstrate collateral sprout-ing 4 to 5 days after injury. These sprouts replace the dam-aged synaptic area, thus increasing input to other neurons.Although these collateral sprouts do not replace original cir-cuits, they do develop from systems most closely associatedwith the injured area.

Conversely, peripheral nerve injuries often result frommeans other than vascular compromise. Common causes ofperipheral nerve injuries include stretching, laceration, com-pression, traction, disease, chemical toxicity, and nutritionaldeficiencies. The response of a peripheral nerve to the injuryis different from that in the CNS. If the cell body isdestroyed, regeneration is not possible. The axon undergoesnecrosis distal to the site of injury, the myelin sheath beginsto pull away, and the Schwann cells phagocytize the area,producing wallerian degeneration (Fig. 2-22). If the damageto the peripheral nerve is not too significant and occursonly to the axon, regeneration is possible. Axonal sprouting

from the proximal end of the damaged axon can occur. Theaxon regrows at the rate of 1.0 mm per day depending onthe size of the nerve fiber (Lundy-Ekman, 2002). To have areturn of function, the axon must grow and reinnervate theappropriate muscle. Failure to do so results in degenerationof the axonal sprout. The rate of recovery from a peripheralnerve injury depends on the age of the patient and the dis-tance between the lesion and the destination of the regen-erating nerve fibers. A discussion of the physical therapymanagement of peripheral nerve injuries is beyond thescope of this text.

Injury to a motor neuron can result in variable findings.If an individual experiences damage to the corticospinaltract from its origin in the frontal lobe to its end within thespinal cord, the patient is classified as having an uppermotor neuron injury. Clinical signs of an upper motor neu-ron injury include spasticity (increased resistance to passivestretch), hyperreflexia, the presence of a Babinski sign, andpossible clonus. Clonus is a repetitive stretch reflex that iselicited by passive dorsiflexion of the ankle or passive wristextension. If the injury is to the anterior horn cell, themotor nerve cells of the brain stem, the spinal root, or thespinal nerve, the patient is recognized as having a lower


Anterior cerebral artery

Posteriorcerebral artery

Anterior cerebral artery

Middle cerebral artery

Posteriorcerebral artery

A

BFIGURE 2-21. The large cerebral arteries: anterior, middle, andposterior. (From Lundy-Ekman L. Neuroscience: Fundamentals forRehabilitation, 2nd edition. Philadelphia, WB Saunders, 2002.)

Presynaptic axonterminals retract

Chromatolysis ofcell body

Axon lesion

Myelin degeneration

Muscle fibersatrophy

A B

Distal axon andterminal degenerates

FIGURE 2-22. Wallerian degeneration. A, Normal connectionsbefore an axon is severed. B, Degeneration following severance ofan axon. Degeneration following axonal injury involves severalchanges: (1) the axon terminal degenerates, (2) myelin breaksdown and forms debris, and (3) the cell body undergoes metabolicchanges. Subsequently, (4) presynaptic terminals retract from thedying cell body, and (5) postsynaptic cells degenerate. (FromLundy-Ekman L. Neuroscience: Fundamentals for Rehabilitation,2nd edition. Philadelphia, WB Saunders, 2002.)

motor neuron injury. Clinical findings of this type of injuryinclude flaccidity, marked muscle atrophy, muscle fascicula-tions, and hyporeflexia.

REFERENCESBranch EF. The neuropathology of stroke. In Duncan PW, Badke

MB (eds). Stroke Rehabilitation: The Recovery of Motor Control. St.Louis, Year Book, 1987, pp 49–77.

Cohen H. Neuroscience for Rehabilitation, 2nd edition. Philadelphia,Lippincott Williams & Wilkins, 1999.

Curtis BA. Neurosciences: The Basics. Philadelphia, Lea & Febiger,1990.

Farber SD. Neurorehabilitation: A Multisensory Approach.Philadelphia, WB Saunders, 1982, pp 1–59.

FitzGerald MJT. Neuroanatomy Basic and Clinical, 3rd edition.Philadelphia, WB Saunders, 1996.

Geschwind N, Levitsky W. Human brain: Left-right asymmetries intemporal speech regions. Science 161:186–187, 1968.

Gilman S, Newman SW. Manter and Gatz’s Essentials of ClinicalNeuroanatomy and Neurophysiology, 10th edition. Philadelphia,FA Davis, 2003, pp 1–11, 61–63, 147–154, 190–203, 225–226.

Guyton AC. Basic Neuroscience: Anatomy and Physiology, 2nd edition.Philadelphia, WB Saunders, 1991, pp 1–24, 39–54, 244–245.

Horak FB. Assumptions underlying motor control for neurologicrehabilitation. In Contemporary Management of Motor ControlProblems: Proceedings of the II Step Conference. Alexandria, VA,Foundation for Physical Therapy, 1991, pp 11–27.

Littell EH. Basic Neuroscience for the Health Professions. Thorofare,NJ, Slack Incorporated, 1990.

Lundy-Ekman L. Neuroscience: Fundamentals for Rehabilitation, 2ndedition. Philadelphia, WB Saunders, 2002, pp 55–63, 70–77,153–167, 220–228, 253–256, 328–339, 383–396.

O’Sullivan SB. Stroke. In O’Sullivan SB, Schmitz TJ (eds). PhysicalRehabilitation Assessment and Treatment, 4th ed. Philadelphia, FADavis, 2001, p 536.


CHAPTER SUMMARY

An understanding of the structures and functions of the nerv-ous system is necessary for physical therapists and physicaltherapist assistants. This knowledge assists practitioners inworking with patients with neuromuscular dysfunctionbecause it allows the therapist to have a better appreciationof the patient’s pathologic condition, deficits, and potentialcapabilities. In addition, an understanding of neuroanatomyis helpful when educating patients and their families regard-ing the patient’s condition and possible prognosis. ■

REVIEW QUESTIONS

1. Describe the major components of the nervous system.

2. What is the function of the white matter?

3. What are some of the primary functions of the parietallobe?

4. What is Broca’s aphasia?

5. Discuss the primary function of the thalamus.

6. What is the primary function of the corticospinal tract?

7. What is an anterior horn cell? Where are these cellslocated?

8. Discuss the components of the PNS.

9. Where is the most common site of cerebral infarction?

10. What are some clinical signs of an upper motor neuroninjury?

Documents

CHAPTER 2 Neuroanatomy · brain, and major motor or efferent tracts relay transmis-sions from the brain to smooth and skeletal muscles. Sensory information enters the CNS through