central nervous system - midlandisd.net

CHAPTER 13

CENTRALNERVOUS SYSTEM

KEY TERMSbrainstemcerebellumcerebrospinal fluid (CSF)cerebrumdiencephalonelectroencephalogram

hypothalamuslimbic systemmeningesreticular formationthalamusventricles

Recall from Chapter 12 that the nervous system is saidto be composed of two major divisions: the centralnervous system (CNS) and the peripheral nervous sys-

tem (PNS). The reason for designating two distinct divisionsis to make the study of the nervous system easier. In thischapter, we discuss the part of the nervous system that lies atthe center of the regulatory process: the central nervous sys-tem. Comprising both the brain and the spinal cord, the cen-tral nervous system is the principal integrator of sensory in-put and motor output. Thus the central nervous system iscapable of evaluating incoming information and formulatingresponses to changes that threaten our homeostatic balance.

This chapter begins with a description of the protectivecoverings of the brain and spinal cord. After that, we brieflydiscuss the watery cerebrospinal fluid (CSF) and the spaces inwhich it is found. We then outline the overall structure andfunction of the major organs of the central nervous system,beginning at the bottom with the spinal cord; this is the sim-plest and least complex part of the CNS. Then our focusmoves upward to the more complex brain, beginning firstwith the narrow brainstem (Figure 13-1) and the roughlyspherical cerebellum attached to its dorsal surface. Againshifting our attention upward, we describe the structure andfunction of the diencephalon and then move on to a discus-sion of the cerebrum. As we move up the central nervoussystem, the complexity of both structure and function in-

CHAPTER OUTLINECoverings of the Brain and Spinal Cord, 375Cerebrospinal Fluid, 377

Fluid Spaces, 377Formation and Circulation of Cerebrospinal Fluid, 377

Spinal Cord, 380Structure of the Spinal Cord, 380Functions of the Spinal Cord, 381

Brain, 383Structure of the Brainstem, 385

Medulla Oblongata, 385Pons, 385Midbrain, 386

Functions of the Brainstem, 386Structure of the Cerebellum, 387Functions of the Cerebellum, 388Diencephalon, 388

Thalamus, 389Hypothalamus, 389Pineal Body, 390

Structure of the Cerebrum, 390Cerebral Cortex, 390Cerebral Tracts and Cerebral Nuclei, 391

Functions of the Cerebral Cortex, 393Functional Areas of the Cortex, 393Sensory Functions of the Cortex, 394Motor Functions of the Cortex, 394Integrative Functions of the Cortex, 395Specialization of Cerebral Hemispheres, 396The Electroencephalogram (EEG), 397

Somatic Sensory Pathways in the Central Nervous System, 399

Somatic Motor Pathways in the Central Nervous System, 400

Cycle of Life, 403The Big Picture, 403Mechanisms of Disease, 404Case Study, 406

374

creases. The spinal cord mediates simple reflexes, whereasthe brainstem and diencephalon are involved in the regula-tion of the more complex maintenance functions such asregulation of heart rate and breathing. The cerebral hemi-spheres, which together form the largest part of the brain,perform complex integrative functions such as consciousthought, learning, memory, language, and problem solving.We end the chapter with a discussion of the somatic sensorypathways and the somatic motor pathways. This prepares usfor Chapter 14, which covers the peripheral nervous system,and Chapter 15, which covers the sense organs.

COVERINGS OF THE BRAIN AND SPINAL CORDBecause the brain and spinal cord are both delicate and vital,nature has provided them with two protective coverings. Theouter covering consists of bone: cranial bones encase thebrain; vertebrae encase the spinal cord. The inner coveringconsists of membranes known as meninges. Three distinctlayers compose the meninges:

1. Dura mater2. Arachnoid membrane3. Pia mater

Observe their respective locations in Figures 13-2 and 13-3.The dura mater, made of strong white fibrous tissue, serves asthe outer layer of the meninges and also as the inner perios-teum of the cranial bones. The arachnoid membrane, a deli-cate, cobweb-like layer, lies between the dura mater and the piamater or innermost layer of the meninges. The transparent piamater adheres to the outer surface of the brain and spinal cordand contains blood vessels.

The dura mater has three important inward extensions:

1. Falx cerebri. The falx cerebri projects downward into thelongitudinal fissure to form a kind of partition betweenthe two cerebral hemispheres. The Latin word falxmeans “sickle” and refers to the curving sickle shape ofthis partition as it extends from the roof of the cranialcavity (Figure 13-2, B).

2. Falx cerebelli. The falx cerebelli is a sickle-shaped exten-sion that separates the two halves, or hemispheres, of thecerebellum.

3. Tentorium cerebelli. The tentorium cerebelli separatesthe cerebellum from the cerebrum. It is called a tento-rium (meaning “tent”) because it forms a tentlike cover-ing over the cerebellum.

Figure 13-2 shows a large space within the dura, wherethe falx cerebri begins to descend between the left and rightcerebral hemispheres. This space, called the superior sagittalsinus, is one of several dural sinuses. Dural sinuses functionas venous reservoirs, collecting blood from brain tissues forthe return trip to the heart.

There are several spaces between and around themeninges (see Figure 13-2). Three of these spaces are thefollowing:

1. Epidural space. The epidural (“on the dura”) space is immediately outside the dura mater but inside thebony coverings of the brain and spinal cord. It con-tains a supporting cushion of fat and other connectivetissues.

2. Subdural space. The subdural (“under the dura”) spaceis between the dura mater and arachnoid membrane.The subdural space contains a small amount of lubricat-ing serous fluid.

3. Subarachnoid space. As its name suggests, the subarach-noid space is under the arachnoid and outside the piamater. This space contains a significant amount of cere-brospinal fluid.

The meninges of the cord (see Figure 13-3) continue ondown inside the spinal cavity for some distance below theend of the spinal cord. The pia mater forms a slender fila-ment known as the filum terminale (see Figure 13-1). At thelevel of the third segment of the sacrum, the filum terminaleblends with the dura mater to form a fibrous cord that dis-appears in the periosteum of the coccyx.

Infections of the meninges are discussed in Box 13-1.

Central Nervous System Chapter 13 375

Hypothalamus

ThalamusDiencephalon

Cervical enlargement(of spinal cord)

CerebellumMidbrain

PonsMedulla

Brainstem

Spinalcord Membranous

covering(meninges)

Bony covering(vertebral column)

Lumbar enlargement(of spinal cord)

Cerebrum

Filumterminale

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Figure 13-1 The central nervous system. Details of both the brainand the spinal cord are easily seen in this figure.

376 Unit 3 Communication, Control, and Integration

Muscle

Skin

Subarachnoidspace

Arachnoid

Periosteum

Dura mater

Epiduralspace

Superiorsagittal sinus

(of dura)

Periosteum

Subduralspace

Skull

Falxcerebri

Piamater

One functionallayer

A

Superior sagittalsinus of dura

Transverse sinusof dura

Falx cerebri

Inferior sagittalsinus of dura

Free margin oftentorium cerebelli

Margin of foramenmagnum

Attached margin oftentorium cerebelli

Tentorium cerebelli

Spinal cord

Medulla oblongata

B

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Figure 13-2 Coverings of the brain. A, Frontal section of the superior portion of the head, as viewed fromthe front. Both the bony and the membranous coverings of the brain can be seen. B, Transverse section of theskull, viewed from below. The dura mater has been retained in this specimen to show how it lines the innerroof of the cranium and the falx cerebri extending inward.

CEREBROSPINAL FLUIDIn addition to its bony and membranous coverings, naturehas further protected the brain and spinal cord against in-jury by providing a cushion of fluid both around the organsand within them. This fluid is the cerebrospinal fluid (CSF).The cerebrospinal fluid does more than simply provide asupportive, protective cushion, however. It is also a reservoirof circulating fluid that, along with blood, the brain moni-tors for changes in the internal environment. For example,changes in the carbon dioxide (CO2) content of CSF triggerhomeostatic responses in the respiratory control centers ofthe brainstem that help regulate the overall CO2 content andpH of the body.

FLUID SPACESCerebrospinal fluid is found in the subarachnoid spacearound the brain and spinal cord and within the cavities andcanals of the brain and spinal cord.

The large, fluid-filled spaces within the brain are calledventricles. There are four of them. Two of them, the lateral(or first and second) ventricles, are located one in eachhemisphere of the cerebrum. As you can see in Figure 13-4,the third ventricle is little more than a thin, vertical pocket offluid below and medial to the lateral ventricles. The fourthventricle is a tiny, diamond-shaped space where the cerebel-lum attaches to the back of the brainstem. Actually, thefourth ventricle is simply a slight expansion of the centralcanal extending up from the spinal cord.

FORMATION AND CIRCULATION OF CEREBROSPINAL FLUIDFormation of cerebrospinal fluid occurs mainly by separa-tion of fluid from blood in the choroid plexuses. Choroidplexuses are networks of capillaries that project from the piamater into the lateral ventricles and into the roofs of thethird and fourth ventricles. Each choroid plexus is coveredwith a sheet of a special type of ependymal (glial) cell thatreleases the cerebrospinal fluid into the fluid spaces. Fromeach lateral ventricle the fluid seeps through an opening, theinterventricular foramen (of Monro), into the third ventri-cle, then through a narrow channel, the cerebral aqueduct(or aqueduct of Sylvius), into the fourth ventricle (Fig-ure 13-5). Some of the fluid moves from the fourth ventricledirectly into the central canal of the cord. Some of it movesout of the fourth ventricle through openings in its roof, twolateral foramina (foramina of Luschka) and one medianforamen (foramen of Magendie). These openings allow cere-brospinal fluid to move into the cisterna magna, a space be-


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Figure 13-3 Coverings of the spinal cord. The dura mater is shown in purple. Notice how it extends tocover the spinal nerve roots and nerves. The arachnoid is highlighted in pink and the pia mater in orange.

Box 13-1 HEALTH MATTERS

Meningitis

Infection or inflammation of the meninges is termedmeningitis. It most often involves the arachnoid and pia

mater, or the leptomeninges (“thin meninges”). Meningitis ismost commonly caused by bacteria such as Neisseriameningitidis (meningococcus), Streptococcus pneumoniae,or Haemophilus influenzae. However, viral infections, my-coses (fungal infections), and tumors may also cause in-flammation of the meninges. Individuals with meningitisusually complain of fever and severe headaches, as well asneck stiffness and pain. Depending on the primary cause,meningitis may be mild and self-limiting or may progressto a severe, perhaps fatal, condition. If only the spinalmeninges are involved, the condition is called spinalmeningitis.

hind the medulla that is continuous with the subarachnoidspace around the brain and cord. The fluid circulates in thesubarachnoid space, then is absorbed into venous bloodthrough the arachnoid villi (fingerlike projections of thearachnoid membrane into the brain’s venous sinuses).

Briefly, here is the circulation route of cerebrospinal fluid: itis formed by separation of fluid from blood in the choroidplexuses into the ventricles of the brain, circulates throughthe ventricles and into the central canal and subarachnoidspaces, and is absorbed back into blood.


A B

Figure 13-4 Fluid spaces of the brain. The large figure shows the ventricles highlighted within the brain in aleft lateral view. The small figure shows the ventricles from above.

Box 13-2 DIAGNOSTIC STUDY

Lumbar Puncture

The extension of the meninges beyond the cord is conve-nient for performing lumbar punctures without danger of

injuring the spinal cord. A lumbar puncture is a withdrawalof some of the cerebrospinal fluid from the subarachnoidspace in the lumbar region of the spinal cord. The physicianinserts a needle just above or below the fourth lumbar ver-tebra, knowing that the spinal cord ends an inch or moreabove that level. The fourth lumbar vertebra can be easily lo-cated because it lies on a line with the iliac crest. Placing a pa-tient on the side and arching the back by drawing the kneesand chest together separates the vertebrae sufficiently to in-troduce the needle.

Cerebrospinal fluid removed through a lumbar puncturecan be tested for the presence of blood cells, bacteria, or otherabnormal characteristics that may indicate an injury or infec-tion. A sensor called a manometer is sometimes attached tothe needle to determine the pressure of the cerebrospinal fluidwithin the subarachnoid space. The lumbar puncture can alsobe used to introduce diagnostic agents such as radiopaquedyes for x-ray photography into the subarachnoid space.

Spinalcord

Thirdlumbar

vertebra

Subarachnoidspace

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The amount of cerebrospinal fluid in the average adult isabout 140 ml (about 23 ml in the ventricles and 117 ml inthe subarachnoid space of brain and cord). Box 13-2 ex-plains the diagnostic value of testing a patient’s cere-brospinal fluid. Box 13-3 shows what happens when the cir-culation of cerebrospinal fluid is blocked.

Subarachnoidspace

Choroid plexusof lateral ventricle

Superiorsagittal

sinus

Arachnoidvillus

Arachnoidvillus

Superiorsagittal sinus

Falx cerebri(dura mater)

Arachnoid layer

Piamater

Subarachnoidspace

Cerebral cortex

LateralforamenCisterna

magna

Dura mater

Interventricularforamen

Choroid plexusof third ventricle

Cerebral aqueduct

Choroid plexusof fourth ventricle

Medianforamen

Central canalof spinal cord

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Figure 13-5 Flow of cerebrospinal fluid. The fluid produced by filtration of blood by the choroid plexus ofeach ventricle flows inferiorly through the lateral ventricles, interventricular foramen, third ventricle, cerebralaqueduct, fourth ventricle, and subarachnoid space and to the blood.

1. Name the three membranous coverings of the centralnervous system in order, beginning with the outermostlayer.

2. Trace the path of cerebrospinal fluid from its formation by achoroid plexus to its reabsorption into the blood.

SPINAL CORD

STRUCTURE OF THE SPINAL CORDThe spinal cord lies within the spinal cavity, extending fromthe foramen magnum to the lower border of the first lumbarvertebra (Figure 13-6), a distance of about 45 cm (18 inches)in the average body. The spinal cord does not completely fill

the spinal cavity—it also contains the meninges, cerebrospinalfluid, a cushion of adipose tissue, and blood vessels.

The spinal cord is an oval-shaped cylinder that tapersslightly as it descends and has two bulges, one in the cervicalregion and the other in the lumbar region (see Figure 13-6).Two deep grooves, the anterior median fissure and the poste-



Hydrocephalus

Occasionally, some condition interferes with the circula-tion of cerebrospinal fluid. For example, a brain tumor

may press against the cerebral aqueduct, shutting off theflow of fluid from the third to the fourth ventricle. In such anevent the fluid accumulates within the lateral and third ven-tricles because it continues to form even though its drainageis blocked. This condition is known as internal hydro-cephalus. If the fluid accumulates in the subarachnoid spacearound the brain, external hydrocephalus results. Subarach-noid hemorrhage, for example, may lead to the formation ofblood clots that block drainage of the cerebrospinal fluid

from the subarachnoid space. With decreased drainage, anincreased amount of fluid remains in the space.

When internal hydrocephalus occurs in an infant whoseskull has not completely ossified, the increasing fluid pres-sure in the ventricles causes the cranium to swell (see figurebelow). This condition can be treated by surgical placementof a shunt, or tube, to drain the excess cerebrospinal fluid(see figure below). When this condition occurs in an olderchild or adult, the skull will not yield to the increasing fluidpressure. The pressure instead compresses the soft nervoustissue of the brain, potentially leading to coma or even death.

Hydrocephalus. A, Ventricles become swollen when the flow of cerebrospinal fluid is blocked. B, Surgicalplacement of a shunt can restore flow of CSF and reduce swelling.

rior median sulcus, just miss dividing the cord into separatesymmetrical halves. The anterior fissure is the deeper andthe wider of the two grooves—a useful factor to rememberwhen you examine spinal cord diagrams. It enables you totell at a glance which part of the cord is anterior and whichis posterior.

Two bundles of nerve fibers called nerve roots projectfrom each side of the spinal cord (see Figure 13-6). Fiberscomprising the dorsal nerve root carry sensory informationinto the spinal cord. Cell bodies of these unipolar, sensoryneurons make up a small region of gray matter in the dorsalnerve root called the dorsal root ganglion. Fibers of the ven-tral nerve root carry motor information out of the spinalcord. Cell bodies of these multipolar, motor neurons are inthe gray matter that composes the inner core of the spinalcord. Numerous interneurons are also located in the spinalcord’s gray matter core. On each side of the spinal cord, thedorsal and ventral nerve roots join together to form a singlemixed nerve called, simply, a spinal nerve. Spinal nerves,components of the peripheral nervous system, are consid-ered in more detail in the next chapter.

Although the spinal cord’s core of gray matter looks like aflat letter H in transverse sections of the cord, it actually hasthree dimensions, since the gray matter extends the length ofthe cord. The limbs of the H are called anterior, posterior, andlateral horns of gray matter, or gray columns (see Figure 13-6).They consist predominantly of cell bodies of interneuronsand motor neurons.

White matter surrounding the gray matter is subdividedin each half of the cord into three columns (or funiculi): theanterior, posterior, and lateral white columns. Each whitecolumn, or funiculus, consists of a large bundle of nerve

fibers (axons) divided into smaller bundles called tracts,shown in Figure 13-7. The names of most spinal cord tractsindicate the white column in which the tract is located, thestructure in which the axons that make up the tract origi-nate, and the structure in which they terminate. Examples:the lateral corticospinal tract is located in the lateral whitecolumn of the cord. The axons that compose it originatefrom neuron cell bodies in the spinal cortex (of the cere-brum) and terminate in the spinal cord. The anteriorspinothalamic tract lies in the anterior white column. Theaxons that compose it originate from neuron cell bodies inthe spinal cord and terminate in a portion of the brain calledthe thalamus.

You may wish to refer to pp. 1003-1004 of the Mini-Atlasat the end of this book, where you will find detailed photo-graphs of a human spinal cord. How many structures canyou identify without looking at the key?

FUNCTIONS OF THE SPINAL CORDThe spinal cord performs two general functions. Briefly, itprovides conduction routes to and from the brain and servesas the integrator, or reflex center, for all spinal reflexes.

Spinal cord tracts provide conduction paths to and fromthe brain. Ascending tracts conduct sensory impulses up thecord to the brain. Descending tracts conduct motor im-pulses down the cord from the brain. Bundles of axons com-pose all tracts. Tracts are both structural and functional or-ganizations of these nerve fibers. They are structuralorganizations: all of the axons of any one tract originatefrom neuron cell bodies located in the same area of the cen-tral nervous system, and all of the axons terminate in singlestructure elsewhere in the central nervous system. For exam-


Figure 13-6 Spinal cord. The inset illustrates a transverse section of the spinal cord shown in the broader view.

ple, all the fibers of the spinothalamic tract are axons origi-nating from neuron cell bodies located in the spinal cord andterminating in the thalamus. Tracts are functional organiza-tions: all the axons that compose one tract serve one generalfunction. For instance, fibers of the spinothalamic tractsserve a sensory function. They transmit impulses that pro-duce the sensations of crude touch, pain, and temperature.

Because so many different tracts make up the whitecolumns of the cord, we mention only a few of the moreimportant ones. Locate each tract in Figure 13-7. ConsultTables 13-1 and 13-2 for a brief summary of these tracts.

Five important ascending, or sensory, tracts and theirfunctions, stated very briefly, are as follows:

1. Lateral spinothalamic tracts. Crude touch, pain, andtemperature

2. Anterior spinothalamic tracts. Crude touch and pressure3. Fasciculi gracilis and cuneatus. Discriminating touch

and conscious sensation of position and movement ofbody parts (kinesthesia)

4. Spinocerebellar tracts. Subconscious kinesthesia5. Spinotectal. Touch that triggers visual reflexes


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Figure 13-7 Major tracts of the spinal cord. The major ascending (sensory) are highlighted in blue. Themajor descending (motor) tracts are highlighted in red.

Table 13-1 Major Ascending Tracts of Spinal Cord

Name Function Location Origin* Termination†

Lateral spinothalamic

Anterior spinothalamic

Fasciculi gracilis and cuneatus

Anterior and posterior spinocerebellar

Spinotectal

*Location of cell bodies of neurons from which axons of tract arise.†Structure in which axons of tract terminate.

Pain, temperature, and crude touch opposite

side

Crude touch and pressure

Discriminating touch and pressure sensations, in-

cluding vibration, stereognosis, and two-point

discrimination; also conscious kinesthesia

Unconscious kinesthesia

Touch related to visual reflexes

Lateral white

columns

Anterior white

columns

Posterior white

columns

Lateral white

columns

Lateral white

columns

Posterior gray column

opposite side

Posterior gray column

opposite side

Spinal ganglia same

side

Anterior or posterior

gray column

Posterior gray

columns

Thalamus

Thalamus

Medulla

Cerebellum

Superior collicu-

lus (midbrain)

Further discussion of the sensory neural pathways may befound on pp. 399-400.

Six important descending or motor tracts and their func-tions in brief are as follows:

1. Lateral corticospinal tracts. Voluntary movement; con-traction of individual or small groups of muscles, par-ticularly those moving hands, fingers, feet, and toes onopposite side of body

2. Anterior corticospinal tracts. Same as preceding exceptmainly muscles of same side of body

3. Reticulospinal tracts. Help maintain posture duringskeletal muscle movements

4. Rubrospinal tracts. Transmit impulses that coordinatebody movements and maintenance of posture

5. Tectospinal tracts. Head and neck movement related tovisual reflexes

6. Vestibulospinal tracts. Coordination of posture andbalance

Further discussion of motor neural pathways may befound on pp. 400-402.

The spinal cord also serves as the reflex center for allspinal reflexes. The term reflex center means the center of areflex arc or the place in the arc where incoming sensory im-pulses become outgoing motor impulses. They are structuresthat switch impulses from afferent to efferent neurons. Intwo-neuron arcs, reflex centers are merely synapses betweenneurons. In all other arcs, reflex centers consist of interneu-rons interposed between afferent and efferent neurons.

Spinal reflex centers are located in the gray matter of thecord. Box 13-4 discusses how reflex centers can act as paincontrol areas.

BRAINThe brain is one of the largest organs in adults (Box 13-5).It consists, in round numbers, of 100 billion neurons and900 billion glia. In most adults, it weighs about 1.4 kg (3 pounds). Neurons of the brain undergo mitotic cell divi-sion only during the prenatal period and the first fewmonths of postnatal life. Although they grow in size afterthat, they do not increase in number. Malnutrition duringthe crucial prenatal months of neuron multiplication is re-ported to hinder the process and result in fewer brain cells.The brain attains full size by about the eighteenth year butgrows rapidly only during the first 9 years or so.

Six major divisions of the brain, named from below, up-ward, are as follows: medulla oblongata, pons, midbrain, cere-bellum, diencephalon, and cerebrum. Very often the medullaoblongata, pons, and midbrain are referred to collectively asthe brainstem. Look at these three structures in Figure 13-8.Do you agree that they seem to form a stem for the rest ofthe brain?


Table 13-2 Major Descending Tracts of Spinal Cord

Name Function Location Origin* Termination†

Lateral corticospinal (or crossed pyramidal)

Anterior corticospinal (direct pyramidal)

Reticulospinal

Rubrospinal

Tectospinal

Vestibulospinal

*Location of cell bodies of neurons from which axons of tract arise.†Structure in which axons of tract terminate.

Voluntary movement, contraction of in-

dividual or small groups of muscles,

particularly those moving hands,

fingers, feet, and toes of opposite side

Same as lateral corticospinal except

mainly muscles of same side

Maintain posture during movement

Coordination of body movement and

posture

Head and neck movement during visual

reflexes

Coordination of posture/balance

Lateral white

columns

Anterior white

columns

Anterior white

columns

Lateral white

columns

Anterior white

columns

Anterior white

columns

Motor areas or cerebral

cortex opposite side

from tract location in

cord

Motor cortex but on same

side as location in cord

Reticular formation

(midbrain, pons, and

medulla)

Red nucleus (of midbrain)

Superior colliculus (mid-

brain)

Vestibular nucleus (pons,

medulla)

Lateral or ante-

rior gray

columns

Lateral or ante-

rior gray

columns

Anterior gray

columns

Anterior gray

columns

Medulla and

anterior gray

columns

Anterior gray

columns

1. What are spinal nerve roots? How does the dorsal rootdiffer from the ventral root?

2. Name the regions of the white and gray matter seen ina horizontal section of the spinal cord.

3. Contrast ascending tracts and descending tracts of the spinalcord. Can you give an example of each?


The term pain control area means a place in the pain con-duction pathway where impulses from pain receptors

can be inhibited. The first pain control area suggested wasa segment of the posterior gray horns of the spinal cord.Substantia gelatinosa is the name of this segment. Here,axon terminals of neurons that conduct from pain recep-tors to the spinal cord synapse with neurons that conductpain impulses up the spinal cord to the brain. Severaldecades ago, researchers made a surprising discoveryabout these synapses in the substantia gelatinosa. Theyfound they could inhibit pain conduction across them bystimulating skin touch receptors in a painful area. From thisknowledge, a new theory about pain developed—the aptlynamed gate-control theory of pain. According to this theory,the substantia gelatinosa functions as a gate that can closeand thus bar the entry of pain impulses into ascendingpaths to the brain. One way to close the gate is to stimulateskin touch receptors in a painful area. Today this is usuallydone by a device called the transcutaneous electrical nervestimulation (TENS) unit (see figure below). A patient usesthe TENS unit to apply a low level of stimulation for a longperiod. This results in closure of the spinal cord pain gateand relief of pain.

In recent years, pain control areas have been identified inthe brain, notably in the gray matter around the cerebralaqueduct and around the third ventricle. Neurons in theseareas send their axons down the spinal cord to terminate in

the substantia gelatinosa, where they release enkephalins.These chemicals act to prevent pain impulse conductionacross synapses in the substantia gelatinosa. In short,enkephalins tend to close the spinal cord pain gate. Brief, in-tense transcutaneous stimulation at trigger or acupuncturepoints has been found to relieve pain in distant sites. The in-tense stimulation is postulated to activate brain pain controlareas. They then send impulses down the cord to the sub-stantia gelatinosa, closing the spinal cord pain gate.

Box 13-4

Pain Control Areas

Transcutaneous electrical nerve stimulation (TENS) unit.

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CortexCorpuscallosum

Cerebellum

Thalamus

Pineal bodyHypothalamus

Midbrain

Pons

Medullaoblongata

Diencephalon

Brain-stem

Cerebrum

Figure 13-8 Divisions of the brain. A midsagittal section of the brain reveals features of its major divisions.

STRUCTURE OF THE BRAINSTEMThree divisions of the brain make up the brainstem. Themedulla oblongata forms the lowest part of the brainstem,the midbrain forms the uppermost part, and the pons liesbetween them, that is, above the medulla and below themidbrain.

Medulla OblongataThe medulla oblongata is the part of the brain that attachesto the spinal cord. It is, in fact, an enlarged extension of thespinal cord located just above the foramen magnum. It mea-sures only a few centimeters (about an inch) in length and isseparated from the pons above by a horizontal groove. It iscomposed of white matter (projection tracts) and a networkof gray and white matter called the reticular formation (seeFigure 13-18).

The pyramids (Figure 13-9) are two bulges of white mat-ter located on the ventral surface of the medulla. Fibers ofthe so-called pyramidal tracts form the pyramids.

The olive (see Figure 13-9) is an oval projection appear-ing one on each side of the ventral surface of the medulla,lateral to the pyramids.

Located in the medulla’s reticular formation are variousnuclei, or clusters of neuron cell bodies. Some nuclei arecalled control centers—for example, the cardiac, respiratory,and vasomotor control centers (Box 13-6).

PonsJust above the medulla lies the pons, composed, like themedulla, of white matter and reticular formation. Fibers thatrun transversely across the pons and through the middlecerebellar peduncles into the cerebellum make up the exter-


Box 13-5 FYI

Studying Human Brains

As with any part of the body, the best way to learn theanatomy of the brain is to look at actual specimens. Re-

call from Chapter 1 (Figure 1-1, p. 5) that this is the traditionalmethod of learning human anatomy. When cadavers are notavailable for this purpose, one useful alternative is usingphotographs of well-dissected cadavers.

In Figure A below is an oblique frontal (coronal) section ofthe human brain, as seen from the front of the subject. Thisphotograph shows details of the internal features of all themajor brain divisions. Compare this view of the brain with Fig-ure B, which shows the brain cut on horizontal (transverse)

planes at two slightly different levels, as seen from above. Thencompare these photographs with those shown in the Mini-Atlas at the back of this book, p. 1002. How do sections in dif-ferent planes of the brain benefit your understanding of thethree-dimensional aspects of brain anatomy? What benefitsare there to studying the brains of human cadavers, ratherthan relying solely on artist’s renderings of the brain? Are thereany advantages to using artist’s renderings of brain anatomy?

Refer to these photographs—and those in the Mini-Atlasat the back of this book—often as you study the details ofbrain anatomy.

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Human brain specimens. A, Oblique frontal section. B, Horizontal sections (left section is slightly inferior toright section).

nal white matter of the pons and give it its arching, bridge-like appearance.

MidbrainThe midbrain (mesencephalon) is appropriately named. Itforms the midsection of the brain, because it lies above thepons and below the cerebrum. Both white matter (tracts)and reticular formation compose the midbrain. Extendingdivergently through it are two ropelike masses of white mat-ter named cerebral peduncles (see Figure 13-9). Tracts in thepeduncles conduct impulses between the midbrain and cere-brum. In addition to the cerebral peduncles, another land-

mark of the midbrain is the corpora quadrigemina (literally,“body of four twins”). The corpora quadrigemina are twoinferior colliculi and two superior colliculi. Notice the locationof the two sets of twin colliculi, or the corpora quadrigem-ina, in Figure 13-9, B. They form the posterior, upper part ofthe midbrain, the part that lies just above the cerebellum.Certain auditory centers are located in the inferior collicu-lus. The superior colliculus contains visual centers. Twoother midbrain structures are the red nucleus and the sub-stantia nigra. Each of these consists of clusters of cell bodiesof neurons involved in muscular control. The substantia ni-gra (literally, “black matter”) gets its name from the darkpigment in some of its cells.

FUNCTIONS OF THE BRAINSTEMThe brainstem, like the spinal cord, performs sensory, motor,and reflex functions. The spinothalamic tracts are importantsensory tracts that pass through the brainstem on their wayto the thalamus in the diencephalon. The fasciculi cuneatusand gracilis and the spinoreticular tracts are sensory tractswhose axons terminate in the gray matter of the brainstem.Corticospinal and reticulospinal tracts are two of the majortracts present in the white matter of the brainstem.

Nuclei in the medulla contain a number of reflex centers.Of first importance are the cardiac, vasomotor, and respira-tory centers. Other centers present in the medulla are forvarious nonvital reflexes such as vomiting, coughing, sneez-ing, hiccupping, and swallowing.



Vital Centers

Because the cardiac, vasomotor, and respiratory controlcenters are essential for survival, they are called the vi-

tal centers. They serve as the centers for various reflexescontrolling heart action, blood vessel diameter, and respi-ration. Because the medulla contains these centers, it is themost vital part of the entire brain—so vital that injury ordisease of the medulla often proves fatal. Blows at the baseof the skull and bulbar poliomyelitis, for example, causedeath if they interrupt impulse conduction in the vital res-piratory centers.

S

R L

I

Intermediate mass

Cerebralpeduncle

Medullaoblongata

Thalamus

Thalamus

Optic chiasma

Hypothalamus

Pyramid Diencephalon

Diencephalon

Diencephalon

Pineal body

Midbrain

Superior colliculi

Inferior colliculi

Cerebral peduncle

Superior cerebellar peduncles

Fourth ventricle

Inferior cerebellarpeduncles

Medulla oblongata

Brain-stem

Middlecerebellarpeduncle

Midbrain

Pons

Pons

Olive

Olive

A

B

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L R

I

Figure 13-9 The brainstem and diencephalon. A, Anterior aspect. B, Posterior aspect (shifted slightly to lateral).

The pons contains centers for reflexes mediated by thefifth, sixth, seventh, and eighth cranial nerves. The locationsand functions of these peripheral nerves are discussed inChapter 14. In addition, the pons contains the pneumotaxiccenters that help regulate respiration.

The midbrain, like the pons, contains reflex centers forcertain cranial nerve reflexes, for example, pupillary reflexesand eye movements, mediated by the third and fourth cra-nial nerves, respectively.

STRUCTURE OF THE CEREBELLUMThe cerebellum, the second largest part of the brain, is lo-cated just below the posterior portion of the cerebrum andis partially covered by it (Figure 13-10). A transverse fissureseparates the cerebellum from the cerebrum. These twoparts of the brain have several characteristics in common.For instance, gray matter makes up the outer portion, or cor-tex, of each. White matter predominates in the interior ofeach. Observe the arbor vitae, that is, the internal white mat-ter of the cerebellum in Figure 13-10. Note its distinctivepattern, similar to the veins of a leaf. Note, too, that the sur-faces of both the cerebellum and the cerebrum have numer-ous grooves (sulci) and raised areas (gyri). The gyri of thecerebellum, however, are much more slender and less promi-nent than those of the cerebrum. Like the cerebrum, thecerebellum consists of two large lateral masses, the cerebellarhemispheres, and a central section called the vermis.

The internal white matter of the cerebellum is composedof some short and some long tracts. The shorter tracts con-duct impulses from neuron cell bodies located in the cere-bellar cortex to neurons whose dendrites and cell bodiescompose nuclei located in the interior of the cerebellum.The longer tracts conduct impulses to and from the cerebel-lum. Fibers of the longer tracts enter or leave the cerebellumby way of its three pairs of peduncles (see Figure 13-9), asfollows:

1. Inferior cerebellar peduncles. Composed chiefly oftracts into the cerebellum from the medulla and cord

(notably, spinocerebellar, vestibulocerebellar, and reticu-locerebellar tracts)

2. Middle cerebellar peduncles. Composed almost entirelyof tracts into the cerebellum from the pons, that is, pon-tocerebellar tracts

3. Superior cerebellar peduncles. Composed principally oftracts from dentate nuclei in the cerebellum through thered nucleus of the midbrain to the thalamus

An important pair of cerebellar nuclei are the dentate nu-clei, one of which lies in each hemisphere. Tracts connectthese nuclei with the thalamus and with motor areas of thecerebral cortex. By means of these tracts, cerebellar impulsesinfluence the motor cortex. Impulses in other tracts enablethe motor cortex to influence the cerebellum.


S

PA

I

Diencephalon

Cerebellum

Brain-stem

Fourth ventricle

Vermis

Lateral hemisphere

GyrusSulcus

Arbor vitae

Cerebrum

Figure 13-10 The cerebellum. This midsagittal section shows features of the cerebrum and surroundingstructures of the brain.


Parkinson Disease

The importance of the cerebral nuclei in regulating vol-untary motor functions is made clear in cases of Parkin-

son disease. Normally, neurons that lead from the sub-stantia nigra to the cerebral nuclei secrete dopamine.Dopamine inhibits the excitatory effects of acetylcholineproduced by other neurons in the cerebral nuclei. Such in-hibition produces a balanced, restrained output of muscle-regulating signals from the cerebral nuclei. In Parkinsondisease, however, neurons leading from the substantia ni-gra degenerate and thus do not release normal amountsof dopamine. Without dopamine, the excitatory effects ofacetylcholine are not restrained, and the cerebral nucleiproduce an excess of signals that affect voluntary musclesin several areas of the body. Overstimulation of posturalmuscles in the neck, trunk, and upper limbs produces thesyndrome of effects that typify this disease: rigidity andtremors of the head and limbs; an abnormal, shuffling gait;absence of relaxed arm-swinging while walking; and a for-ward tilting of the trunk.

FUNCTIONS OF THE CEREBELLUMThe cerebellum performs three general functions, all ofwhich have to do with the control of skeletal muscles:

1. Acts with the cerebral cortex to produce skilled move-ments by coordinating the activities of groups ofmuscles

2. Helps control posture. It functions below the level ofconsciousness to make movements smooth instead ofjerky, steady instead of trembling, and efficient and coordinated instead of ineffective, awkward, and uncoordinated.

3. Controls skeletal muscles to maintain balance

There have been many theories about cerebellar functions.One theory, based on comparative anatomy studies and sub-stantiated by experimental methods, regards the cerebellumas three organs, each with a somewhat different function: co-ordinated control of muscle action, excitation and inhibitionof postural reflexes, and maintenance of balance.

Coordinated control of muscle action, which is ascribedto the neocerebellum (superior vermis and hemispheres), isclosely associated with cerebral motor activity. Normal mus-cle action involves groups of muscles, the various membersof which function together as a unit. In any given action, forexample, the prime mover contracts and the antagonist re-laxes but then contracts weakly at the proper moment to act

as a brake, checking the action of the prime mover. Also, thesynergists contract to assist the prime mover, and the fixa-tion muscles of the neighboring joint contract. Throughsuch harmonious, coordinated group action, normal move-ments are smooth, steady, and precise as to force, rate, andextent.

Achievement of such movements results from cerebellaractivity added to cerebral activity. Impulses from the cere-brum may start the action, but those from the cerebellumsynergize or coordinate the contractions and relaxations ofthe various muscles once they have begun. Figure 13-11shows how the cerebrum and cerebellum work together. Im-pulses from the motor control areas of the cerebrum traveldown the corticospinal tract and, through peripheral nerves,to skeletal muscle tissue. At the same time, the impulses goto the cerebellum. The cerebellum compares the motor com-mands of the cerebrum to information coming from recep-tors in the muscle. In effect, the cerebellum compares the in-tended movement to the actual movement. Impulses thentravel from the cerebellum to both the cerebrum and themuscle tissue to adjust or coordinate the movements to pro-duce the intended action. Most physiologists consider thisthe main function of the cerebellum.

One part of the cerebellum is also thought to be con-cerned with both exciting and inhibiting postural reflexes.

In addition, part of the cerebellum presumably dischargesimpulses important to the maintenance of balance. Sensoryimpulses from equilibrium receptors in the ear reach thecerebellum. Here, connections are made with the propermotor fibers for contraction of the necessary muscles for sta-bilizing the body.

Cerebellar disease (e.g., abscess, hemorrhage, tumors,trauma) produces certain characteristic symptoms. Predom-inant among them are ataxia (muscle incoordination), hy-potonia, tremors, and disturbances of gait and balance. Oneexample of ataxia is overshooting a mark or stopping beforereaching it when trying to touch a given point on the body(finger-to-nose test). Drawling, scanning, and singsongspeech are also examples of ataxia. Tremors are particularlypronounced toward the end of the movements and with theexertion of effort. Disturbances of gait and balance vary, de-pending on the muscle groups involved. The walk, for in-stance, is often characterized by staggering or lurching andby a clumsy manner of raising the foot too high and bring-ing it down with a clap. Paralysis does not result from loss ofcerebellar function.

DIENCEPHALONThe diencephalon (literally, “between-brain”) is the part ofthe brain located between the cerebrum and the midbrain(mesencephalon). Although the diencephalon consists of


Uppermotor neuron

Corticospinaltract

Rubrospinal tract

Spinal cord

Lower motor neuron

Peripheralnerve

Muscle

Proprioceptor

Spinocerebellar tract

Cerebellum

Rednucleus

Thalamus

Cerebrum(motor cortex)

cortex

Figure 13-11 Coordinating function of the cerebellum. Impulsesfrom the motor control areas of the cerebrum travel down to skeletalmuscle tissue and to the cerebellum at the same time. The cerebellum,which also receives sensory information from the muscle tissue, com-pares the intended movement to the actual movement. It then sendsimpulses to both the cerebrum and the muscle tissue, thus coordinat-ing and “smoothing” muscle activity.

1. Name the three major divisions of the brainstem andbriefly describe the function of each.

2. What are gyri? What are sulci?3. Describe how the cerebellum works with the cerebrum to coor-

dinate muscle activity.

several structures located around the third ventricle, themain ones are the thalamus and hypothalamus. The dien-cephalon also includes the optic chiasma, the pineal body,and several other small but important structures.

ThalamusThe thalamus is a dumbbell-shaped mass of gray mattermade up of many nuclei. As Figures 13-9 and 13-12 show,each lateral mass of the thalamus forms one lateral wall ofthe third ventricle. Extending through the third ventricle,and thus joining the two lateral masses of the thalamus, isthe intermediate mass. Two important groups of nuclei thatmake up the thalamus are the geniculate bodies, located inthe posterior region of each lateral mass. The geniculatebodies play a role in processing auditory and visual input.

Large numbers of axons conduct impulses into the thala-mus from the spinal cord, brainstem, cerebellum, cerebralnuclei, and various parts of the cerebrum. These axons ter-minate in thalamic nuclei, where they synapse with neuronswhose axons conduct impulses out of the thalamus to virtu-ally all areas of the cerebral cortex. Thus the thalamus servesas the major relay station for sensory impulses on their wayto the cerebral cortex.

The thalamus performs the following primary functions:

1. Plays two parts in the mechanism responsible forsensationsa. Impulses from appropriate receptors, on reaching

the thalamus, produce conscious recognition ofthe crude, less critical sensations of pain, tempera-ture, and touch

b. Neurons whose dendrites and cell bodies lie incertain nuclei of the thalamus relay all kinds of

sensory impulses, except possibly olfactory, to thecerebrum

2. Plays a part in the mechanism responsible for emo-tions by associating sensory impulses with feelings ofpleasantness and unpleasantness

3. Plays a part in the arousal or alerting mechanism4. Plays a part in mechanisms that produce complex

reflex movements

HypothalamusThe hypothalamus consists of several structures that liebeneath the thalamus and form the floor of the third ven-tricle and the lower part of its lateral walls. Prominentamong the structures composing the hypothalamus arethe supraoptic nuclei, the paraventricular nuclei, and themamillary bodies. The supraoptic nuclei consist of graymatter located just above and on either side of the opticchiasma. The paraventricular nuclei of the hypothalamusare named for their location close to the wall of the thirdventricle. The midportion of the hypothalamus gives riseto the infundibulum, the stalk leading to the posteriorlobe of the pituitary gland (neurohypophysis). The poste-rior part of the hypothalamus consists mainly of themamillary bodies (see Figure 13-19), which are involvedwith the olfactory sense (smell).

The hypothalamus is a small but functionally importantarea of the brain. It weighs little more than 7 g (1⁄4 oz), yet itperforms many functions of the greatest importance bothfor survival and for the enjoyment of life. For instance, itfunctions as a link between the psyche (mind) and the soma(body). It also links the nervous system to the endocrine sys-tem. Certain areas of the hypothalamus function as pleasurecenters or reward centers for the primary drives such as eat-


Intermediate mass of thalamus

Right lateral mass ofthalamus

Pineal body

CorpusCallosum

Fornix

Hypothalamus

Third ventricle

Optic chiasma

Cerebralcortex

Pituitary gland Infundibulum

Cerebrum

Midbrain

DiencephalonA

S

P

I

Figure 13-12 Diencephalon and surrounding structures. This midsagittal section highlights the largestregions of the diencephalon, the thalamus and hypothalamus, but also shows the smaller optic chiasma andpineal body. Note the position of the diencephalon between the midbrain and the cerebrum. Compare this viewof the diencephalon with that in Figure 13-9.

ing, drinking, and sex. The following paragraphs give a briefsummary of hypothalamic functions.

1. The hypothalamus functions as a higher autonomiccenter or, rather, as several higher autonomic centers. Bythis we mean that axons of neurons whose dendrites andcell bodies lie in nuclei of the hypothalamus extend intracts from the hypothalamus to both parasympatheticand sympathetic centers in the brainstem and cord.Thus impulses from the hypothalamus can simultane-ously or successively stimulate or inhibit few or manylower autonomic centers. In other words, the hypothala-mus serves as a regulator and coordinator of autonomicactivities. It helps control and integrate the responsesmade by autonomic (visceral) effectors all over the body.

2. The hypothalamus functions as the major relay stationbetween the cerebral cortex and lower autonomiccenters. Tracts conduct impulses from various centers inthe cortex to the hypothalamus. Then, by way of numer-ous synapses in the hypothalamus, these impulses arerelayed to other tracts that conduct them on down toautonomic centers in the brainstem and cord and also to spinal cord somatic centers (anterior horn motorneurons). Thus the hypothalamus functions as the link between the cerebral cortex and lower centers—hence between the psyche and the soma. It provides a crucial part of the route by which emotions canexpress themselves in changed bodily functions. It is the all-important relay station in the neural pathwaysthat makes possible the mind’s influence over thebody—sometimes, unfortunately, even to the profounddegree of producing “psychosomatic disease.” The posi-tive benefits of this mind-body link are the dramatic influences our conscious mind can have in healing thebody of various illnesses.

3. Neurons in the supraoptic and paraventricular nuclei of the hypothalamus synthesize the hormones releasedby the posterior pituitary gland (neurohypophysis).Because one of these hormones affects the volume ofurine excreted, the hypothalamus plays an indirect but essential role in maintaining water balance (seeChapters 28 and 29).

4. Some neurons in the hypothalamus have endocrinefunctions. Their axons secrete chemicals, releasing hor-mones, into blood, which circulate to the anterior pitu-itary gland. Releasing hormones control the release ofcertain anterior pituitary hormones—specifically growthhormone and hormones that control hormone secretionby sex glands, thyroid gland, and the adrenal cortex (dis-cussed in Chapter 16). Thus indirectly the hypothalamushelps control the functioning of every cell in the body.

5. The hypothalamus plays an essential role in maintainingthe waking state. Presumably it functions as part of anarousal or alerting mechanism. Clinical evidence of thisis that somnolence (sleepiness) characterizes some hy-pothalamic disorders.

6. The hypothalamus functions as a crucial part of themechanism for regulating appetite and therefore theamount of food intake. Experimental and clinical find-ings indicate the presence of an “appetite center” in thelateral part of the hypothalamus and a “satiety center”located medially. For example, an animal with an experi-mental lesion in the ventromedial nucleus of the hypo-thalamus consumes tremendous amounts of food.Similarly, a human being with a tumor in this region of the hypothalamus may eat insatiably and gain anenormous amount of weight.

7. The hypothalamus functions as a crucial part of themechanism for maintaining normal body temperature.Hypothalamus neurons whose fibers connect with auto-nomic centers for vasoconstriction, dilation, and sweat-ing and with somatic centers for shivering constituteheat-regulating centers. Marked elevation of body tem-perature frequently characterizes injuries or other ab-normalities of the hypothalamus.

Pineal BodyAlthough the thalamus and hypothalamus account for mostof the tissue that makes up the diencephalon, there are sev-eral smaller structures of importance. For example, the opticchiasma is a region where the right and left optic nerves crosseach other before entering the brain—exchanging fibers asthey do so. The resulting bundles of fibers are called the op-tic tracts. Various small nuclei just outside the thalamus andhypothalamus, collectively referred to as the epithalamus, arealso included among the structures of the diencephalon.One of the most intriguing of the epithalamic structures isthe pineal body, or epiphysis.

As Figures 13-12 and 13-15 show, the pineal body is locatedjust above the corpora quadrigemina of the midbrain. Itsname comes from the fact that it resembles a tiny pine cone.The functions of the pineal body are still not well understood,but it seems to be involved in regulating the body’s biologicalclock. It does produce some hormones, most notably mela-tonin, so it is also called the pineal gland. Melatonin is thoughtto synchronize various body functions with each other andwith external stimuli. More discussion regarding the functionof the pineal body appears in Chapter 16.

STRUCTURE OF THE CEREBRUMCerebral CortexThe cerebrum, the largest and uppermost division of thebrain, consists of two halves, the right and left cerebralhemispheres. The surface of the cerebrum—called the cere-bral cortex—is made up of gray matter only 2 to 4 mm(roughly 1⁄12 to 1⁄6 inch) thick. But despite its thinness, the


1. What are the two main components of the dien-cephalon? Where are they located?

2. Name three general functions of the thalamus.3. Name three general functions of the hypothalamus.

cortex has six layers, each composed of millions of axon ter-minals synapsing with millions of dendrites and cell bodiesof other neurons.

If one uses a little imagination, the surface of the cerebralcortex looks like a group of small sausages. Each “sausage”represents a convolution, or gyrus. Names of some of theseare the precentral gyrus, postcentral gyrus, cingulate gyrus,and hippocampal gyrus.

Between adjacent gyri lie either shallow grooves calledsulci or deeper grooves called fissures. Fissures, as well as afew, largely imaginary boundaries, divide each cerebralhemisphere into five lobes. Four of the lobes are named forthe bones that lie over them: frontal lobe, parietal lobe, tem-poral lobe, and occipital lobe (Figure 13-13). A fifth lobe,the insula (island of Reil), lies hidden from view in the lateralfissure. The lobes are highlighted in Figure 13-13. The insulacan also be seen in the photographs on p. 385. Names and lo-cations of prominent cerebral fissures are the following (seeFigure 13-13):

1. Longitudinal fissure. The deepest groove in the cere-brum; divides the cerebrum into two hemispheres

2. Central sulcus (fissure of Rolando). Groove between thefrontal and parietal lobes

3. Lateral fissure (fissure of Sylvius). A deep groovebetween the temporal lobe below and the frontal andparietal lobes above; island of Reil lies deep in the lateralfissure

4. Parietooccipital fissure. Groove that separates the occip-ital lobe from the parietal lobe

Cerebral Tracts and Cerebral NucleiBeneath the cerebral cortex lies the large interior of the cere-brum. It is mostly white matter made up of numerous tracts.

A few islands of gray matter, however, lie deep inside thewhite matter of each hemisphere. Collectively these arecalled cerebral nuclei (or historically, basal ganglia). Tractsthat make up the cerebrum’s internal white matter are ofthree types: projection tracts, association tracts, and com-missural tracts (Figure 13-14). Projection tracts are exten-sions of the ascending, or sensory, spinothalamic tracts anddescending, or motor, corticospinal tracts. Association tractsare the most numerous of cerebral tracts; they extend fromone convolution to another in the same hemisphere. Com-missural tracts, in contrast, extend from a point in one hemi-sphere to a point in the other hemisphere. Commissuraltracts compose the corpus callosum (prominent whitecurved structure seen in Figure 13-12) and the anterior andposterior commissures.

Cerebral nuclei, seen in Figure 13-15, include the follow-ing masses of gray matter in the interior of each cerebralhemisphere:

1. Caudate nucleus. Observe the curving “tail” shape ofthis cerebral nucleus

2. Lentiform nucleus. So named because of its lenslikeshape; note in Figure 13-18 that the lentiform nucleusconsists of two structures, the putamen and the globuspallidus; the putamen lies lateral to the globus pallidus(also called the pallidum)

3. Amygdaloid nucleus. Observe the location of thisalmond-shaped structure at the tip of the caudatenucleus (the term amygdaloid literally means “like analmond”)

A structure associated with the cerebral nuclei is the in-ternal capsule. It is a large mass of white matter located, asFigure 13-15 shows, between the caudate and lentiform nu-


Insula(island of

Reil)

Temporal lobe

Central sulcus

Superior frontal gyrus

Frontallobe

Lateralfissure

Occipitallobe

Parietooccipitalfissure

Parietal lobe

Postcentral gyrus

A

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Figure 13-13 Left hemisphere of cerebrum, lateral surface.


Cerebralcortex

Commissural fibers(corpus callosum)

Cerebralnuclei

White matter

Thalamus

Projection fibers

Association fibers

S

I

LR

S

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PA

A

B

B

A

Amygdaloid nucleus

Substantia nigra(in midbrain)

Thalamus

Caudate nucleus

Lentiform nucleusCerebral nuclei

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Body ofcaudate nucleus

Internal capsule

Putamen

Globuspallidus

Thalamus

Mamillary body

Head of caudate nucleus Putamen

Lentiformnucleus

Corpusstriatum

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R L

A

B

Figure 13-14 Cerebral tracts. A, Lateral perspective, showing various association fibers. B, Frontal (coronal)perspective, showing commissural fibers that make up the corpus callosum and the projection fibers that com-municate with lower regions of the nervous system.

Figure 13-15 Cerebral nuclei. A, The cerebral nucleiseen through the cortex of the left cerebral hemisphere.B, The cerebral nuclei seen in a frontal (coronal) sectionof the brain.

clei and between the lentiform nucleus and thalamus. Thecaudate nucleus, internal capsule, and lentiform nucleusconstitute the corpus striatum. The term means “stripedbody.”

Researchers are still investigating the exact functions ofthe cerebral nuclei, but we already know that this part of thecerebrum plays an important role in regulating voluntarymotor functions. For example, most of the muscle contrac-tions involved in maintaining posture, walking, and per-forming other gross or repetitive movements seem to be ini-tiated or modulated in the cerebral nuclei.

FUNCTIONS OF THE CEREBRAL CORTEXFunctional Areas of the CortexDuring the past decade or so, research scientists in variousfields—neurophysiology, neurosurgery, neuropsychiatry,and others—have added mountains of information to ourknowledge about the brain. However, questions come fasterthan answers, and clear, complete understanding of thebrain’s mechanisms still eludes us. Perhaps it always will.

Perhaps the capacity of the human brain falls short of theability to fully understand its own complexity.

We do know that certain areas of the cortex in each hemi-sphere of the cerebrum engage predominantly in one partic-ular function—at least on the average. Differences betweengenders and among individuals of both genders are not un-common. The fact that many cerebral functions have a typi-cal location is known as the concept of cerebral localization.The fact that localization of function varies from person toperson, and even at different times in an individual when thebrain is damaged, is called cerebral plasticity.

The function of each region of the cerebral cortex de-pends on the structures with which it communicates. For ex-ample, the postcentral gyrus (Figures 13-16 and 13-17)functions mainly as a general somatic sensory area. It re-ceives impulses from receptors activated by heat, cold, andtouch stimuli. The precentral gyrus, on the other hand, func-tions chiefly as the somatic motor area (see Figures 13-16and 13-17). Impulses from neurons in this area descend overmotor tracts and eventually stimulate somatic effectors, theskeletal muscles. The transverse gyrus of the temporal lobeserves as the primary auditory area. The primary visual areasare in the occipital lobe. It is important to remember that nopart of the brain functions alone. Many structures of thecentral nervous system must function together for any onepart of the brain to function normally.


Premotorarea

Precentralgyrus

(primarysomatic

motor area)

Central sulcus

Postcentral gyrus(primary somatic

sensory area)

Primarytaste area

Somaticsensoryassociation area

Visual associationarea

Visualcortex

Wernicke’s area(sensory speech area)

Transverse gyrus

Auditoryassociation area

Primaryauditory area

Broca’s area(motor speech area)

Prefrontalarea

A

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Figure 13-16 Functional areas of the cerebral cortex.

1. Name the five lobes that comprise each cerebral hemi-sphere. Where is each located?

2. Name the cerebral nuclei and describe where they arelocated within the cerebrum.

Sensory Functions of the CortexVarious areas of the cerebral cortex are essential for normalfunctioning of the somatic, or “general,” senses, as well as theso-called “special senses.” The somatic senses include sensa-tions of touch, pressure, temperature, body position (pro-prioception), and similar perceptions that do not requirecomplex sensory organs. The special senses include vision,hearing, and other types of perception that require complexsensory organs, for example, the eye and the ear.

As stated earlier, the postcentral gyrus serves as a primaryarea for the general somatic senses. As Figure 13-17, A,shows, sensory fibers carrying information from receptors inspecific parts of the body terminate in specific regions of thesomatic sensory area. In other words, the cortex contains asort of “somatic sensory map” of the body. Areas such as theface and hand have a proportionally larger number of sen-sory receptors, so their part of the somatic sensory map islarger. Likewise, information regarding vision is mapped inthe visual cortex, and auditory information is mapped in theprimary auditory area (see Figure 13-16).

The cortex does more than just register separate and sim-ple sensations, however. Information sent to the primary sen-sory areas is in turn relayed to the various sensory associationareas, as well as to other parts of the brain. There the sensoryinformation is compared and evaluated. Eventually, the cortexintegrates separate bits of information into whole perceptions.

Suppose, for example, that someone put an ice cube inyour hand. You would, of course, see it and sense somethingcold touching your hand. But also, you would probablyknow that it was an ice cube because you would perceive atotal impression compounded of many sensations such astemperature, shape, size, color, weight, texture, and move-ment and position of your hand and arm.

Discussion of somatic sensory pathways begins on p. 399.The special senses are discussed in Chapter 15.

Motor Functions of the CortexMechanisms that control voluntary movements are ex-tremely complex and imperfectly understood. It is known,however, that for normal movements to take place, manyparts of the nervous system—including certain areas of thecerebral cortex—must function.

The precentral gyrus, that is, the most posterior gyrus ofthe frontal lobe, constitutes the primary somatic motor area(see Figures 13-16 and 13-17, B). A secondary motor arealies in the gyrus immediately anterior to the precentralgyrus. Neurons in the precentral gyrus are said to control in-dividual muscles, especially those that produce movementsof distal joints (wrist, hand, finger, ankle, foot, and toemovements). Notice in Figure 13-17 that the primary so-matic motor area is mapped according to the specific areasof the body it controls. Neurons in the premotor area justanterior to the precentral gyrus are thought to activategroups of muscles simultaneously.

Motor pathways descending from the cerebrum throughthe brainstem and spinal cord are discussed on pp. 400-402.Autonomic motor pathways are discussed in Chapter 14.


Figure 13-17 Primary somatic sensory (A) and motor (B) areasof the cortex. The body parts illustrated here show which parts ofthe body are “mapped” to specific areas of each cortical area. The ex-aggerated face indicates that more cortical area is devoted to pro-cessing information to/from the many receptors and motor units ofthe face than for the leg or arm, for example.

Integrative Functions of the Cortex“Integrative functions” is a nebulous phrase. Even more ob-scure, however, are the neural processes it designates. Theyconsist of all events that take place in the cerebrum betweenits reception of sensory impulses and its sending out of mo-tor impulses. Integrative functions of the cerebrum includeconsciousness and mental activities of all kinds. Conscious-ness, use of language, emotions, and memory are the inte-grative cerebral functions that we shall discuss—but onlybriefly.

Consciousness. Consciousness may be defined as a stateof awareness of one’s self, one’s environment, and other be-ings. Very little is known about the neural mechanisms thatproduce consciousness. We do know, however, that con-sciousness depends on excitation of cortical neurons by im-pulses conducted to them by a network of neurons known asthe reticular activating system. The reticular activating sys-tem consists of centers in the brainstem’s reticular formationthat receive impulses from the spinal cord and relay them tothe thalamus and from the thalamus to all parts of the cere-bral cortex (Figure 13-18). Both direct spinal reticular tractsand collateral fibers from the specialized sensory tracts(spinothalamic, lemniscal, auditory, and visual) relay im-pulses over the reticular activating system to the cortex.Without continual excitation of cortical neurons by reticularactivating impulses, an individual is unconscious and cannotbe aroused. Here, then, are two current concepts about thereticular activating system: (1) it functions as the arousal oralerting system for the cerebral cortex, and (2) its function-ing is crucial for maintaining consciousness. Drugs knownto depress the reticular activating system decrease alertnessand induce sleep. Barbiturates, for example, act this way.Amphetamine, on the other hand, a drug known to stimu-late the cerebrum and to enhance alertness and producewakefulness, probably acts by stimulating the reticular acti-vating system.

Certain variations in the levels or state of consciousnessare normal. All of us, for example, experience different levelsof wakefulness. At times, we are highly alert and attentive. Atother times, we are relaxed and nonattentive. All of us alsoexperience different levels of sleep. Two of the best-knownstages are called slow-wave sleep (SWS) and rapid eye move-ment (REM) sleep. Slow-wave sleep takes its name from theslow frequency, high-voltage brain waves that identify it. It isalmost entirely a dreamless sleep. Rapid eye movement sleep,on the other hand, is associated with dreaming.

In addition to the various normal states of consciousness,altered states of consciousness also occur under certain con-ditions. Anesthetic drugs produce an altered state of con-sciousness, namely, anesthesia. Disease or injury of the brainmay produce an altered state called coma.

Peoples of various cultures have long been familiar withan altered state called meditation. Meditation is a wakingstate but differs markedly in certain respects from the usualwaking state. According to some, meditation is a “higher” or“expanded” level of consciousness. This higher conscious-

ness is accompanied, almost paradoxically, by a high degreeof both relaxation and alertness. With training in meditationtechniques and practice, an individual can enter the medita-tive state at will and remain in it for an extended period oftime.

Language. Language functions consist of the ability tospeak and write words and the ability to understand spokenand written words. Certain areas in the frontal, parietal, andtemporal lobes serve as speech centers—as crucial areas, thatis, for language functions. The left cerebral hemisphere con-tains these areas in about 90% of the population; in the re-maining 10%, either the right hemisphere or both hemi-spheres contain them. Lesions in speech centers give rise tolanguage defects called aphasias. For example, with damageto an area in the inferior gyrus of the frontal lobe (Broca’sarea, see Figure 13-16), a person becomes unable to articu-late words but can still make vocal sounds and understandwords heard and read.

Emotions. Emotions—both the subjective experiencingand objective expression of them—involve functioning of thecerebrum’s limbic system. The name limbic (Latin for “borderor fringe”) suggests the shape of the cortical structures thatmake up the system. They form a curving border around thecorpus callosum, the structure that connects the two cerebralhemispheres. Look now at Figure 13-19. Here on the medialsurface of the cerebrum lie most of the structures of the lim-bic system. They are the cingulate gyrus and the hippocam-pus (the extension of the hippocampal gyrus that protrudesinto the floor of the inferior horn of the lateral ventricle).These limbic system structures have primary connectionswith various other parts of the brain, notably the thalamus,


Radiations to cortex

Visual impulses

Reticular formation Auditory impulses

Projection tospinal cord

Ascending sensory tracts

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Figure 13-18 Reticular activating system. Consists of centers inthe brainstem reticular formation plus fibers that conduct to thecenters from below and fibers that conduct from the centers to wide-spread areas of the cerebral cortex. Functioning of the reticular acti-vating system is essential for consciousness.

fornix, septal nucleus, amygdaloid nucleus (the tip of the cau-date nucleus, one of the cerebral nuclei), and the hypothala-mus. Some physiologists, therefore, include these connectedstructures as parts of the limbic system.

The limbic system (or to use its more descriptive name,the emotional brain) functions in some way to make us ex-perience many kinds of emotions—anger, fear, sexual feel-ings, pleasure, and sorrow, for example. To bring about thenormal expression of emotions, parts of the cerebral cortexother than the limbic system must also function. Consider-able evidence exists that limbic activity without the modu-lating influence of the other cortical areas may bring on theattacks of abnormal, uncontrollable rage suffered periodi-cally by some unfortunate individuals.

Memory. Memory is one of our major mental activities.The cortex is capable of storing and retrieving both short-term memory and long-term memory. Short-term memoryinvolves the storage of information over a few seconds orminutes. Short-term memories can be somehow consoli-dated by the brain and stored as long-term memories thatcan be retrieved days—or even years—later.

Both short-term and long-term memory are functions ofmany parts of the cerebral cortex, especially of the temporal,parietal, and occipital lobes. Findings made by Dr. WilderPenfield, a noted Canadian neurosurgeon, first gave evidenceof this in the 1920s. He electrically stimulated the temporallobes of epileptic patients undergoing brain surgery. Theyresponded, much to his surprise, by recalling in the mostminute detail songs and events from their past. Such long-

term memories are believed to consist of some kind of struc-tural traces—called engrams—in the cerebral cortex. Widelyaccepted today is the theory that an engram consists of somekind of permanent change in the synapses in a specific cir-cuit of neurons. Repeated impulse conduction over a givenneuronal circuit produces the synaptic change. What thechange is still is a matter of speculation. Two suggestions arethat it represents an increase in the number of presynapticaxon terminals or an increase in the number of receptor pro-teins in the postsynaptic neuron’s membrane. Other sugges-tions involve changes in the average concentrations of neu-rotransmitters at certain synapses or changes in thefunctions of astrocytes. Whatever the change is, it facilitatesimpulse transmission at the synapses.

Many research findings indicate that the cerebrum’s lim-bic system—the “emotional brain”—plays a key role inmemory. To mention one role, when the hippocampus (partof the limbic system) is removed, the patient loses the abilityto recall new information. Personal experience substantiatesa relationship between emotion and memory.

Specialization of Cerebral HemispheresThe right and left hemispheres of the cerebrum specialize indifferent functions. For example, as already noted, the lefthemisphere specializes in language functions—it does thetalking, so to speak. The left hemisphere also appears todominate the control of certain kinds of hand movements,notably skilled and gesturing movements. Most people usetheir right hands for performing skilled movements, and theleft side of the cerebrum controls the muscles on the right


Fornix

Anterior thalamic nucleus

Anterior commissure

Septal nucleus

Cingulate gyrus

Olfactory bulb

Olfactory cortex

Mamillary bodyof hypothalamus

Amygdaloid nucleus

Corpus callosum

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Figure 13-19 Structures of the limbic system.

side that execute these movements. The next time you arewith a group of people who are talking, observe their ges-tures. The chances are about 9 to 1 that they will gesturemostly with their right hands—indicative of left cerebralcontrol.

Evidence that the right hemisphere of the cerebrum spe-cializes in certain functions has also been reported. It seemsthat one of the right hemisphere’s specialties is the percep-tion of certain kinds of auditory stimuli. For instance, somestudies show that the right hemisphere perceives nonspeechsounds such as melodies, coughing, crying, and laughingbetter than the left hemisphere. The right hemisphere mayalso function better at tactual perception and for perceivingand visualizing spatial relationships.

Despite the specializations of each cerebral hemisphere,both sides of a normal person’s brain communicate witheach other via the corpus callosum to accomplish the manycomplex functions of the brain.

The Electroencephalogram (EEG)Cerebral activity goes on as long as life itself. Only when lifeceases (or moments before) does the cerebrum cease itsfunctioning. Only then do all of its neurons stop conductingimpulses. Proof of this has come from records of brain elec-trical potentials known as electroencephalograms, or EEGs.These records are usually made from a number of electrodesplaced on different regions of the scalp, and they consist ofwaves—brain waves (Figure 13-20).

Four types of brain waves are recognized based on fre-quency and amplitude of the waves. Frequency, or the num-ber of wave cycles per second, is usually referred to as Hertz(Hz, from Hertz, a German physicist). Amplitude meansvoltage. Listed in order of frequency from fastest to slowest,brain wave names are beta, alpha, theta, and delta. Beta waveshave a frequency of more than 13 Hz and a relatively lowvoltage. Alpha waves have a frequency of 8 to 13 Hz and a rel-atively high voltage. Theta waves have both a relatively lowfrequency—4 to 7 Hz—and a low voltage. Delta waves havethe slowest frequency—less than 4 Hz—but a high voltage.Brain waves vary in different regions of the brain, in differ-ent states of awareness, and in abnormal conditions of thecerebrum.

Fast, low-voltage beta waves characterize EEGs recordedfrom the frontal and central regions of the cerebrum whenan individual is awake, alert, and attentive, with eyes open.Beta waves predominate when the cerebrum is busiest, thatis, when it is engaged with sensory stimulation or mental ac-tivities. In short, beta waves are “busy waves.” Alpha waves, incontrast, are “relaxed waves.” They are moderately fast, rela-tively high-voltage waves that dominate EEGs recorded fromthe parietal lobe, occipital lobe, and posterior parts of thetemporal lobes when the cerebrum is idling, so to speak. Theindividual is awake but has eyes closed and is in a relaxed,nonattentive state. This state is sometimes called the “alphastate.” When drowsiness descends, moderately slow, low-voltage theta waves appear. Theta waves are “drowsy waves.”“Deep sleep waves,” on the other hand, are delta waves.These slowest brain waves characterize the deep sleep fromwhich one is not easily aroused. For this reason, deep sleep isreferred to as slow-wave sleep.


A

B

Figure 13-20 The electroencephalogram (EEG). A, Examples ofalpha, beta, theta, and delta waves seen on an EEG. B, Photographshowing a person undergoing an EEG test. Notice the scalp elec-trodes that detect voltage fluctuations within the cranium.

Box 13-8 FYI

Using EEGs

Physicians use electroencephalograms (EEGs) to help lo-calize areas of brain dysfunction, to identify altered

states of consciousness, and often to establish death. Twoflat EEG recordings (no brain waves) taken 24 hours apartin conjunction with no spontaneous respiration and totalabsence of somatic reflexes are criteria accepted as evi-dence of brain death.


Box 13-9 DIAGNOSTIC STUDY

Brain Studies

Neurobiologists have adapted many methods of medicalimaging to the diagnosis of brain disorders without the

trauma of extensive exploratory surgery. A few of the manyapproaches to studying the brain are listed here:

X-ray photography. Traditional radiography (x-ray photog-raphy) of the head sometimes reveals tumors or injuriesbut does not show the detail of soft tissue necessary todiagnose many brain problems.

Computed tomography (CT). This radiographic imagingtechnique involves scanning a person’s head with a re-volving x-ray generator. X rays that pass through tissuehit x-ray sensors, which send the information to a com-puter that constructs an image that appears as a “slice ofbrain” on a video screen. CT scans can be “stacked” by thecomputer to give a three-dimensional view not possiblewith traditional radiography. Besides revealing the normalstructure of the brain, CT scanning can often detect he-morrhages, tumors, and other abnormalities.

Positron-emission tomography (PET). PET scanning is avariation of CT scanning in which a radioactive substanceis introduced into the blood supply of the brain. The ra-dioactive material shows up as a bright spot on the im-age. Different substances are taken up by the brain in dif-ferent amounts, depending on the type of tissue and thelevel of activity, enabling radiologists to determine thefunctional characteristics of specific parts of the brain.

Single-photon emission computed tomography (SPECT).SPECT is a method of scanning similar to PET, but usingmore stable substances and different detectors. SPECT isused to visualize blood flow patterns in the brain—making

it useful in diagnosing cerebrovascular accidents (CVAs orstrokes) and brain tumors.

Ultrasonography. In this method, high-frequency sound(ultrasound) waves are reflected off anatomical structuresto form images. This is similar to the way radar works. Be-cause it does not use harmful radiation, ultrasonographyis often used in diagnosing hydrocephalus or brain tu-mors in infants. When used on the brain, ultrasonogra-phy is often called echoencephalography.

Magnetic resonance imaging (MRI). Also called nuclearmagnetic resonance (NMR) imaging, this scanning methodalso has the advantage of avoiding the use of harmful ra-diation. In MRI, a magnetic field surrounding the head in-duces brain tissues to emit radio waves that can be usedby a computer to construct a sectional image. MRI has theadded advantage of producing sharper images than CTscanning and ultrasound. This makes it very useful in de-tecting small brain abnormalities.

Evoked potential (EP). As discussed in this chapter, elec-troencephalography is the measurement of the electricalactivity of the brain. The EP test is similar to the elec-troencephalogram (EEG), but the brain waves observedare caused (evoked) by specific stimuli such as a flash oflight or a sudden sound. Recently, this information hasbeen analyzed by a computer that then produces acolor-coded graphic image of the brain generated on avideo screen, a brain electrical activity map (BEAM).Changes in color on the BEAM represent changes inbrain activity evoked by each stimulus that is given. Thistechnique is useful in diagnosing abnormalities of the vi-sual or auditory systems because it reveals whether asensory impulse is reaching the appropriate part of thebrain.

Magnetoencephalography (MEG). This new method ofmeasuring brain activity uses a sensitivity machine calleda biomagnetometer, which detects the very small mag-netic fields generated by neural activity. It can accuratelypinpoint the part of the brain involved in a CVA (stroke),seizure, or other disorder or injury. Based on technologyfirst developed by the military to locate submarines, MEGpromises to improve our ability to diagnose epilepsy,Alzheimer’s disease, and perhaps even certain types ofaddiction.

PET scan.

CT scan.MRI scan.

SOMATIC SENSORY PATHWAYS IN THE CENTRAL NERVOUS SYSTEMFor the cerebral cortex to perform its sensory functions, im-pulses must first be conducted to its sensory areas by way ofrelays of neurons referred to as sensory pathways. Most im-pulses that reach the sensory areas of the cerebral cortexhave traveled over at least three pools of sensory neurons. Weshall designate these as primary sensory neurons, secondarysensory neurons, and tertiary sensory neurons (Figure 13-21).

Primary sensory neurons of the relay conduct from theperiphery to the central nervous system. Secondary sensoryneurons conduct from the cord or brainstem up to the thal-amus. Their dendrites and cell bodies are located in spinalcord or brainstem gray matter. Their axons ascend in as-cending tracts up the cord, through the brainstem, and ter-minate in the thalamus. Here they synapse with dendrites orcell bodies of tertiary sensory neurons (see Figure 13-21).Tertiary sensory neurons conduct from the thalamus to thepostcentral gyrus of the parietal lobe, the somaticosensoryarea. Bundles of axons of tertiary sensory neurons form thal-amocortical tracts. They extend through the portion of cere-bral white matter known as the internal capsule to the cere-bral cortex (see Figure 13-21).

For the most part, sensory pathways to the cerebral cor-tex are crossed pathways. This means that each side of thebrain registers sensations from the opposite side of the body.Look again at Figure 13-21. The axons that decussate (cross


1. Where is the primary somatic motor area of the cerebralcortex? Where is the primary somatic sensory area?

2. What does the reticular activating system have to dowith alertness?

3. What is the function of the limbic system?4. What kind of information can be gained from an EEG?

Somaticsensory area of

cerebral cortex

Tertiarysensoryneuron

Thalamus

Mediallemniscus

Secondarysensoryneuron

Mediallemniscus

Fasciculusgracilis

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Dorsal rootganglion

Decussation ofmedial lemniscus

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Midbrain

Pons

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Receptor

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Collateralfibers toreticularformation

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sensoryneuron

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Figure 13-21 Examples of somatic sensory pathways. A, A pathway of the medial lemniscal system thatconducts information about discriminating touch and kinesthesia. B, A spinothalamic pathway that conducts in-formation about pain and temperature.

from one side to the other) in these sensory pathways arewhich sensory neurons: primary, secondary, or tertiary?Usually it is the axon of a secondary sensory neuron that de-cussates at some level in its ascent to the thalamus. Thus gen-eral sensations of the right side of the body are predomi-nantly experienced by the left somatic sensory area. Generalsensations of the left side of the body are predominantly ex-perienced by the right somatic sensory area.

Two sensory pathways conduct impulses that producesensations of touch and pressure, namely, the medial lemnis-cal system and the spinothalamic pathway (see Figure 13-21).The medial lemniscal system consists of the tracts that makeup the posterior white columns of the cord (the fasciculicuneatus and gracilis) plus the medial lemniscus, a flat bandof white fibers extending through the medulla, pons, andmidbrain. (The term lemniscus literally means “ribbon,” re-ferring to this tract’s flattened shape.)

The fibers of the medial lemniscus, like those of thespinothalamic tracts, are axons of secondary sensory neu-rons. They originate from cell bodies in the medulla, decus-sate, and then extend upward to terminate in the thalamuson the opposite side. The function of the medial lemniscalsystem is to transmit impulses that produce our more dis-criminating touch and pressure sensations, including stere-ognosis (awareness of an object’s size, shape, and texture),precise localization, two-point discrimination, weight dis-crimination, and sense of vibrations. The sensory pathwayfor kinesthesia (sense of movement and position of bodyparts) is also part of the medial lemniscal system.

Crude touch and pressure sensations are functions of thespinothalamic pathway. Knowing that something touchesthe skin is a crude touch sensation, whereas knowing its pre-cise location, size, shape, or texture involves the discriminat-ing touch sensations of the medial lemniscal system.

SOMATIC MOTOR PATHWAYS IN THE CENTRAL NERVOUS SYSTEMFor the cerebral cortex to perform its motor functions, im-pulses must be conducted from its motor areas to skeletalmuscles by relays of neurons referred to as somatic motorpathways. Somatic motor pathways consist of motor neu-rons that conduct impulses from the central nervous systemto somatic effectors, that is, skeletal muscles. Some motorpathways are extremely complex and not at all clearly de-fined. Others, notably spinal cord reflex arcs, are simple andwell established. You read about these in Chapter 12. Lookback now at Figure 13-11. From this diagram you can derivea cardinal principle about somatic motor pathways—theprinciple of the final common path. It is this: only one finalcommon path, namely, each single motor neuron from the

anterior gray horn of the spinal cord, conducts impulses to aspecific motor unit within a skeletal muscle. Axons from theanterior gray horn are the only ones that terminate in skele-tal muscle cells. This principle of the final common path toskeletal muscles has important practical implications. Forexample, it means that any condition that makes anteriorhorn motor neurons unable to conduct impulses also makesskeletal muscle cells supplied by these neurons unable tocontract. They cannot be willed to contract nor can theycontract reflexively. They are, in short, so flaccid that they areparalyzed. Most famous of the diseases that produce flaccidparalysis by destroying anterior horn motor neurons is po-liomyelitis. Numerous somatic motor paths conduct im-pulses from motor areas of the cerebrum down to anteriorhorn motor neurons at all levels of the cord.

Two methods are used to classify somatic motor path-ways—one based on the location of their fibers in themedulla and the other on their influence on the lower motorneurons. The first method divides them into pyramidal andextrapyramidal tracts. The second classifies them as facilita-tory and inhibitory tracts.

Pyramidal tracts are those whose fibers come to-gether in the medulla to form the pyramids, hence their name.Because axons composing the pyramidal tracts originate fromneuron cell bodies located in the cerebral cortex, they alsobear another name—corticospinal tracts (Figure 13-22). Aboutthree fourths of their fibers decussate (cross over from oneside to the other) in the medulla. After decussating, they ex-tend down the spinal cord in the crossed corticospinal tractlocated on the opposite side of the spinal cord in the lateralwhite column. About one fourth of the corticospinal fibers donot decussate. Instead, they extend down the same side of thespinal cord as the cerebral area from which they came. Onepair of uncrossed tracts lies in the anterior white columns ofthe cord, namely, the anterior corticospinal tracts. The otheruncrossed corticospinal tracts form part of the lateral corti-cospinal tracts. About 60% of corticospinal fibers are axonsthat arise from neuron cell bodies in the precentral (frontallobe) region of the cortex. About 40% of corticospinal fibersoriginate from neuron cell bodies located in postcentral areasof the cortex, areas classified as sensory; now, more accurately,they are often called sensorimotor areas.

Relatively few corticospinal tract fibers synapse directlywith anterior horn motor neurons. Most of them synapsewith interneurons, which in turn synapse with anterior hornmotor neurons. All corticospinal fibers conduct impulsesthat depolarize resting anterior horn motor neurons. The ef-fects of depolarizations occurring rapidly in any one neuronadd up, or summate. Each impulse, in other words, depolar-izes an anterior horn motor neuron’s resting potential a lit-tle bit more. If sufficient numbers of impulses impingerapidly enough on a neuron, its potential reaches the thresh-old level. At that moment the neuron starts conducting impulses—it is stimulated. Stimulation of anterior hornmotor neurons by corticospinal tract impulses results instimulation of individual muscle groups (mainly of the


1. Over how many afferent neurons does somatic sensoryinformation usually pass?

2. Explain why stimuli on the left side of the body are per-ceived by the right side of the cerebral cortex.

hands and feet). Precise control of their contractions is, inshort, the function of the corticospinal tracts. Without stim-ulation of anterior horn motor neurons by impulses overcorticospinal fibers, willed movements cannot occur. Thismeans that paralysis results whenever pyramidal corti-cospinal tract conduction is interrupted. For instance, theparalysis that so often follows cerebral vascular accidents(CVAs, or strokes) comes from pyramidal neuron injury—sometimes of their cell bodies in the motor areas, sometimesof their axons in the internal capsule (see Figure 13-22).

Extrapyramidal tracts are much more complex thanpyramidal tracts. They consist of all motor tracts from thebrain to the spinal cord anterior horn motor neurons exceptthe corticospinal (pyramidal) tracts. Within the brain, ex-trapyramidal tracts consist of numerous relays of motorneurons between motor areas of the cortex, cerebral nuclei,

thalamus, cerebellum, and brainstem. In the cord, some ofthe most important extrapyramidal tracts are the reticu-lospinal tracts.

Fibers of the reticulospinal tracts originate from cell bodiesin the reticular formation of the brainstem and terminate ingray matter of the spinal cord, where they synapse with in-terneurons that synapse with lower (anterior horn) motorneurons. Some reticulospinal tracts function as facilitatorytracts, others as inhibitory tracts. Summation of these oppos-ing influences determines the lower motor neuron’s response.It initiates impulse conduction only when facilitatory im-pulses exceed inhibitory impulses sufficiently to decrease thelower motor neuron’s negativity to its threshold level.

Conduction by extrapyramidal tracts plays a crucial partin producing our larger, more automatic movements be-cause extrapyramidal impulses bring about contractions of


Somaticmotorarea

of cerebralcortex

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Pons

Medulla

Spinalcord

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Spinalcord

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tract

Interneuron

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Figure 13-22 Examples of somatic motor pathways. A, A pyramidal pathway, through the lateral corti-cospinal tract. B, Extrapyramidal pathways, through the rubrospinal and reticulospinal tracts.

groups of muscles in sequence or simultaneously. Such mus-cle action occurs, for example, in swimming and walkingand, in fact, in all normal voluntary movements.

Conduction by extrapyramidal tracts plays an importantpart in our emotional expressions. For instance, most of ussmile automatically at things that amuse us and frown atthings that irritate us. It is extrapyramidal, not pyramidal,impulses that produce the smiles or frowns.

Axons of many different neurons converge on, that is,synapse with each anterior horn motor neuron (see Fig-ure 13-22). Hence many impulses from diverse sources—some facilitatory and some inhibitory—continually bom-bard this final common path to skeletal muscles. Together

the added, or summated, effect of these opposing influ-ences determines lower motor neuron functioning. Facili-tatory impulses reach these cells via sensory neurons(whose axons, you will recall, lie in the posterior roots ofspinal nerves), pyramidal (corticospinal) tracts, and ex-trapyramidal facilitatory reticulospinal tracts. Impulsesover facilitatory reticulospinal fibers facilitate the lowermotor neurons that supply extensor muscles. At the sametime, they reciprocally inhibit the lower motor neuronsthat supply flexor muscles. Hence facilitatory reticu-lospinal impulses tend to increase the tone of extensormuscles and decrease the tone of flexor muscles.

Inhibitory impulses reach lower motor neurons mainlyvia inhibitory reticulospinal fibers that originate from cellbodies located in the bulbar inhibitory area in the medulla.They inhibit the lower motor neurons to extensor muscles(and reciprocally stimulate those to flexor muscles). Henceinhibitory reticulospinal impulses tend to decrease extensormuscle tone and increase flexor muscle tone—note thatthese are opposite effects from facilitatory reticulospinalimpulses.

The set of coordinated commands that control the pro-grammed muscle activity mediated by extrapyramidal path-ways is often called a motor program. Traditionally, the pri-mary somatic motor areas of the cerebral cortex werethought to be the principal organizer of motor programssent along the extrapyramidal pathway. That view has beenreplaced by the concept illustrated in Figure 13-23. Thisnewer concept holds that motor programs result from theinteraction of several different centers in the brain. Appar-ently, many voluntary motor programs are organized in thecerebral nuclei and cerebellum—perhaps in response to awilled command by the cerebral cortex. Impulses that con-stitute the motor program are then channeled through thethalamus and back to the cortex, specifically to the primarymotor area. From there, the motor program is sent down tothe inhibitory and facilitatory regions of the brainstem. Sig-nals from the brainstem then continue on down one or morespinal tracts and out to the muscles by way of the lower (an-terior horn) motor neurons. All along the way, neural con-nections among the various motor control centers allow re-finement and adjustment of the motor program. If all thissounds complicated and confusing, imagine what it must belike to be a neurobiologist trying to figure out how all thisworks! In fact, scientists working in this field admit that theyhave not worked out all the details of the extrapyramidal circuits—or exactly how they control muscle activity. How-ever, the model shown in Figure 13-23 summarizes the cur-rent notion that it is a complex, interactive process.


Figure 13-23 Concept of extrapyramidal motor control. Pro-grammed movements result from a set of impulses called a motorprogram. The motor program organized in the cerebral nuclei andcerebellum in response to a command from the cortex is sent back tothe primary motor control area of the cortex. From there, it is sent tothe brainstem, then on through the spinal cord to the skeletalmuscles. All along the motor pathway, the motor program can berefined by various components of this complex pathway.

1. What is the “principle of final common path” as it per-tains to somatic motor pathways?

2. Distinguish between pyramidal and extrapyramidalpathways.



Signs of Motor Pathway Injury

Injury of upper motor neurons (those whose axons lie in ei-ther pyramidal or extrapyramidal tracts) produces symp-

toms frequently referred to as “pyramidal signs,” notably aspastic type of paralysis, exaggerated deep reflexes, and apositive Babinski reflex (p. 431). Actually, pyramidal signs re-sult from interruption of both pyramidal and extrapyramidalpathways. The paralysis stems from interruption of pyrami-dal tracts, whereas the spasticity (rigidity) and exaggeratedreflexes come from interruption of inhibitory extrapyramidalpathways.

Injury to lower motor neurons produces symptoms dif-ferent from those of upper motor neuron injury. Anteriorhorn cells or lower motor neurons, you will recall, constitutethe final common path by which impulses reach skeletalmuscles. This means that if they are injured, impulses can nolonger reach the skeletal muscles they supply. This, in turn,results in the absence of all reflex and willed movements pro-duced by contraction of the muscles involved. Unused, themuscles soon lose their normal tone and become soft andflabby (flaccid). In short, absence of reflexes and flaccid paral-ysis are the chief “lower” motor neuron signs.

If the most obvious structural change over the life span is theoverall growth, then degeneration, of the skeleton and other

body parts, then the most obvious functional change is thedevelopment, then degeneration, of the complex integrativecapacity of the central nervous system.

Although the development of the brain and spinal cordbegins in the womb, further development is required by thetime a baby is born. The lack of development of the centralnervous system in a newborn is evidenced by lack of themore complex integrative functions such as language, com-plex memory, comprehension of spatial relationships, andcomplex motor skills such as walking. As childhood proceeds,one can easily see evidence of the increasing capacity of thecentral nervous system for complex function. A child learns touse language, to remember both concrete and abstract ideas,to walk, and even to behave in ways that conform to the

norms of society. By the time a person reaches adulthood,most, if not all, of these complex functions have become fullydeveloped. We use them throughout adult life to help usmaintain internal stability in an unstable external world.

As we enter very late adulthood, the tissues of the brainand spinal cord may degenerate. If they do degenerate—andthey may not—the degree of change varies from one individ-ual to the next. In some cases, the degeneration is profound—or it occurs in a critical part of the brain—and an older personbecomes unable to communicate, to walk, or to performsome other complex functions. In many cases, however, thedegeneration produces milder effects such as temporarylapses in memory or fumbling with certain very complex mo-tor tasks. As our understanding of this process increases, weare finding ways to avoid entirely such changes associatedwith aging.

CYCLE OF LIFE

Central Nervous System

The central nervous system is the ultimate regulator of theentire body. It serves as the anatomical and functional

center of the countless feedback loops that maintain the rel-ative constancy of the internal environment. The central ner-vous system directly or indirectly regulates, or at least influ-ences, nearly every organ in the body.

The intriguing thing about the way in which the centralnervous system regulates the whole body is that it is able tointegrate, or bring together, literally millions of bits of infor-

mation from all over the body and make sense of it all. Notonly does the central nervous system make sense of all thisinformation, it compares it to previously stored memoriesand makes decisions based on its own conclusions about thedata. The complex integrative functions of human language,consciousness, learning, and memory enable us to adapt tosituations that less complex organisms could not. Thus ourwonderfully complex central nervous system is essential toour survival.

THE BIG PICTURE

The Central Nervous System and the Whole Body


Disorders of the Central Nervous SystemDestruction of Brain TissueInjury or disease can destroy neurons. A common example isthe destruction of neurons of the motor area of the cere-brum that results from a cerebrovascular accident (CVA). ACVA, or stroke, is a hemorrhage from or cessation of bloodflow through cerebral blood vessels. When this happens, theoxygen supply to portions of the brain is disrupted and neu-rons cease functioning. If the lack of oxygen is prolonged,the neurons die. If the damage occurs in a motor controlarea of the brain (see Figures 13-16 and 13-17), a person canno longer voluntarily move the parts of the body controlledby the affected area(s). Because motor neurons cross overfrom side to side in the brainstem (see Figure 13-22), flaccidparalysis appears on the side of the body opposite the side ofthe brain on which the CVA occurred. The term hemiplegia(hem-i-PLEE-ja) refers to paralysis (loss of voluntary musclecontrol) of one whole side of the body.

One of the most common crippling diseases that appearsduring childhood, cerebral palsy, also results from damageto brain tissue. Cerebral palsy involves permanent, nonpro-gressive damage to motor control areas of the brain. Suchdamage is present at birth or occurs shortly after birth andremains throughout life. Possible causes of brain damage in-clude prenatal infections or diseases of the mother; mechan-ical trauma to the head before, during, or after birth; nerve-damaging poisons; reduced oxygen supply to the brain; andother factors. The resulting impairment to voluntary musclecontrol can manifest in various ways. Many people withcerebral palsy exhibit spastic paralysis, a type of paralysischaracterized by involuntary contractions of affected mus-cles. In cerebral palsy, spastic paralysis often affects one en-tire side of the body (hemiplegia), or both legs (paraplegia),both legs and one arm (triplegia), or all four extremities(quadriplegia).

DementiaVarious degenerative diseases can result in destruction ofneurons in the brain. This degeneration can progress to ad-versely affect memory, attention span, intellectual capacity,personality, and motor control. The general term for thissyndrome is dementia (de-MEN-sha).

Alzheimer’s (ALZ-hye-merz) disease (AD) is character-ized by dementia. Its characteristic lesions develop in thecortex during the middle to late adult years. Exactly whatcauses dementia-producing lesions to develop in the brainsof individuals with Alzheimer’s disease is not known. Thereis some evidence that this disease has a genetic basis—atleast in some families. A current theory is that more than oneof the four or five different genes associated with AD has tobe abnormal before AD occurs. Other evidence indicatesthat environmental factors may have a role. Because the ex-act cause of Alzheimer’s disease is still not known, develop-ment of an effective treatment has proven difficult. Cur-

rently, people diagnosed with this disease are treated byhelping them maintain their remaining mental abilities andlooking after their hygiene, nutrition, and other aspects ofpersonal health management.

Huntington disease (HD) is an inherited disease charac-terized by chorea (involuntary, purposeless movements) thatprogresses to severe dementia and death. The initial symp-toms of this disease first appear between ages 30 and 40, withdeath generally occurring by age 55. The gene responsiblefor Huntington disease causes the body to make the proteinhuntingtin incorrectly. In brain cells, the abnormal form ofhuntingtin apparently clings to molecules too tightly andthus prevents normal function.

Acquired immune deficiency syndrome (AIDS), causedby HIV (human immunodeficiency virus) infection, can alsocause dementia. The immune deficiency characteristic ofAIDS results from HIV infection of white blood cells that arecritical to the proper function of the immune system (seeChapter 21). However, HIV also infects neurons and cancause progressive degeneration of the brain—resulting indementia.

Diseases caused by prions, pathogenic protein molecules,can also cause dementia. For example, bovine spongiform en-cephalopathy also known as BSE or “mad cow disease” is adegenerative disease of the central nervous system caused byprions that convert normal proteins of the nervous systeminto abnormal proteins, causing loss of nervous system func-tion, including dementia. Creutzfeldt-Jakob (KROYTS-feltYAH-kobe) Disease (CJD) is another prion disease that sim-ilarly reduces brain function, causing dementia. These dis-eases have caused controversy recently because animalbrains (the tissue that carries prions to other organisms)were fed to other animals in the human food chain, increas-ing the risk of infecting large numbers of humans. Themechanism of prion disease was outlined in Chapter 1, p. 28.

Seizure DisordersSome of the most common nervous system abnormalitiesbelong to the group of conditions called seizure disorders.These disorders are characterized by seizures—suddenbursts of abnormal neuron activity that result in temporarychanges in brain function. Seizures may be very mild, caus-ing subtle changes in level of consciousness, motor control,or sensory perception. On the other hand, seizures may bequite severe—resulting in jerky, involuntary muscle contrac-tions called convulsions or even unconsciousness.

Recurring or chronic seizure episodes constitute a condi-tion called epilepsy. Although some cases of epilepsy can betraced to specific causes such as tumors or chemical imbal-ances, most epilepsy is idiopathic (of unknown cause).Epilepsy is often treated with anticonvulsive drugs such asphenobarbital, phenytoin, or valproic acid that block neuro-transmitters in affected areas of the brain. By thus blocking

MECHANISMS OF DISEASE


synaptic transmission, such drugs inhibit the explosivebursts of neuron activity associated with seizures. Withproper medication, many people with epilepsy lead normallives without the fear of experiencing uncontrollableseizures. Even those who have not responded well to drugtherapies have often gained some relief from surgeries thatcut or destroy areas of the brain prone to severe seizures.

Diagnosis and evaluation of epilepsy or any seizure disor-der often rely on electroencephalography (see Figure 13-20).As Figure 13-24 illustrates a normal EEG that shows themoderate rise and fall of voltage in various parts of thebrain, but a seizure manifests as an explosive increase in the size and frequency of voltage fluctuations. Different clas-sifications of epilepsy are based on the location(s) and theduration of these changes in brain activity.

Normal ECG

1

1 second

2

3

4

Seizure

Vol

tage

Figure 13-24 ECG.


C. Anxiety related to the injuryD. Lack of mobility during transport

2. Emilio asks why he can feel a little sensation in someparts of his arms. The nurse’s response would be basedon the understanding that:A. A concussion is causing these transient signs.B. His injury affected the autonomic nervous center

in the cerebellum.C. These findings are consistent with a C5 and C6 in-

jury level.D. The brain pathways are too complex to depict the

exact etiology.3. In considering Emilio’s respiratory status you would be

concerned if:A. He continues to take shallow breaths.B. His anxiety causes episodes of shallow breathing.C. He is breathing only with his diaphragm.D. His breath sounds are bronchovesicular.

4. Because of the nature of Emilio’s injury, all of the fol-lowing central nervous system functions will be affectedexcept:A. Control of postureB. Conduction route to the brainC. Reflex activity for the spineD. Conduction route from the brain

Emilio Hernandez is a 19-year-old college student whowas rock climbing and fell 30 feet to the ground. He is

6 feet 7 inches tall and very active on his college basketballteam. His medical history is negative except for the usualchildhood illnesses and minor accidents. At the time of hisinjury, he was unable to move any extremities and com-plained of neck discomfort. He was awake, alert, and able toanswer questions appropriately. He had no feeling in hisarms or legs. On physical examination, Emilio’s pupils wereequal and reactive to light. There were several scrapes on hisarms. His blood pressure and heart rate were 110/72 and 86,respectively. His respirations were 18, unlabored, and regu-lar but shallow. Paramedics applied a cervical collar at thescene, placed him on a backboard, immobilized his head,and transported him to the medical center by helicopter.

In the emergency room, further physical examinationrevealed no deep tendon reflexes. Emilio did have sensoryperception from his head to just above the nipple line of hischest. He was unable to expand his chest wall. His color waspoor, but his skin was warm and dry. His blood pressureand heart rate were now 110/60 and 68, respectively, withrespirations at 24.

1. Which one of the following explanations best describesthe changes in Emilio’s vital signs?A. Lying on his back during transportB. The level and extent of injury

CASE STUDY

COVERINGS OF THE BRAIN AND SPINAL CORDA. Two protective coverings (Figure 13-2)

1. Outer covering is bone; cranial bones encase the brain, and vertebrae encase the spinal cord (Figure 13-1)

2. Inner covering is the meninges; the meninges of thecord continue inside the spinal cavity beyond the endof the spinal cord

B. Meninges—three membranous layers (Figure 13-3)1. Dura mater—strong, white fibrous tissue; outer layer

of meninges and inner periosteum of the cranialbones; has three important extensionsa. Falx cerebri

(1) Projects downward into the longitudinal fis-sure between the two cerebral hemispheres

(2) Dural sinuses—function as veins, collectingblood from brain tissues for return to the heart

(3) Superior sagittal sinus—one of several duralsinuses

b. Falx cerebelli—separates the two hemispheres ofthe cerebellum

c. Tentorium cerebelli—separates the cerebellumfrom the cerebrum

2. Arachnoid membrane—delicate, cobwebby layer be-tween the dura mater and pia mater

3. Pia mater—innermost, transparent layer; adheres tothe outer surface of the brain and spinal cord; con-tains blood vessels; beyond the spinal cord, forms aslender filament called filum terminale, at level ofsacrum, blends with dura mater to form a fibrous cordthat disappears into the periosteum of the coccyx

4. Several spaces exist between and around themeningesa. Epidural space—located between the dura mater

and inside the bony covering of the brain andspinal cord; contains a supporting cushion of fatand other connective tissues

b. Subdural space—located between the dura materand arachnoid membrane; contains lubricatingserous fluid

c. Subarachnoid space—located between the arach-noid and pia mater; contains a significant amountof cerebrospinal fluid

CHAPTER SUMMARY

CEREBROSPINAL FLUIDA. Functions

1. Provides a supportive, protective cushion2. Reservoir of circulating fluid, which is monitored by the

brain to detect changes in the internal environmentB. Fluid spaces

1. Cerebrospinal fluid—found within the subarachnoidspace around the brain and spinal cord and withinthe cavities and canals of the brain and spinal cord

2. Ventricles—fluid-filled spaces within the brain; fourventricles within the brain (Figure 13-4)a. First and second ventricles (lateral)—one located

in each hemisphere of the cerebrumb. Third ventricle—thin, vertical pocket of fluid be-

low and medial to the lateral ventriclesc. Fourth ventricle—tiny, diamond-shaped space

where the cerebellum attaches to the back of thebrainstem

C. Formation and circulation of cerebrospinal fluid(Figure 13-5)1. Occurs by separation of fluid from blood in the

choroid plexusesa. Fluid from the lateral ventricles seeps through the

interventricular foramen (of Monro) into thethird ventricle

b. From the third ventricle goes through the cerebralaqueduct into the fourth ventricle

c. From the fourth ventricle fluid goes to two differ-ent areas(1) Some fluid flows directly into the central

canal of the spinal cord(2) Some fluid leaves the fourth ventricle through

openings in its roof into the cisterna magna, aspace that is continuous with the subarach-noid space

d. Fluid circulates in the subarachnoid space andthen is absorbed into venous blood through thearachnoid villi

SPINAL CORDA. Structure of the spinal cord (Figure 13-6)

1. Lies within the spinal cavity and extends from theforamen magnum to the lower border of the firstlumbar vertebra

2. Oval-shaped cylinder that tapers slightly from abovedownward

3. Two bulges, one in the cervical region and one in thelumbar region

4. Anterior median fissure and posterior median sulcusare two deep grooves; anterior fissure is deeper andwider

5. Nerve rootsa. Fibers of dorsal nerve root

(1) Carry sensory information into the spinal canal(2) Dorsal root ganglion—cell bodies of unipolar,

sensory neurons make up a small region ofgray matter in the dorsal nerve root

b. Fibers of ventral nerve root(1) Carry motor information out of the spinal

cord(2) Cell bodies of multipolar, motor neurons are

in the gray matter of the spinal cord6. Interneurons are located in the spinal cord’s gray

matter core7. Spinal nerve—a single mixed nerve on each side of

the spinal cord where the dorsal and ventral nerveroots join together

8. Gray mattera. Extends the length of the cordb. Consists predominantly of cell bodies of interneu-

rons and motor neuronsc. In transverse section, looks like an H with the

limbs being called the anterior, posterior, and lat-eral horns of gray matter

9. White mattera. Surrounds the gray matter and is subdivided in

each half on the cord into three funiculi: anterior,posterior, and lateral white columns

b. Each funiculus consists of a large bundle of axonsdivided into tracts

c. Names of spinal tracts indicate the location of thetract, the structure in which the axons originate,and the structure in which they terminate

B. Functions of the spinal cord1. Provides conduction routes to and from the brain

a. Ascending tracts—conduct impulses up the cordto the brain

b. Descending tracts—conduct impulses down thecord from the brain

c. Bundles of axons compose all tractsd. Tracts are both structural and functional organi-

zations of nerve fibers(1) Structural—all axons of any one tract origi-

nate in the same structure and terminate inthe same structure

(2) Functional—all axons that compose one tractserve one general function

e. Important ascending (sensory) tracts (Figure 13-7)(1) Lateral spinothalamic tracts—crude touch,

pain, and temperature(2) Anterior spinothalamic tracts—crude touch,

pressure(3) Fasciculi gracilis and cuneatus—discriminating

touch and conscious kinesthesia(4) Spinocerebellar tracts—subconscious

kinesthesia(5) Spinotectal—touch related to visual reflexes

f. Important descending (motor) tracts (Figure 13-7)(1) Lateral corticospinal tracts—voluntary move-

ments on opposite side of the body(2) Anterior corticospinal tracts—voluntary

movements on same side of body(3) Reticulospinal tracts—maintain posture dur-

ing movement


(4) Rubrospinal tracts—transmits impulses thatcoordinate body movements and mainte-nance of posture

(5) Tectospinal tracts—head and neck move-ments during visual reflexes

(6) Vestibulospinal tracts—coordination of pos-ture and balance

g. Spinal cord—reflex center for all spinal reflexes;spinal reflex centers are located in the gray matterof the cord

THE BRAINA. Structures of the brainstem (Figure 13-9)

1. Medulla oblongataa. Lowest part of the brainstemb. Part of the brain that attaches to spinal cord, lo-

cated just above the foramen magnumc. A few centimeters in length and separated from

the pons above by a horizontal grooved. Composed of white matter and a network of gray

and white matter called the reticular formationnetwork

e. Pyramids—two bulges of white matter located onthe ventral side of the medulla; formed by fibersof the pyramidal tracts

f. Olive—oval projection located lateral to the pyramids

g. Nuclei—clusters of neuron cell bodies located inthe reticular formation

2. Ponsa. Located above the medulla and below the

midbrainb. Composed of white matter and reticular

formation3. Midbrain

a. Located above the pons and below the cerebrum;forms the midsection of the brain

b. Composed of white tracts and reticular formationc. Extending divergently through the midbrain are

cerebral peduncles; conduct impulses between themidbrain and cerebrum

d. Corpora quadrigemina—landmark in midbrain(1) Made up of two inferior colliculi and two su-

perior colliculi(2) Forms the posterior, upper part of the mid-

brain that lies just above the cerebellum(3) Inferior colliculus—contains auditory centers(4) Superior colliculus—contains visual centers

e. Red nucleus and substantia nigra—clusters of cellbodies of neurons involved in muscular control

B. Functions of the brainstem1. Performs sensory, motor, and reflex functions2. Spinothalamic tracts—important sensory tracts that

pass through the brainstem3. Fasciculi cuneatus and gracilis and spinoreticular

tracts—sensory tracts whose axons terminate in thegray matter of the brainstem

4. Corticospinal and reticulospinal tracts—two ofthe major tracts present in the white matter of thebrainstem

5. Nuclei in medulla—contain reflex centersa. Of primary importance—cardiac, vasomotor, and

respiratory centersb. Nonvital reflexes—vomiting, coughing, sneezing,

etc.6. Pons—contains reflexes mediated by fifth, sixth, sev-

enth, and eighth cranial nerves and pneumotaxic cen-ters that help regulate respiration

7. Midbrain—contains centers for certain cranial nervereflexes

C. Structure of the cerebellum (Figure 13-10)1. Second largest part of the brain2. Located just below the posterior portion of the cere-

brum; transverse fissure separates the two parts of thebrain

3. Gray matter makes up the cortex, and white matterpredominates in the interior

4. Arbor vitae—internal white matter of the cerebellum;distinctive pattern similar to the veins of a leaf

5. Cerebellum has numerous sulci and gyri6. Consists of the cerebellar hemispheres and the vermis7. Internal white matter—composed of short and long

tractsa. Shorter tracts—conduct impulses from neuron

cell bodies located in the cerebellar cortex to neu-rons whose dendrites and cell bodies composenuclei located in the interior of the cerebellum

b. Longer tracts—conduct impulses to and from thecerebellum; fibers enter or leave by way of threepairs of peduncles(1) Inferior cerebellar peduncles—composed

chiefly of tracts into the cerebellum from themedulla and cord

(2) Middle cerebellar peduncles—composed al-most entirely of tracts into the cerebellumfrom the pons

(3) Superior cerebellar peduncles—composedprincipally of tracts from dentate nuclei inthe cerebellum through the red nucleus of themidbrain to the thalamus

8. Dentate nucleia. Important pair of cerebellar nuclei, one of which

is located in each hemisphereb. Nuclei connected with thalamus and with motor

areas of the cerebral cortex by tractsc. By means of the tracts, cerebellar impulses influ-

ence the motor cortex, and the motor cortex in-fluences the cerebellum

D. Functions of the cerebellum1. Three general functions, all of which have to do with

the control of skeletal musclesa. Acts with cerebral cortex to produce skilled move-

ments by coordinating the activities of groups ofmuscles


b. Controls skeletal muscles to maintain balancec. Controls posture; operates at subconscious level

to smooth movements and make movements effi-cient and coordinated

2. Cerebellum compares the motor commands of thecerebrum to the information coming from proprio-ceptors in the muscle; impulses travel from the cere-bellum to both the cerebrum and muscles to coordi-nate movements to produce the intended action(Figure 13-11)

E. The diencephalon (Figure 13-12)1. Located between the cerebrum and the midbrain2. Consists of several structures located around the third

ventricle: thalamus, hypothalamus, optic chiasma,pineal body, and several others

3. The thalamusa. Dumbbell-shaped mass of gray matter made up

of many nucleib. Each lateral mass forms one lateral wall of the

third ventriclec. Intermediate mass—extends through the third

ventricle and joins the two lateral massesd. Geniculate bodies—two of the most important

groups of nuclei comprising the thalamus; locatedin posterior region of each lateral mass; play rolein processing auditory and visual input

e. Serves as a major relay station for sensory im-pulses on their way to the cerebral cortex

f. Performs the following primary functions:(1) Plays two parts in mechanism responsible for

sensations(a) Impulses produce conscious recognition

of the crude, less critical sensations ofpain, temperature, and touch

(b) Neurons relay all kinds of sensory im-pulses, except possibly olfactory, to thecerebrum

(2) Plays part in the mechanism responsible for emotions by associating sensory impulses with feeling of pleasantness and unpleasantness

(3) Plays part in arousal mechanism(4) Plays part in mechanisms that produce com-

plex reflex movements4. The hypothalamus

a. Consists of several structures that lie beneath thethalamus

b. Forms floor of the third ventricle and lower partof lateral walls

c. Prominent structures found in the hypothalamus(1) Supraoptic nuclei—gray matter located

just above and on either side of the optic chiasma

(2) Paraventricular nuclei—located close to thewall of the third ventricle

(3) Mamillary bodies—posterior part of hypo-thalamus, involved with olfactory sense

d. Infundibulum—the stalk leading to the posteriorlobe of the pituitary gland

e. Small but functionally important area of thebrain, performs many functions of greatest im-portance for survival and enjoyment

f. Links mind and bodyg. Links nervous system to endocrine systemh. Summary of hypothalamic functions

(1) Regulator and coordinator of autonomic activities

(2) Major relay station between the cerebral cor-tex and lower autonomic centers; crucial partof the route by which emotions can expressthemselves in changed bodily functions

(3) Synthesizes hormones secreted by posteriorpituitary and plays an essential role in main-taining water balance

(4) Some neurons function as endocrine glands(5) Plays crucial role in arousal mechanism(6) Crucial part of mechanism regulating

appetite(7) Crucial part of mechanism maintaining nor-

mal body temperature5. Pineal body

a. Located just above the corpora quadrigemina ofthe midbrain

b. Involved in regulating the body’s biological clockc. Produces some hormones—most notable hor-

mone is melatoninF. Structure of the cerebrum

1. Cerebral cortexa. Largest and uppermost division of the brain;

consists of right and left cerebral hemispheres;each hemisphere is divided into five lobes (Figure 13-13)(1) Frontal lobe(2) Parietal lobe(3) Temporal lobe(4) Occipital lobe(5) Insula (island of Riel)

b. Cerebral cortex—outer surface made up of sixlayers of gray matter

c. Gyri—convolutions; some are named: precentralgyrus, postcentral gyrus, cingulate gyrus, and hip-pocampal gyrus

d. Sulci—shallow groovese. Fissures—deeper grooves, divide each cerebral

hemisphere into lobes; four prominent cerebralfissures(1) Longitudinal fissure—deepest fissure; divides

cerebrum into two hemispheres(2) Central sulcus (fissure of Rolando)—groove

between frontal and parietal lobes(3) Lateral fissure (fissure of Sylvius) —groove

between temporal lobe below and parietallobes above; island of Reil lies deep in lateralfissure


(4) Parietooccipital fissure—groove that separatesoccipital lobe from parietal lobes

2. Cerebral tracts and cerebral nucleia. Cerebral nuclei—islands of gray matter located

deep inside the white matter of each hemisphere(Figure 13-15); include the following:(1) Caudate nucleus(2) Lentiform nucleus—consists of putamen and

globus pallidus(3) Amygdaloid nucleus

b. Cerebral tracts make up cerebrum’s white matter;there are three types (Figure 13-14)(1) Projection tracts—extensions of the sensory

spinothalamic tracts and motor corticospinaltracts

(2) Association tracts—most numerous cerebraltracts; extend from one convolution to an-other in the same hemisphere

(3) Commissural tracts—extend from one convolution to a corresponding convolutionin the other hemisphere; compose the cor-pus callosum and anterior and posteriorcommissures

c. Corpus striatum—composed of caudate nucleus,internal capsule, and lentiform nucleus

G. Functions of the cerebral cortex1. Functional areas of the cortex—certain areas of the

cerebral cortex engage in predominantly one particu-lar function (Figures 13-16 and 13-17)a. Postcentral gyrus—mainly general somatic sen-

sory area; receives impulses from receptors acti-vated by heat, cold, and touch stimuli

b. Precentral gyrus—chiefly somatic motor area; im-pulses from neurons in this area descend overmotor tracts and stimulate skeletal muscles

c. Transverse gyrus—primary auditory aread. Occipital lobe—primary visual areas

2. Sensory functions of the cortexa. Somatic senses—sensations of touch, pressure,

temperature, proprioception, and similar percep-tions that require complex sensory organs

b. Cortex contains a “somatic sensory map” of thebody

c. Information sent to primary sensory areas is re-layed to sensory association areas, as well as toother parts of the brain

d. The sensory information is compared and evalu-ated, and the cortex integrates separate bits of in-formation into whole perceptions

3. Motor functions of the cortexa. For normal movements to occur, many parts of

the nervous system must functionb. Precentral gyrus—primary somatic motor area;

controls individual musclesc. Secondary motor area—in the gyrus immediately

anterior to the precentral gyrus; activates groupsof muscles simultaneously

4. Integrative functions of the cortexa. Consciousness (Figure 13-18)

(1) State of awareness of one’s self, one’s environ-ment, and other beings

(2) Depends on excitation of cortical neurons byimpulses conducted to them by the reticularactivating system

(3) There are two current concepts about thereticular activating system(a) Functions as the arousal system for the

cerebral cortex(b) Its functioning is crucial for maintaining

consciousnessb. Language

(1) Ability to speak and write words and abilityto understand spoken and written words

(2) Speech centers—areas in the frontal, parietal,and temporal lobes

(3) Left cerebral hemisphere contains speech cen-ters in approximately 90% of the population;in the remaining 10%, contained in either theright hemisphere or both

(4) Aphasias—lesions in speech centersc. Emotions (Figure 13-19)

(1) Subjective experiencing and objective express-ing of emotions involve functioning of thelimbic system

(2) Limbic system—also known as the “emo-tional brain”(a) Most structures of limbic system lie on

the medial surface of the cerebrum; theyare the cingulate gyrus and hippocampus

(b) Have primary connections with otherparts of the brain, such as the thalamus,fornix, septal nuclei, amygdaloid nucleus,and hypothalamus

d. Memory(1) One of our major mental activities(2) Cortex is capable of storing and retrieving

both short- and long-term memory(3) Temporal, parietal, and occipital lobes are

among the areas responsible for short- andlong-term memory

(4) Engrams—structural traces in the cerebralcortex that comprise long-term memories

(5) Cerebrum’s limbic system plays a key role inmemory

5. Specialization of cerebral hemispheresa. Right and left hemispheres of the cerebrum

specialize in different functions; however,both sides of a normal person’s brain communicate with each other to accomplishcomplex functions

b. Left hemisphere is responsible for:(1) Language functions(2) Dominating control of certain hand

movements


c. Right hemisphere is responsible for:(1) Perception of certain kinds of auditory

material(2) Tactual perception(3) Perceiving and visualizing spatial

relationships6. The electroencephalogram (EEG)

a. Records of brain electrical potentials obtained byrecording brain waves

b. Four types of brain waves based on frequency andamplitude (Figure 13-20)(1) Beta waves—frequency 13 Hz and relatively

low voltage; “busy waves”(2) Alpha waves—frequency of 8 to 13 Hz and

relatively low voltage; “relaxed waves”(3) Theta waves—frequency of 4 to 7 Hz and low

voltage; “drowsy waves”(4) Delta waves—frequency of 4 Hz and high

voltage; “deep sleep waves”

SOMATIC SENSORY PATHWAYS IN THE CENTRAL NERVOUS SYSTEM A. For the cerebral cortex to perform its sensory functions,

impulses must first be conducted to the sensory areasby sensory pathways (Figure 13-21)

B. Three main pools of sensory neurons1. Primary sensory neurons—conduct impulses from

the periphery to the central nervous system2. Secondary sensory neurons

a. Conduct impulses from the cord or brainstem tothe thalamus

b. Dendrites and cell bodies are located in the graymatter of the cord and brainstem

c. Axons ascend in ascending tracts up the cord,through the brainstem, and terminate in the thal-amus, where they synapse with dendrites or cellbodies of tertiary sensory neurons

3. Tertiary sensory neuronsa. Conduct impulses from thalamus to the postcen-

tral gyrus of the parietal lobeb. Bundle of axons of tertiary sensory neurons form

the thalamocortical tractsc. Extend through the internal capsule to the cere-

bral cortexC. Sensory pathways to the cerebral cortex are crossedD. Two sensory pathways conduct impulses that produce

sensations of touch and pressure1. Medial lemniscal system

a. Consists of tracts that make up the fasciculicuneatus and gracilis, and the medial lemniscus

b. Axons of secondary sensory neurons make upmedial lemniscus

c. Functions—transmit impulses that produce dis-criminating touch and pressure sensations andkinesthesia

2. Spinothalamic pathway—functions are crude touchand pressure sensations

SOMATIC MOTOR PATHWAYS IN THE CENTRAL NERVOUS SYSTEMA. For the cerebral cortex to perform its motor functions,

impulses are conducted from its motor areas to skeletalmuscles by somatic motor pathways

B. Consist of motor neurons that conduct impulses fromthe central nervous system to skeletal muscles; somemotor pathways are extremely complex and others arevery simple

C. Principle of the final common path—cardinal principleabout somatic motor pathways; only one final commonpath, the motor neuron from the anterior gray horn ofthe spinal cord, conducts impulses to skeletal muscles

D. Two methods used to classify somatic motor pathways1. Divides pathways into pyramidal and extrapyramidal

tracts (Figure 13-22)a. Pyramidal tracts—also known as corticospinal tracts

(1) Approximately three quarters of the fibers de-cussate in the medulla and extend down thecord in the crossed corticospinal tract locatedon the opposite side of the spinal cord in thelateral white column

(2) Approximately one quarter of the fibers donot decussate but extend down the same sideof the spinal cord as the cerebral area fromwhich they came

b. Extrapyramidal tracts—much more complex thanpyramidal tracts(1) Consist of all motor tracts from the brain to

the spinal cord anterior horn motor neuronsexcept the corticospinal tracts

(2) Within the brain, consist of numerous relaysof motor neurons between motor areas of thecortex, cerebral nuclei, thalamus, cerebellum,and brainstem

(3) Within the spinal cord, some important tractsare the reticulospinal tracts

(4) Conduction by extrapyramidal tracts plays acrucial part in producing large, automaticmovements

(5) Conduction by extrapyramidal tracts plays animportant part in emotional expressions

(6) Motor program—set of coordinated com-mands that control the programmed motoractivity mediated by extrapyramidal pathways(Figure 13-23)

CYCLE OF LIFE: CENTRAL NERVOUS SYSTEMA. The development and degeneration of the central

nervous system is the most obvious functional changeover the life span

B. Development of brain and spinal cord begins in thewomb

C. Lack of development in the newborn is evidenced bylack of complex integrative functions1. Language2. Complex memory


3. Comprehension of spatial relationships4. Complex motor skills

D. Complex functions develop by adulthoodE. Late adulthood—tissues degenerate

1. Profound degeneration—unable to perform complexfunctions

2. Milder degeneration—temporary memory lapse ordifficulty with complex motor tasks

THE BIG PICTURE: THE CENTRAL NERVOUSSYSTEM AND THE WHOLE BODYA. Central nervous system—ultimate regulator of the

body; essential to survivalB. Able to integrate bits of information from all over the

body, make sense of it, and make decisions


1. What term means the membranous covering ofthe brain and cord? What three layers compose thiscovering?

2. What are the large fluid-filled spaces within the braincalled? How many are there? What do they contain?

3. Describe the formation and circulation of cere-brospinal fluid.

4. Describe the spinal cord’s structure and general functions.

5. List the major components of the brainstem and iden-tify their general functions.

6. Describe the general functions of the cerebellum.7. Describe the general functions of the thalamus.8. Describe the general functions of the hypothalamus.9. Describe the general functions of the cerebrum.

10. What general functions does the cerebral cortexperform?

11. Define consciousness. Name the normal states, orlevels, of consciousness.

12. Name some altered states of consciousness.

REVIEW QUESTIONS

1. Explain what the term reflex center means. Is an in-terneuron necessary for a reflex center? Explain youranswer.

2. Explain briefly what is meant by the arousal or alertingmechanism.

3. Some people claim meditation has a wide range ofbenefits. What benefits can be supported by scientificevidence?

4. If a researcher discovered that a substantial reductionin neurotransmitter concentration caused difficulty informing memory, what theory of memory formationwould be refuted?

5. In the previous chapter, an action potential was ex-plained in terms of electrical activity. Explain theprocess of measuring that activity to differentiate thetypes of brain waves.

6. A person having an absence of any reflex and a personhaving exaggerated deep tendon reflexes are showingsigns of different motor pathway injuries. Using thesesymptoms, explain the motor pathway that was damagedand what other symptoms each person might have.

7. Compare pyramidal tract and extrapyramidal tractfunctions.

8. A patient with a brain infection can be diagnosed byculturing cerebrospinal fluid. The greatest concentra-tion of the disease-causing organism can be drawnfrom the fluid as soon as it leaves the brain at the levelof the third or fourth vertebrae. Why would this be anunwise place to take the sample? Where would a betterlocation be? Explain your answer.

CRITICAL THINKING QUESTIONS

13. Identify the following kinds of brain waves according totheir frequency, voltage, and the level of consciousnessin which they predominate: alpha, beta, delta, theta.

14. Locate the dendrite, cell body, and axon of primary,secondary, and tertiary sensory neurons.

central nervous system - midlandisd.net

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