Introduction to the Structure and Function of the Central Nervous System · 2015-06-04 · CHAPTER 3 Introduction to the Structure and Function of the Central Nervous System 49 ferring

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The human brain has been called the most complicatedorganization of matter in the known universe. Know-ing the most important structures of the brain and theirspatial relationships is important for understandinghow the brain works. In addition, knowing how differ-ent structures are interconnected often provides valu-able hints about how the activity of different brain areasis integrated to form a network that supports complexcognitive and emotional functions.

This chapter presents some basic information aboutthe major structures of the nervous system and their in-terconnections, an area of study called neuroanatomy.It also includes a general discussion of the relationshipbetween structure and function, an area of investigationknown as functional neuroanatomy.

C H A P T E R 3

Introduction to theStructure and Function ofthe Central Nervous System

GENERAL TERMINOLOGYAN OVERVIEW OF THE CENTRAL NERVOUS SYSTEM

The Central and Peripheral Nervous SystemsMajor Divisions of the BrainThe MeningesThe Cerebral VentriclesGray Matter and White Matter

THE FOREBRAINThe Cerebral CortexThe Basal Ganglia

The Limbic SystemThe Diencephalon

THE BRAIN STEMThe MidbrainThe Hindbrain

THE CEREBELLUMTHE SPINAL CORDSUMMARY

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GENERAL TERMINOLOGY

To understand neuroanatomy, you will need to knowsome general terminology. Many of the terms in neu-roanatomy are actually Greek or Latin words. It is

often useful to look up the literal translations of thesewords because many very imposing neuroanatomicalterms end up having rather straightforward literalmeanings. For example, the names of two structures inthe brain, the substantia nigra and the locus ceruleus,

seemingly highly technical terms, translate from theLatin to “black stuff” and “blue place,” respectively.Translating neuroanatomical terms makes them lessmystifying and sometimes more memorable.

To find our way around the nervous system, wemust first know some of the conventional terminol-ogy used in anatomy to indicate where a structure islocated relative to other structures and relative to thewhole brain. The most important terms are superior(above), inferior (below), anterior (front), posterior(behind), lateral (side), and medial (middle). Withthese six terms we can specify relative position withinthe three-dimensional framework of the brain and thebody (Figure 3.1). Two or more of these terms can becombined to provide even more specific locations.

Although logically these terms are all we need tospecify location in three dimensions, some additionalterms are also used to denote relative location. Ter-minology differs to some extent when referring to thebrain versus the brain stem and spinal cord. In the

46 PART I Foundations

brain, the superior/inferior axis is often referred toas the dorsal/ventral (literally “back” and “belly,” re-spectively) axis, whereas in the brain stem and spinalcord, dorsal (or posterior) refers to the direction to-ward the back and ventral (or anterior) the directiontoward the front. In addition, in the brain stem andspinal cord, the direction toward the brain is referredto as rostral (“beak”); the direction away from thebrain is referred to as caudal (“tail”) (see Figure 3.1).

These terms originated from the study of theanatomy of four-legged animals, such as dogs. Inthese animals, the head and brain are aligned withthe central axis of the body, so the bottom parts of thebrain have the same position relative to the upperparts of the brain as the belly of the animal has to itsback. A similar relationship holds for the rostral/cau-dal areas of the body and the anterior/posterior axisof the brain. If you imagine that you are down on allfours looking straight ahead, the dorsal/ventral androstral/caudal axes will make sense in a way that

(A) (B)

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3 Cervicalflexure

Cephalicflexure

3-Vesicle stage

ForebrainMidbrainHindbrain

Spinal cord

Dorsal

Ventral

Rostral Caudal

(D)

(C)

Anterior(rostral)

Ventral(inferior)

Ventral(anterior)

Posterior

Dorsal(superior)

Dorsal(posterior)

Caudal

FIGURE 3.1 (A) The human brain develops from the embryonic neural tube, its ros-tral end giving rise to the brain and its caudal end becoming the spinal cord. (B) As itdevelops, the human nervous system flexes at the junction of the midbrain and the di-encephalon. (C) Because of this flexure, the terms dorsal and ventral and the terms ante-rior and posterior refer to different directions when applied to the brain than to the brainstem and spinal cord. (D) In lower vertebrates the nervous system in organized in astraight line, and directional nomenclature is consistent throughout its length. (FromKandel et al., 1995, p. 78)

they don’t when you imagine yourself standing. Thisis because over the course of evolution (and of indi-vidual development) the proliferation of the forebrainhas caused the human brain to bend forward 90° rel-ative to the central axis of the body (see Figure 3.1).

Because these terms indicate the location of struc-tures relative to other structures, it is possible for astructure in the anterior portion of the brain to be pos-terior to a structure that is even further anterior, justas Greenland, north relative to most of the rest of theworld, is south of the North Pole. In addition, theseterms are used to denote different areas within a sin-gle structure. For example, the names of various areaswithin the thalamus, an egg-shaped structure, are de-rived from this nomenclature, as illustrated by thedorsomedial nucleus and the ventrolateral nucleus.

The terms contralateral (opposite side) and ipsi-lateral (same side), which we encountered in chapter1, are used to designate the side of the body or brainon which a structure or connection occurs in relationto a reference point. For example, although about 90%of the fibers originating in the motor cortex of onecerebral hemisphere cross over to the contralateralside of the body, about 10% do not cross over and thusform ipsilateral connections. When structures or con-nections occur on both sides of the body, they are saidto be bilateral. Unilateral refers to a structure or con-nection on only one side of the body.

The term afferent (from the Latin ad + ferre, “tocarry inward”), encountered in chapter 1, refers toneural input feeding into a structure. Conversely, theterm efferent (from the Latin ex + ferre, “to carry out-ward”) denotes neural output leaving a structure. Aswith the other directional terms we have already dis-cussed, these are relative terms. The same fiber tractwill be efferent to one structure and afferent to an-other. For example, the fornix, an arching fiber tractdeep within the base of the forebrain that conveys in-formation from the hippocampus to the hypothala-mus, is a hippocampal efferent and a hypothalamicafferent.

Finally, several terms are used to describe the dif-ferent planes of anatomical section (cut) along whichthe brain can be dissected or visualized (Figure 3.2).These planes are typically seen in photographs anddiagrams of the brain. Sagittal sections (from the

CHAPTER 3 Introduction to the Structure and Function of the Central Nervous System 47

Latin sagitta, “arrow”) divide the brain into right andleft parts. Coronal sections (from the Latin coronalis,“crown”) divide the brain into anterior and posteriorparts. These are also called frontal sections. By con-vention, sections in this plane are viewed from be-hind, so that the left side of the section is the left sideof the brain. Horizontal (or axial) sections divide thebrain into upper (superior) and lower (inferior)parts. Again, by convention, sections in this plane aretypically viewed from above.

AN OVERVIEW OF THECENTRAL NERVOUS SYSTEM

The Central and Peripheral Nervous Systems

The most basic structural subdivisions of the humannervous system are the central nervous system (CNS)and the peripheral nervous system (PNS). The cen-tral nervous system consists of the brain and spinalcord, and the peripheral nervous system consists ofthe sensory and motor nerves that are distributedthroughout the body and that convey information toand from the brain (via 12 pairs of cranial nerves) andthe spinal cord (via 31 pairs of spinal nerves). The pe-ripheral nervous system is divided into the somaticnervous system and the autonomic nervous system.The somatic nervous system is the part of the PNSthat innervates the skin, joints, and skeletal muscles.The autonomic nervous system (ANS) is the part ofthe PNS that innervates internal organs, blood vessels,and glands. Each of these systems has related motorand sensory components. The somatic motor systemincludes skeletal muscles and the parts of the nervoussystem that control them; the somatosensory systeminvolves the senses of touch, temperature, pain, bodyposition, and body movement. The autonomic ner-vous system also has a motor component, sendingmotor output to regulate and control the smooth mus-cles of internal organs, cardiac muscle, and glands (au-tonomic motor). The autonomic nervous system alsoincludes sensory input from these internal structuresthat is used to monitor their status (autonomic sen-sory). The major sensory modalities other than touch(vision, audition, smell, and taste) are sometimes re-ferred to as special sensory.

Major Divisions of the Brain

The brain is divided into three parts: the forebrain(prosencephalon), the midbrain (mesencephalon),and the hindbrain (rhombencephalon) (Table 3.1and Figure 3.3). The forebrain includes the cerebralcortex, basal ganglia, limbic system (together mak-ing up the telencephalon), and diencephalon. Themidbrain and hindbrain together constitute the brainstem, with the hindbrain being further subdividedinto the pons and cerebellum (metencephalon) andthe medulla oblongata (myelencephalon). The me-dulla oblongata is often called simply the medulla.

In the course of evolution (and recapitulated inthe course of individual human fetal development)these divisions developed from the enlargement ofthe rostral end of the primordial neural tube. In thisprocess, the most rostral region expanded to becomethe forebrain, with its two subdivisions, the telen-


cephalon and the diencephalon, while more caudalregions expanded to become the hindbrain, with itstwo subdivisions, the pons (including the cerebel-lum) and the medulla. We consider each of these di-visions in the remainder of this chapter, workingfrom the most rostral to the most caudal, and endwith a discussion of the spinal cord. First, however,we consider some general aspects of the brain.

The Meninges

The brain and the spinal cord are encased in threelayers of protective membrane, the dura mater, thearachnoid, and the pia mater (Figure 3.4). These arecollectively called the meninges. The outermostlayer, the dura mater (Latin for “hard mother”), is atough, inelastic membrane following the contour ofthe skull. Between the dura mater and the pia materis the arachnoid membrane (Greek for “spider,” re-

Sagittal section

Coronal (frontal) section

Horizontal (axial) section

Anterior Posterior

FIGURE 3.2 Terms used todescribe the planes of section(cut) through the brain. (FromRosenzweig & Leiman, 1989, p. 29)


ferring to its weblike structure). This consists of twolayers of fibrous and elastic tissue and does not fol-low the sulci and gyri of the cortex. Between the twolayers of the arachnoid membrane is the subarach-noid space, a spongy structure containing cerebro-spinal fluid (CSF). The innermost membrane, the piamater (Latin for “pious mother”), adheres to the sur-face of the cortex, following the contours of its fold-ings and fissures.

The Cerebral Ventricles

The brain has four cavities filled with cerebrospinalfluid called cerebral ventricles (from the Latin for“stomach”). They are portions of the phylogeneti-cally ancient neural canal that enlarged and changedtheir shape as a result of the massive expansion of

Cerebralhemisphere

Cerebellum

TelencephalonProsencephalon

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

Mesencephalon

Rhombencephalon

FIGURE 3.3 The major divisions of the human brain.(From Kolb & Whishaw, 1996, p. 43)

Table 3.1 Major Divisions of the Brain

Primitive Mammalian Regions of Alternative Brain Divisions Brain Divisions Human Brain Terminology

Neocortex

Telencephalon Basal gangliaLateral ventricles

Forebrain

Forebrain Limbic system(prosencephalon)

ThalamusDiencephalon Epithalamus

HypothalamusPineal bodyThird ventricle

TectumMidbrain Mesencephalon Tegmentum(mesencephalon) Aqueduct of Sylvius Brain stem

PonsMetencephalon Cerebellum

Fourth ventricleHindbrain(rhombencephalon)

Myelencephalon Medulla oblongataFourth ventricle

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(From Kolb & Whishaw, 1996, p. 43)

the forebrain in the course of evolution. This processhas caused the ventricles to have a rather compli-cated shape (Figure 3.5).

There are two lateral ventricles (one in each hemi-sphere), the third ventricle in the diencephalon andthe fourth ventricle in the hindbrain. Each ventriclecontains a tuft of capillary vessels, called a choroidplexus, through which CSF enters the ventricles. Nar-row channels connect all four ventricles, and CSF cir-culates through the lateral ventricles to the third andthen to the fourth ventricle. From the fourth ventricleCSF leaves the ventricular system and circulatesaround the surface of the brain and spinal cord in thesubarachnoid space. From there CSF is eventually ab-sorbed into the venous circulation.

The brain is thus surrounded by CSF. This pro-vides it with structural support (much as our body issupported by the water we swim in) and also withan added measure of protection from the effects of ablow to the head. If the circulation of CSF throughthe ventricles is blocked at one of the narrow chan-nels that interconnect them, fluid builds up in frontof the blockage. This condition, known as hydro-cephalus (from the Greek for “water” + “brain”), isserious because as CSF accumulates it displaces andkills neighboring neurons.

Although, as mentioned in chapter 1, in medievaltimes the ventricles were thought to be directly in-volved in cognitive function, it is now recognizedthat there is no evidence that this is the case. Theymay, however, contain neuroactive molecules thatexert some influence over brain function. In our dis-


cussions the cerebral ventricles will serve most oftenas descriptive reference points for location within thebrain.

Gray Matter and White Matter

Parts of the central nervous system appear gray orwhite depending on the parts of the neuron that theycontain (Figure 3.6). Where there is a concentrationof cell bodies, the tissue appears gray. These areas,known as gray matter, have this color because thecell bodies contain the nucleus of the cell, which, inturn, contains darkly colored genetic material calledchromatin. Gray matter is where almost all interac-tions between neurons take place, processes that un-derlie the complex functioning of the CNS.

Tissue containing axons, the extended part of thenerve cell, appears white because of the fatty myelinsheath surrounding each axon. As we saw in chapter2, axons convey information from one area of theCNS to another. A large number of axons bundled to-gether and conveying information from one regionof gray matter to another is called a tract or fiber tract(from the Latin tractus, “extension” or “track”).

The most elaborate concentration of cell bodies inthe brain is arranged in a massive sheet, the cerebralcortex. In addition to the cortex, there are other con-centrations of cell bodies in the central nervous sys-tem. Each of these is called a nucleus (plural, nuclei),not to be confused with the nucleus of an individualcell. The term nucleus is derived from the Latin wordfor “nut,” reflecting the nutlike shape of many of

FIGURE 3.4 The membranessurrounding the brain, collec-tively termed the meninges.(From Netter, 1974, p. 35)

Skin

Arachnoid membrane

Subarachnoid space

Pia mater

Cortex

Dura mater

Bone

these clumps of cell bodies. Nuclei come in vari-ous sizes and shapes, ranging from the small androughly spherical brain-stem nuclei (e.g., the ab-ducens nucleus in the pons) to those that are rela-tively large and exotically shaped, such as the long,arching caudate nucleus in the forebrain.

Concentrations of cell bodies outside the centralnervous system are called ganglia rather than nuclei.However, the inconsistent processes of naming andidentifying structures have generated several excep-tions to this general guideline. Thus, for example, alarge and important group of gray-matter structuresdeep within the forebrain is collectively called thebasal ganglia.

THE FOREBRAIN

The structures most identified with higher cognitivefunction are found in the forebrain. Foremost amongthese is the cerebral cortex.

The Cerebral Cortex

The cerebral cortex, forming the outer covering of thecerebral hemispheres (cortex comes from the Latin


Dorsal(A) (B)

Anterior

Thirdventricle

Thirdventricle

Dorsal

Massaintermedia

Cerebralaqueduct Fourth

ventricle

Leftlateralventricle

Cerebralaqueduct

Leftlateralventricle

Rightlateralventricle

Fourthventricle

FIGURE 3.5 The cerebral ventricles. (A) Lateral view of the left hemisphere. (B) Frontal view. (From Carlson, 1999, p. 63)

Gray matter

White matter

FIGURE 3.6 Horizontal section through the cerebralhemispheres showing gray matter and white matter. Inaddition to the cerebral cortex, the continuous enfoldedsheet of gray matter on the surface of the cerebral hemi-spheres, this section also shows groups of gray-matterstructures located deep within the hemispheres. (FromDeArmond, Fusco, & Dewey, 1974, p. 10)

word for “bark”) is truly vast, particularly in humans,where it is estimated to contain 70% of all the neuronsof the CNS. And if one considers that the medulla ob-longata—with a diameter little more than that of adime and a length of only a few inches—can mediatephysiological functioning sufficient to sustain life, therelative enormity of the cerebral cortex, estimated tohave an area of about 2,300 square centimeters (cm2),can be appreciated.


THE FOUR LOBES Each cerebral hemisphere istraditionally divided into four lobes: the occipital,parietal, temporal, and frontal lobes (Figure 3.7).These areas, taking their names simply from thebones of the skull that overlie them, were definedlong before anything significant was known aboutthe functional specialization of the cerebral cortex.Nevertheless, it turns out that these general areas areoften useful in describing areas of the cortex that are

Lateral Medial

Lateral sulcus

Frontal

Frontal

Parietal

Parietal

Occipital

Temporal

VentralDorsal

Frontal

Temporal

Frontal

Longitudinal fissure

Central sulcus

Parietal

Occipital

Temporal

Occipital

Cingulate gyrus

Central sulcus

FIGURE 3.7 The four lobes of the cerebral hemispheres, as seen from the lateral,medial, dorsal, and ventral views. The central sulcus is the dividing line between the frontal and parietal lobes. The boundaries between the other three lobes are notprecisely defined. Cortex in these border areas is often described in terms of the twoadjacent lobes, thus generating such terms as parieto-occipital and parietotemporal. Thecingulate gyrus (dark area on the medial surface) is usually identified specifically, ratherthan being classified as part of any of the four lobes. (From Kolb & Whishaw, 1996, p. 52)

involved in particular behaviors. Because we willmost often be considering functions mediated by thecortex, we will frequently use the names of theselobes to specify cortical location.

MAJOR CORTICAL FEATURES The characteristicfolding of the cortex, which allows the enormous cor-tical surface to fit in the relatively small space en-closed by our skull, creates deep furrows or grooveson its surface. Each of these is called a sulcus (plural,sulci) and a few of the deeper ones are called fissures.Each outfolding is called a gyrus (plural, gyri). Someof the more important of these features, together withsome major brain structures, are shown in Figures 3.8,3.9, and 3.10.

THE LAYERED ORGANIZATION OF THE CORTEXThe vast majority of cerebral cortex in humans has sixlayers: five layers of neurons and an outermost layerof fibers, termed the plexiform layer. This six-layeredcortex, which appeared relatively late in evolution, is


called neocortex. It is also termed isocortex (from theGreek iso, meaning “same”) because all of it is com-posed of six layers, although, as we will see shortly,the relative thickness of the different layers variesacross the cortex. Areas of cortex with fewer than sixlayers are known as allocortex (from the Greek allos,meaning “other”). Allocortex has two major compo-nents, paleocortex and archicortex. Paleocortex in-cludes olfactory cortex and has only two layers.Archicortex has only one layer and is found in areasof the hippocampus, including Ammon’s horn andthe dentate gyrus.

VARIATIONS IN CORTICAL LAYERS: CYTO-ARCHITECTONICS The relative thickness and cellcomposition of each of the six cortical layers variesacross the neocortex, with different areas of the neo-cortex having characteristic patterns. The study ofthese patterns, called cytoarchitectonics (literally,“cell architecture”), was begun in the early 20th cen-tury. The two most widely accepted cytoarchitectonic

Superior frontalgyrus

Central sulcusPrecentralgyrus

Postcentralgyrus

Supramarginalgyrus

Angular gyrus

Lateralsulcus(Sylvian fissure)

Cerebellum

Medulla

Middlefrontalgyrus

Inferiorfrontalgyrus

MiddleTemporalgyrus

Superiortemporalgyrus

Inferiortemporalgyrus

Temporalpole

Frontalpole

Occipitalpole

FIGURE 3.8 Lateral view ofthe left hemisphere, showingsome major gyri and sulci. (From DeArmond et al., 1974, p. 4)


Corpuscallosum

Cingulate gyrus

Central sulcus

Calcarinesulcus

Pineal gland

Midbrain

Cerebellum

Medulla

Pons

Anteriorcommissure

Posteriorcommissure

Frontalpole

Occipitalpole

Subcallosal area

Uncus

Parahippocampalgyrus

MedullaCerebellum

Pons

Inferior temporalgyrus

Midbrain

Mamillary bodies

Olfactory tract

Olfactorybulb

Longitudinalfissure

Optic nerve

Optic chiasm

FIGURE 3.9 Medial view ofthe right hemisphere, showingsome important gyri and sulciand the major divisions of thebrain stem. (From DeArmond et al.,1974, p. 8)

FIGURE 3.10 Ventral viewof the brain, showing some im-portant cortical features andbrain-stem structures. (FromDeArmond et al., 1974, p. 6)

maps of the cortex are those developed by KorbinianBroadmann (Figure 3.11) and by von Economo andKoskinas. Broadmann’s map is used most widely.The two systems have a fair amount of agreement,and yet they also differ significantly. This disagree-ment indicates that there is a significant amount ofinterpretation implicit in cytoarchitectonics, as thereis in the development of any system of classification.The Broadmann numbering system is often used sim-ply to identify a particular area of cortex, such as area17 at the occipital pole of the cerebral hemispheres. Itturns out that there is significant correspondence be-tween areas defined by cytoarchitectonic studies andareas identified as having specialized function byother methods. Area 17, for example, turns out to bethe primary visual cortex. This and other correspon-dences suggest that at least some of the areas definedby Broadmann are also areas specialized for particu-lar psychological processes, although this is not al-ways the case.

CORTICOCORTICAL FIBER CONNECTIONS Let’snow consider the interconnections between corticalareas, termed corticocortical connections. Some ofthese intracortical connections are short, the fiberstaking a U-shaped course that connects adjacent gyri.Others are very long, connecting distant parts of thecortex within the same hemisphere (ipsilateral cor-


ticocortical fibers). Other long fibers connect homo-topic fields (corresponding areas) in the right and lefthemispheres (cortical commissural fibers).

Ipsilateral Corticocortical Fibers Figure 3.12 showsthe major ipsilateral corticocortical fibers includingthe arcuate fasciculus, the uncinate fasciculus, the su-perior longitudinal fasciculus, the inferior longitu-dinal fasciculus, the cingulate fasciculus, and thesuperior occipitofrontal fasciculus.

Commissural Fibers The most prominent exampleof cortical commissural fibers, the fiber tracts con-necting the left and right hemispheres, is the corpuscallosum (Figure 3.12A; see also Figure 3.9). Thereare, however, other commissural fibers connectingthe two hemispheres. These include the anteriorcommissure and the posterior commissure (see Fig-ure 3.9). In chapter 1 we saw how knowledge of cor-ticocortical connections can provide a conceptualframework for understanding patterns of cognitiveimpairment in terms of the disconnection of brainareas, that is, as disconnection syndromes.

FUNCTIONAL DIVISIONS OF THE CORTEX Themajor functional divisions of the cerebral cortex areshown schematically in Figure 3.13. In this section webriefly review these divisions.

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FIGURE 3.11 The most generally accepted cytoarchitectonic map of the cerebralcortex, developed by Korbinian Brodmann in 1909. (From Nauta & Feirtag, 1986, p. 301)


FIGURE 3.12 Some major commissural fibers and ipsilateral corticocortical fiberslinking neocortical areas. (A) A sagittal cut made near the midline showing the twomost important commissural fibers linking the two hemispheres: the corpus callosumand the anterior commissure. (B) A more lateral sagittal cut showing the major ipsilat-eral corticocortical fibers. (From Nauta & Feirtag, 1986, p. 304)

Motor Cortex The frontal lobes play a major role inthe planning and execution of movement. The precen-tral gyrus, just anterior to the central sulcus, is knownas the motor strip, motor cortex, or M1 and is involvedin the execution of movement. Lesions of the motorcortex result in a loss of voluntary movement on thecontralateral side of the body, a condition known ashemiplegia. Electrical stimulation of this cortex revealsa complete motor representation of the body on theprecentral gyrus, the so-called motor homunculus(“little man”; Figure 3.14). Such mapping of a neuralstructure in terms of associated behaviors is called afunctional map. There are many functional maps inthe cortex and in other brain regions.

Just anterior to the motor cortex are the premotorarea, on the lateral surface of the hemisphere, and thesupplementary motor area, on the medial surface.These areas are involved in the coordination of se-quences of movement. Frontal areas anterior to thepremotor cortex, the prefrontal cortex, are involvedin the higher-order control of movement, includingplanning and the modification of behavior in re-sponse to feedback about its consquences.


Somatosensory Cortex The somatosensory cortex,or SI, in the postcentral gyrus of each hemisphere re-ceives sensory information from the contralateral sideof the body about touch, pain, temperature, vibration,proprioception (body position), and kinesthesis(body movement). As with the motor cortex, there isan orderly mapping of the body surface representedon the postcentral gyrus, a mapping termed the sen-sory homunculus. Ventral to SI is the secondary so-matosensory cortex, or SII, which receives inputmainly from SI. Both SI and SII project to posteriorparietal areas where higher-order somatosensory andspatial processing take place.

Information from sensory receptors in the bodyarrives at SI via two major systems, both relayingthrough the thalamus. The spinothalamic systemconveys information about pain and temperature viaa multisynaptic pathway, whereas the lemniscal sys-tem conveys more precise information about touch,proprioception, and movement via a more directpathway. A comparison of the anatomy and functionof these two systems exemplifies how different pat-terns of neuroanatomical interconnection underlie

Primary somatosensory cortex

Primary auditorycortex (mostlyhidden from view)

Parietal lobeFrontal lobe

Primary motor cortex

Temporal lobe Occipital lobe

Primaryvisualcortex

Motorassociation

cortex

Auditory associationcortex

Somatosensoryassociation

cortex

Visualassociation

cortex

FIGURE 3.13 The mainfunctional divisions of thecortex as seen from a lateralview. Note that most of theprimary visual cortex is onthe medial surface of eachcerebral hemisphere, andso only the small lateralportion is visible in thisfigure. (From Carlson, 1998, p. 69)

specialization of function, a correspondence that wewill find throughout the nervous system.

Visual Cortex Visual information is conveyed fromthe retina to the lateral geniculate nucleus of the thal-amus before being projected to the banks of the cal-carine sulcus in the occipital lobes (see Figure 3.9).This route is called the primary visual pathway, andits initial cortical destination in the calcarine sulcus isknown by several names, including the primary vi-sual cortex, V1, and striate cortex. The last term is de-rived from the fact that a thin “thread” (Latin stria) ofdark tissue is visible in layer IV of this cortical region.As we noted earlier, the primary visual cortex is alsoknown by the area of Brodmann’s cytoarchitectonicmap corresponding to it: area 17.


The cortical visual system is organized analogouslyto the somatosensory system in that there is a con-tralateral relationship between the side (right-left) ofthe body that is stimulated (or, in the visual system,the side of space in which a stimulus appears) and thehemisphere to which the input is projected. Thus,stimuli from the right side of space (actually stimulat-ing the left side of the retina because of right-left re-versal caused by the lens of the eye) are projected tothe left hemisphere. Therefore a lesion in the left vi-sual cortex will result in a loss of vision in the right vi-sual field, an impairment known as right hemianopia(literally, half-not-seeing).

The cortical visual system supports such criti-cal functions as visual acuity, pattern vision, shapediscrimination, and figure-ground discrimination.

Toes

Ankle

Knee

Hip

Trun

kSh

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bow

Wris

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Thumb

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Face

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Leg

ArmHand

Lips

Tongue

Mouth

FIGURE 3.14 The motor homunculus on the precentral gyrus. The motor cortex canbe seen in the lateral view of the cerebral cortex. (From Netter, 1974, p. 68)

Damage to the cortical visual system results in corti-cal blindness. This is experienced as complete blind-ness, just like that resulting from damage to the twoeyes. These patients thus experience no knowledgeof their immediate visual world. Nevertheless, un-like in blindness due to damage to the eyes, someresidual visual functioning can be demonstrated incortical blindness. We will discuss this residual func-tion later in this chapter.

Surrounding V1 is cortex known as extrastriatecortex (“outside the striate”). It is also sometimestermed prestriate cortex, referring to the fact that it isanterior to the striate cortex, or visual association cor-tex. Literally dozens of visual areas have been discov-ered in extrastriate cortex, each with its own topo-graphically organized map of visual space. These arespecialized for the processing of specific aspects of thevisual world. Some of these visual areas will be dis-cussed in greater detail in chapter 5.

In addition to the primary visual pathways, thereare several secondary visual pathways. The most im-portant of these are the direct retinal projections to thesuperior colliculus in the midbrain, a structure in-volved in the mediation of visuomotor coordination.There are also direct retinal projections to the hypothal-amus in the diencephalon, which are involved in theprocess of entrainment of light-dark activity cycles.

Auditory Cortex The primary auditory cortex (AI)and the auditory association cortex (AII) surround-ing it are located in Heschl’s gyrus in the lower lip ofthe Sylvian fissure in each temporal lobe (see Figure3.13). These regions of auditory cortex contain severalorderly representations of sound frequency, known astonotopic maps, and are important for the perceptionof such auditory properties as pitch, location, rhythm,and timbre.

As mentioned in chapter 1, input reaches the audi-tory cortex in an unusual way. As we have seen, in vi-sion and somesthesis (body sense) there is an orderlycontralateral relationship between the side of spacefrom which a stimulus originates and the cerebralhemisphere to which input is projected. This exclu-sive contralaterality of direct input is not the case in the auditory system. Instead, efferents from the


cochlear nucleus, the synaptic destination of the au-ditory nerve, cross the midline to form connectionswith the contralateral cochlear nucleus, as well ascontinuing ipsilaterally. These fibers course to the su-perior olive and then to the inferior colliculus. Ateach of these levels fibers also cross to the corre-sponding structure on the contralateral side of thebrain stem. From the inferior colliculus these fibersreach the medial geniculate nucleus of the thalamusand from there project, via the auditory radiations, tothe auditory cortex in the ipsilateral temporal lobe.

The result of all this crossing over at several lev-els of the auditory system is that each ear projects toboth hemispheres and each hemisphere receives pro-jections from both ears. This pattern of connectivityexplains why damage to one auditory cortex doesnot produce noticeable impairment (as an analogousunilateral lesion in the visual or somatosensory cor-tex would), although, as we will see in chapter 4, spe-cial tests can detect a mild impairment in such cases.It also explains why cortical deafness (deafness dueto cortical damage) is extremely rare. Such a condi-tion would require damage to both auditory cortices.Because of the widely separated locations of the cor-tices in the right and left temporal lobes, such a pairof lesions is extremely rare, although cases of corticaldeafness have been reported.

Olfactory Cortex The cortical representation forsmell is different from that of the three major sensorymodalities just discussed. First, rather than being de-rived from the transduction of a specific form of en-ergy into a pattern of neural impulses, as is the casein somesthesis, vision, and audition, olfactory inputto cortex is derived from the coding of the presenceof particular molecules in the environment of the or-ganism. In addition, although input from each of thethree major sensory modalities is relayed to the thal-amus before reaching its respective primary corticalarea, olfactory information gains access to the olfac-tory cortex on the ventral surface of the frontal lobesdirectly, without thalamic mediation. It has been sug-gested that the directness of this cortical input con-tributes to the emotional intensity so often associatedwith olfactory experience.


the cortex (somatosensory, visual, and auditory)projects to a neighboring area of cortex for further pro-cessing. These areas are known as the sensory associ-ation area or secondary sensory area for the particularmodality in question. For example, the visual associa-tion area is a ring of cortex on the lateral and medialsurface of the hemisphere adjacent to the primary vi-sual cortex in the occipital lobes. Whereas lesions in aprimary sensory projection area are associated withimpairments in elementary sensory function, such asvisual acuity, lesions to sensory association areas re-sult in impairments in higher-order perceptual pro-cesses that are termed agnosias. Agnosia can takedifferent forms depending on the sensory modality af-fected and on other factors. We will consider visualagnosia in greater detail in chapters 5 and 8.

Luria’s model goes on to propose that each of themodality-specific sensory association areas then pro-jects to an area in the inferior parietal lobe, the ter-tiary sensory area, where the already highly pro-cessed information associated with each modality isintegrated to yield a multimodal representation.From here information is projected to the tertiarymotor area in the dorsolateral frontal cortex. Thus,dorsolateral frontal cortex receives input that is theproduct of extensive processing throughout the cere-bral cortex. In terms of sensory processing, the dor-sal region of the frontal lobes is, in the words of theneuroanatomist Walle Nauta, a “neocortical end ofthe line” (Nauta & Feirtag, 1986).

The frontal lobes then subject this highly processedsensory information to executive control in the formof superordinate motor organization such as planning.The specific behavioral manifestations of this super-ordinate organization are then implemented by thesecondary motor area, which coordinates sequencesof movements, and the primary motor area, which fi-nally effects movement (Figure 3.15).

The Basal Ganglia

The basal ganglia are a group of subcortical gray-matter structures in the forebrain. The three mainsubdivisions of the basal ganglia are the putamen, theglobus pallidus, and the caudate nucleus (Figure3.16). The caudate and putamen are collectively called

The Cortical Representation for Taste The corticalrepresentation for taste is the least understood of allcortical sensory areas. Its precise location is unclear,but it is thought to be in the upper lip of the Sylvianfissure. Like the three major sensory modalities, andunlike olfaction, information initially coded in tastebuds on the tongue and on neighboring areas reachesits cortical destination after synapsing in the thalamus.

Association Cortex Association cortex is tradition-ally defined as areas of cortex that do not mediateelementary sensory or motor function. This term con-notes that elaborate associative or cognitive processesare mediated by these areas, including the higher-order processing of sensory information that under-lies perception and the higher-order planning andorganization underlying complex behavior. Althoughin some general sense this is accurate, it is now clearthat specific areas within what had been termed asso-ciation cortex have highly specialized functional roles.As a result, the sketchy notion of association cortexhas given way to a more delineated picture of the con-tribution made by specific cortical areas to higher-order processing. For example, it is now clear thatareas outside V1 and extrastriate cortex, in the pari-etal and temporal lobes, although not involved in theearly stages of visual processing, are critical for theperception of specific aspects of the visual world. Sim-ilarly, prefrontal cortex, not directly involved in theexecution or coordination of specific movements, iscritical to the higher-order organization of action, in-cluding planning and the regulation of behavior in re-sponse to feedback about ongoing behavior. Thelanguage areas also exemplify the more specific iden-tification of the functional characteristics of areas notinvolved in elementary sensory or motor processing.

LURIA’S MODEL OF SEQUENTIAL HIERARCHICALCORTICAL PROCESSING The Soviet neuropsy-chologist Aleksandr Luria (1902–1977) has proposed amodel for cortical processing. Although in certainrespects it is highly oversimplified, retaining, for ex-ample, the vague notion of association cortex, it nev-ertheless serves as a useful framework for thinkingabout cortical processing. According to Luria’s model,each of the major primary sensory projection areas of

the neostriatum, a term that reflects the fact that theyare phylogenetically most recent and that they arestructurally related, being connected by cell bridges.Together, the three structures are referred to as thecorpus striatum. Because of their functional relation-ship with the corpus striatum, the subthalamic nu-


cleus and the substantia nigra are also generally con-sidered to be components of the basal ganglia.

The caudate nucleus and the putamen are themajor recipients of afferents to the corpus striatum.These afferents originate from areas throughout thecortex, including areas of cortex mediating motorfunction, and from the substantia nigra. The globuspallidus is the major source of efferents from thebasal ganglia, projecting mainly to thalamic nuclei.These, in turn, project to motor cortex, premotor cor-tex, and prefrontal cortex. Thus, rather than beingpart of the stream from motor cortex to spinal cordthat directly executes motor activity, the basal gan-glia and their connections with the thalamus form a cortical-subcortical-cortical loop that appears tomonitor and adjust motor activity. The regulatoryrole of the basal ganglia in motor function is also sug-gested by the fact that lesions to these structures fre-quently result in disorders that have unwantedmovement as their major symptom. These are dis-cussed in more detail in chapter 9. Some structuresof the basal ganglia also have been shown to play arole in certain cognitive processes.

The Limbic System

In the 19th century Paul Broca called the corticalstructures at the border between forebrain and brainstem le grand lobe limbique, the great limbic lobe (fromthe Latin limbus, meaning “border”). These struc-tures include the cingulate gyrus (arching aroundthe superior margin of the corpus callosum), the sub-callosal gyrus, the parahippocampal gyrus (Figure3.17), and the hippocampus.

In 1952 Paul MacLean hypothesized that a numberof structures, including this cortical ring, constituted afunctional system, which he named the limbic sys-tem. Although there is some disagreement about thebasis for including a particular structure in this sys-tem, and even about the very validity of the limbicsystem concept (see chapter 11), structures tradition-ally considered to be part of the limbic system include,in addition to the cortical areas just mentioned, theseptum, the amygdala, the hypothalamus (includingits posterior portion, the mamillary bodies), and theanterior nucleus of the thalamus (Figure 3.18).

(A)

(B)

12

2

3

3

3

2

2

2

1

1

1

FIGURE 3.15 Luria’s model of sequential processing inthe cerebral cortex. (A) Information arriving at the primarysensory areas (dark shading) is processed and conveyed tosecondary (medium shading) and tertiary (light shading)sensory areas for further perceptual and symbolic elabora-tion. (B) From the tertiary sensory area, information is thenconveyed to the tertiary motor area (light shading) in thefrontal lobes, which mediates higher-order motor process-ing such as planning and other forms of executive control.Specific motor sequences are then organized by the sec-ondary motor area, (medium shading), which then sendsthis input on to the primary motor cortex (dark shading)for final implementation. (From Kolb & Whishaw, 1996, p. 170[based on information in Luria, 1973])


Putamen and globus pallidus

Cerebral cortex Caudate nucleus

Thalamus

Hypothalamus

FIGURE 3.16 The three mainstructures of the basal ganglia:the putamen, the globus pal-lidus, and the caudate nucleusas viewed through a transpar-ent cerebral hemisphere. Thecell bridges connecting theputamen and the caudate nu-cleus are seen. (From Gleitman,Fridlung, & Reisberg, 1999, p. 26)

Frontalpole

Occipitalpole

Subcallosalgyrus

Parahippocampalgyrus

Cingulate gyrus FIGURE 3.17 Medial view ofthe right hemisphere, with thebrain stem and cerebellum re-moved, showing limbic cortex.The hippocampus is hiddenbehind the parahippocampalgyrus. (From Carlson, 1999, p. 69)

The limbic system receives three main sources ofcortical input: (a) from posterior association cortexvia the cingulate gyrus, hippocampus, and fornix(the pathway connecting the hippocampus with themamillary bodies in the posterior hypothalamus); (b)from inferotemporal cortex via entorhinal cortex (theanterior portion of the parahippocampal gyrus) andhippocampus; and (c) from prefrontal cortex (Figure3.19). Each of these sources of input carry informa-tion from association cortices, providing the limbicsystem with highly processed information about theenvironment.

There are three major sources of limbic efferentsto cortex. The cingulate gyrus receives input fromthe mamillary bodies via the anterior thalamus. Inaddition, prefrontal cortex receives limbic inputfrom the hypothalamus and from the amygdala (seeFigure 3.19).

Limbic structures have been shown to be impor-tant in emotional function and in memory. The hypo-thalamus, in addition to its role in the regulation ofautonomic and endocrine function, plays an impor-tant role in the regulation of emotional behavior, in-cluding rage behavior. In addition, the septum (andalso parts of the hypothalamus) produces intensepleasure when electrically stimulated. The amygdala


is involved in emotional processing, particularly con-ditioned fear. It is also involved in social behavior.

Finally, although there is little evidence that thehippocampus is involved directly in emotion, the hip-pocampus is critical for normal memory. Bilateraldamage to this structure results in a profound inabil-ity to remember anything new, a condition known asanterograde amnesia. It is not surprising that thesame structures or system of structures might medi-ate both memory and emotion because these two do-mains of function are closely related.

The Diencephalon

The most important structures in the diencephalonare the thalamus and the hypothalamus (Figure3.20). The word thalamus comes from the Greek for“inner room,” presumably a reference to the fact thatthe thalamus is situated deep in the brain. Althoughit is usually referred to in the singular, as if therewere only one, there are actually two thalami, one ineach hemisphere. Each is an ovoid (roughly egg-shaped) group of nuclei, bordered laterally by thethird ventricle, dorsally by the lateral ventricles, andlaterally by the internal capsule (fibers from motorcortex to brain-stem nuclei and the spinal cord).

Cingulatecortex

Fornix

Thalamus

Mamillarybody

Hippocampus

Amygdala

Hypothalamus

FIGURE 3.18 Some majorstructures of the limbic systemas viewed through transparentcerebral hemispheres. (FromGleitman et al., 1999, Fig. 2.13, p. 27)


The thalamus contains many specific nuclei. Someof these are points where neurons carrying incomingsensory information from each sensory modality (ex-cept olfaction) synapse on their way to the cortex. Al-though these are sometimes referred to as relay nu-clei, a great deal of processing takes place in thesenuclei, so it would be a mistake to think of them assimply passing information on to the next link in thechain. The sensory input from each modality reachesa specific thalamic nucleus. These nuclei include thelateral geniculate nucleus (for vision), the medialgeniculate nucleus (for audition), and specific nucleifor the relay of somatosensory input (Figure 3.21).

The thalamus also receives input from the basalganglia, cerebellum, motor cortex, and medial tem-poral cortex and sends projections back to theseareas. It thus serves as a pivotal structure in the for-mation of many cortical-subcortical-cortical loopsthat are involved in a broad range of functions. Forexample, returning connections from primary corti-cal sensory areas to the thalamus have been impli-cated in the modulation of thalamocortical input. Inthe motor domain, a loop involving motor cortex–basal ganglia–thalamus–motor cortex and a loop in-volving motor cortex–cerebellum–thalamus–motor

Prefrontal cortex

Septem

Subcallosalgyrus

Hypothalamus

Amygdala

Cortex

Anteriorthalamicnucleus

Mamillarybody

Hippocampus

Entorhinalcortex

Inferotemporalcortex

Posterior association

cortex

Cingulate gyrus

Fornix

FIGURE 3.19 Major connections within thelimbic system and between the limbic systemand other regions of the brain. Limbic structuresare shown within the rectangle. Bold arrowsindicate major sources of input to the limbic sys-tem and major targets of output from the limbicsystem. (Adapted from Kandel et al., 1995, p. 606)

Hypothalamus

Thalamus

FIGURE 3.20 Medial view of the right hemisphereshowing the medial surface of the right thalamus and, just below it, the right half of the hypothalamus. (FromDeArmond et al., 1974, p. 8)


nections to prefrontal cortex, the amygdala, the retic-ular formation, and the spinal cord. The fact that thehypothalamus receives input from and sends outputto both higher cortical centers and lower brain-stemand spinal centers suggests that it serves as an inter-face between these two neural domains, a notion thatis consistent with the influence of the hypothalamusover both behavior and internal physiological state.

The influence of the hypothalamus on inner phys-iological state is most evident in its central role in themaintenance of homeostasis, the internal biologicalsteady state that every living organism must con-stantly maintain in order to remain alive. It plays itsrole both by activating specific behaviors (such asdrinking and eating) and by directly regulatingphysiological processes that maintain the integrity ofthe body’s internal environment. Internal regulationis achieved partly through hypothalamic control ofthe autonomic nervous system, that division of thenervous system that controls smooth muscle, cardiacmuscle, and glands. Thus, the hypothalamus may bethought of as the highest regulatory center of the au-tonomic nervous system. However, its control is farfrom absolute; it is influenced both by higher corticalcenters processing information about the external

cortex have been shown to be important in the con-trol and regulation of movement. The thalamus isalso involved in cognitive function, including lan-guage and memory.

The hypothalamus, true to its name, is locatedjust below the thalamus, comprising the tissue form-ing lower walls of the third ventricle. This small andcompact group of nuclei (Figure 3.22) is involved inan extremely broad range of functions. As we havealready noted, the hypothalamus is a central compo-nent of the limbic system. In fact, Nauta has sug-gested that the limbic system should be defined asthose structures that are in close synaptic contactwith the hypothalamus (Nauta & Feirtag, 1986). It isthus critically involved in limbic-mediated emotionaland motivational processes.

Structures sending input to the hypothalamus in-clude limbic cortex, via the hippocampus and fornix,orbital (ventral) prefrontal cortex, the amygdala, andthe reticular formation. Thus, the hypothalamus re-ceives both highly processed input from the cortexand information from the internal environment. Italso receives direct projections from the retina thatare involved in hypothalamic control of circadianrhythms. Major hypothalamic output includes con-

FIGURE 3.21 Majorthalamic nuclei, theirsources of input, andtheir cortical projections.(From Netter, 1974, p. 72)

Inter

nal medullary lamina

Pulvinar

Parietalcortex

Cingulatecortex

Prefrontalcortex

Premotorcortex andsupplementarymotor cortex

Motorcortex

Cerebellum and basal ganglia

Arcuate

Auditorycortex

Visualcortex

Medial

Posteriorlateral

Dorsallateral

Anterior

Ventralanterior Ventral

lateralVentralposteriorlateral

Lateralgeniculatenucleus

AcousticpathwayOptic

tract

Medialgeniculatenucleus

Somatosensorycortex

Somesthetic(from head)

Somesthetic(from body)

Medullary lamina

Mamillarybodies

Basal ganglia

Amygdala andtemporalneocortex

Superior colliculus and extrastriate cortex

environment and lower brain-stem centers partici-pating in physiological regulation.

The other mechanism by which the hypothalamusexerts control over internal physiology is via regula-tion of endocrine function. Hormonal regulation ofphysiological function has certain advantages overneural mechanisms, particularly when slower, morediffuse, and more prolonged influence is required.The hypothalamus regulates endocrine function bycontrolling the release of hormones from the anteriorpituitary gland, which protrudes from the base ofthe hypothalamus (see Figure 3.22). The anteriorpituitary has been conceptualized as the “mastergland” of the endocrine system in that it triggers therelease of hormones by a number of endocrineglands, including the adrenal cortex, the thyroidgland, and the gonads. This anterior-pituitary regu-lation of endocrine function is, in turn, regulated bythe hypothalamus. Interestingly, the hypothalamusachieves this regulation by secreting its own peptidehormones into a vascular portal system that deliversthem directly to the anterior pituitary. The hypothal-amus also monitors the consequences of this regula-tory activity by assessing the levels of circulatinghormones present in its blood supply.


Although the anterior pituitary gland is not partof the nervous system, the posterior pituitary is com-posed of axons of neurons whose cell bodies are inspecific nuclei in the hypothalamus and whose axonterminals release hormones into the general circu-lation. These hormones include vasopressin (alsocalled antidiuretic hormone), which regulates waterretention, and oxytocin, which is involved in milkproduction and in uterine contractions. Thus, thehypothalamus is a gland, perhaps a startling conclu-sion until we realize the kinship between the releaseof neurotransmitter by neurons throughout the ner-vous system and the neurosecretory function ofhypothalamic cells. As we saw in chapter 2, in asense all neurons are secretory.

THE BRAIN STEM

The brain stem lies between the diencephalon andthe spinal cord (Figure 3.23). It is composed of themidbrain (mesencephalon), the pons (metencepha-lon), and the medulla oblongata (myelencephalon).Much of the brain stem is primarily concerned withregulating and maintaining life-sustaining processessuch as respiration, cardiac function, and homeosta-

Dorsomedial nucleus Thalamus

Anteriorcommissure

Massaintermedia

Posteriornucleus

Ventromedialnucleus

Mamillarynuclei

Midbrain

Paraventricularnucleus

Anteriornucleus

Suprachiasmaticnucleus

Supraopticnucleus

Opticchiasm

Anterior pituitary

Posterior pituitary

Arcuatenucleus

Pons

Preoptic nucleus

FIGURE 3.22 Midsagittalview of the hypothalamus,showing some major hypotha-lamic nuclei and some neigh-boring structures. The opticchiasm is the point at whichsome fibers from each opticnerve cross over to the otherside of the brain. (From Netter,1974, p. 76)

sis. It is also involved in the control of sleep andwakefulness, emotion, attention, and consciousness.

The brain stem contains groups of motor and sen-sory nuclei, as well as nuclei that exert a modulatory(regulatory) effect on higher brain centers by releasingspecific neurotransmitters. An example of such a nu-cleus is the substantia nigra, a major site of dopaminesynthesis. Axons projecting from this nucleus releasedopamine at synapses in widespread areas of the an-terior forebrain, including the basal ganglia and pre-frontal cortex. In addition to its many nuclei, the brainstem has major ascending (sensory) and descending(motor) white-matter tracts running through it.

It may be tempting, in the context of consideringhigher-order brain function, to consider the brainstem as a simple conduit through which informationflows back and forth between the forebrain and thespinal cord. This would be an error, however. As wehave indicated, the brain stem plays a central role inthe neural control of some extremely complex func-tions that are essential for survival and also partici-pates in the regulation of a number of higher-orderfunctions (Damasio, 1999).


The Midbrain

The midbrain lies between the diencephalon and thepons. The part of the midbrain dorsal to the cerebralaqueduct is called the tectum (Latin, “roof”) and con-sists of two paired structures, the superior and infe-rior colliculi. The tegmentum (Latin, “covering”) liesventral to the aqueduct. The most ventral region ofthe midbrain contains larger fiber tracts carrying in-formation from the forebrain to the spinal cord (thecorticospinal tract) and from the forebrain to thebrain stem (corticobulbar tract). Connections be-tween the forebrain and the cerebellum, running inboth directions, also pass through the midbrain.

The midbrain contains several structures involvedwith vision. These include the superior colliculus,which is involved in the mediation of visuomotorfunction. Earlier in this chapter we mentioned thatpatients with cortical blindness nevertheless havesome preserved visual function. For example, theyare able to accurately point to the location of a spotof light even though they have no conscious experi-ence of seeing the stimulus. This paradoxical ability,

(A) Cerebral hemisphere

Corpus callosum

Diencephalon

Cerebellum

Pons

Medulla

Spinal cord

Midbrain

FIGURE 3.23 (A) Midsagittal section of the brain, brainstem, and spinal cord. (B) An MRI of the same structures inthe living brain. (From Kandel et al., 1995, p. 9)

which has been termed blindsight (Weiskrantz,1986), may be mediated by the superior colliculus.

The midbrain is also the site of two of the threecranial nerve nuclei that control eye movement, theocculomotor nucleus and the trochlear nucleus. Thepupillary reflex (constriction of the pupil of the eyein response to intense light) is mediated by a nucleusincorporated in the occulomotor nucleus. The pupil-lary reflex is tested to assess the integrity of midbrainstructures in patients who are unconscious.

The inferior colliculus, an auditory relay, is lo-cated in the midbrain, just below the superior collicu-lus. The mesencephalic tegmentum contains a num-ber of nuclei involved in motor function, includingthe red nucleus and the substantia nigra. Finally, themesencephalic reticular formation, part of the diffuseset of nuclei extending from the lower medulla to thethalamus, takes up much of the midbrain. The retic-ular formation is involved in a number of functions,including arousal, autonomic regulation, and theregulation of pain.

The Hindbrain

The hindbrain (rhombencephalon) is made up of thepons (metencephalon) and the medulla (myelen-cephalon). The cerebellum is also considered to bepart of the metencephalon, but it will be consideredin a separate section.

PONS The pons is composed of the pontine tegmen-tal region, forming the floor of the fourth ventricle,and, more ventrally, a number of fiber tracts inter-spersed with pontine nuclei. In addition to the contin-uation of cortical projections to the brain stem andspinal cord, pontine fibers include massive connec-tions between cortex and cerebellum, the cerebellarpeduncles (Latin for “little feet”). Nuclei in the ponsinclude those having auditory and vestibular (senseof head position and motion) function, as well as sen-sory and motor nuclei for the face and mouth. One ofthe three nuclei controlling the extraocular muscles ofthe eyes is also found in the pons. The pons also con-tains a large portion of the reticular formation.

MEDULLA The medulla lies just caudal to thepons and rostral to the spinal cord. As the medulla


extends caudally, the fourth ventricle narrows andshifts ventrally until it becomes a narrow, centrallylocated tube as the medulla merges with the spinalcord. The medulla contains a number of importantnuclei, fiber tracts, and the most caudal portion ofthe reticular formation.

At the dorsal edge of its rostral end, the medullacontains two pairs of large nuclei, the gracile nucleiand the cuneate nuclei, relay nuclei for the ascend-ing lemniscal pathways, the somatosensory path-ways carrying information about touch, body posi-tion sense, and kinesthesis from the spinal cord to thethalamus and then on to the cortical somatosensoryarea. In the ventral portion of its rostral part is theolivary complex, including the inferior and medialaccessory olivary nuclei. This highly enfolded nu-cleus receives input from the cortex and the red nu-cleus and projects to the cerebellum. The medullaalso contains several sensory nuclei, including thoseprocessing vestibular input and sensory input fromthe face, mouth, throat, and abdomen. The medullaalso contains motor nuclei innervating the neck,tongue, and throat. Finally, motor nuclei in themedulla innervate organs of the body that are crucialfor the maintenance of life, including the viscera, theheart, and muscles involved in respiration.

In the ventral medulla are found the continua-tions of the corticospinal motor projections. Abouthalfway down the length of the medulla, 90% of thecorticospinal fibers on each side cross over to thecontralateral side. This point of crossover (decussa-tion) is called the pyramidal decussation.

THE CEREBELLUM

Bulging out of the dorsal surface of the pons, thecerebellum is part of the hindbrain. However, itsfunction is different from that of the brain-stem struc-tures we have just discussed. Although the term cere-bellum comes from the Latin for “little brain,” it is ac-tually a massive structure. The cerebellum overliesthe dorsal surface of the brain at the level of the pons,forming the roof of the fourth ventricle and sittingon the massive cerebellar peduncles. The cerebellumhas a cortex, the surface of which, because of its in-tricate infoldings, has an area equal to that of thecerebral cortex. In the depths of the cerebellum are

four pairs of deep nuclei. Between the cortex and thedeep nuclei is internal white matter.

Most input to the cerebellum reaches the cerebel-lar cortex via relays in the deep nuclei, althoughsome arrives at the cortex directly. Cerebellar affer-ents bring information about the state of the bodyfrom sensory areas processing somatosensory, visual,auditory, and vestibular information. In addition, thecerebellum also receives afferents from structures in-volved in motor function, including the motor andpremotor cortices.

Cerebellar output, almost entirely from the deepnuclei, projects, via the thalamus, to motor and pre-motor cortex. Cerebellar efferents also project tobrain-stem nuclei that send descending projectionsto the spinal cord, and to the rhombencephalic retic-ular formation.

The multisensory and motor input to the cerebel-lum, together with its output to motor-related cortexand to spinal centers mediating posture and gait, sug-gest that the cerebellum plays a central role in themodulation, adjustment, and coordination of bodymovement on the basis of information about currentbody state and current and intended movement. Thisnotion is supported by the fact that, despite the ex-tensive sensory and motor input the cerebellum re-ceives, lesions to the cerebellum do not cause sensorydeficits or paralysis. Instead, cerebellar lesions causedisruptions in the maintenance of posture and in the

sequential coordination of movement on the side ofthe body ipsilateral to the lesion. (The ipsilateral ef-fect is due to the fact that cerebellar efferents crossover to the other side of the brain stem, affectingbrain-stem fibers that then themselves cross overagain at the pyramidal decussation.) These disrup-tions are generally termed cerebellar ataxia (literally,“lack of order”) and are exemplified by the disrup-tion of the attempt to carry out a command, such as“extend your arm in front of you and then bring yourfinger to your nose.” A patient with cerebellar ataxiawho attempted this would evidence a marked in-stability of movement, with the hand increasinglylurching from side to side as the finger approachedthe nose.

THE SPINAL CORD

The spinal cord extends from the medulla, rostrally,to the cauda equina (“horse tail”) caudally. Thirty-one pairs of spinal nerves leave the spinal cord,passing through small openings in the spinal col-umn. The cauda equina is composed of the lowestspinal nerves as they continue to course caudally be-yond the end of the spinal cord before leaving thespinal column.

In cross section the spinal cord has a butterfly-shaped center of gray matter, surrounded by whitematter (Figure 3.24). The white matter consists of

Ventralhorn

Dorsalhorn Gray

matter

Whitematter

Spinalnerve

Dorsalroot

Ventralroot

Dorsalrootganglion

FIGURE 3.24 Ventral view of asection of the spinal cord. (From Netter,1974, p. 52)

69

ascending somatosensory tracts, descending motortracts, and intraspinal projection fibers connecting dif-ferent regions within the spinal cord. On each side thegray matter is divided into a ventral horn and a dor-sal horn. The ventral horn is the site of cell bodies of �motor neurons, the final link in the neuronal chainthat activates muscle contraction. Axons of � motorneurons leave the spinal cord in the ventral root. Thedorsal horn contains cell bodies of interneurons thatconvey input from primary somatosensory neuronsto the brain and to motor neurons at the same and atother levels of the spinal cord. These latter pathwaysconnecting somatosensory input with motor outputare the basis for spinal reflexes. Axons of primary so-matosensory neurons enter the spinal cord in the dor-sal root. However, the cell bodies of these neurons arenot in the dorsal horn, but remain outside the centralnervous system in the dorsal root ganglia.

SUMMARY

There are several general terms that help us orientourselves within the nervous system. Anterior (ros-tral) refers to the front, and posterior (caudal) refersto the back. Superior (dorsal) denotes the top or upperportion of a structure, and inferior (ventral) the bot-tom or underside. Medial refers to the middle, andlateral to the side. These terms are used to indicatethe position of a structure relative to the whole brainand also relative to other structures.

The central nervous system consists of the brainand the spinal cord. The peripheral nervous systemincludes all of the sensory and motor nerves cours-ing through the body and conveying signals to andfrom the central nervous system.

The brain is traditionally divided into three parts:the forebrain (prosencephalon), the midbrain (mes-encephalon), and the hindbrain (rhombencephalon).The forebrain includes the cerebral cortex, the basalganglia, the lateral ventricles, and the limbic system.The cerebral cortex, the most highly evolved regionof the human brain, is traditionally divided into fourlobes: the occipital, parietal, temporal, and frontal.The surface of the cortex has many sulci and gyri thatincrease the area of cortex that can be contained inthe limited volume of the skull. They also conve-


niently serve as landmarks for identifying cortical re-gions. Most cortex has six layers of cells, although afew regions have only one or two layers. Differentcortical regions vary with respect to the thicknessand cellular composition of these layers, and thestudy of this variation, called cytoarchitectonics, hasyielded cortical maps that have turned out to be use-ful in identifying functional areas of the cortex.

Fibers tracts of varying length connect areaswithin each hemisphere and between the two hemi-spheres. Many patterns of cognitive impairment canbe understood in terms of interruption of specific fi-ber tracts and the resulting disconnection of specificcortical regions.

Different cortical regions have been shown to bespecialized for specific functions. These include theprimary motor cortex and cortical areas that are spe-cialized for the processing of elementary sensory in-formation from each sensory modality. The termassociation cortex was long used to indicate cortexmediating higher-order processing. This concept hasgiven way to a more delineated understanding of thespecialized functions mediated by specific cortical re-gions outside the primary sensory and motor areas.

The basal ganglia are gray-matter structures lo-cated deep in the base of the forebrain. They are im-portant for the regulation and control of movement.Structures within the basal ganglia have also beenshown to play a role in some areas of cognition.

The limbic system includes a number of corticaland subcortical structures at the border between theforebrain and the brain stem. The limbic systemserves as a useful term to describe structures involvedin emotion, motivation, and the maintenance ofhomeostasis, although the validity of this concept hasbeen questioned. Some structures within this system,including the hippocampus and the mamillary bod-ies, have been shown to be important for memory.

The diencephalon includes the thalamus and thehypothalamus. The thalamus receives informationfrom each sensory modality (except olfaction) andconveys this information on to the respective pri-mary cortical sensory area. The thalamus also re-ceives input from the basal ganglia, cerebellum, andmotor cortex and sends projections back to theseareas. It thus serves as a central component in many

cortical-subcortical-cortical loops. The hypothalamusis involved in an extremely broad range of functions,many of which have the goal of maintaining homeo-stasis, both through the regulation of behavior andthe regulation of the body’s internal state. The hypo-thalamus mediates control of the organism’s internalstate through the regulation of the autonomic nervoussystem and through the control of endocrine function.

The brain stem includes the midbrain, the pons,and the medulla. These structures contain groups ofmotor and sensory nuclei and neurons that exert amodulatory effect on higher brain centers. They alsocontain ascending and descending fiber tracts thatmediate somatosensory and motor function. Manyof the nuclei in the medulla mediate processes thatare essential for life, such as cardiac function andblood pressure. For this reason, major lesions of themedulla often result in death.


The cerebellum plays a central role in the modu-lation and coordination of body movement on thebasis of information about current body state andcurrent and intended movement.

The spinal cord conveys motor commands fromthe brain to the muscles of the body via � motor neu-rons that leave the spinal cord in the ventral root.Axons of primary somatosensory neurons, convey-ing information about the body to the brain, enter thespinal cord in the dorsal root. The spinal cord alsomediates many reflexes that allow for rapid and au-tomatic responses to a variety of stimuli.

Now that we have surveyed the major structuresof the central nervous system, we turn to a discus-sion of experimental methods in neuropsychology.

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Introduction to the Structure and Function of the Central Nervous System · 2015-06-04 · CHAPTER 3 Introduction to the Structure and Function of the Central Nervous System 49 ferring