··

Note that the clinical case at the beginning of the chapterrefers to a patient who loses consciousness subsequent to

head trauma.

1 Why is it important to do a CT scan or MRI in the case of a head injurythat involves loss of consciousness?

2 What happened to the patient’s brain as his head was struck by thesteel beam?

3 Most tissues swell when they are exposed to blunt trauma. Does thathappen to the brain?

A 28-year-old man was struckin the back of the head by a steel

beam while at work. He apparently lostconsciousness for approximately 30 minutes. He was hospitalized since hewas still confused and disorientedthroughout his stay in the emergencyroom. By the next day he was felt to beback to normal except for a headache and was discharged. The patient had nomemory of the event, and he had no

memory of an important conversation hehad had with his boss 5 minutes beforethe head trauma. He also had no memoryof being in the emergency room and nomemories of events subsequent to theinjury until the next day. He also noted,besides persistent headache, that he was somewhat forgetful since the injury. Neurologic examination was unremarkable.

CLINICAL CASE

The brain, a bilaterally symmetric, soft, gelatinous struc-ture surrounded by its meninges and enclosed in its bony cranium, is continuous with the spinal cord at the foramenmagnum at the base of the skull. At birth the brain weighsless than 400 g, but by the beginning of the second year of lifeit has more than doubled in weight to 900 g. The adult brainweighs between 1,250 and 1,450 g, and demonstrates a gen-der differential, since brains of males generally weigh morethan those of females. This statement, however, should betempered by the evidence that, in adults, the ratio of brain tobody weight is greater in females than in males and that theincrease in weight is due more to the proliferation of neuro-glia than to the mitotic activity of neurons. An additionalpoint of interest is that there does not appear to be a relation-ship between brain weight and intelligence.

As detailed in Chapter 2, it is evident during embryo-genesis that the brain is subdivided into five continuousregions, from rostral to caudal: the telencephalon, dienceph-alon, mesencephalon, metencephalon, and myelencephalon.As the brain grows in size and complexity, these regions

fold upon and over one another, so that in the adult the evidence of these subdivisions is no longer clearly apparent.

The present chapter will not discuss the functional aspectsof the brain; instead its basic morphology and architecture

C H A P T E R 6

Gross Anatomy of the Brain

CLINICAL CASE

CEREBRUM

DIENCEPHALON

CEREBELLUM

BRAINSTEM

CLINICAL CONSIDERATIONS

SYNONYMS AND EPONYMS

FOLLOW-UP TO CLINICAL CASE

QUESTIONS TO PONDER

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are detailed to provide an anatomical framework of referencefor the chapters that follow, and many of the major topicsintroduced in this chapter are discussed in further detail inspecific chapters in this textbook. Much of this terminologyshould be memorized so that when, in later chapters, func-tional connections among regions of the brain are discussedthe student has a visual image of the location of the variousstructures and the pathways the connections take.

If the adult brain is viewed in three dimensions, only threeregions are clearly visible, and these are the cerebrum, cere-bellum, and part of the brainstem.

CEREBRUMThe cerebral hemispheresare narrower posteriorly, atthe occipital pole, than an-teriorly, at the frontal pole.They are large, oval struc-

tures that superficially resemble the surface of a shelled walnut (Fig. 6.1). The midline longitudinal cerebral fissure,occupied in life by the falx cerebri, incompletely separates thetwo cerebral hemispheres from one another. The floor of thecerebral fissure is formed by the corpus callosum, a largemyelinated fiber tract that forms an anatomical and func-tional connection between the right and left hemispheres.

The surface few millimeters of the cerebral hemisphere arecomposed of a highly folded collection of gray matter, knownas the cerebral cortex. This folding increases the surface areaand presents elevations, gyri, and depressions, sulci. Deep tothe cortex is a central core of white matter that forms the bulkof the cerebrum and represents fiber tracts, supported byneuroglia, ferrying information destined for the cortex andcortical responses to other regions of the central nervous system (CNS). Buried within the mass of white matter are collections of neuron cell bodies, some of which are lumpedtogether under the rubric of basal ganglia, even though, tech-nically, they are nuclei. Large collections of gray matter are

GROSS ANATOMY OF THE BRAIN === 69

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The cerebrum, observed from above,hides the remainder of the brain fromview, and is composed of two large,oval, cerebral hemispheres

Central sulcus

Precentral gyrus (motor area)

Postcentral gyrus (sensory area)

Sensory speech area

Angular gyrus

Superior parietal lobule

Occipital lobe

Cerebellum

Pons

Medulla oblongata

Parietal lobe

Lateral sulcus (fissure)

Superior temporal gyrus

Middle temporal gyrus

Inferior temporal gyrus

Supramarginal gyrus

Intraparietal sulcus

Parieto-occiptal sulcus

Postcentral sulcus

Precentral sulcus

Frontal lobe

Motor speech area

Temporal lobe

Inferior frontalgyrus

Middle frontal gyrus

Superior frontal gyrus

Preoccipital notch

Figure 6.1 = Diagram of the brain from a lateral view.

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also present in the diencephalons, namely, the epithalamus,thalamus, hypothalamus, and subthalamus.

The cerebrum is a hollow structure and the cavities withinthe cerebral hemispheres are called the right and left lateralventricles, which communicate with the third ventricle viathe interventricular foramen (foramen of Monro) (Fig. 6.2A).The two lateral ventricles are separated from one another bytwo closely adjoined non-nervous membranes, each knownas a septum pellucidum. Ependymal cells line each lateralventricle, and protruding into each ventricle is a choroid plexusthat functions in the manufacture of cerebrospinal fluid.

Lobes of the cerebral hemispheresEach cerebral hemisphere issubdivided into five lobes:the frontal, parietal, tem-poral, and occipital lobes,and the insula (Table 6.1).

Additionally, the cortical constituents of the limbic systemare also considered to be a region of the cerebral hemisphereand some consider it to be the sixth lobe, the limbic lobe.

Viewed from the side, each cerebral hemisphere re-sembles the shape of a boxing glove, where the thumb is the temporal lobe and is separated from the parietal lobeby the lateral fissure (fissure of Sylvius) (Fig. 6.1). The floor ofthe lateral fissure is formed by the insula (island of Reil) that

is hidden by the frontal, parietal, and temporal opercula(L., “lids”), regions of the same named lobes. Although the geographic distributions of many of the sulci and gyri are relatively inconsistent, some regularly occupy specific locations, are recognizable in most brains, and are named.The sulci are generally smaller and shallower than thefissures, and one of these, the central sulcus (central sulcus ofRolando), separates the frontal lobe from the parietal lobe.The division between the parietal and occipital lobes is notreadily evident when viewed from the lateral aspect becauseit is defined as the imaginary line between the preoccipitalnotch and the parieto-occipital notch. However, it is clearlydelimited on the medial aspect of the cerebral hemisphere,where the boundary between these two structures is theparieto-occipital sulcus and its continuation, the calcarinefissure (Fig. 6.3).

Frontal lobe

The frontal lobe extends from the frontal pole to the central sulcus

On its lateral aspect, the frontal lobe extends from the frontalpole to the central sulcus, constituting the anterior one-thirdof the cerebral cortex. Its posteriormost gyrus, the precentralgyrus, consists of the primary motor area and is borderedanteriorly by the precentral sulcus and posteriorly by the

70 === CHAPTER 6

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The five lobes of the cerebralhemispheres are the frontal, parietal,temporal, and occipital lobes, and theinsula

Lobe

Frontal

Parietal

Temporal

Occipital

Insula

Limbic

Table 6.1 = Lobes of the cerebral hemispheres.

Surface

Lateral

MedialInferior

Lateral

Medial

Lateral

SuperiorInferior

LateralMedial

Lateral

Medial

Major gyri

Precentral gyrusInferior frontal gyrusAnterior paracentral lobuleGyrus rectus and orbital gyri

Postcentral gyrusSuperior parietal lobuleInferior parietal lobule

Supramarginal gyrusAngular gyrus

Posterior paracentral lobule precuneus

Superior temporal gyrusMiddle and inferior temporal gyriTransverse temporal gyri (of Heschl)Fusiform gyrus

Superior and inferior occipital gyriCuneate gyrus (cuneus)Lingual gyrus

Short and long gyri

Cingulate gyrusParahippocampal gyrusHippocampal formationSubcallosal, parolfactory, and

preterminal gyri

Function/comment

Primary motor areaBroca’s area in dominant hemisphere; functions in speech productionContinuation of precentral gyrusOlfactory bulb and tract in the olfactory sulcus

Primary somesthetic areaAssociation area involved in somatosensory function

Integrates auditory, visual, and somatosensory informationReceives visual inputContinuation of the postcentral gyrus

Wernicke’s area in dominant hemisphere; ability to read, understand, and speak the written word

Primary auditory cortexBorders the parahippocampal gyrus of the limbic lobe

Separated from each other by the lateral occipital sulcusSeparated from each other by the calcarine fissure; striate cortex

(primary visual cortex) is on the banks of this fissure

Forms the floor of the lateral sulcus; associated with taste

Above the body of the corpus callosum and continues as the isthmusAnterior continuation of isthmus; ends in the uncusComposed of hippocampus, subiculum, and dentate gyrusCollectively known as the subcallosal area

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GROSS ANATOMY OF THE BRAIN === 71

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Anterior horn

Inferior horn

Body

Trigone

Third ventricle

Cerebralaqueduct

Posterior horn

Fourth ventricle

Lateral aperture

Lateral ventricle

Left and right ventricles

Interventricular foramen

Third ventricle

Cerebral aqueduct

Fourth ventricle

Median aperture of MagendieLateral aperturesof Luschka

B

ACentral canal

Subarachnoid space

Arachnoid granulations

Superior sagittal sinus

Great cerebral vein

Straight sinus

Confluence of sinuses

Figure 6.2 = (A) Diagram of the ventricles of the brain and central canal of the spinal cord in situ. (B) A three-dimensional representation of the ventricles of the brain.

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central sulcus. The region of the frontal lobe located anteriorto the precentral sulcus is subdivided into the superior, middle, and inferior frontal gyri. This subdivision is due to the presence, though inconsistent, of two longitudinallydisposed sulci, the superior and inferior frontal sulci. Theinferior frontal gyrus is demarcated by extensions of the lat-eral fissure into three subregions: the pars triangularis, parsopercularis, and pars orbitalis. In the dominant hemisphere, aregion of the inferior frontal gyrus is known as Broca’s area,which functions in the production of speech.

On its inferior aspect, the frontal lobe presents the longitu-dinally disposed olfactory sulcus. Medial to this sulcus is thegyrus rectus (also known as the straight gyrus), and lateral toit are the orbital gyri. The olfactory sulcus is partly occupiedby the olfactory bulb and olfactory tract (Figs 6.4, 6.5). At its posterior extent, the olfactory tract bifurcates to form the lateral and medial olfactory striae. The intervening area be-tween the two striae is triangular in shape and is known asthe olfactory trigone and it abuts the anterior perforatedsubstance.

On its medial aspect, the frontal lobe is bordered by thearched cingulate sulcus, which forms the boundary of the sup-erior aspect of the cingulate gyrus. The quadrangular-shaped

cortical tissue anterior to the central sulcus is a continua-tion of the precentral gyrus and is known as the anteriorparacentral lobule.

Parietal lobe

The parietal lobe extends from the central sulcus to the parieto-occipitalsulcus

The parietal lobe is interposed between the frontal and occipital lobes and is situated above the temporal lobe. On itslateral aspect, its anteriormost gyrus, the postcentral gyrus, isthe primary somesthetic area to which primary somatosens-ory information is channeled from the contralateral half ofthe body. The remainder of the parietal lobe, separated fromthe postcentral gyrus by the postcentral sulcus, is subdividedby the inconsistent intraparietal sulcus, into the superior andinferior parietal lobules. The former is an association areainvolved in somatosensory function, whereas the latter isseparated into the supramarginal gyrus, which integratesauditory, visual, and somatosensory information, and theangular gyrus, which receives visual input.

72 === CHAPTER 6

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Choroid plexus

Central sulcus

Thalamus

Splenium of corpus callosum

Parieto-occipitalsulcus (fissure)

Cuneus

Pineal body (gland)

Calcarine fissure(visual area)

Lingualgyrus

Cerebral aqueduct

CerebellumMidbrain

Massa intermedia

Interventricular foramen

Lamina terminalis

Body of corpuscallosum

Septum pellucidum

Genu of corpus callosum

Fornix

Cingulate gyrus

Anterior commissure

Hypothalamus

Optic chiasma

Pituitary gland

Mammillary body

Hypothalamic sulcus

BrainstemPons

Medulla oblongata

Fourth ventricle

Figure 6.3 = Diagram of the medial view of a sagittal section of the brain.

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On its medial aspect, the parietal lobe is separated from theoccipital lobe by the parieto-occipital sulcus and its inferiorcontinuation, the calcarine fissure. This region of the parietallobe is subdivided into two major structures, the anteriorlypositioned posterior paracentral lobule (a continuation ofthe postcentral gyrus) and the posteriorly situated precuneus.

Temporal lobe

The temporal lobe, the “thumb of the boxing glove,” is situated inferiorly tothe lateral fissure and anterior to the parieto-occipital sulcus

The temporal lobe is separated from the frontal and parietallobes by the lateral fissure and from the occipital lobe by animaginary plane that passes through the parieto-occipital sulcus. The anteriormost aspect of the temporal lobe isknown as the temporal pole. On its lateral aspect, the temporallobe exhibits three parallel gyri, the superior, middle, andinferior temporal gyri, separated from each other by theinconsistently present superior and middle temporal sulci.The superior temporal gyrus of the dominant hemispherecontains Wernicke’s area, which is responsible for the individual’s ability to speak and understand the spoken andwritten word.

GROSS ANATOMY OF THE BRAIN === 73

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Olfactory bulb

Olfactory tract

Corpus callosum

Optic nerve (CN II)

Internal carotid artery

Posterior cerebralartery

Middle cerebralartery

Superior cerebral artery

Abducent nerve (CN VI)

Facial nerve (CN VII)

Vestibulocochlearnerve (CN VIII)

Hypoglossal nerve (CN XII)

Glossopharyngeal nerve (CN IX)

Vagus nerve (CN X)

Accessory nerve (CN XI)

Temporal lobe

Frontal lobe

Anterior cerebral artery

Anteriorcommunicatingartery

Anterior cerebralartery

Posterior communicatingartery

Oculomotor nerve (CN III)

Trigeminal nerve(CN V)

Basilar artery Labyrinthine artery

Anterior inferiorcerebellar artery

Vertebral artery

Posterior inferiorcerebellar artery

Anterior spinalartery

Figure 6.4 = Diagram of the base of the brain displaying the cranial nerves and the arterial supply. Note that the frontal lobes are pulled apart slightly to show thecorpus callosum and the anterior cerebral arteries; also the right temporal lobe is sectioned to demonstrate the middle cerebral artery.

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Hidden within the lateral fissure is the superior aspect of the temporal lobe whose surface is marked by the obliquelyrunning transverse temporal gyri (of Heschl), the primaryauditory cortex.

The inferior aspect of the temporal lobe is grooved by theinferior temporal sulcus that is interposed between the inferior temporal gyrus and the lateral occipitotemporalgyrus (fusiform gyrus). The collateral sulcus separates the fusiform gyrus from the parahippocampal gyrus of thelimbic lobe.

Occipital lobe

The occipital lobe extends from the parieto-occipital sulcus to the occipitalpole

The occipital lobe extends from the occipital pole to the parieto-occipital sulcus. On its lateral aspect, the occipital lobepresents the superior and inferior occipital gyri, separatedfrom each other by the horizontally running lateral occipitalsulcus.

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Longitudinal cerebral fissure Olfactory bulb

Olfactory tractTemporal pole

Lateral sulcus (fissure)

Anterior perforated substance

Infundibulum

Mammillary body

Midbrain

Optic nerve (CN II)

Optic chiasma

Optic tract

Oculomotor nerve (CN III)

Trochlear nerve (CN IV)

Sensory root of trigeminal nerve (CN V)

Motor root oftrigeminal nerve (CN V)

Abducent nerve (CN VI)

Facial nerve (CN VII)

Nervus intermedius (CN VII)

Vestibulocochlear nerve (CN VIII)

Pyramid

Glossopharyngeal nerve (CN IX)

Vagus nerve (CN X)

Cranial root of accessory nerve (CN XI)

Spinal root of accessory nerve (CN XI)

Olive

Pons

Hypoglossal nerve(CN XII)

Cerebellum

Figure 6.5 = Diagram of the base of the brain displaying the location of the cranial nerves.

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On its medial aspect, the occipital lobe is subdivided into thesuperiorly located cuneate gyrus (cuneus) and the inferiorlypositioned lingual gyrus, separated from each other by thecalcarine fissure. The cortical tissue on each bank of thisfissure is known collectively as the striate cortex (calcarinecortex), and forms the primary visual cortex.

Insula

The insula forms the floor of the lateral sulcus

In order to view the insula the frontal, temporal, and parietalopercula have to be pulled apart, since this lobe is submergedwithin and forms the floor of the lateral sulcus. It is com-pletely circumscribed by the circular sulcus. The lateral sur-face of the insula is subdivided into several short and longgyri, the most prominent of which is located posteriorly. Theinsula is believed to be associated with taste, and perhapsother visceral functions.

Limbic lobeThe limbic lobe is a complexregion and includes the cingulate gyrus, parahippo-campal gyrus, hippocampalformation, subcallosal gyrus,

parolfactory gyrus, and the preterminal gyrus.The following description is a view of the medial aspect of

the hemisected brain and the various regions of the corpuscallosum are obvious landmarks. Therefore, the corpus callo-sum will now be described, even though it is not a part of thelimbic lobe. The anterior extent of the corpus callosum,known as the genu, bends inferiorly and turns posteriorly,where it forms a slender connection, the rostrum, with theanterior commissure. The posterior extent of the corpus callo-sum is bulbous in shape, and is known as the splenium (seeFig. 6.3).

The cingulate gyrus is located above the corpus callosumand is separated from it by the callossal sulcus. As the cingulate gyrus continues posteriorly, it follows the curva-ture of the corpus callosum and dips beneath the splenium tocontinue anteriorly as the isthmus of the cingulate gyrus. Theanterior continuation of the isthmus is the parahippocampalgyrus whose anteriormost extent is known as the uncus.Above the parahippocampal gyrus is the hippocampal sulcus, which separates the parahippocampal gyrus from thehippocampal formation (composed of the hippocampus,subiculum, and dentate gyrus).

Just beneath the rostrum of the corpus callosum is the subcallosal gyrus. The connection between the anterior com-missure and the optic chiasma is the lamina terminalis andthe cortical tissue anterior to the lamina terminalis is theparolfactory gyrus and preterminal gyrus. The subcallosal,parolfactory, and preterminal gyri are referred to as the subcallosal area.

Brodmann’s classification of the cerebral cortex

The best accepted system of functional regionalization of thecerebral cortex was developed by the neuroanatomist, K.Brodmann, who in the late nineteenth and early twentiethcenturies mapped the cortex into 47 unique areas, each asso-ciated with specific morphological characteristics. Althoughlater investigators refined and expanded his map into morethan 200 areas and assigned functional characteristics tothem, Brodmann’s original classification is still widely used.The major areas, their location, and function are presented inTable 6.2.

Histology of the cerebral cortexThe cerebral cortex is wellendowed with neurons, neuroglia, nerve fibers, and a rich vascular supply. The

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The limbic lobe is a hemisphericalregion on the medial aspect of thecerebral cortex that surrounds thecorpus callosum and the diencephalon

Area number Area name Location

1, 2, and 3 Primary somatosensory cortex Parietal lobe4 Primary motor cortex Frontal lobe5 and 7 Somatosensory association cortex Parietal lobe6 Supplementary motor area and Frontal lobe

premotor cortex8 Frontal eye field Frontal lobe9–12 and 46–47 Prefrontal cortex Frontal lobe17 Primary visual cortex Occipital lobe18 and 19 Visual association cortex Occipital lobe22 Wernicke’s speech area (dominant Temporal lobe (and perhaps

hemisphere) into the parietal lobe also)Auditory association cortex

41 and 42 Primary auditory cortex Temporal lobe44 and 45 Broca’s speech area (dominant Frontal lobe

hemisphere)

Table 6.2 = Selected Brodmann’s areas.

The histological organization of thecerebral cortex permits its subdivisioninto three regions: the archicortex,mesocortex, and neocortex

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arrangement of the three types of neurons that populate thecortex—pyramidal cells, stellate neurons, and fusiform neurons—permit the classification of the cortex into threetypes: the archicortex (allocortex), mesocortex ( juxtallocor-tex), and neocortex (isocortex).

The archicortex, phylogenetically the oldest region, iscomposed of only three layers and is located in the limbic sys-tem. The mesocortex, phylogenetically younger, is composedof three to six layers, and is located predominantly in theinsula and cingulate gyrus. The neocortex, phylogeneticallythe youngest region of the cerebral cortex, is composed of sixlayers and comprises the bulk of the cerebral cortex.

Although the cerebral cortex is arranged in layers, super-imposed upon this cytoarchitecture is a functional organiza-tion of cell columns. Each cell column is less than 0.1 mm indiameter, is perpendicular to the superficial surface of thecortex, passes through each of the six cortical layers, and iscomposed of neurons with similar functions. All neurons of asingle column respond to like stimuli from the same region ofthe body.

The organization of the six layers of the neocortex isknown as its cytoarchitecture, where each layer has a nameand an associated Roman numeral (Table 6.3).

White matter of the cerebral hemispheresThe central core of whitematter that forms the sub-stance of the cerebrum iscomposed of myelinatednerve fibers of varied sizes

and their supporting neuroglia. These fibers may be classifiedinto the following three categories: commissural, projection,and association fibers.

Commissural fibers

Commissural fibers are bundles of axons that connect the right and leftcerebral hemispheres

Commissural fibers (transverse fibers) interconnect the rightand left cerebral hemispheres. There are four bundles of com-missural fibers, the corpus callosum, anterior commissure,posterior commissure, and hippocampal commissure (seeFig. 6.3).

The largest group of the commissural fibers, the corpuscallosum, is comprised of four regions: the anteriormost rostrum, the curved genu, the relatively flattened body, andits posteriormost region, the splenium. The corpus callosumconnects the neocortex of the right hemisphere with that ofthe left.

The anterior commissure connects the right and leftamygdalas, the olfactory bulbs, and several cortical regionsof the two temporal lobes.

The posterior commissure connects the right and left pre-tectal region and related cell groups of the mesencephalon.

The hippocampal commissure (commissure of thefornix) joins the right and left hippocampi to one another.

Projection fibers

Projection fibers are restricted to a single hemisphere and connect thecerebral hemispheres with lower levels

Projection fibers are restricted to a single hemisphere andconnect the cerebral cortex with lower levels, namely the corpus striatum, diencephalon, brainstem, and spinal cord.The majority of these fibers are axons of pyramidal cells andfusiform neurons. These fibers are component parts of theinternal capsule, which is subdivided into the anterior limb,genu, posterior limb, retrolentiform, and sublentiformregions. The projection fibers may be subdivided into corti-copetal and corticofugal fibers.

Corticopetal fibers are afferent fibers that bring informa-tion from the thalamus to the cerebral cortex. They consist ofthalamocortical fibers.

Corticofugal fibers are efferent fibers that transmit informa-tion from the cerebral cortex to lower centers of the brain andspinal cord. They consist of the corticobulbar, corticopontine,corticospinal, and corticothalamic fibers.

Association fibers

Association fibers connect regions of a hemisphere to other regions of thesame hemisphere

Association fibers, also known as arcuate fibers, arerestricted to a single hemisphere and are subdivided into twomajor categories, short arcuate fibers and long arcuate fibers.

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There are three categories of fibercomponents in the cerebralhemispheres: commissural, projection,and association fibers

Name Numeral Components

Molecular layer I Afferent fibers from the thalamus or from the cerebral cortex

External granular layer II Stellate neuronsExternal pyramidal layer III Small pyramidal cellsInternal granular layer IV Fusiform neuronsInternal pyramidal layer V Larger pyramidal cellsFusiform layer VI Fusiform neurons

Table 6.3 = Layers of the neocortex.

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They are the axons of pyramidal cells and fusiform neurons.Short arcuate fibers, which connect adjacent gyri, do not usu-ally reach the subcortical white matter of the cerebral cortex;most of them are confined to the cortical gray matter. Thelong arcuate fibers, which connect nonadjacent gyri, consistof the following fiber tracts (Table 6.4): the uncinate fascicu-lus, cingulum, superior longitudinal fasciculus, inferior longitudinal fasciculus, and fronto-occipital fasciculus.

Basal gangliaThe basal ganglia, calledganglia even though they arenuclei, are large collectionsof cell bodies that are embed-ded deep in the white matter

of the brain (Fig. 6.6). These soma include those deep nuclei ofthe brain and brainstem which, when damaged, producemovement disorders. Thus the basal ganglia are composed ofthe caudate nucleus, lenticular nucleus (putamen and globuspallidus), subthalamic nucleus of the ventral thalamus, andthe substantia nigra of the mesencephalon (the caudatenucleus and the putamen together are referred to as the stria-tum). These nuclei have numerous connections with variousregions of the CNS; some receive input and are categorized asinput nuclei, some project to other regions and are referred to as output nuclei, whereas some receive input, project toother regions of the CNS, and have local interconnections andthese are known as intrinsic nuclei.

DIENCEPHALONThe diencephalon, inter-posed between the cerebrumand the midbrain, has fourregions: the epithalamus,

thalamus (Gr., “bed, bedroom”), hypothalamus, and subtha-lamus. The right and left halves of the diencephalon are separated from one another by a narrow slit-like space, theependymal-lined third ventricle. Rostrally, the interventricu-lar foramina (of Monro) leads from the lateral ventricles intothe third ventricle, whereas caudally, the third ventricle is

connected to the fourth ventricle by the cerebral aqueduct(of Sylvius).

The epithalamus, composed of the pineal body, striamedullaris, and habenular trigone, constitutes the dorsalsurface of the diencephalon. The right and left thalami com-pose the bulk of the diencephalon and form the superioraspect of the lateral walls of the third ventricle. The two tha-lami, structures composed of numerous nuclei, are connectedto each other by a bridge of gray matter, the interthalamicadhesion (massa intermedia). Some of the nuclei of the thalamus form distinctive bulges on its surface, namely thepulvinar (L., “cushion”) and the medial and lateral genicu-late bodies. The boundary between the thalamus and thehypothalamus is marked by a groove, the hypothalamic sulcus, located along the lateral walls of the third ventricle.Structures associated with the hypothalamus are the pituit-ary gland and its infundibulum, the tuber cinereum, and the two mammillary bodies. The subthalamic nuclei andfiber tract form the subthalamus.

CEREBELLUMThe cerebellum is located in the posterior aspect of the brain, just below theoccipital lobes of the cere-brum (Figs 6.7–6.8). It is separated from the cerebrum

via a horizontal dural reflection, the tentorium cerebelli. The cerebellum is connected to the midbrain, pons, and medullaof the brainstem via three pairs of fiber bundles, the superior,middle, and inferior cerebellar peduncles, respectively.Viewing the cerebellum, it can be seen that it is composed ofthe right and left cerebellar hemispheres and the narrow,intervening vermis. The vermis is also subdivided into asuperior and an inferior portion, where the superior portion is visible between the two hemispheres, while its inferior portion is buried between the two hemispheres.

The surface of the cerebellum has horizontal elevations,known as folia, and indentations between the folia, known assulci. Some of these sulci are deeper than others and they are said to subdivide each hemisphere into three lobes, the

GROSS ANATOMY OF THE BRAIN === 77

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Fiber group

Uncinate fasciculus

Cingulum

Superior longitudinalfasciculus

Inferior longitudinal fasciculus

Fronto-occipital fasciculusTable 6.4 = Extent of the long arcuatefiber groups.

Extends to

Motor speech area andorbital gyri of frontal lobe

Parahippocampal gyrus andparts of temporal lobe

Occipital, parietal, andtemporal lobes

Anterior region of theoccipital lobe

Temporal and occipital lobes

Extends from

Anterior temporal lobe and uncus

Medial cortex below rostrum

Anterior frontal lobe

Posterior region of the parietal andtemporal lobes

Ventrolateral regions of the frontal lobe

The basal ganglia consist of some deepcerebral nuclei and brainstem nucleithat, when damaged, producemovement disorders

The cerebellum is located below theoccipital lobe of the cerebralhemispheres. It is connected to thebrainstem via the superior, middle, andinferior cerebellar peduncles

The diencephalon is that portion of theprosencephalon that surrounds thethird ventricle

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small anterior lobe, the much larger posterior lobe, and theinferiorly positioned flocculonodular lobe (formed from the nodule of the vermis and the flocculus of each cere-bellar hemisphere). The anterior lobe is separated from theposterior lobe by the primary fissure, and the postero-lateral fissure separates the flocculonodular lobe from theposterior lobe (Figs 6.7–6.9).

Similar to the cerebrum, the cerebellum has an outer rim of gray matter, the cortex, an inner core of nerve fibers, themedullary white matter, and the deep cerebellar nuclei,located within the white matter. The cortex and white matter

are easily distinguished from each other in a midsagittal sec-tion of the cerebellum, where the white matter arborizes,forming the core of what appears to be a tree-like architec-ture, known as the arbor vitae.

Histologically, the cerebellar cortex is a three-layeredstructure, the outermost molecular layer, the middlePurkinje layer, and the innermost granular layer. The granular layer is well defined due to the presence of nucleicacids in the nuclei of its numerous, small cells. The Purkinjelayer, composed of a single layer of large Purkinje cell peri-karyons, is also easily recognizable. The molecular layer is

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Corpus callosum

Dorsal view Ventral view

Head of caudate nucleus

Insula

Claustrum

External capsule

Lateral fissure

Thalamus

Globus pallidus

Putamen

Posterior limb ofinternal capsule

Anterior limb ofInternal capsule

Genu ofinternal capsule

Posterior limb of theinternal capsule

Thalamus

Anterior horn of lateral ventricle

Septum pellucidum

Fornix

Interventricular foramen

Third ventricle

Habenularnucleus

Superior colliculus

Pineal body

Tail of caudatenucleus

Posterior horn oflateral ventricle

Figure 6.6 = Diagram of a coronal section of the brain displaying the basal ganglia.

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GROSS ANATOMY OF THE BRAIN === 79

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Substantia nigra

Red nucleus

Superior colliculus

Anterior lobe

Primary fissure

Posteriorlobe

Superior vermis

Horizontal fissure

Posterior lobe

Inferior vermis

Superior view

Inferior view

Tonsil

Nodule

Flocculus Cerebellar peduncle

Figure 6.7 = Superior and inferior views of thecerebellum.

Middle cerebellar peduncle

Inferior cerebellar peduncle

Superior cerebellar peduncle

Flocculus

Vestibulocochlear nerve (CN VIII)

Facial nerve (CN VII)

Nervus intermedius (CN VII)

Sensory root oftrigeminal nerve (CN V)

Motor root oftrigeminal nerve (CN V)

Superior colliculus

Inferior colliculus

Olive

PyramidFigure 6.8 = Diagram of a lateral view of thecerebellum and medulla.

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rich in axons and dendrites as well as capillaries that pene-trate deep into this layer.

Four pairs of nuclei are located within the substance of thecerebellar white matter. These are the fastigial, dentate,emboliform, and globose nuclei. The connections betweenthe cortical regions and the deep nuclei of the cerebellum permit the subdivision of the cerebellum into three zones—the vermal, paravermal, and hemispheric—where each zoneis composed of deep cerebellar nuclei, white matter, and cortex.

BRAINSTEMThe brainstem, the oldestpart of the CNS, is composedof the mesencephalon,metencephalon, and myel-

encephalon (although some authors also include the dien-cephalon) (Figs 6.9, 6.10). Since these are embryologic terms, one may also state that the brainstem is composed of the

mesencephalon, pons, cerebellum, and medulla oblongata.As parts of it have been overgrown by the cerebrum and thecerebellum, its dorsal aspect is mostly hidden from view inthe whole brain, whereas its ventral and lateral aspects arevisible. Removal of the cerebral and cerebellar hemispheresexposes the brainstem in its entirety and it is usually exam-ined in that fashion as well as by hemisecting the entire brain.

MesencephalonThe mesencephalon (mid-brain) is a relatively narrowband of the brainstem sur-rounding the cerebral aque-

duct, extending from the diencephalon to the pons. Thedorsal aspect of the midbrain is known as the tectum (L.,“roof”) and incorporates the paired superior and inferiorcolliculi (also known as the corpora quadrigemina). Thesestructures are associated with the lateral and medial genicu-late bodies, respectively, and they are all associated with

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Right and left fornix

Medial geniculate body

Third ventricle

Habenular trigone

Thalamus Pulvinar

Brachium of superiorcolliculus

Cerebral peduncle

Brachium of inferior colliculus

Medial lemniscus

Lateral lemniscus

Facial colliculus

Vestibular nerve

Cochlear nerve

Sulcus limitans

Tuberculum cinereum

Tuberculum cuneatus

Obex

Tuberculum gracilis

Fasciculus gracilis

Fasciculus cuneatus

Lateral geniculate body

Superior colliculus

Inferior colliculus

Median eminence

Middle cerebellarpeduncle

Inferior cerebellarpeduncle

Superior cerebellarpeduncle

Vestibular nuclei

Hypoglossal trigone

Vagal trigone

Tuberculum cinereum

Dorsal view Figure 6.9 = Diagram of the dorsal view of the brainstem.

The brainstem is composed of themesencephalon, metencephalon, andmyelencephalon

The mesencephalon is that region of thebrainstem that surrounds the cerebralaqueduct (of Sylvius)

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visual and auditory functions. The trochlear nerve (CN IV)exits the dorsal aspect of the mesencephalon just below theinferior colliculus. All other cranial nerves exit the ventralaspect of the brainstem. The region of the mesencephalonbelow the cerebral aqueduct is known as the midbrain(mesencephalic) tegmentum (L., “cover”). The cerebralhemispheres are connected to the brainstem by two largefiber tracts, the cerebral peduncles, and the depressionbetween the peduncles is known as the interpeduncularfossa, the site of origin of the oculomotor nerve (CN III).

MetencephalonThe cerebellum overlies andhides the dorsal aspect of thebrainstem, but its ventralaspect, the pons, is clearly

evident. Rostrally, the superior pontine sulcus acts as theboundary between the metencephalon and the midbrain andthe inferior pontine sulcus as the boundary between themetencephalon and the myelencephalon. Part of the floor ofthe fourth ventricle is formed by the dorsal aspect of the pons,

and is known as the pontine tegmentum, the structure thathouses the nuclei of the trigeminal, abducent, facial, andvestibulocochlear nerves. Cranial nerves VI, VII, and VIIIleave the brainstem at the inferior pontine sulcus, whereasthe trigeminal nerve exits the brainstem through the middlecerebellar peduncle.

MyelencephalonThe caudal-most portion ofthe brainstem, the myelen-cephalon, also known as themedulla oblongata, extends

from the inferior pontine sulcus to the spinal cord. Theboundary between them is the region where the lateral wallsof the fourth ventricle converge in a V shape at the midlineobex (at the level of the foramen magnum).

The ventral surface of the myelencephalon displays the anterior midline fissure, bordered on each side by thepyramids and crossed by the pyramidal decussations, connecting the right and left pyramids to each other. Theolives are olivepit-shaped swellings lateral to each pyramid.

GROSS ANATOMY OF THE BRAIN === 81

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Caudate nucleus

Right and left fornix

Optic chiasma

Optic nerve (CN II)

Infundibulum

Optic tract

Tuber cinereum

Oculomotor nerve (CN III)

Lentiform nucleus

Anterior commissure

Posterior perforated area

Optic radiations

Lateral geniculate body

Crus cerebri (midbrain)

Vestibulocochlear nerve(CN VIII)

Vagus nerve (X)

Olive

Pyramid

Decussation of pyramids

Pons

Medulla oblongata

Mammillary body

Motor root of trigeminal nerve (CN V)

Sensory root of trigeminal nerve (CN V)

Facial nerve (CN VII)

Glossopharyngeal nerve (CN IX)

Hypoglossal nerve (X)

Accessory nerve (XI)

Ventral viewFigure 6.10 = Diagram of the ventral view of thebrainstem.

The metencephalon is located belowthe cerebellum; its ventral bulge, thepons, is clearly visible

The myelencephalon, the caudal-mostportion of the brainstem, houses thefourth ventricle

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A review of the head CT thatwas performed in the emergency

room revealed the extent of the patient’sinjuries. There were modest contusions ofthe bilateral anterior and orbital regionsof the frontal lobes and the left anteriortemporal lobe. Contusions are brain“bruises.” The patient had a concussion,meaning any alteration of consciousnessresulting from head trauma. Persistentheadache and memory difficulties areoften characteristics of “postconcussionsyndrome,” which can also include dizzi-ness, poor balance, poor concentration,and other vague symptoms.

There are several ways in which headtrauma can cause brain injury. As in theabove case, contusion can result. The loca-tions of these are often predictable. Coupand contrecoup are terms used to definethe location of contusion related to thesite of head trauma. A coup injury refers toa contusion that occurs to the part of thebrain that directly underlies the site ofhead trauma. A contrecoup injury refers toa contusion that occurs in parts of thebrain directly opposite the site of headtrauma. The patient above suffered contrecoup contusions. The anterior andorbital regions of the frontal lobes and theanterior temporal lobes are the most com-mon locations of contusions. The skull issomewhat rigid and is the first to have

contact with the object of injury. There is avery sudden acceleration or decelerationof the skull. The brain within is somewhatmobile. As the skull accelerates or decel-erates, the brain crashes into the skull atcertain points. The damage occurs mostlyto those parts of the brain surface that aremost angular and also in close proximityto the bones of the skull.

There are other locations and mechanisms of injury that are often more important, especially in cases where disability or death occur from head trauma. There is also a rotationalcomponent to brain movement inside the skull at the moment of impact. Thiscauses shearing stresses, especially to the upper brainstem region, which candamage the reticular system, which maintains consciousness and awareness.This is probably the mechanism by whichhead injury causes loss or alteration ofconsciousness. Damage to the corpus callosum is also often demonstratedpathologically, secondary to its rotationalmomentum against the rigid falx. Diffuseaxonal injury refers to diffuse damage towhite matter, specifically to axons, whichhas been demonstrated pathologically.This is also thought to be due to shearingor stretching injury to the long and thin axons due to sudden and severe rotational and angular stresses.

FOLLOW-UP TO CLINICAL CASE

Archicortex AllocortexCentral sulcus Central sulcus of

RolandoCommissural fibers Transverse fibersCuneate gyrus CuneusHippocampal Commissure of the

commissure fornixInterthalamic adhesion Massa intermediaInterventricular foramen Foramen of MonroLateral fissure Fissure of SylviusLateral occipitotemporal Fusiform gyrus

gyrusMesocortex JuxtallocortexMyelencephalon Medulla oblongataNeocortex Isocortex

NeopalliumHomogenetic cortex

Striate cortex Calcarine cortexSuperior and inferior Corpora

colliculi quadrigeminaTransverse temporal Transverse temporal

gyri gyri of Heschl

Name of structure Synonym(s)/or term eponym(s)

The hypoglossal nerve is evident as a number of thinfilaments on each side of the brainstem, arising from the anterior lateral sulcus between the pyramids and olives. The glossopharyngeal, vagus, and accessory nerves arisefrom the groove dorsal to the olives.

The dorsal surface of the myelencephalon presents theposterior median fissure, which is interposed between theright and left tuberculum gracilis, swellings formed by the nucleus gracilis. Just lateral to the tuberculum gracilis isanother swelling, the tuberculum cuneatus, a bulge formedby the underlying nucleus cuneatus. The caudal continuationof the tuberculum gracilis is the fasciculus gracilis, and thecontinuation of the tuberculum cuneatus is the fasciculuscuneatus. Just lateral to the tuberculum cuneatus is another

swelling, the tuberculum cinereum, formed by the descend-ing tract of the trigeminal nerve.

SYNONYMS AND EPONYMS OF THE BRAIN

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As indicated at the beginning of this chapter, most of the major topics discussed here are presented in detail in subsequent chapters. Thereforethe pertinent clinical considerations are presented in the chapters dealingwith the specific topics.

CLINICAL CONSIDERATIONS

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QUESTIONS TO PONDER1. How are the categories of the three fiber components of the cerebralwhite matter classified?

2. Why is it inaccurate to call the basal ganglia, “ganglia?”

3. What are peduncles?

4. Describe the reason why the trochlear nerve is an unusual cranialnerve.

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