Chapter 16 Cerebral Hemispheres
The cerebral hemispheres are the largest part of the human brain. They each have a highly convoluted external cortex, beneath which lies an extensive internal mass of white matter that contains the basal ganglia. Each hemisphere also contains a lateral ventricle, continuous with the third ventricle through the interventricular foramen. The two hemispheres are linked by the commissural fibres of the corpus callosum.
Gyri, Sulci, and Lobes
On the superolateral cerebral surface, two prominent furrows—the lateral (Sylvian) fissure and the central sulcus—are the main features that determine its surface divisions (Figs 16.1, 16.3). The lateral fissure is a deep cleft on the lateral and inferior surfaces. It separates the frontal and parietal lobes above from the temporal lobe below. It has a short stem that divides into three rami. The stem commences inferiorly at the anterior perforated substance, extending laterally between the orbital surface of the frontal lobe and the anterior pole of the temporal lobe and accommodating the sphenoparietal venous sinus. Upon reaching the lateral surface of the hemisphere, it divides into anterior horizontal, anterior ascending and posterior rami. The anterior ramus runs forward for 2.5 cm or less into the inferior frontal gyrus, and the ascending ramus ascends for an equal distance into the same gyrus. The posterior ramus is the largest. It runs posteriorly and slightly upward, across the lateral surface of the hemisphere for approximately 7 cm, and turns up to end in the parietal lobe. Its floor is the insula, and it accommodates the middle cerebral vessels.
The central sulcus (see Figs 16.1, 16.3) is the boundary between the frontal and parietal lobes. It starts in or near the superomedial border of the hemisphere, a little behind the midpoint between the frontal and occipital poles. It runs sinuously downward and forward for 8 to 10 cm to end a little above the posterior ramus of the lateral sulcus, from which it is always separated by an arched gyrus. Its general direction makes an angle of approximately 70° with the median plane. It demarcates the primary motor and somatosensory areas of the cortex, located in the precentral and postcentral gyri, respectively.
The medial cerebral surface (Figs 16.2, 16.4) lies within the great longitudinal fissure. The commissural fibres of the corpus callosum lie in the depths of the fissure. The curved anterior part of the corpus callosum is the genu, continuous below with the rostrum and narrowing rapidly as it passes back to the upper end of the lamina terminalis. The genu continues above into the trunk or body, the main part of the commissure, which arches up and back to a thick, rounded posterior extremity, the splenium. The bilateral vertical laminae of the septum pellucidum are attached to the concave surfaces of the trunk, genu and rostrum, occupying the interval between them and the fornix. In front of the lamina terminalis, and almost coextensive with it, is the paraterminal gyrus, a narrow triangle of grey matter separated from the rest of the cortex by a shallow posterior paraolfactory sulcus. A short vertical sulcus, the anterior paraolfactory sulcus, may occur a little anterior to the paraterminal gyrus. The cortex between these two sulci is the subcallosal area (paraolfactory gyrus).
Fig. 16.3 Left lateral aspect of the brain.
(Dissection by E. L. Rees; photograph by Kevin Fitzpatrick on behalf of GKT School of Medicine, London.)
Fig. 16.4 Medial surface of the left cerebral hemisphere after sagittal section of the brain, followed by removal of the brain stem and septum pellucidum.
(Photograph by Kevin Fitzpatrick on behalf of GKT School of Medicine, London.)
The posterior region of the medial surface is traversed by the parieto-occipital and calcarine sulci. These two deep sulci converge anteriorly to meet a little posterior to the splenium. The parieto-occipital sulcus marks the boundary between the parietal and occipital lobes. It starts on the superomedial margin of the hemisphere approximately 5 cm anterior to the occipital pole, sloping down and slightly forward to the calcarine sulcus. The calcarine sulcus starts near the occipital pole. Although usually restricted to the medial surface, its posterior end may reach the lateral surface. Directed anteriorly, it joins the parieto-occipital sulcus at an acute angle behind the splenium. Continuing forward, it crosses the inferomedial margin of the hemisphere and forms the inferolateral boundary of the isthmus, which connects the cingulate with the parahippocampal gyrus. The visual cortex lies above and below the posterior part of the calcarine sulcus, behind the junction with the parieto-occipital sulcus. The calcarine is deep and produces an elevation, the calcar avis, in the wall of the posterior horn of the lateral ventricle.
The inferior cerebral surface is divided by the stem of the lateral fissure into a small anterior part and a larger posterior part (Figs 1.8, 16.5, 16.6). The anterior part is the orbital region of the inferior surface. It is transversely concave and lies above the cribriform plate of the ethmoid, the orbital plate of the frontal and the lesser wing of the sphenoid. A rostrocaudal olfactory sulcus traverses the region near its medial margin, overlapped by the olfactory bulb and tract. The medial strip thus demarcated is the gyrus rectus. The rest of this surface bears irregular orbital sulci, generally H-shaped, that divide it into the anterior, medial, posterior and lateral orbital gyri.
The larger posterior region of the inferior cerebral surface is partly superior to the tentorium as well as to the middle cranial fossa and is traversed by the anteroposterior collateral and occipitotemporal sulci (see Figs 16.2, 16.4). The collateral sulcus starts near the occipital pole and extends anteriorly and parallel to the calcarine sulcus, separated from it by the lingual gyrus. Anteriorly, it may continue into the rhinal sulcus, but the two are usually separate. The rhinal sulcus (fissure) runs forward in the line of the collateral sulcus, separating the temporal pole from the hook-shaped uncus posteromedial to it. This sulcus is the lateral limit of the piriform lobe (Fig. 16.7).
Cerebral Cortex
Microstructure
The neocortex essentially consists of three neuronal cell types. The most abundant are pyramidal cells. Non-pyramidal cells, also called stellate or granule cells, are divided into spiny and non-spiny neurones. All types have been subdivided on the basis of size and shape (Fig. 16.8).
Pyramidal cells (Fig. 16.9) have a flask-shaped or triangular cell body ranging from 10 to 80 µm in diameter. The soma gives rise to a single thick apical dendrite and multiple basal dendrites. The apical dendrite ascends toward the cortical surface, tapering and branching, to end in a spray of terminal twigs in the most superficial lamina, the molecular layer. From the basal surface of the cell body, dendrites spread more horizontally, for distances up to 1 mm for the largest pyramidal cells. Like the apical dendrite, the basal dendrites branch profusely along their length. All pyramidal cell dendrites are studded with myriad dendritic spines. These become more numerous as the distance from the parent cell soma increases. A single slender axon arises from the axon hillock, which is usually situated centrally on the basal surface of the pyramidal neurone. Ultimately, in the vast majority of (if not all) cases, the axon leaves the cortical grey matter to enter the white matter. Pyramidal cells are thus, perhaps universally, projection neurones. They appear to use excitatory amino acids, either glutamate or aspartate, exclusively as their neurotransmitters.
Spiny stellate cells are the second most numerous cell type in the neocortex and, for the most part, occupy lamina IV. They have relatively small multipolar cell bodies, commonly 6 to 10 µm in diameter. Several primary dendrites, profusely covered in spines, radiate for varying distances from the cell body. Their axons ramify within the grey matter, predominantly in the vertical plane. Spiny stellate cells are likely to use glutamate as their neurotransmitter.
Neurones with mainly horizontally dispersed axons include basket and horizontal cells. Basket cells have a short, vertical axon that rapidly divides into horizontal collaterals; these end in large terminal sprays synapsing with the somata and proximal dendrites of pyramidal cells. The cell bodies of horizontal cells lie mainly at the superficial border of lamina II and occasionally deep in lamina I (the molecular or plexiform layer). They are small and fusiform, and their dendrites spread short distances in two opposite directions in lamina I. Their axons often stem from a dendrite, then divide into two branches that travel away from each other for great distances in the same layer.
Laminar Organization
Typical neocortex is described as having six layers or laminae lying parallel to the surface (Figs 16.10–16.12).
Fig. 16.12 Lateral (A) and medial (B) surfaces of the left cerebral hemisphere depicting Brodmann’s areas.
Neocortical Structure
Five regional variations are described in neocortical structure (see Fig. 16.11). Although all are said to develop from the same six-layered pattern, two types—granular and agranular—are regarded as virtually lacking certain laminae and are referred to as heterotypical. Homotypical variants, in which all six laminae are found, are called frontal, parietal and polar—names that link them with specific cortical regions in a somewhat misleading manner (e.g. the frontal type also occurs in parietal and temporal lobes).
The other three types of cortex are intermediate forms. In the frontal type, large numbers of small and medium-sized pyramidal neurones appear in laminae III and V, and the granular layers (II and IV) are less prominent. The relative prominence of these major forms of neurone vary reciprocally wherever this form of cortex exists.
For almost 100 years it has been customary to refer to discrete cortical territories not only by their anatomical location in relation to gyri and sulci but also in relation to their cytoarchitectonic characteristics as originally descried by Brodmann (see Fig. 16.12). Some of the areas so defined (e.g. the primary sensory and motor cortices) have clear relevance in terms of anatomical connections and functional significance; others less so.
Overview of Cortical Connectivity
All neocortical areas are connected with subcortical regions, although the density of these connections varies among areas. First among these are connections with the thalamus (Ch. 15). All areas of the neocortex receive afferents from more than one thalamic nucleus, and all such connections are reciprocal. The vast majority of (if not all) cortical areas project to the striatum, tectum, pons and brain stem reticular formation. Additionally, all cortical areas are reciprocally connected with the claustrum; the frontal cortex connects with the anterior part, and the occipital lobe with the posterior part.
Widely separated but functionally interconnected areas of cortex share common patterns of connections with subcortical nuclei and within the neocortex. For example, contiguous zones of the striatum, thalamus, claustrum, cholinergic basal forebrain, superior colliculus and pontine nuclei connect with anatomically widely separated areas in the prefrontal and parietal cortices, which are themselves interconnected. In contrast, other cortical regions that are functionally distinct (e.g. areas in the temporal and parietal cortices) do not share such contiguity in their subcortical connections. (See Case 12.)
Frontal Lobe
The frontal lobe is the rostral region of the hemisphere, anterior to the central sulcus and above the lateral fissure. On the superolateral surface, extending onto the medial surface, is the precentral gyrus, running parallel to the central sulcus and limited anteriorly by the precentral sulcus. The area of the frontal lobe anterior to the precentral sulcus is divided into the superior, middle and inferior frontal gyri (see Figs 16.1, 16.2). In front of these gyri lies the frontal pole. The ventral surface of the frontal lobe overlies the bony orbit and is the orbitofrontal cortex. The medial surface extends from the frontal pole anteriorly to the paracentral lobule behind. It consists of the medial frontal cortex and the anterior cingulate cortex.
Primary Motor Cortex
The primary motor cortex (MI) corresponds to the precentral gyrus (area 4). It is the area of cortex with the lowest threshold for eliciting contralateral muscle contraction by electrical stimulation. The primary motor cortex contains a detailed, topographically organized map (motor homunculus) of the opposite body half, with the head represented most laterally and the legs and feet represented on the medial surface of the hemisphere in the paracentral lobule (Fig. 16.13). A striking feature is the disproportionate representation of body parts in relation to their physical size. Thus, large areas represent the muscles of the hand and face, which are capable of finely controlled or fractionated movements.
The ipsilateral somatosensory cortex (SI) projects in a topographically organized way to area 4, and the connection is reciprocal. The projection to the motor cortex arises in areas 1 and 2, with little or no contribution from area 3b. Fibres from SI terminate in layers II and III of area 4, where they contact mainly pyramidal neurones. Evidence suggests that neurones activated monosynaptically by fibres from SI, as well as those activated polysynaptically, make contact with layer V pyramidal cells, which give rise to corticospinal fibres, including Betz cells. Movement-related neurones in the motor cortex that can be activated from SI tend to have a late onset of activity, mainly during the execution of movement. It has been suggested that this pathway plays a role primarily in the making of motor adjustments during a movement. Additional ipsilateral corticocortical fibres to area 4 from behind the central sulcus come from the second somatosensory area (SII).
Premotor Cortex
Immediately in front of the primary motor cortex lies Brodmann’s area 6 (Fig. 16.14). Area 6 extends onto the medial surface, where it becomes contiguous with area 24 in the cingulate gyrus, anterior and inferior to the paracentral lobule. A number of functional motor areas are contained in this cortical region. Lateral area 6, the area over most of the lateral surface of the hemisphere, corresponds to the premotor cortex. The premotor cortex is divided into dorsal and ventral areas on functional grounds and on the basis of ipsilateral corticocortical association connections.
Frontal Eye Field
The frontal eye field lies predominantly within Brodmann’s area 8, anterior to the superior premotor cortex (Fig. 16.15). It receives its major thalamic projection from the parvocellular mediodorsal nucleus, with additional afferents from the medial pulvinar, the ventral anterior nucleus and the suprageniculate–limitans complex. It connects with the paracentral nucleus of the intralaminar group. The thalamocortical pathways to the frontal eye field form part of a pathway from the superior colliculus, the substantia nigra and the dentate nucleus of the cerebellum. The frontal eye field has extensive ipsilateral corticocortical connections, receiving fibres from several visual areas in the occipital, parietal and temporal lobes, including the medial temporal area (V5) and area 7a. There is also a projection from the superior temporal gyrus, which is auditory rather than visual in function. From within the frontal lobe, the frontal eye field receives fibres from the ventrolateral and dorsolateral prefrontal cortices. It projects to the dorsal and ventral premotor cortices and to the medial motor area, probably to the supplementary eye field adjacent to the supplementary motor area proper. It projects prominently to the superior colliculus, the pontine gaze centre within the pontine reticular formation and other oculomotor-related nuclei in the brain stem. As its name implies, it is important in the control of eye movements. Destructive lesions of the frontal eye field cause ipsilateral conjugate deviation of the eyes, whereas stimulation, such as with an epileptic discharge, induces contralateral deviation.
Supplementary Motor Cortex
The supplementary motor area contains a representation of the body in which the leg is posterior and the face anterior, with the upper limb between them. Its role in the control of movement involves primarily complex tasks, which require temporal organization of sequential movements and retrieval of motor memory. The consequences of damage to the supplementary motor area bear some striking similarities to the effects of basal ganglia dysfunction; akinesia is common, and there may be problems with the performance of sequential, complex movements. Stimulation of the supplementary motor area in conscious patients has been reported to elicit the sensation of an urge to move or the feeling that a movement is about to occur. A region anterior to the supplementary motor area for face representation is important in vocalization and speech production.
Prefrontal Cortex
The prefrontal cortex on the lateral surface of the hemisphere comprises predominantly Brodmann’s areas 9, 46 and 45 (see Figs 16.12, 16.14, 16.15). In non-human primates, two subdivisions of the lateral prefrontal cortex are recognized: a dorsal area equivalent to area 9 and perhaps including the superior part of area 46, and a ventral area consisting of the inferior part of area 46 and area 45. Areas 44 and 45 are particularly notable in humans because, in the dominant hemisphere, they constitute the motor speech area (Broca’s area; see Case 12). Both the dorsolateral and ventrolateral prefrontal areas receive their major thalamic afferents from the mediodorsal nucleus, and there are additional contributions from the medial pulvinar, the ventral anterior nucleus and the paracentral nucleus of the anterior intralaminar group. The dorsolateral area receives long association fibres from the posterior and middle superior temporal gyrus, including auditory association areas; from parietal area 7a; and from much of the middle temporal cortex. From within the frontal lobe it also receives projections from the frontal pole (area 10) and from the medial prefrontal cortex (area 32) on the medial surface of the hemisphere. It projects to the supplementary motor area, the dorsal premotor cortex and the frontal eye field. All these thalamic and corticocortical connections are reciprocal. Commissural connections are with the homologous area and with the contralateral inferior parietal cortex. The ventrolateral prefrontal area receives long association fibres from areas 7a and 7b of the parietal lobe, auditory association areas of the temporal operculum, the insula and the anterior part of the lower bank of the superior temporal sulcus. From within the frontal lobe it receives fibres from the anterior orbitofrontal cortex and projects to the frontal eye field and the ventral premotor cortex. It connects with the contralateral homologous area via the corpus callosum. These connections are probably all reciprocal.
Parietal Lobe
The lateral aspect of the parietal lobe is divided into three areas by postcentral and intraparietal sulci (see Fig. 16.1). The postcentral sulcus, often divided into upper and lower parts, is posterior and parallel to the central sulcus. Inferiorly, it ends above the posterior ramus of the lateral fissure. The postcentral gyrus or primary somatosensory cortex lies between the central and postcentral sulci. Posterior to the postcentral sulcus there is a large area, subdivided by the intraparietal sulcus. It usually starts in the postcentral sulcus near its midpoint and extends posteroinferiorly across the parietal lobe, dividing it into superior and inferior parietal lobules. Posteriorly, its occipital ramus extends into the occipital lobe, joining the transverse occipital sulcus at right angles.