Cerebral hemisphere

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CHAPTER 23 Cerebral hemisphere

The two cerebral hemispheres are the largest major divisions of the brain. Each hemisphere consists of an external highly-convoluted cortex, beneath which lies an extensive internal mass of white matter that partly encloses the basal ganglia. Each hemisphere also contains a lateral ventricle that is continuous with the third ventricle through the interventricular foramen. The two hemispheres are linked by the commissural fibres of the corpus callosum.

The cerebral hemisphere contains primary motor and sensory cortical areas. These represent the highest level at which motor activities are controlled and the highest level to which general and special sensory systems project, providing the neural substrate for conscious experience of sensory stimuli. Association areas are modality-specific and also multimodal, and they enable complex analysis of the internal and external environment and the relationship of the individual to the external world. Parts of the hemisphere, termed the limbic system, have an ancient lineage. They are concerned with memory and the emotional aspects of behaviour, and provide an affective overtone to conscious experience as well as interfacing with subcortical areas, such as the hypothalamus, through which widespread physiological activities are integrated. Other cortical areas, primarily within the frontal region, are concerned with the highest aspects of cognitive function, and contribute to personality, foresight and planning.

The cerebral hemispheres are separated by a deep median cleft, the great longitudinal fissure, which contains a crescent-shaped fold of the dura mater, the falx cerebri (see Ch. 27).

Each cerebral hemisphere may be considered to have superolateral, medial and inferior (basal) surfaces or aspects. The superolateral surface follows the concavity of the cranial vault. The medial surface is flat and vertical, separated from the opposite hemisphere by the great longitudinal fissure and falx cerebri. The inferior surface is irregular and divided into orbital and tentorial regions. The orbital part is concave and lies above the orbital and nasal roofs. The tentorial part lies in the middle cranial fossa; posteriorly it lies above the tentorium cerebelli, which separates it from the superior surface of the cerebellum.

GYRI, SULCI AND LOBES

The surface of the cerebral hemisphere shows a complex pattern of convolutions, or gyri, which are separated by furrows of varying depth known as fissures, or sulci (Fig. 23.1, Fig. 23.2). Some of these are consistently located, others less so. They provide part of the basis for the division of the hemisphere into lobes. The frontal, parietal, temporal and occipital lobes approximately correspond in surface extent to the overlying cranial bones from which they take their names. The anterior and posterior extremities of the hemispheres form the frontal and occipital poles respectively, and the temporal pole is the anterior extremity of the temporal lobe.

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Fig. 23.1 Lateral aspect of the left cerebral hemisphere indicating the major gyri and sulci.

(Dissection by EL Rees; photograph by Kevin Fitzpatrick on behalf of GKT School of Medicine, London; figure enhanced by B. Crossman.)

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Fig. 23.2 Sagittal section of the brain, with the brain stem removed, showing the major gyri and sulci on the medial aspect of the left cerebral hemisphere.

(Photograph by Kevin Fitzpatrick on behalf of GKT School of Medicine, London; figure enhanced by B. Crossman.)

The insula is a cortical region hidden within the depths of the lateral fissure by overhanging parts (opercula) of the frontal, parietal and temporal lobes. A complex of gyri on the medial aspect of the hemisphere makes up the limbic lobe.

The area of the adult cerebral cortex is approximately 2200 cm2: its convoluted form increases the cortical volume to three times greater than it would be if the surface were smooth.

Two prominent furrows, the lateral (Sylvian) fissure and the central sulcus, are the main features on the superolateral cerebral surface, and they determine its surface divisions.

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 and accommodates the middle cerebral vessels. It 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. Reaching the lateral surface of the hemisphere it gives rise to an ascending ramus which runs into the inferior frontal gyrus. The larger posterior ramus runs posteriorly and slightly upwards, across the lateral surface of the hemisphere to end in the parietal lobe. The insula lies in the depths of the lateral fissure.

The central sulcus is the boundary between the frontal and parietal lobes and demarcates the primary motor and somatosensory areas of the cortex, located in the precentral and postcentral gyri, respectively. 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 downwards and forwards to end a little above the posterior ramus of the lateral sulcus.

The superior frontal gyrus, above the superior frontal sulcus, is continuous over the superomedial margin with the medial frontal gyrus and may be incompletely divided. The middle frontal gyrus lies between the superior and inferior frontal sulci. The inferior frontal gyrus lies below the inferior frontal sulcus which is invaded by the ascending ramus of the lateral fissure. In the left hemisphere, the cortical areas around this ramus make up the motor speech area (Broca’s area; areas 44 and 45).

The medial cerebral surface 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, the genu, is continuous below with the rostrum and narrows rapidly as it passes back to the upper end of the lamina terminalis. The cortex immediately below the rostrum is the subcallosal area (parolfactory gyrus). The genu of the corpus callosum continues above into the trunk or body, 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.

The anterior region of the medial surface of the hemisphere is divided into outer and inner zones by the curved cingulate sulcus, which starts below the rostrum and passes first forwards, then up and finally backwards, conforming to the callosal curvature. Its posterior end turns up to the superomedial margin of the hemisphere approximately 4 cm behind its midpoint, and is posterior to the upper end of the central sulcus. The outer zone, except for its posterior extremity, is part of the frontal lobe, and is subdivided into anterior and posterior areas by a short sulcus, which ascends from the cingulate sulcus above the midpoint of the corpus callosum. The larger anterior area is the medial frontal gyrus; the posterior is the paracentral lobule. The superior end of the central sulcus usually invades the paracentral lobule posteriorly and the precentral gyrus is continuous with the lobule. This area is concerned with movements of the contralateral lower limb and perineal region – clinical evidence suggests that it exercises voluntary control over defaecation and micturition.

The zone under the cingulate sulcus is the cingulate gyrus. Starting below the rostrum, this gyrus follows the callosal curve, separated by the callosal sulcus. It continues round the splenium to the inferior surface, and then into the parahippocampal gyrus, through the narrow isthmus.

The posterior region of the medial surface is traversed by the parieto-occipital and the calcarine sulci. These two deep sulci converge anteriorly to meet a little posterior to the splenium. The parieto-occipital sulcus marks the boundary between 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 forwards to the calcarine sulcus. The calcarine sulcus starts near the occipital pole. Though 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 forwards, it crosses the inferomedial margin of the hemisphere, and forms the inferior 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. The calcarine sulcus is deep and produces an elevation, the calcar avis, in the wall of the posterior horn of the lateral ventricle.

The area posterior to the upturned end of the cingulate sulcus, and anterior to the parieto-occipital sulcus, is the precuneus. It forms the medial surface of the parietal lobe with the part of the paracentral lobule behind the central sulcus. The medial surface of the occipital lobe is formed by the cuneus, a wedge of cortex bounded in front by the parieto-occipital sulcus, below by the calcarine sulcus, and above by the superomedial margin.

The inferior cerebral surface is divided by the stem of the lateral fissure into a small anterior and larger posterior part (Fig. 23.3, Fig. 23.4). The anterior part constitutes the orbital region of the frontal lobe. It is concave and lies above the cribriform plate of the ethmoid, the orbital plate of the frontal, and the lesser wing of the sphenoid bone. A rostrocaudal olfactory sulcus traverses it near its medial margin, overlapped by the olfactory bulb and tract. The medial strip thus demarked is the gyrus rectus. The rest of this surface bears irregular orbital sulci, generally H-shaped, which 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 but also to the middle cranial fossa. The collateral sulcus starts near the occipital pole, and extends anteriorly and parallel to the calcarine sulcus, separated from it by the lingual gyrus. The lingual gyrus, between the calcarine and collateral sulci, passes into the parahippocampal gyrus, which begins at the isthmus where it is continuous with the cingulate gyrus. Anteriorly, the parahippocampal gyrus continues into the hook-shaped uncus, which lies lateral to the midbrain and posterolateral to the anterior perforated substance. The uncus is part of the piriform cortex of the olfactory system, phylogenetically one of the oldest parts of the cortex, and is separated from the temporal pole by the rhinal sulcus (fissure) which marks the lateral limit of the entorhinal cortex (area) (Fig. 23.4).

The occipitotemporal sulcus is parallel and lateral to the collateral sulcus, does not usually reach the occipital pole, and is frequently divided. The medial occipitotemporal gyrus extends from the occipital to the temporal pole and is limited medially by the collateral and rhinal sulci and laterally by the occipitotemporal sulcus. The lateral occipitotemporal gyrus is continuous, round the inferolateral margin of the hemisphere, with the inferior temporal gyrus.

CEREBRAL CORTEX

The cerebral cortex is sometimes described as consisting of a phylogenetically old allocortex, which includes archicortex and palaeocortex, and a newer neocortex. These distinctions are reflected in the organizational arrangement of neuronal elements in different regions.

MICROSTRUCTURE

The microscopic structure of the cerebral cortex is an intricate blend of nerve cells and fibres, neuroglia and blood vessels. The neocortex essentially consists of three neuronal cell types. Pyramidal cells are the most abundant. 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. 23.5).

Pyramidal cells 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 towards 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 a myriad of dendritic spines. These become more numerous as 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 in all, cases, the axon leaves the cortical grey matter to enter the white matter. Pyramidal cells are thus, perhaps universally, projection neurones. They use an excitatory amino acid, either glutamate or aspartate, as their neurotransmitter.

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 variable distances from the cell body. Their axons ramify within the grey matter predominantly in the vertical plane. Spiny stellate cells probably use glutamate as their neurotransmitter.

The smallest group comprises the heterogeneous non-spiny or sparsely spinous stellate cells. All are interneurones, and their axons are confined to grey matter. In morphological terms, this is not a single class of cell, but a multitude of different forms, including basket, chandelier, double bouquet, neurogliaform, bipolar/fusiform and horizontal cells. Various types may have horizontally, vertically or radially ramifying axons.

Neurones with mainly horizontally dispersed axons include basket and horizontal cells. Basket cells have a short, vertical axon, which rapidly divides into horizontal collaterals, and 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, 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, which travel away from each other for great distances in the same layer.

Neurones with an axonal arborization predominantly perpendicular to the pial surface include chandelier, double bouquet and bipolar/fusiform cells. Chandelier cells have a variable morphology, although most are ovoid or fusiform and their dendrites arise from the upper and lower poles of the cell body. The axonal arborization, which emerges from the cell body or a proximal dendrite, is characteristic and identifies these neurones. A few cells in the more superficial laminae (II and IIIa) have descending axons, deeper cells (laminae IIIc and IV) have ascending axons, and intermediate neurones (IIIb) often have both. The axons ramify close to the parent cell body and terminate in numerous vertically oriented strings, which run alongside the axon hillocks of pyramidal cells with which they synapse. Double bouquet (or bitufted) cells are found in laminae II and III and their axons traverse laminae II and V. Generally, these neurones have two or three main dendrites, which give rise to a superficial and deep dendritic tuft. A single axon arises usually from the oval or spindle-shaped cell soma and rapidly divides into an ascending and descending branch. These branches collateralize extensively, but the axonal arbor is confined to a perpendicularly extended, but horizontally confined, cylinder, approximately 50 to 80 μm across. Bipolar cells are ovoid with a single ascending and a single descending dendrite, which arise from the upper and lower poles, respectively. These primary dendrites branch sparsely and their branches run vertically to produce a narrow dendritic tree, rarely more than 10 μm across, which may extend through most of the cortical thickness. Commonly, the axon originates from one of the primary dendrites, and rapidly branches to give a vertically elongated, horizontally confined, axonal arbor, which closely parallels the dendritic tree in extent.

The principal recognizable neuronal type is the neurogliaform or spiderweb cell. These small spherical cells, 10 to 12 μm in diameter, are found mainly in laminae II to IV, depending on cortical area. Seven to ten thin dendrites typically radiate out from the cell soma, some branching once or twice to form a spherical dendritic field of approximately 100 to 150 μm diameter. The slender axon arises from the cell body or a proximal dendrite. Almost immediately, it branches profusely within the vicinity of the dendritic field (and usually somewhat beyond), to give a spherical axonal arbor up to 350 μm in diameter.

The majority of non-spiny or sparsely spinous non-pyramidal cells probably use GABA as their principal neurotransmitter. This is almost certainly the case for basket, chandelier, double bouquet, neurogliaform and bipolar cells. Some are also characterized by the coexistence of one or more neuropeptides, including neuropeptide Y, vasoactive intestinal polypeptide (VIP), cholecystokinin, somatostatin and substance P. Acetylcholine is present in a subpopulation of bipolar cells, which may additionally be GABAergic and contain VIP.

Laminar organization

The most apparent microscopical feature of the neocortex when it has been stained for cell bodies or for fibres is its horizontal lamination. The significance of this for understanding cortical functional organization is debatable, but the use of cytoarchitectonic description to identify regions of cortex is common.

Typical neocortex is described as having six layers or laminae lying parallel to the surface (Fig. 23.6).

I The molecular or plexiform layer is cell sparse, containing only scattered horizontal cells and their processes enmeshed in a compacted mass of tangential, principally horizontal axons and dendrites. These are afferent fibres, which arise from outside the cortical area, together with intrinsic fibres from cortical interneurones, and the apical dendritic arbors of virtually all pyramidal neurones of the cerebral cortex. In histological sections stained to show myelin, layer I appears as a narrow horizontal band of fibres.

II The external granular lamina contains a varying density of small neuronal cell bodies including both small pyramidal and non-pyramidal cells; the latter may predominate. Myelin fibre stains show mainly vertically arranged processes traversing the layer.

III The external pyramidal lamina contains pyramidal cells of varying sizes, together with scattered non-pyramidal neurones. The size of the pyramidal cells is smallest in the most superficial part of the layer and greatest in the deepest part. This lamina is frequently further subdivided into IIIa, IIIb and IIIc, with IIIa most superficial and IIIc deepest. As in layer II, myelin stains reveal a mostly vertical organization of fibres.

IV The internal granular lamina is usually the narrowest of the cellular laminae. It contains densely packed, small, round cell bodies of non-pyramidal cells, notably spiny-stellate cells and some small pyramidal cells. Within the lamina, in myelin stained sections, a prominent band of horizontal fibres (outer band of Baillarger) is seen.

V The internal pyramidal (ganglionic) lamina typically contains the largest pyramidal cells in any cortical area, though actual sizes vary considerably from area to area. Scattered non-pyramidal cells are also present. In myelin stains, the lamina is traversed by ascending and descending vertical fibres, and also contains a prominent central band of horizontal fibres (inner band of Baillarger).

VI The multiform (or fusiform/pleiomorphic) layer consists of neurones with a variety of shapes, including recognizable pyramidal, spindle, ovoid and many other shaped somata. Typically, most cells are small to medium in size. This lamina blends gradually with the underlying white matter, and a clear demarcation of its deeper boundary is not always possible.

Five regional variations are described in neocortical structure (Fig. 23.7). While 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 agranular type is considered to have diminished, or absent, granular laminae (II and IV), but always contains scattered stellate somata. Large pyramidal neurones are found in the greatest densities in agranular cortex, which is typified by the numerous efferent projections of pyramidal cell axons. Although it is often equated with motor cortical areas such as the precentral gyrus (area 4), agranular cortex also occurs elsewhere, e.g. areas 6, 8 and 44 and parts of the limbic system.

In the granular type of cortex the granular layers are maximally developed, and contain densely packed stellate cells, among which small pyramidal neurones are dispersed. Laminae III and IV are poorly developed or unidentifiable. This type of cortex is particularly associated with afferent projections. However, it does receive efferent fibres, derived from the scattered pyramidal cells, although they are less numerous than elsewhere. Granular cortex occurs in the postcentral gyrus (somatosensory area), striate area (visual area) and superior temporal gyrus (acoustic area), and in small areas of the parahippocampal gyrus. Despite its very high density of stellate cells, especially in the striate area, it is almost the thinnest of the five main types. In the striate cortex the external band of Baillarger (lamina IV) is well defined as the stria (white line) of Gennari.

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 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.

The parietal type of cortex contains pyramidal cells, which are mostly smaller in size than in the frontal type. In marked contrast, the granular laminae are wider and contain more of the stellate cells: this kind of cortex occupies large areas in the parietal and temporal lobes. The polar type is classically identified with small areas near the frontal and occipital poles and is the thinnest form of cortex. All six laminae are represented, but the pyramidal layer (III) is reduced in thickness and not so extensively invaded by stellate cells as it is in the granular type of cortex. In both polar and granular types, the multiform layer (VI) is more highly organized than in other types.

It is customary to refer to some discrete cortical territories not only by their anatomical location in relation to gyri and sulci, but also in relation to their cytoarchitectonic characteristics (Brodmann’s areas) (Fig. 23.8). 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 have axonal connections with other cortical areas on the same side (association fibres), the opposite side (commissural fibres), and with subcortical structures (projection fibres).

The primary somatosensory, visual and auditory areas give rise to ipsilateral corticocortical connections to the association areas of the parietal, occipital and temporal lobes, respectively, which then progressively project towards the medial temporal limbic areas, notably the parahippocampal gyrus, entorhinal cortex and hippocampus. Thus, the first (primary) somatic sensory area (SI) projects to the superior parietal cortex (Brodmann’s area 5), which in turn projects to the inferior parietal cortex (area 7). From here connections pass to cortex in the walls of the superior temporal sulcus, and so on to the posterior parahippocampal gyrus, and on into limbic cortex. Similarly, for the visual system, the primary visual cortex (area 17) projects to the parastriate cortex (area 18), which in turn projects to the peristriate region (area 19). Information then flows to inferotemporal cortex (area 20), to cortex in the walls of the superior temporal sulcus, then to medial temporal cortex in the posterior parahippocampal gyrus, and so to limbic areas. The auditory system shows a similar progression from primary auditory cortex to temporal association cortex and so to the medial temporal lobe.

In addition to this stepwise outward progression from sensory areas through posterior association cortex, connections also occur at each stage with parts of the frontal cortex. Thus, taking the somatic sensory system as an example, primary somatic sensory cortex (SI) in the postcentral gyrus is reciprocally connected with the primary motor cortex (area 4) in the precentral gyrus. The next step in the outward progression, the superior parietal lobule (area 5), is interconnected with the premotor cortex (area 6), and this in turn is connected with area 7 in the inferior parietal lobule. This has reciprocal connections with prefrontal association cortex on the lateral surface of the hemisphere (areas 9 and 46), and temporal association areas, which connect with more anterior prefrontal association areas, and, ultimately in the sequence, with orbitofrontal cortex. Similar stepwise links exist between areas on the visual and auditory association pathways in the occipitotemporal lobe and areas of the frontal association cortex. The connections between sensory and association areas are reciprocal.

All neocortical areas are connected with subcortical regions although their density varies between areas. First among these are connections with the thalamus. 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.

All cortical areas receive topographically organized cholinergic projections from the basal forebrain, noradrenergic fibres from the locus coeruleus, serotoninergic fibres from the midbrain raphe nuclei, dopaminergic fibres from the ventral midbrain and histaminergic fibres from the posterior hypothalamus.

Different cortical areas have widely different afferent and efferent connections. Some have connections that are unique, e.g. the corticospinal tract arises from pyramidal cells in a restricted area around the central sulcus.

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 cortex, which are themselves interconnected. In contrast, other functionally distinct cortical regions, e.g. areas in the temporal and parietal cortex, do not share such contiguity in their subcortical connections.

Cortical lamination and cortical connections

The cortical laminae represent, to some extent, horizontal aggregations of neurones with common connections. This is most clearly seen in the lamination of cortical efferent (pyramidal) cells. The internal pyramidal lamina, layer V, gives rise to cortical projection fibres, most notably corticostriate, corticobulbar (including corticopontine) and corticospinal axons. In addition, a significant proportion of feedback corticocortical axons arise from cells in this layer, as do some corticothalamic fibres. Layer VI, the multiform lamina, is the major source of corticothalamic fibres. Supragranular pyramidal cells, predominantly layer III, but also lamina II, give rise primarily to both association and commissural corticocortical pathways. Generally, short corticocortical fibres arise more superficially, and long corticocortical (both association and commissural) axons come from cells in the deeper parts of layer III. Major afferents to a cortical area tend to terminate in layers I, IV and VI. Quantitatively lesser projections end either in the intervening laminae II/III and V, or sparsely throughout the depth of the cortex. Numerically, the largest input to a cortical area tends to terminate mainly in layer IV. This pattern of termination is seen in the major thalamic input to visual and somatic sensory cortex. In general, non-thalamic subcortical afferents to the neocortex, which are shared by widespread areas, tend to terminate throughout all cortical layers, but the laminar pattern of their endings still varies considerably from area to area.

FRONTAL LOBE

The frontal lobe is the rostral region of the hemisphere, anterior to the central sulcus and above the lateral fissure. The precentral gyrus runs parallel to the central sulcus on the superolateral surface and extends onto the medial surface, and is 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 (Fig. 23.1). The frontal pole lies in front of these gyri. The ventral surface of the frontal lobe, the orbitofrontal cortex, overlies the bony orbit. The medial surface extends from the frontal pole to the paracentral lobule and 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), and is the area of cortex with the lowest threshold for eliciting contralateral muscle contraction by electrical stimulation. It contains a detailed topographically organized map (motor homunculus) of the opposite body half, with the head represented most laterally, and the leg and foot represented on the medial surface of the hemisphere in the paracentral lobule (Fig. 23.9). A striking feature is the disproportionate representation of body parts in relation to their physical size: large areas represent the muscles of the face and hand, which are capable of finely controlled or fractionated movements.

The cortex of area 4 is agranular, and layers II and IV are difficult to identify. The most characteristic feature is the presence in lamina V of some extremely large pyramidal cell bodies, Betz cells, which may approach 80 μm in diameter. These neurones project their axons into the corticospinal and corticobulbar tracts.

The major thalamic connections of area 4 are with the ventral posterolateral nucleus (VPL), which in turn receives afferents from the deep cerebellar nuclei. The VPL nucleus also contains a topographic representation of the contralateral body, which is preserved in its point-to-point projection to area 4, where it terminates largely in lamina IV. Other thalamic connections of area 4 are with the centromedian and parafascicular nuclei. These appear to provide the only route through which output from the basal ganglia, routed via the thalamus, reaches the primary motor cortex, since the projection of the internal segment of the globus pallidus to the ventrolateral nucleus of the thalamus is confined to the anterior division, and there is no overlap with cerebellothalamic territory. The anterior part of the ventrolateral nucleus projects to the premotor and supplementary motor areas of cortex with no projection to area 4.

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, including Betz cells, which give rise to corticospinal fibres. Movement-related neurones in the motor cortex which 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 making motor adjustments during a movement. Additional ipsilateral corticocortical fibres to area 4 from behind the central sulcus come from the second somatic sensory area (SII).

Neurones in area 4 are responsive to peripheral stimulation, and have receptive fields similar to those in the primary sensory cortex. Cells located posteriorly in the motor cortex have cutaneous receptive fields, whereas more anteriorly situated neurones respond to stimulation of deep tissues.

The motor cortex receives major frontal lobe association fibres from the premotor cortex and the supplementary motor area and also fibres from the insula. It is probable that these pathways modulate motor cortical activity in relation to the preparation, guidance and temporal organization of movements. Area 4 sends fibres to, and receives fibres from, its contralateral counterpart, and also projects to the contralateral supplementary motor cortex.

Apart from its contribution to the corticospinal tract, the motor cortex has diverse subcortical projections. The connections to the striatum and pontine nuclei are heavy. It also projects to the subthalamic nucleus. The motor cortex sends projections to all nuclei in the brain stem, which are themselves the origin of descending pathways to the spinal cord, namely the reticular formation, the red nucleus, the superior colliculus, the vestibular nuclei and the inferior olivary nucleus.

Corticospinal tract

The corticospinal or pyramidal tract provides direct control by the cerebral cortex over motor centres of the spinal cord. An homologous pathway to the brain stem, the corticobulbar projection, fulfils a similar function in relation to motor nuclei of the brain stem. The corticospinal tract does not originate solely from the motor cortex, but is conveniently considered in conjunction with it.

The percentage of corticospinal fibres that arise from the primary motor cortex may actually be quite small, probably in the region of 20 to 30%. They arise from pyramidal cells in layer V and give rise to the largest diameter corticospinal axons. There is also a widespread origin from other parts of the frontal lobe, including the premotor cortex and the supplementary motor area. Many axons from the frontal cortex, notably the motor cortex, terminate in the ventral horn of the spinal cord. In cord segments mediating dexterous hand and finger movements they terminate in the lateral part of the ventral horn, in close relationship to motor neuronal groups. A small percentage establish direct monosynaptic connections with α motor neurones.

Between 40 and 60% of pyramidal tract axons arise from parietal areas, including area 3a, area 5 of the superior parietal lobe, and SII in the parietal operculum. The majority of parietal fibres to the spinal cord terminate in the deeper layers of the dorsal horn.

Motor cortical neurones are active in relation to the force of contraction of agonist muscles; their relation to amplitude of movement is less clear. Their activity precedes the onset of electromyographic activity by 50 to 100 milliseconds, suggesting a role for cortical activation in generating rather than monitoring movement.

Premotor cortex

Immediately in front of the primary motor cortex lies Brodmann’s area 6. It 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 within 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 a dorsal and a ventral area (PMd and PMv respectively) on functional grounds, and on the basis of ipsilateral corticocortical association connections.

The major thalamic connections of the premotor cortex are with the anterior division of the ventrolateral nucleus and with the centromedian, parafascicular and centrolateral components of the intralaminar nuclei. Subcortical projections to the striatum and pontine nuclei are prominent, and this area also projects to the superior colliculus and the reticular formation. Both dorsal and ventral areas contribute to the corticospinal tract. Commissural connections are with the contralateral premotor, motor and superior parietal (area 5) cortex. Ipsilateral corticocortical connections with area 5 in the superior parietal cortex, and inferior parietal area 7b are common to both dorsal and ventral subdivisions of the premotor cortex, and both send a major projection to the primary motor cortex. The dorsal premotor area also receives fibres from the posterior superior temporal cortex and projects to the supplementary motor cortex. The frontal eye field (area 8) projects to the dorsal subdivision. Perhaps the greatest functionally significant difference in connectivity between the two premotor area subdivisions is that the dorsal premotor area receives fibres from the dorsolateral prefrontal cortex, whereas the ventral subdivision receives fibres from the ventrolateral prefrontal cortex. All of these association connections are likely to be, or are known to be, reciprocal.

Neuronal activity in the premotor cortex in relation to both preparation for movement and movement itself has been extensively studied experimentally. Direction selectivity for movement is a common feature of many premotor neurones. In behavioural tasks, neurones in the dorsal premotor cortex show anticipatory activity and task-related discharge as well as direction selectivity, but little or no stimulus-related changes. The dorsal premotor cortex is probably important in establishing a motor set or intention, contributing to motor preparation in relation to internally guided movement. In contrast, ventral premotor cortex is more related to the execution of externally (especially visually) guided movements in relation to a specific external stimulus.

Frontal eye field

The frontal eye field corresponds to parts of Brodmann’s areas 6, 8 and 9 (Fig. 23.10). 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, and 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, to the pontine gaze centre within the pontine reticular formation, and to other oculomotor related nuclei in the brain stem. As its name implies, it is important in the control of eye movements.

Supplementary motor cortex

The supplementary motor area (SMA; MII) lies medial to area 6, and extends from the most superolateral part to the medial surface of the hemisphere. Area 24 in the cingulate gyrus adjacent to area 6 contains several motor areas, which are termed cingulate motor areas. An additional functional subdivision, the preSMA, lies anterior to the supplementary motor area on the medial surface of the cortex. In the present discussion, these additional medial motor areas are included with the supplementary motor cortex.

The supplementary motor area receives its major thalamic input from the anterior part of the ventral lateral nucleus, which in turn is the major recipient of fibres from the internal segment of the globus pallidus. Additional thalamic afferents are from the ventral anterior nucleus, the intralaminar nuclei, notably the centrolateral and centromedial nuclei, and also from the mediodorsal nucleus. The connections with the thalamus are reciprocal. The supplementary motor cortex receives connections from widespread regions of the ipsilateral frontal lobe, including from the primary motor cortex, the dorsal premotor area, the dorsolateral and ventrolateral prefrontal, medial prefrontal and orbitofrontal cortex and the frontal eye field. These connections are reciprocal, but the major ipsilateral efferent pathway is to the motor cortex. Parietal lobe connections of the supplementary motor cortex are with the superior parietal area 5 and possibly inferior parietal area 7b. Contralateral connections are with the supplementary motor area, and motor and premotor cortices of the contralateral hemisphere. Subcortical connections, other than with the thalamus, pass to the striatum, subthalamic nucleus and pontine nuclei, the brain stem reticular formation and the inferior olivary nucleus. The supplementary motor area makes a substantial contribution to the corticospinal tract, contributing as much as 40% of the fibres from the frontal lobe.

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 is primarily in complex tasks which require temporal organization of sequential movements and in the retrieval of motor memory.

Stimulation of the supplementary motor area in conscious patients has been reported to elicit the sensation of an urge to move, or of anticipation that a movement is about to occur. A region anterior to the supplementary motor area for face representation (areas 44, 45) is important in vocalization and speech production (Fig. 23.10).

Prefrontal cortex

The prefrontal cortex on the lateral surface of the hemisphere comprises predominantly Brodmann’s areas 9, 46 and 45 (Fig. 23.8). 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 man since, in the dominant hemisphere, they constitute the motor speech area (Broca’s area). 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 from 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 both area 7a and area 7b of the parietal lobe, from auditory association areas of the temporal operculum, from the insula and from 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.

The cortex of the frontal pole (area 10) receives thalamic input from the mediodorsal nucleus, the medial pulvinar and the paracentral nucleus. It is reciprocally connected with the cortex of the temporal pole, the anterior orbitofrontal cortex, and the dorsolateral prefrontal cortex. The orbitofrontal cortex connects with the mediodorsal, anteromedial, ventral anterior, medial pulvinar, paracentral and midline nuclei of the thalamus. Cortical association pathways come from the inferotemporal cortex, the anterior superior temporal gyrus and the temporal pole. Within the frontal lobe it has connections with the medial prefrontal cortex, the ventrolateral prefrontal cortex and medial motor areas. Commissural and other connections follow the general pattern for all neocortical areas.

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