Grays Anatomy: Cerebral Hemispheres PDF

CHAPTER Cerebral hemispheres 25 The cerebral hemispheres are the largest and most developed part trostimulation and behavioural analy...

CHAPTER Cerebral hemispheres 25 The cerebral hemispheres are the largest and most developed part trostimulation and behavioural analyses (Alarcon et al 2014, Jin et al of the human brain. They contain the primary motor and sensory cor- 2014). Some details of the anatomy of the gyri, sulci, association fibres tices, the highest levels at which motor activities are controlled and to and the amygdaloid nuclear complex are described online only (Figs which general and special sensory systems project, and which provide 25.2; 25.9–25.13; 25.14; 25.17; 25.18; 25.20; 25.22; 25.27; 25.36; the neural substrate for the conscious experience of sensory stimuli. 25.37; 25.46–25.49). Association areas are both modality-specific and multimodal, enabling complex analyses of the internal and external environment and of the relationship of the individual with the external world. The elements of CEREBRAL HEMISPHERE SURFACES, the limbic system are particularly concerned with memory and the SULCI AND GYRI emotional aspects of behaviour, and provide an affective overtone to conscious experience as well as an interface with subcortical areas such Each hemisphere has superolateral, medial and inferior (basal) surfaces, as the hypothalamus, through which widespread physiological activities separated by superomedial, inferolateral, medial orbital and medial are integrated. Other cortical areas, primarily within the frontal region, occipital margins respectively. are concerned with the highest aspects of cognitive function and con- The superolateral surface is convex and lies beneath the bones of the tribute to personality, judgment, foresight and planning. cranial vault; the frontal, parietal, temporal and occipital lobes corre- The external surface of each hemisphere is highly convoluted into a spond approximately in surface extent to the overlying bones from series of folds or gyri, separated by furrows or sulci (Fig. 25.1). The which they take their names. The frontal and parietal lobes are sepa- configuration of the main cerebral sulci and gyri provides the basis for rated from the temporal lobe by the prominent lateral (Sylvian) fissure. dividing the hemispheres into frontal, parietal, occipital, temporal, The inferior surface is divided by the anterior part of the lateral insular and limbic lobes. The internal white matter contains associa- fissure into a small anterior orbital part and a larger posterior tentorial tion fibres limited to each hemisphere, commissural fibres linking part. The orbital part is the concave orbital surface of the frontal lobe corresponding areas of both hemispheres, and projection fibres con- and rests on the floor of the anterior cranial fossa. The posterior part is necting the cerebral cortex of each hemisphere with subcortical, brain- formed by the basal aspects of the temporal and occipital lobes, and stem and spinal cord nuclei. Some of these bundles (tracts, fasciculi) rests on the floor of the middle cranial fossa and the upper surface of are relatively well defined macroscopically and microscopically, while the tentorium cerebelli, which separates it from the superior surface others are less easy to identify. A detailed knowledge of the three- of the cerebellum. The medial surface is flat and vertical, separated from dimensional anatomical interrelationships of white matter tracts is a the opposite hemisphere by the longitudinal fissure and the falx cerebri. requisite for the planning, intraoperative monitoring and execution of Anteriorly, the cerebral hemisphere terminates at the frontal and tem- neurosurgical resective procedures, e.g. for tumour surgery, and epi- poral poles, and posteriorly at the occipital pole. lepsy and deep brain stimulation procedures. Current understanding The cerebral sulci delineate the brain gyri and are extensions of the of these relationships owes much to the seminal work of Josef Klingler subarachnoid space (Butler and Hodos 2005, Sarnat and Netsky 1981, and his meticulous dissection of white matter tracts using formalin- Park et al 2007, Chi et al 1977, Nishikuni and Ribas 2013, Ono et al fixed, freeze-thawed brains (Agrawal et al 2011). Contemporary neuro- 1990, Catani and Thiebaut de Schotten 2012, Duvernoy 1991, Naidich surgical anatomical studies seek to define and delineate these fibre et al 2013). When they are deep and anatomically constant, they are bundles, particularly in areas of complexity such as fibre crossing, by referred to as fissures. The main sulci have depths of 1–3 cm, and their correlating anatomical findings obtained using Klingler’s dissection walls harbour small gyri that connect with each other (transverse gyri). techniques with the results obtained from diffusion-weighted mag- Sulci that separate the transverse gyri vary in length and depth, and may netic resonance imaging, functional MRI (fMRI), intraoperative elec- become visible as incisures at the surface of the brain. The indentations Precentral gyrus Postcentral sulcus Fig. 25.1 The lateral aspect of the left cerebral Central sulcus hemisphere, indicating the major gyri and sulci. Precentral sulcus (Dissection by EL Rees; photograph by Kevin Postcentral gyrus Fitzpatrick on behalf of GKT School of Medicine, Superior Supramarginal London; figure enhanced by B Crossman.) frontal gyrus gyrus Middle frontal gyrus Intraparietal Inferior sulcus frontal gyrus Angular gyrus Ascending ramus of lateral fissure Lateral fissure Superior Posterior ramus temporal gyrus of lateral fissure Middle Inferior temporal temporal gyrus gyrus 373 Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral hemispheres Paracentral Body of corpus callosum Fig. 25.3 A sagittal section of the brain, with the lobule brainstem removed, showing the major gyri and Fornix Cingulate sulcus sulci on the medial aspect of the left cerebral Precuneus Superior frontal hemisphere. (Photograph by Kevin Fitzpatrick on gyrus behalf of GKT School of Medicine, London; figure Parieto-occipital enhanced by B Crossman.) sulcus Cingulate gyrus Cuneus Genu of corpus Splenium of callosum corpus callosum Calcarine sulcus Lingual gyrus Rostrum of 3 Lateral occipitotemporal gyrus corpus callosum SECTION Isthmus Subcallosal area Parahippocampal gyrus Uncus caused by cortical arteries can have an appearance similar to that of the two types – granular and agranular – are regarded as virtually lacking incisures. The sulci of the superolateral and inferior surfaces of the certain laminae, and are referred to as heterotypical. Homotypical vari- hemisphere are usually orientated towards the nearest ventricular cavity. ants, in which all six laminae are found, are called frontal, parietal and On the brain surface, the sulci can be long or short, interrupted or polar, names that link them with specific cortical regions in a somewhat continuous. Sulci that are usually continuous include the lateral fissure misleading manner (e.g. the frontal type also occurs in parietal and and the callosal, calcarine, parieto-occipital, collateral and, generally, temporal lobes). the central sulcus. The agranular type is considered to have diminished, or absent, On the superolateral surface of the hemisphere, the frontal and granular laminae (II and IV) but always contains scattered stellate temporal regions are each composed of three horizontal gyri (superior, somata. Large pyramidal neurones are found in the greatest densities middle and inferior frontal and temporal gyri). The central area is in agranular cortex, which is typified by the numerous efferent projec- composed of two slightly oblique gyri (pre- and postcentral gyri). The tions of pyramidal cell axons. Although it is often equated with motor parietal region is comprised of two semicircular lobules (superior and cortical areas such as the precentral gyrus (area 4), agranular cortex also inferior parietal lobules, the inferior being formed by the supramarginal occurs elsewhere, e.g. areas 6, 8 and 44, and parts of the limbic system. and angular gyri) (see Fig. 25.1). The occipital region is composed of In the granular type of cortex the granular layers are maximally two or three less well-defined gyri (superior, middle and inferior occipi- developed and contain densely packed stellate cells, among which small tal gyri). The insula, which lies deep in the floor of the lateral fissure, pyramidal neurones are dispersed. Laminae III and IV are poorly devel- consists of 4–5 diagonal gyri (short and long insular gyri) (Fig. 25.2). oped or unidentifiable. This type of cortex is particularly associated with The orbital part of the inferior surface is covered by the orbital gyri afferent projections. However, it does receive efferent fibres, derived and the basal aspect of the rectus gyri, and the tentorial part of the from the scattered pyramidal cells, although they are less numerous inferior surface is covered by the basal aspects of the inferior temporal, than elsewhere. Granular cortex occurs in the postcentral gyrus (soma- inferior occipital and lingual gyri, and the fusiform gyrus. The medial tosensory area), striate area (visual area) and superior temporal gyrus surface of the hemisphere is characterized by a very well defined (acoustic area), and in small areas of the parahippocampal gyrus. C-shaped inner ring composed primarily of two continuous gyri (cin- Despite its very high density of stellate cells, especially in the striate gulate and parahippocampal gyri), surrounded by a much less well area, it is almost the thinnest of the five main types. In the striate cortex, defined outer ring of gyri (medial aspects of rectus and superior frontal the external band of Baillarger (lamina IV) is well defined as the stria gyri, paracentral lobule, precuneus, cuneus, and medial aspect of lingual (white line) of Gennari. gyrus) (Fig. 25.3). 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 MICROSTRUCTURE OF THE CORTEX prominent. The relative prominence of these major forms of neurone varies reciprocally wherever this form of cortex exists. The microscopic structure of the cerebral cortex is an intricate complex The parietal type of cortex contains pyramidal cells, which are mostly of nerve cells and fibres, neuroglia and blood vessels. The neocortex smaller in size than in the frontal type. In marked contrast, the granular essentially consists of three neuronal cell types. Pyramidal cells are the laminae are wider and contain more of the stellate cells: this kind of most abundant. Non-pyramidal cells, also called stellate or granule cortex occupies large areas in the parietal and temporal lobes. The polar cells, are divided into spiny and non-spiny types. All types have been type is classically identified with small areas near the frontal and occipi- further subdivided on the basis of size and shape (Fig. 25.4; see tal poles, and is the thinnest form of cortex. All six laminae are repre- Fig. 3.3). sented, 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 LAMINAR ORGANIZATION highly organized than in other types. It is customary to refer to some discrete cortical territories not only The most obvious microscopic feature of a thin section of the neocor- by their anatomical location in relation to gyri and sulci, but also in tex stained to demonstrate cell bodies or fibres is its horizontal lami- relation to their cytoarchitectonic characteristics (Brodmann’s areas) nation. The extent to which this organization aids the understanding (Fig. 25.7). Some of the areas so defined, e.g. the primary sensory and of cortical functional organization is debatable, but the use of cytoar- motor cortices, have clear relevance in terms of anatomical connections chitectonic description to identify regions of cortex is common. Typical and functional significance, others less so. neocortex is described as having six layers, or laminae, lying parallel to the surface (Fig. 25.5). These are the molecular or plexiform layer; external granular lamina; external pyramidal lamina; internal granular CORTICAL LAMINATION AND lamina; internal pyramidal (ganglionic) lamina; and multiform (or CORTICAL CONNECTIONS fusiform/pleiomorphic) layer. The cortical laminae represent, to some extent, horizontal aggregations of neurones with common connections. This is most clearly seen in NEOCORTICAL STRUCTURE the lamination of cortical efferent (pyramidal) cells. The internal pyramidal lamina, layer V, gives rise to cortical projection fibres, most Five regional variations in neocortical structure are described (Fig. notably corticostriate, corticobulbar (including corticopontine) and 374 25.6). While all are said to develop from the same six-layered pattern, corticospinal axons. In addition, a significant proportion of feedback Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral hemispheres Four main types of sulci have been described: large primary sulci (e.g. central, precentral, postcentral and continuous sulci); short primary sulci (e.g. rhinal, olfactory, lateral and occipital sulci); short sulci com- posed of several branches (e.g. orbital and subparietal sulci); and short, free supplementary sulci (e.g. medial frontal and lunate sulci) (Ono et al 1990). Sulci often have side branches that may be unconnected or connected (with end-to-side, end-to-end or side-to-side connections that can also join two neighbouring parallel sulci). 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. 25 Like the apical dendrite, the basal dendrites branch profusely along their length. All pyramidal cell dendrites are studded with a myriad of A CHAPTER 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 cases, if not in all, 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 dis- tances 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 B class of cell but a multitude of different forms, including basket, chan- delier, double bouquet, neurogliaform, bipolar/fusiform and horizon- Fig. 25.2 The basic organization of the main cerebral gyri. tal cells. Various types may have horizontally, vertically or radially A, Superolateral surface, left side. B, Medial and basal surfaces, right side. Red lines indicate constant gyri. The frontal and temporal regions ramifying axons. each consist of three horizontal gyri; the central area consists of two Neurones with mainly horizontally dispersed axons include basket slightly oblique gyri; the parietal region consists of two lobules (a and horizontal cells. Basket cells have a short, vertical axon, which quadrangular superior lobule and an inferior lobule consisting of two rapidly divides into horizontal collaterals, and these end in large termi- semicircular gyri); the occipital region consists of three irregular, less well nal sprays synapsing with the somata and proximal dendrites of pyrami- defined, predominantly longitudinal gyri that converge towards the dal cells. The cell bodies of horizontal cells lie mainly at the superficial occipital pole; and the insula is composed of four or five diagonal gyri. border of lamina II, occasionally deep in lamina I (the molecular or Medially, the external lateral gyri and lobules extend along the superior plexiform layer). They are small and fusiform, and their dendrites and inferolateral borders of each hemisphere. Together, they constitute spread short distances in two opposite directions in lamina I. Their an outer medial ring that surrounds a well-defined, C-shaped inner ring axons often stem from a dendrite, then divide into two branches, which composed of two continuous gyri. (With permission from Ribas GC, The travel away from each other for great distances in the same layer. cerebral sulci and gyri, Neurosurg Focus 2010 Feb;28(2):E2.) 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 The principal recognizable neuronal type is the neurogliaform or most are ovoid or fusiform and their dendrites arise from the upper and spiderweb cell. These small spherical cells, 10–12 µm in diameter, are lower poles of the cell body. The axonal arborization, which emerges found mainly in laminae II–IV, depending on cortical area. Seven to from the cell body or a proximal dendrite, is characteristic and identifies ten thin dendrites typically radiate out from the cell soma, some these neurones. A few cells in the more superficial laminae (II and IIIa) branching once or twice to form a spherical dendritic field of approxi- have descending axons, deeper cells (laminae IIIc and IV) have ascend- mately 100–150 µm diameter. The slender axon arises from the cell ing axons, and intermediate neurones (IIIb) often have both. The axons body or a proximal dendrite. Almost immediately, it branches profusely ramify close to the parent cell body and terminate in numerous verti- within the vicinity of the dendritic field (and usually somewhat cally orientated strings, which run alongside the axon hillocks of beyond), to give a spherical axonal arbor up to 350 µm in diameter. pyramidal cells, with which they synapse. Double bouquet (or bitufted) The majority of non-spiny or sparsely spinous non-pyramidal cells cells are found in laminae II and III and their axons traverse laminae II probably use γ-aminobutyric acid (GABA) as their principal neurotrans- and V. Generally, these neurones have two or three main dendrites, mitter. This is almost certainly the case for basket, chandelier, double which give rise to a superficial and deep dendritic tuft. A single axon bouquet, neurogliaform and bipolar cells. Some are also characterized arises usually from the oval or spindle-shaped cell soma and rapidly by the coexistence of one or more neuropeptides, including neuropep- divides into an ascending and descending branch. These branches col- tide Y, vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), lateralize extensively, but the axonal arbor is confined to a perpendicu- somatostatin and substance P. Acetylcholine is present in a subpopula- larly extended, but horizontally confined, cylinder, 50–80 µm across. tion of bipolar cells, which may additionally be GABAergic and contain Bipolar cells are ovoid with a single ascending and a single descending VIP. dendrite, which arise from the upper and lower poles, respectively. These primary dendrites branch sparsely and their branches run verti- cally 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. 374.e1 Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral hemispheres The molecular or plexiform layer is cell-sparse, containing only scat- tered 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 den- dritic 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. 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. 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 3 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. The SECTION 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. The internal pyramidal (gan- glionic) 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). The multiform (or fusiform/pleiomorphic) layer consists of neurones with a variety of shapes, including recognizable pyramidal, spindle, ovoid and many other shapes of 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 bound- ary is not always possible. 374.e2 Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral lobes A Fig. 25.4 A, Characteristic H neocortical neurones. I From left to right are shown Martinotti, neurogliaform, basket, II horizontal, fusiform, S stellate and pyramidal N types of neurone. B, The III most frequent types of neocortical neurone, showing typical connections with each IV F other and with afferent B P fibres. The right and left afferent fibres are association or 25 V corticocortical connections; the central CHAPTER afferent is a specific sensory fibre. Neurones VI M are shown in their characteristic lamina but many have somata in B Afferent fibres more than one layer. H Efferent neurones I Neurones limited to cortex II Neurone types: B Basket F Fusiform P H Horizontal III S M Martinotti F N Neurogliaform P Pyramidal S Stellate IV B P P V P N M VI corticocortical axons arise from cells in this layer, as do some cortico- (ocular dominance columns). Adjacent orientation columns aggregate thalamic fibres. Layer VI, the multiform lamina, is the major source of within an ocular dominance column to form a hypercolumn, respond- corticothalamic fibres. Supragranular pyramidal cells, predominantly ing to all orientations of stimulus for both eyes for one point in the layer III but also lamina II, give rise primarily to both association and visual field. Similar functional columnar organization has been commissural corticocortical pathways. Generally, short corticocortical described in widespread areas of neocortex, including motor cortex and fibres arise more superficially, and long corticocortical (both association association areas. 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 CEREBRAL LOBES laminae II/III and V, or sparsely throughout the depth of the cortex. Numerically, the largest input to a cortical area tends to terminate Each cerebral hemisphere is divided into six lobes: frontal, parietal, mainly in layer IV. This pattern of termination is seen in the major occipital, temporal, insular and limbic lobes. The surface features of the thalamic input to visual and somatic sensory cortex. In general, non- hemispheres exhibit considerable inter-individual variation in terms of thalamic subcortical afferents to the neocortex, which are shared by the depth and size of their sulci and the resulting patterns of gyral sepa- widespread areas, tend to terminate throughout all cortical layers, but ration (Ribas 2010). Connections between sulci are common; differing the laminar pattern of their endings still varies considerably from area interpretations of these patterns of connectivity continue to contribute to area. to inconsistencies in the literature, e.g. the use of different boundaries to demarcate the temporal, parietal and occipital lobes (Fig. 25.8). In what follows, each lobe will be described in terms of its external COLUMNS AND MODULES sulci and gyri, internal cortical structure and connectivity. Unless other­ wise indicated in the caption, the dissections in this chapter display Experimental physiological and connectional studies have demon- features in left cerebral hemispheres. strated an internal organization of the cortex, which is at right angles to the pial surface, with vertical columns or modules running through the depth of the cortex. The term ‘column’ refers to the observation that FRONTAL LOBE all cells encountered by a microelectrode penetrating and passing per- pendicularly through the cortex respond to a single peripheral stimulus, The frontal lobe is the largest part of the cerebral hemisphere. It con- a phenomenon first identified in the somatosensory cortex. In the visual tains the primary motor area (MI) within the precentral gyrus, the cortex, narrow (50 µm) vertical strips of neurones respond to a bar supplementary motor area (SMA) anteriorly and medially, and the stimulus of the same orientation (orientation columns), and wider premotor areas anteriorly and laterally. While movement is thought to strips (500 µm) respond preferentially to stimuli detected by one eye be initiated from within MI, the supplementary motor and premotor 375 Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral hemispheres Plexiform (molecular) Internal granular and external band of Baillarger superior, middle and inferior frontal gyri are disposed longitudinally, anterior to the precentral gyrus, and are separated by superior and External granular Ganglionic layer, containing inner band of Baillarger inferior frontal sulci (see Fig. 25.1); they are frequently referred to as Pyramidal Multiform (polymorphous) F1, F2 and F3 respectively. On its medial surface, the frontal lobe is limited inferiorly by the cingulate sulcus, which starts within the subcallosal region and extends over the cingulate gyrus. The cingulate sulcus may occasionally be double anteriorly and enclose a connection of the anterior aspect of the cingulate gyrus and the medial and anterior aspects of the superior frontal gyrus. It frequently has side branches that are directed inferiorly (see Fig. 25.10). Frontal lobe internal structure and connectivity 3 Primary motor cortex SECTION 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. 25.15). A striking feature is the dispropor- tionate 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 corticonuclear tracts. The major thalamic connections of area 4 are with the posterior part of the ventral lateral nucleus, which in turn receives afferents from the deep cerebellar nuclei. The posterior part of the ventral lateral nucleus also contains a topographic representation of the contralateral body half, 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, Golgi Nissl Weigert routed via the thalamus, reaches the primary motor cortex, since the projection of the internal segment of the globus pallidus to the ventral Fig. 25.5 The layers of the cerebral cortex. The three vertical columns lateral nucleus of the thalamus is confined to the anterior division, and represent the disposition of cellular elements, as revealed by the staining there is no overlap with cerebellothalamic territory. The anterior part of techniques of Golgi (impregnating whole neurones), Nissl (staining cell the ventral lateral nucleus projects to the premotor and supplementary bodies) and Weigert (staining nerve fibres). motor areas of cortex with no projection to area 4. The ipsilateral somatosensory cortex (SI) projects in a topographi- areas are believed to instruct the MI area. The most anterior and basal cally organized way to area 4, and the connection is reciprocal. The aspects of the frontal lobes are related to judgement and complex projection to the motor cortex arises in areas 1 and 2, with little or no aspects of volitional behaviour. contribution from area 3b. Fibres from SI terminate in layers II and III of area 4, where they contact mainly pyramidal neurones. Evidence Frontal lobe sulci and gyri suggests that neurones activated monosynaptically by fibres from SI, as well as those activated polysynaptically, make contact with layer V The frontal lobe is delimited posteriorly by the central sulcus, medially pyramidal cells, including Betz cells, which give rise to corticospinal by the (great) longitudinal fissure and inferolaterally by the lateral fibres. Movement-related neurones in the motor cortex that can be fissure (Figs 25.9–25.10; see Figs 25.1, 25.8). The area of the frontal activated from SI tend to have a late onset of activity, mainly during the lobe anterior to the precentral gyrus is divided into longitudinal supe- execution of movement. It has been suggested that this pathway plays rior, middle and inferior frontal gyri; the frontal pole lies anterior to a role primarily in making motor adjustments during a movement. these gyri (see Figs 25.1, 25.8, 25.21B). Its superolateral (dorsal) surface Additional ipsilateral corticocortical fibres to area 4 from behind the is covered by the frontal bone. Its basal (ventral) surface lies over the central sulcus come from the second somatic sensory area (SII). orbital part of the frontal bone and the cribriform plate of the ethmoid Neurones in area 4 are responsive to peripheral stimulation, and bone, and displays the orbital and rectus gyri. The medial surface faces have receptive fields similar to those in the primary sensory cortex. Cells the falx cerebri. located posteriorly in the motor cortex have cutaneous receptive fields, The central sulcus is the boundary between the frontal and parietal whereas more anteriorly situated neurones respond to stimulation of lobes. It demarcates the primary motor and somatosensory areas of deep tissues. the cortex, located in the precentral and postcentral gyri respectively. It The motor cortex receives major frontal lobe association fibres from starts in or near the superomedial border of the hemisphere, a little the premotor cortex and the supplementary motor area, and also fibres behind the midpoint between the frontal and occipital poles, and from the insula. It is probable that these pathways modulate motor runs sinuously, resembling a lengthened letter S, downwards and for- cortical activity in relation to the preparation, guidance and temporal wards, to end usually a little above the posterior ramus of the lateral organization of movements. Area 4 sends fibres to, and receives fibres sulcus. The central sulcus is usually a continuous sulcus in both from, its contralateral counterpart, and also projects to the contralateral hemispheres. supplementary motor cortex. The precentral gyrus lies obliquely over the superolateral surface of Apart from its contribution to the corticospinal tract, the motor the cerebral hemisphere, its upper aspect extending on to the medial cortex has diverse subcortical projections. The connections to the stria- surface. It is continuous superiorly and inferiorly with the postcentral tum and pontine nuclei are heavy. It also projects to the subthalamic gyrus along connections that encircle both extremities of the central nucleus. The motor cortex sends projections to all nuclei in the brain- sulcus. stem, which are themselves the origin of descending pathways to the The pre- and the postcentral gyri are roughly parallel to the coronal spinal cord: namely, the reticular formation, the red nucleus, the supe- 376 suture; the precentral sulcus is located slightly posterior to it. The rior colliculus, the vestibular nuclei and the inferior olivary nucleus. Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral hemispheres A SFS PreCS CS IPS MFS PostCS IFS ISJ ASCR SyF 25 SOS CHAPTER IOS STS ITS PSCR B SFG MFG SPLob PreCG PostCG SMG IFG Tr AG SOG Orb Op MOG IFG STG MTG ITG Fig. 25.9 The main sulci (A) and gyri (B) of the superolateral surface of the brain. Abbreviations: AG, angular gyrus; ASCR, anterior subcentral ramus of Sylvian fissure; CS, central sulcus; IFG, inferior frontal gyrus; IFS, inferior frontal sulcus; IOS, inferior occipital sulcus; IPS, intraparietal sulcus; ISJ, intermediary sulcus of Jensen; ITG, inferior temporal gyrus; ITS, inferior temporal sulcus; MFG, middle frontal gyrus; MFS, middle frontal sulcus; MOG, middle occipital gyrus; MTG, middle temporal gyrus; Op, opercular part of inferior frontal gyrus; Orb, orbital part of inferior frontal gyrus; PostCG, postcentral gyrus; PostCS, postcentral sulcus; PreCG, precentral gyrus; PreCS, precentral sulcus; PSCR, posterior subcentral ramus of Sylvian fissure; SFG, superior frontal gyrus; SFS, superior frontal sulcus; SMG, supramarginal gyrus; SOG, superior occipital gyrus; SOS, superior occipital sulcus; SPLob, superior parietal lobule; STG, superior temporal gyrus; STS, superior temporal sulcus; SyF, lateral or Sylvian fissure; Tr, triangular part of inferior frontal gyrus. (Adapted with permission from Ribas GC. The cerebral sulci and gyri. Neurosurg Focus 2010, 28(2):E2.) The inferior connection corresponds to the subcentral gyrus, deline- The localization of motor and sensory hand areas has been studied ated anteriorly and posteriorly by the anterior and posterior subcentral by correlating imaging cortical stimulation and postmortem cadaveric rami of the lateral fissure. It can either be situated completely over the studies. The motor hand area has been localized by fMRI to a protrusion lateral fissure or be in part internal to the fissure, in this situation giving of the precentral gyrus that corresponded precisely to the middle genu the false impression that the central sulcus is a branch of the lateral of the central sulcus, at the distal end of the superior frontal sulcus fissure. The superior connection corresponds to the paracentral lobule (Yousry et al 1997, Ribas 2010) (Figs 25.11–25.12). Postmortem studies (of Ecker) disposed along the medial surface of the hemisphere inside revealed that this protrusion was delimited by two anteriorly directed the interhemispheric fissure, delineated anteriorly by the paracentral fissures that deepened towards the base of the protrusion. Hand sensory sulcus and posteriorly by the ascending and distal part (marginal function has been localized to the postcentral component of the middle ramus) of the cingulate sulcus. Broca described a middle connection connection of the pre- and postcentral gyri (Boling and Olivier 2004, between the pre- and postcentral gyri (pli de passage moyen of Broca) Boling et al 2008). that may be present as a gyral bridge, usually hidden within the central The precentral gyrus is delimited anteriorly by the precentral sulcus, sulcus; on the cortical surface, this corresponds to the classic, posteriorly itself divided into superior and inferior precentral sulci by the connec- convex, middle genu of the central sulcus. When this middle connec- tion of the middle frontal gyrus with the precentral gyrus. Further con- tion is sufficiently developed so that it reaches the brain surface, it nections of the superior, middle and inferior frontal gyri may divide the interrupts the central sulcus (Régis et al 2005). superior and the inferior precentral sulci into additional segments. The 376.e1 Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral hemispheres A RoCC CaS CiS PreCS PaCS CS MaCiS POS SubPS SupRosS CaF InfRosS 3SECTION Spl Ant and PostOlfS PaTeG RhiS OTS ColS B SFG (MedFG) CaN CC CiG Fo IVeFo Tha PaCLob PreCu Cu LatV CiG RoCC PaOlfG GRe PaTeG CiPo AntCom TePo ITG Un IIIV PHG FuG Spl Ist LiG Fig. 25.10 The main sulci (A) and gyri (B) of the medial and basal temporo-occipital surfaces of the right side of the brain. Abbreviations: AntCom, anterior commissure; Ant and PostOlfS, anterior and posterior paraolfactory sulcus; CaF, calcarine fissure; CaN, caudate nucleus; CaS, callosal sulcus; CC, corpus callosum; CiG, cingulate gyrus; CiPo, cingulate pole; CiS, cingulate sulcus; ColS, collateral sulcus; CS, central sulcus; Cu, cuneus; Fo, fornix; FuG, fusiform gyrus; GRe, gyrus rectus; IIIV, third ventricle; InfRosS, inferior rostral sulcus; Ist, isthmus of cingulate gyrus; ITG, inferior temporal gyrus; IVeFo, interventricular foramen of Monro; LatV, lateral ventricle; LiG, lingual gyrus; MaCiS, marginal ramus of the cingulate sulcus; MedFG, medial frontal gyrus; OTS, occipitotemporal sulcus; PaCLob, paracentral lobule; PaCS, paracentral sulcus; PaOlfG, paraolfactory gyri; PaTeG, paraterminal gyrus; PHG, parahippocampal gyri; POS, parieto-occipital sulcus; PreCS, precentral sulcus; PreCu, precuneus; RhiS, rhinal sulcus; RoCC, rostrum of the corpus callosum; SFG, superior frontal gyrus; Spl, splenium of corpus callosum; SubPS, subparietal sulcus; SupRosS, superior rostral sulcus; TePo, temporal pole; Tha, thalamus; Un, uncus. (Adapted with permission from Ribas GC. The cerebral sulci and gyri. Neurosurg Focus 2010, 28(2):E2.) superior part of the precentral sulcus is very often interrupted superiorly 2012). Superiorly, the inferior frontal gyrus is crossed by various small by a connection between the superior frontal and precentral gyri, pro- branches of the interrupted inferior frontal sulcus; the triangular sulcus ducing a more medial segment, the medial precentral sulcus, that cor- typically pierces the superior aspect of the triangular part. In the domi- responds to the sulcus precentralis medialis of Eberstaller. More dorsally, nant hemisphere, the opercular and triangular parts of the inferior gyrus within the precentral region, the marginal precentral sulcus (sulcus correspond to Broca’s area, which is responsible for the production of precentralis marginalis of Cunningham) may merge with the superior spoken language (Fig. 25.13) (Quiñones-Hinojosa et al 2003). The precentral or central sulci. The inferior segment of the precentral sulcus most posterior aspect of the inferior frontal gyrus, identifiable by the always ends inside the opercular part of the inferior frontal gyrus, pro- connection of its opercular part with the precentral gyrus, corresponds ducing its characteristic U shape. to the ventral premotor cortical area; its bilateral stimulation causes The superior frontal gyrus is continuous anteriorly and inferiorly speech arrest (Duffau 2011). with the rectus gyrus; it may also be connected to the orbital gyri and The superior frontal sulcus separates the superior and middle frontal the middle frontal gyrus. Posteriorly, it is connected to the precentral gyri. It is very deep and is frequently continuous, ending posteriorly by gyrus by at least one fold, which most commonly lies medially along encroaching on the precentral gyrus at the level of its omega region the interhemispheric fissure. Usually the superior longitudinal gyrus is (corresponding to the motor cortical representation of the contralateral subdivided into two longitudinal portions by a medial frontal sulcus; hand). The superior frontal sulcus therefore tends to point the way to its medial portion is sometimes termed the medial frontal gyrus. The the middle frontoparietal pli de passage, as well as to the middle genu supplementary motor area is located along the most medial portion of of the precentral gyrus, where there is also a motor representation of the superior frontal gyrus, immediately facing the precentral gyrus; it the hand (Boling et al 1999). varies between individuals and has poorly defined borders. The middle The inferior frontal sulcus is always interrupted by the multiple con- frontal gyrus is usually the largest of the frontal gyri, frequently con- nections running between the middle and inferior gyri and usually has nected superficially to the precentral gyrus by a prominent root that lies three parts: orbital, triangular and opercular. The orbital part is the most between the extremities of a marked interruption in the precentral prominent. The triangular part is usually more retracted, such that there sulcus. It harbours a complex of multiple shallow sulcal segments is a small widening of the lateral fissure at its base corresponding to the 376.e2 known collectively as the middle or intermediate frontal sulcus (Petrides anterior Sylvian point. It is characterized by horizontal and anterior Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral hemispheres ascending rami of the lateral fissure that consistently divide the lateral fissure into anterior and posterior branches. The opercular part is always U-shaped and harbours the inferior aspect of the precentral sulcus; it is continuous posteriorly with the basal aspect of the precentral gyrus over the anterior subcentral ramus of the lateral fissure (Fig. 25.14). The anterior basal portion of the opercular part is sometimes divided by another branch of the lateral fissure, the diagonal sulcus of Eberstaller. Inferiorly, the orbital part continues with the lateral orbital gyrus, at times passing under a shallow sulcus known as the fronto-orbital sulcus. The basal apex of the triangular part is always superior to the SFS lateral fissure; the base of the opercular part can be located either supe- PreCS riorly or within the fissure. Anteriorly, the inferior frontal gyrus termi- PreCG nates by merging with the anterior portion of the middle frontal gyrus. All of the frontal gyri are delineated anteriorly by the frontomarginal 25 sulcus (frontomarginal sulcus of Wernicke), which lies superior and parallel to the supraciliary margin, separating the superolateral and CHAPTER orbital frontal surfaces. Posteriorly, the inferior frontal gyrus is con- nected to the precentral gyrus along the posterior aspect of its opercular part. The olfactory sulcus lies longitudinally in a paramedian position on the frontobasal or orbital surface of each frontal lobe. It accommodates the olfactory tract and bulb. Posteriorly, the olfactory tract is divided into medial and lateral striae, which delineate the most anterior aspect of the anterior perforated substance (see Figs 25.20B, 25.32). The narrow gyrus rectus, medial to the olfactory sulcus, is considered to be the most anatomically constant of the cerebral gyri. The orbital gyri, lateral to the olfactory sulcus, account for the greatest proportion of the frontobasal surface. The anterior, posterior, medial and lateral orbital gyri are delineated by the lateral, medial and transverse orbital sulci and the cruciform sulcus of Rolando, which together form a characteristic H shape. The posterior orbital gyrus lies anterior to the anterior perfo- rated substance and typically presents a configuration similar to a Fig. 25.11 The hand motor activation site corresponds to a knob-like tricorn hat, a feature that may facilitate its identification in anatomical cortical area of the contralateral precentral gyrus, which in MRI axial specimens where the H-shaped orbital sulcus is less obvious. The planes usually resembles an inverted omega shape (the area within the remaining orbital gyri are connected to the superior, middle and infe- red circle) and may be identified by its relationship to the posterior end of rior frontal gyri along the frontal pole. the superior frontal sulcus. Abbreviations: PreCG, precentral gyrus; PreCS, precentral sulcus; SFS, superior frontal sulcus (non-continuous, interrupted). (Courtesy of Professor Edson Amaro Jr MD, Department of Radiology, University of São Paulo Medical School.) Fig. 25.12 A reconstruction of the short U-shaped (red) and long projection (green) tracts of the hand-knob motor region in the left hemisphere. A, Left lateral view. B, Top view. C, Posterior view. The connections of the hand region resemble a ‘poppy flower’ with a green stem and four red ‘petals’ (1, posterior; 2, inferior; 3, anterior; 4, superior). The posterior (1) and inferior (2) petals correspond to the frontoparietal U-tracts between the precentral (PrCG) and postcentral (PoCG) gyri (i.e. hand superior and hand middle, respectively). The anterior (3) and superior (4) petals correspond to the U-shaped connections between the precentral gyrus and the middle frontal gyrus (MFG), and the middle frontal gyrus and superior frontal (SFG) gyrus, respectively. The ‘green stem’ is formed by ascending thalamocortical projection fibres and descending projections to the putamen (corticostriatal), pons (corticopontine) and spinal A C cord (corticospinal tract). (With permission from Catani M, Dell’acqua F, Vergani F et al; Short frontal lobe connections of the human brain. Cortex 2012 Feb;48(2):273–91.) Putamen Thalamus Pons Spinal cord B 376.e3 Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral hemispheres MFG SOG Tr STS Orb Op 3SECTION MFG Tr SOG STS Orb Op MFG Op STS Orb A B C Fig. 25.13 Functional magnetic resonance images (fMRI) of the language cortical areas (left cerebral hemisphere) activated by rhyme tasks (A, red), semantic tasks (B, blue) and fluency tasks (C, green). Abbreviations: MFG, middle frontal gyrus; Op, opercular part of inferior frontal gyrus; Orb, orbital part of inferior frontal gyrus; SOG, superior occipital gyrus; STS, superior temporal sulcus; Tr, triangular part of inferior frontal gyrus. (Courtesy of Prof. Edson Amaro Jr MD, Department of Radiology, University of São Paulo Medical School.) The paracentral lobule, bounded posteriorly by the marginal ramus and anteriorly by the paracentral sulcus (a branch of the cingulate sulcus), contains the distal part of the central sulcus and, inferior to it, the so-called paracentral fossa. Anterior to the paracentral lobule, the medial aspect of the superior frontal gyrus lies over the cingulate sulcus and the cingulate gyrus, merging inferiorly with the gyrus rectus. The latter is bounded superiorly by the superior rostral sulcus and accom- modates the shallower inferior rostral sulcus along its surface. The cingulate gyrus systematically connects with the gyrus rectus around the posterior end of the superior rostral sulcus by a prominent U-shaped cortical fold known as the cingulate pole, which is located immediately anterior to the subcallosal gyri. Small supraorbital sulci lie within the medial surface of the frontal pole, superior to the superior rostral sulcus at the level of the genu of the corpus callosum. 376.e4 Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral hemispheres A IFS IFS/PreCS PreCS CS PostCS 25 CHAPTER ASyP IRP B Fig. 25.14 Components of the frontoparietal operculum. A, A cadaveric specimen. B, MRI. The triangular part of the inferior frontal gyrus usually contains a descending branch of the inferior frontal sulcus (IFS). Three U-shaped convolutions are formed by the opercular part of the inferior frontal gyrus, which always harbours the inferior part of the precentral sulcus (PreCS); the subcentral gyrus or Rolandic operculum (the inferior connection of the pre- and postcentral gyri enclosing the inferior part of the central sulcus (CS)); and the connection between the postcentral and supramarginal gyri that contains the inferior part of the postcentral sulcus (PostCS). The most distal component of the operculum is a C-shaped convolution that connects the supramarginal and superior temporal gyri, and encircles the posterior end of the lateral (Sylvian) fissure. The bases of the U-shaped convolutions and their related sulcal extremities may be either superior to the fissure, as indicated in this specimen, or inside the fissure. Other abbreviations: ASyP, anterior Sylvian point; IRP, inferior Rolandic point. (Adapted with permission from Ribas GC, Ribas EC, Rodrigues CJ: The anterior sylvian point and the suprasylvian operculum. Neurosurg Focus 18:E1–E6, 2005.) 376.e5 Downloaded for Elise Barbeau ([email protected]) at McGill University from ClinicalKey.com by Elsevier on June 02, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved.  Cerebral lobes A B 1 Agranular 2 Frontal 3 Parietal 4 Granular 5 Polar I I I I I II II II II II 1 4 2 III 2 III III III 3 III IV 3 5 1 4 4 IV 25 4 IV IV V CHAPTER IV 2 V V V IV VI V VI VI VI VI Fig. 25.6 The distribution (A) and characteristics (B) of the five major types of cerebral cortex. A 3 B 2 1 3 4 4 6 8 5 7A 6 8 7 5 9 7B 31 24 9 23 19 32 46 1 2 30 40 18 33 29 10 10 39 26 44 18 17 45 19 11 18 36 34 11 37 19 22 28 17 37 25 38 38 21 47 42 20 20 41