Physical Address

304 North Cardinal St.
Dorchester Center, MA 02124

Cerebral hemisphere and cerebral cortex


The cerebral hemisphere is derived from the embryological telencephalon ( Chapter 1 ). It is the largest part of the forebrain and it reaches the greatest degree of development in the human brain. Superficially, the cerebral hemisphere consists of a layer of grey matter, the cerebral cortex, which is highly convoluted to form a complex pattern of ridges ( gyri ; singular, gyrus ) and furrows ( sulci ; singular, sulcus ). This serves to maximise the surface area of the cerebral cortex, about 70% of which is hidden within the depths of sulci ( Figs 13.1 , 13.2 ). Beneath the surface, axons running to and from the cells of the cortex form an extensive mass of white matter. Deep within the hemisphere lie additional masses of grey matter, most notably the thalamus and the basal ganglia (consisting of the caudate nucleus, putamen and globus pallidus). Figs 13.3–13.12 show a rostro-caudal series of coronal sections through the brain in which the major internal structures can be identified.

Fig. 13.1
Lateral aspect of the cerebral hemisphere showing major gyri and sulci.

Fig. 13.2
Median sagittal section of the cerebral hemisphere showing major gyri and sulci.
The brainstem and cerebellum have been removed to show the inferomedial aspect of the temporal lobe.

Fig. 13.3
Coronal section of the cerebral hemisphere.

Fig. 13.4
Coronal section of the cerebral hemisphere.

Fig. 13.5
Coronal section of the cerebral hemisphere.

Fig. 13.6
Coronal section of the cerebral hemisphere.

Fig. 13.7
Coronal section of the cerebral hemisphere.

Fig. 13.8
Coronal section of the cerebral hemisphere.

Fig. 13.9
Coronal section of the cerebral hemisphere.

Fig. 13.10
Coronal section of the cerebral hemisphere.

Fig. 13.11
Coronal section of the cerebral hemisphere.

Fig. 13.12
Coronal section of the cerebral hemisphere.

The vast majority of those nerve fibres that pass between the cerebral cortex and subcortical structures are condensed, deep within the hemisphere, into a broad sheet called the internal capsule ( Figs 13.4–13.11, 13.13–13.15 ; see also Fig. 1.25, Fig. 1.26, Fig. 1.27, Fig. 1.28, Fig. 1.30 , 13.24 ). Between the internal capsule and the cortical surface, fibres radiate in and out to produce a fan-like arrangement, the corona radiata . The principal subcortical grey matter is located in close proximity to the internal capsule, the thalamus and caudate nucleus lying medially and the putamen and globus pallidus lying laterally ( Figs 13.1–13.14 ). Within the cerebral hemisphere lies the large C-shaped cavity of the lateral ventricle, which is considered with the rest of the ventricular system in Chapter 6 .

Fig. 13.13
Horizontal (axial) magnetic resonance image of the living brain.

(Courtesy of Professor A Jackson, Wolfson Molecular Imaging Centre, University of Manchester, Manchester, UK.)

Fig. 13.14
Horizontal section of the brain.

Fig. 13.15
Coronal magnetic resonance image of the living brain.

(Courtesy of Professor A Jackson, Wolfson Molecular Imaging Centre, University of Manchester, Manchester, UK.)

The two cerebral hemispheres are separated by a deep cleft, the great longitudinal fissure , which accommodates the meningeal falx cerebri . In the depths of the fissure, the hemispheres are united by the corpus callosum , an enormous sheet of commissural nerve fibres which run between corresponding areas of the two cortices ( Figs 13.2–13.15 ; see also Figs 13.21–13.23, 13.25 ).

Gyri, sulci and lobes of the cerebral hemisphere

Certain gyri and sulci on the surface of the hemisphere are relatively consistently located in different individuals and are the basis for dividing the hemisphere into four lobes, namely the frontal , parietal , temporal and occipital lobes . Their principal topographical features and functional significance are described below. The most conspicuous and deepest cleft on the lateral surface of the hemisphere is the lateral fissure ( Fig. 13.1 ). This separates the temporal lobe below from the frontal and parietal lobes above. Within the depths of the lateral fissure lies a cortical area known as the insula ( Figs 13.6–13.12, 13.14, 13.17 ). The parts of the frontal, parietal and temporal lobes that overlie the insula are called the opercula . Also on the lateral surface of the hemisphere, a single, often uninterrupted sulcus can usually be identified, running between the great longitudinal fissure and the lateral fissure. This is the central sulcus , which marks the boundary between the frontal and parietal lobes ( Figs 13.1 , 13.16 ). The central sulcus extends for a short distance onto the medial surface of the hemisphere, within the great longitudinal fissure ( Figs 13.2 , 13.16 ).

Fig. 13.16, Major functional areas of the cerebral cortex.

The frontal lobe constitutes the entire region in front of the central sulcus. Immediately in front of the sulcus, and running parallel to it, lies the precentral gyrus , which is the primary motor region of the cerebral cortex. In front of the precentral gyrus, the rest of the frontal lobe consists of a more variable pattern of convolutions, of which the superior , middle and inferior frontal gyri can usually be identified ( Fig. 13.1 ).

Behind the central sulcus, and above the lateral fissure, lies the parietal lobe. Its most anterior part is the postcentral gyrus , which is the site of the primary somatosensory cortex . Behind the postcentral gyrus, on the lateral surface of the hemisphere, the intraparietal sulcus divides the rest of the parietal lobe into superior and inferior parietal lobules ( Figs 13.1 , 13.16 ).

The boundary between the parietal lobe and the posteriorly located occipital lobe is not coincident with a single sulcus on the lateral surface of the hemisphere; however, it is clearly marked by the deep parieto-occipital sulcus on the medial surface ( Figs 13.2 , 13.16 ). The occipital lobe does not bear any important landmarks on its lateral surface but, on the medial surface, the prominent calcarine sulcus indicates the location of the primary visual cortex ( Figs 13.2 , 13.16 ).

The temporal lobe lies beneath the lateral fissure, merging posteriorly with the parietal and occipital lobes. On its lateral surface, the temporal lobe is divided into three principal gyri that run roughly parallel to the lateral fissure: the superior , middle and inferior temporal gyri ( Fig. 13.1 ). The superior temporal gyrus includes the primary auditory cortex . Most of this functional region is situated on the superior bank of the gyrus, within the lateral fissure, where the transverse temporal gyri , or Heschl's convolutions , provide a more precise localisation ( Fig. 13.17 ).

Fig. 13.17, Superolateral aspect of the left cerebral hemisphere.

On the medial surface of the hemisphere, certain portions of the frontal, parietal and temporal lobes also constitute components of the limbic system. Curving around the corpus callosum, and running parallel to it, lies the cingulate gyrus ( Figs 13.2 , 13.16 ), separated from the rest of the hemisphere by the cingulate sulcus . The cingulate gyrus passes posteriorly and inferiorly round the posterior portion, or splenium, of the corpus callosum to become continuous with the parahippocampal gyrus of the temporal lobe. Deep to the parahippocampal gyrus, within the temporal lobe, lies the hippocampus ( Figs 13.8–13.12 ). This structure is formed by an in-curling of the inferomedial part of the temporal lobe. The cingulate gyrus, parahippocampal gyrus and hippocampus are sometimes referred to as the limbic lobe of the cerebral hemisphere.

Gyri, sulci and lobes of the cerebral hemisphere

  • The cerebral hemisphere consists of:

    • Superficial cerebral cortex, convoluted to form gyri and sulci

    • Underlying white matter, consisting of cortical afferent and efferent fibres

    • Deep nuclear masses: the thalamus and basal ganglia

  • The two cerebral hemispheres are separated by the great longitudinal fissure and joined by the corpus callosum.

  • The hemisphere is divided into four lobes (frontal, parietal, temporal and occipital) on the basis of surface topography.

  • Principal landmarks that indicate the divisions between lobes are the lateral fissure, central sulcus and parieto-occipital sulcus.

Focal cerebral lesions

Focal cerebral lesions, e.g. a stroke or tumour, produce three kinds of symptoms:

  • 1

    Focal epileptic seizures . The repetitive discharges of groups of neurones in the cerebral cortex produce paroxysmal attacks lasting for brief periods and reflecting the functional properties of the neurones concerned. The patient experiences sudden attacks of abnormal movements or sensations ( simple focal seizures ) or brief alterations in perception, mood and behaviour ( complex partial seizures ). Focal seizures may trigger generalised ( tonic–clonic ) seizures.

  • 2

    Sensory/motor deficits . There is a loss of sensation or movement, detectable on clinical neurological examination.

  • 3

    Psychological deficits . There are breakdowns in psychological processes such as language, perception and memory, demonstrable on psychological evaluation.

If the focal lesion is space-occupying, the syndrome of raised intracranial pressure results (see Fig. 5.2 ).

A unilateral cerebral hemisphere lesion causes mental impairment (e.g. aphasia), a contralateral spastic hemiparesis, hyperreflexia and an extensor plantar response (upper motor neurone lesion) and contralateral hemisensory loss ( Fig. 13.18 ). A vascular insult to the internal capsule, such as an infarction or haemorrhage, leads to the rapid development of this syndrome, known as stroke .

The regional localisation of neuropsychological functions in the cerebral cortex is summarised in Fig. 1.49 .

Fig. 13.18, Unilateral cerebral hemisphere lesion.

Cerebral cortex

Histological structure

The cerebral cortex forms the outer surface of the cerebral hemisphere. It consists of a layer, several millimetres in thickness, of nerve cell bodies, dendritic arborisations and synaptic interconnections. In the early part of the twentieth century, the Swedish anatomist Brodmann produced a numbered, cytoarchitectural map of the cerebral cortex based upon its regional histological characteristics. Although largely superseded by the elucidation of function, in some instances, there is good correspondence between Brodmann's areas and functionally defined regions of the cortex. In such cases, Brodmann's numbers are retained in common use for descriptive purposes.

Long ago in evolutionary history, the cerebral cortex originally arose in relation to olfactory function. Phylogenetically old parts of the cortex (referred to as archicortex and paleocortex ), such as the hippocampus and other parts of the temporal lobe, retain throughout evolution an association with the olfactory system and have a primitive, three-layered cytoarchitecture. These regions have important functions in the emotional aspects of behaviour and in memory. Together with other parts of the cortex and certain subcortical nuclei they constitute the limbic system ( Chapter 16 ). However, most of the cerebral cortex is a more recent acquisition in phylogenetic terms and is referred to as the neocortex . Although its detailed cytological structure varies from region to region, it is generally recognised as consisting of six layers ( Fig. 13.19 ):

  • Layer I , the most superficial layer, contains few nerve cell bodies but many dendritic and axonal processes in synaptic interaction.

  • Layer II contains many small neurones, which establish intracortical connections.

  • Layer III contains medium-sized neurones giving rise to association and commissural fibres.

  • Layer IV is the site of termination of afferent fibres from the specific thalamic nuclei.

  • Layer V is the origin of projection fibres to extracortical targets, such as the basal ganglia, thalamus, brainstem and spinal cord. In the primary motor cortex of the frontal lobe, this layer contains the giant Betz cells, which project fibres into the pyramidal tract.

  • Layer VI also contains association and projection neurones.

Fig. 13.19, Histological structure of the cerebral cortex.

Functional organisation

The cerebral cortex is necessary for conscious awareness and thought, memory and intellect. It is the region to which all sensory modalities ultimately ascend (mostly via the thalamus) and where they are consciously perceived and interpreted in the light of previous experience. The cerebral cortex is also the highest level at which the motor system is represented. It is here that actions are conceived and initiated.

  • The posterior part of the cerebrum receives sensory information from the outside world in the primary sensory areas of the parietal lobe (somatosensory), occipital lobe (vision) and temporal lobe (hearing).

  • In adjacent cortical zones, the information is elaborated and interpreted, thus permitting identification of objects by touch, sight and hearing in a modality-specific process of perception. Areas of cortex at the junction of the three cerebral lobes, known as association cortex, are critical for the multimodal and spatial recognition of the environment.

  • The medial portions of the cerebral hemisphere (limbic system) enable the storage and retrieval of information processed in the posterior hemispheric regions.

  • The anterior part of the cerebrum (frontal lobe) is concerned with the organisation of movement (primary motor area; premotor and supplementary motor areas) and the strategic guidance of complex motor behaviour over time (prefrontal area).

  • In the majority of individuals, areas of association cortex in frontal, parietal and temporal lobes of the left hemisphere are responsible for the comprehension and expression of language. The left hemisphere is, therefore, said to be dominant for language.

The regional localisation of neuropsychological functions in the cerebral cortex is summarised in Fig. 1.49 .

Frontal lobe

The frontal lobe lies anterior to the central sulcus. Immediately anterior to the central sulcus, and running parallel to it, is the precentral gyrus. Functionally, this is known as the primary motor cortex ( Figs 13.1, 13.2, 13.16 ). It corresponds to Brodmann's area 4. Within the cortex of the precentral gyrus, the contralateral half of the body is represented in a precise somatotopic fashion ( Fig. 13.20 ). The representation of the body is inverted, with the head area located in the most inferior part of the precentral gyrus, just above the lateral fissure. Progressing superiorly, successive areas of cortex represent the digits, hand, arm, shoulder and trunk. The lower limb is represented on the medial surface of the hemisphere, above the corpus callosum. The area of cortex devoted to a particular body part is proportional, not to its size, but to the degree of precision with which movements can be executed. Therefore the larynx, tongue, face and digits of the hand are represented by disproportionately large regions.

Fig. 13.20, Schematic coronal section through the cerebral hemisphere illustrating the approximate somatotopic representation of the contralateral body half in the motor and sensory cortices.

Stimulation of the primary motor cortex elicits contraction of discrete muscle groups on the opposite side of the body. The function of this region is the control of voluntary, skilled movements, sometimes referred to as fractionated movements; 30% of corticospinal (pyramidal tract) and corticobulbar fibres arise from neurones of the primary motor cortex, about 3% originating from giant pyramidal (Betz) cells. The principal subcortical afferents to the primary motor cortex originate from the ventral lateral nucleus of the thalamus (see Fig. 12.6 ), which in turn receives input mainly from the dentate nucleus of the cerebellum and from the globus pallidus of the basal ganglia.

The region immediately anterior to the primary motor cortex is known as the premotor cortex (Brodmann's area 6; Fig. 13.16 ). On the lateral surface of the hemisphere, this includes the posterior portions of the superior, middle and inferior frontal gyri. On the medial surface of the hemisphere, the premotor cortex includes a region referred to as the supplementary motor cortex . Here, like the primary motor cortex, there is somatotopic representation of the body although, unlike the primary motor cortex, representation appears to be bilateral in both hemispheres.

Stimulation of premotor cortical areas induces movement that is less focused than that elicited from the primary motor cortex and involves groups of functionally related muscles. Movement evoked from the supplementary motor cortex tends to be postural in nature, involving axial and proximal musculature. Premotor cortical areas are thought to function in the programming of, and preparation for, movement and in the control of posture. The premotor cortex exerts its actions partly via the primary motor cortex, with which it is connected by short association fibres, and partly via corticospinal and corticobulbar fibres. About 30% of the latter originate in the premotor cortex although, unlike the primary motor cortex, giant Betz cells are absent from premotor areas. The principal subcortical input to premotor cortical regions, including the supplementary motor cortex, is the ventral anterior nucleus of the thalamus. This, in turn, receives fibres from the globus pallidus and substantia nigra.

Immediately in front of the premotor cortex, on the lateral surface of the hemisphere, are located two other important regions. In the middle frontal gyrus lies the frontal eye field (Brodmann's area 8). This region controls voluntary conjugate deviation of the eyes, as occur when scanning the visual field. Unilateral damage to this area causes conjugate deviation of the eyes towards the side of the lesion. In the inferior frontal gyrus of the dominant hemisphere (usually the left) lies the motor speech area, also known as Broca's area (Brodmann's areas 44 and 45). Broca's area has important interconnections with parts of the ipsilateral temporal, parietal and occipital lobes that are involved in language function.

The extensive regions of the cortex of the frontal lobe that lie anterior to premotor areas are referred to as the prefrontal cortex . The prefrontal cortex has rich connections with parietal, temporal and occipital cortex through long association fibres running in the subcortical white matter ( Fig. 13.22 ). Subcortical afferents to the prefrontal cortex arise mainly in the mediodorsal and anterior nuclei of the thalamus. The prefrontal cortex has cognitive functions of a high order. These include intellectual, judgemental and predictive faculties and the planning of behaviour.

Frontal lobe lesions

Left frontal lobe lesions cause:

  • Focal seizures. Paroxysmal jerking movements of the contralateral limbs are termed ‘simple motor’ or ‘ Jacksonian ’ seizures .

  • Sensory/motor deficit. There is weakness of the face and upper motor neurone signs in the limbs on the opposite side to the lesion ( contralateral hemiplegia ).

  • Psychological deficit. Speech is produced with great effort and poor articulation, in brief utterances with word errors ( paraphasia ). Repetition of words is impaired but powers of comprehension are relatively preserved. This is known as Broca's aphasia . There is also impairment of reading and writing ( alexia and agraphia ). Degenerative disease of the left frontal lobe results in a progressive non-fluent aphasia , akin to Broca's aphasia.

Premotor areas of the frontal lobes are responsible for the organisation of skilled movements. Unilateral and bilateral lesions of the premotor cortex lead to the inability to carry out skilled movements in the absence of paralysis, sensory loss or cerebellar incoordination ( apraxia ).

Bilateral prefrontal lesions cause:

  • Profound disturbance of behaviour and personality

  • Impaired problem-solving and judgement

Progressive atrophy of the frontal lobes occurs in the degenerative disorder of frontotemporal dementia .

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here

Related Posts