The cerebrum basically consists of two hemispheres that are partially connected with each other along the sagittal plane. Those hemispheres are the right and left cerebral hemispheres which occupy the greater part of the cranial cavity – above the floors of the anterior and middle cranial fossae.
- The cerebrum
- Gross anatomy
- Important Functional Areas
- Blood Supply
- Clinical Aspects
- Related Atlas Images
One hemisphere, usually the left, in a right-handed person, is slightly larger than the other and constitutes the dominant hemisphere. Motor control of speech is also confined to the speech area of Broca (corresponding to Brodmann areas 44 and 45) which is usually found only in the dominant hemisphere. Those and other areas of the cerebrum will be described in detail based on Brodmann’s mapping of the cerebral cortex, under “Functional Areas of the Cerebral Cortex”.
Each hemisphere is composed of gray matter, white matter, blood vessels for example the Circle of Willis, fluids - cerebrospinal fluid (CSF), and, among others, a series of cavities like the ventricles of the brain. However, the gray and white matters are the main neuronal structures of the cerebrum.
The white matter is nerve fibers (axons of the neurons) located within each cerebral hemisphere, deep to the cerebral cortex. Primarily, the white matter connects the cerebral cortex with other brain regions and identical areas in the two hemispheres. There are three categories of white matter:
- Projection fibres – these originate from the grey matter to terminate in other regions of the cortex or in subcortical areas and nuclei.
- Association fibres – these are fibres that interconnect different regions of the cerebral cortex within one hemisphere.
- Commissural fibres – connect identical areas of cortices (pleural of cortex) of right and left cerebral hemispheres (that is, they interconnect identical areas in the two hemispheres). The largest bundle of the commissural fibers is the corpus callosum.
The gray matter is comprised of the bodies of the neurons (the cells of the nervous system). The grey matter includes the cerebral cortex (see below) as well as other subcortical areas like the thalamus, hypothalamus, and the basal ganglia.
The cerebral cortex is the most superficial layer of the cerebrum and makes up the largest portion of the cerebrum’s gray matter. There is also a cerebellar cortex, which forms the superficial layer of the cerebellum (or small brain).
The cerebral cortex is about 2-5mm thick and accounts for about 80% of the brain’s total mass. Its total area has been estimated to be about 2000 cm2. The cerebral cortex is thrown into a complicated series of tortuous folds, the gyri (singular: gyrus) and between these gyri are grooves or indentations called sulci (singular: sulcus).
Sulci are small grooves, but there are also large grooves, and these are called fissures. Fissures divide the cerebral cortex into lobes and also divide the cerebrum into the right and left cerebral hemispheres along the sagittal plane. The fissure involved in this division is called the medial longitudinal fissure.
The highest level of cognitive functions and information processing takes place in the cerebral cortex. Some functions of the cerebral cortex include thinking and reasoning, memory, consciousness, attention, perceptual awareness and language.
The cerebral cortex is one of the first structures to develop during the development of the central nervous system (CNS) in neurulation. At approximately the 18th day, the neural plate invaginates along its central axis to form a longitudinal median neural groove, which has neural folds on each side. The neural folds become particularly prominent at the cranial end of the embryo, and are the first signs of brain development. Neurulation continues till the end of the 3rd week and it is completed at the end of week four of embryonic development. However, during the 5th week, the forebrain partly divides into two secondary brain vesicles, the telencephalon and diencephalon.
Neurons in the neocortex originate from the ventricular zone (VZ) - a pseudostratified proliferative epithelium containing multipotent neural stem cells located at the deep ventricular surface of the telencephalic wall. In addition, excitatory neocortical neurons are produced in the VZ of the dorsal pallium arising from asymmetric divisions of the radial glia progenitors. These excitatory neocortical neurons migrate, mainly, following the radial glia processes.
In the VZ of the cortical neuroepithelium, radial glia first produce excitatory neurons, many of which migrate radially to make up the embryonic preplate and the deepest cortical layers. Later in development, divisions of the radial glia produces cells called intermediate progenitors that detach from the VZ surface and aggregate in a zone overlying the VZ, forming the subventricular zone (SVZ). Under the control of the transcription factor Pax6 and expresses Svet1 and Tbr2, cells of the SVZ undergo one to three more cell divisions and then migrate to make up the neocortex. The neocortex, according to evolution, is the most recently evolved cerebral cortex, and it is the primitive cerebral cortex with six layers. It is the largest part of the cerebral cortex and covers the left and right cerebral hemispheres, with the allocortex making up the rest.
Microscopic examination reveals that the cerebral cortex is predominantly made up of two types of neurons:
- The pyramidal cells – which have a conical cell body of over 30µm in diameter, and with one apical and a lot of basal dendrites, as well as an axon that leaves the base of the cell to the white matter. Those neurons form the output cells of the cerebral cortex.
- The granular cells – these are small cells and have a round cell body of less than 10µm in diameter. Granular cells serve as interneurons, receiving input from cortical afferent fibres and synapsing on output neurons (i.e, pyramidal cells) of the cerebral cortex.
The cerebral cortex is also organized into six horizontal layers or laminae (although layer boundaries are not very obvious in routine sections). Each of those layers have different roles and vary in relative thickness among cortical regions (for example, a somatosensory region like areas 1,2,3 of Brodmann has a thick internal granular layer compared to its pyramidal; a motor region like area 4 of Brodmann has a thick internal pyramidal cell layer compared to its own granular cell layer). The cell layers of the cerebral cortex, from superficial to deep, are:
- Plexiform or Molecular layer – which is a fibre layer with apical dendrites and non-specific afferents.
- External or Outer granular cell layer – which are interneurons for non-specific afferent input.
- Pyramidal or External Pyramidal cell layer – composed of small and medium cells, with short association output.
- Internal granular cell layer – this layer is made up of interneurons for specific afferent input.
- Ganglionic or Internal pyramidal layer – this layer is composed of large cells with projection and long association output.
- Polymorphous or Multiform layer – comprising of variably shaped cells with projection and long association output.
The cerebral cortex is made up of three large surfaces, namely:
- the superolateral surface
- the medial surface
- the inferior surface
Although the cerebral cortex has surfaces, they are not smooth owing that their embryonic development. These surfaces are characterized by elevations or folds called gyri and depressions or grooves called sulci. Those sulci define each gyrus, while the large sulci called fissures (as described above) define and demarcate the cerebral cortex into four major subdivisions called lobes, which are:
- the frontal lobe
- the parietal lobe
- the temporal lobe
- the occipital lobe
Those lobes are named according to their relation to the bones of the skull. Hence the frontal lobe is the part just right under the frontal bone, right next to the parietal bone is the parietal lobe, the temporal lobe is by the temporal bone and occipital lobe is located in relation to the occipital bone of the cranium.
Additional striking external features of the cerebral cortex are the poles. When the cerebrum is viewed from the lateral aspect, each cerebral hemisphere has the appearance, in which, three somewhat pointed ends can be recognised. These pointed ends are the poles of the cerebral cortex. These poles, which are also named in relation to the cranial bones, are the:
- frontal pole – anteriorly
- occipital pole – posteriorly
- temporal pole – lying between the frontal and occipital poles, and points forwards and somewhat downwards.
The superolateral surface of the cerebral cortex is marked with striking features which divide the cortex into the frontal lobe, temporal lobe, parietal lobe, occipital lobe and the insula, and define these lobes as follows:
The frontal lobe is defined on this surface by a fissure called central sulcus (fissure of Rolando) which runs downwards and forwards towards a second fissure called lateral sulcus (fissure of Sylvius) which also define this lobe. It is further subdivided as follows: a sulcus called the precentral sulcus runs downwards and forwards parallel to and a bit anterior to the central sulcus (central fissure). The area between it and the central sulcus is the precentral gyrus. In the region anterior to the precentral gyrus, there are two sulci that run in an anteroposterior direction. Those are the superior and inferior frontal sulci. Those sulci divide this region into superior, middle, and inferior frontal gyri.
The inferior boundary of the frontal lobe, is divided by the rami of the lateral sulcus into 3 parts:
- Pars orbitalis
- Pars triangularis
- Pars opercularis
The part below the anterior ramus is the pars orbitalis, that between the anterior and ascending rami (pleural for ramus) is the pars triangularis (shaped like a triangle), and the part posterior to the ascending ramus is the pars opercularis.
This lobe is defined on the superolateral surface by the lateral sulcus and the inferior temporal sulcus (inferior temporal fissure).It has two sulci that run parallel to the posterior ramus of the lateral sulcus (which is the third branch originating from the stem of the lateral sulcus). Those two sulci are termed the superior and inferior temporal sulci and they divide the superolateral surface of the temporal lobe into the superior, middle and an inferior temporal gyri.
This lobe is defined on the superolateral surface by the central fissure anteriorly, and the parieto-occipital sulcus posteriorly. The parieto-occipital sulcus is joined to the inferior temporal sulcus by an imaginary line. Inferiorly the lobe is defined by the posterior ramus of lateral sulcus and an imaginary line that runs posteriorly from this ramus of lateral sulcus to join the first imaginary line and the inferior temporal sulcus at the point where they (1st imaginary line and the inferior temporal sulcus) meet.
Within the parietal lobe, a sulcus, the postcentral sulcus runs downwards and forwards parallel to and a little behind the central fissure; thus defining a gyrus. That gyrus is the postcentral gyrus and is located between the postcentral sulcus and central fissure. The remaining part of the parietal lobe is divided into a superior parietal lobule and an inferior parietal lobule by the intraparietal sulcus – which runs anteroposteriorly.
NOTE: that those two folds are referred to as lobule as they are slightly larger than a gyrus.
This lobe is defined anteriorly by the parieto-occipital sulcus, the first imaginary line and the upturned posterior end of the inferior temporal sulcus. It shows three short sulci. One of those, the lateral occipital sulcus lies horizontally and divides the lobe into superior and inferior occipital gyri. The second sulcus is the lunate sulcus which runs downwards and slightly forwards just in front of the occipital pole and define a vertical strip, just in front of it. That strip is called the gyrus descendens. The third is the transverse occipital sulcus located in the upermost part of the occipital lobe. The transverse occipital sulcus defines an area just superior to itself. That area is called the arcus parieto-occipitalis (as its name suggests, it belongs partly to the parietal lobe and partly to the occipital lobe). The arcus parieto-occipitalis arches over the parieto-occipital sulcus – which is actually a fissure of the medial surface but just reaches the superolateral surface.
Insula is a Latin word meaning island. It is a part of the cerebral cortex that does not belong to any of the four lobes described above and today it is considered another lobe (insular lobe). It is located deep in the stem and posterior ramus of the lateral sulcus. The surface of the insula is divided into a number of gyri.
During development of the cerebral cortex, the insula grows less than surrounding areas which, therefore, come to overlap it and occlude it from surface view. Those surrounding areas are called opercula (meaning lids; singular of opercula is operculum), and are three in number:
- The frontal operculum: lying between the anterior and ascending rami of lateral sulcus
- The frontoparietal operculum: which lies above the posterior ramus of lateral sulcus
- The temporal operculum: which lies below the posterior ramus of lateral sulcus. This operculum has a superior surface which is hidden in the depth of the lateral sulcus
Like the superolateral surface, this surface is also marked with sulci and gyri. In addition, a few white matter fiber systems are seen on this surface. The corpus callosum is a prominent structure on this surface. The third ventricle of the brain, as well as some other related structures like the interventrcular foramen (of Monro) – through which the third ventricle communicates with the lateral ventricles, can also be seen on the medial surface. Other structures seen on this surface include the thalamus, hypothalamus and cerebral aqueduct (of Sylvius).
Anterior to-, posterior to- and above the corpus callosum are the sulci and gyri of the medial surface of the cerebral cortex. The most prominent of the sulci is the cingulate sulcus (sulcus cinguli) which follows a curved course parallel to the upper convex margin of the corpus callosum. Anteriorly, the sulcus cinguli ends below the rostrum of the corpus callosum, and posteriorly, it turns upwards to reach the superomedial border, a little behind the upper end of the central sulcus. The area between the cingulate sulcus and the corpus callosum is the gyrus cinguli (or cingulate gyrus). Hence that gyrus is defined by the cingulate sulcus superiorly, and the corpus callosum inferiorly (specifically by the callosal sulcus which wounds around the corpus callosum).
Above the cingulate sulcus, up to the limit of the superomedial border (superomedial edge) is an area which consists of two parts. A smaller posterior part, which is wound around the end of the central sulcus. This part is called the paracentral lobule and it is separated from the anterior part by a very short sulcus which is continuous with the cingulate sulcus. The larger anterior part (which lies in front of this short sulcus) is called the medial frontal gyrus.
The part behind the paracentral lobule and gyrus cinguli, shows two major sulci that cut off a triangular area. That area is called the cuneus. This triangular area is defined anteriorly and above by the parieto-occipital sulcus (which crosses the superomedial border to appear as a very short sulcus on the superolateral surface), inferiorly by the calcarine sulcus, and posteriorly by the superomadial border of the cerebral cortex. The calcarine sulcus extends forwards beyond its junction with the parieto-occipital sulcus and ends a little below the splenium of the corpus callosum (posterior part of the corpus callosum) to define a small area called the isthmus which lies between it (calcarine sulcus) and the splenium of corpus callosum.
Between the parieto-occipital sulcus and paracentral lobule is a quadrilateral area called the precuneus. Anteroinferiorly the precuneus is separated from the posterior part of the gyrus cinguli by the suprasplenial (or subparietal) sulcus. The precuneus and the posterior part of the paracentral lobule form the medial surface of the parietal lobe.
This surface of the cerebral cortex is also marked with sulci that define some gyri on the inferior surface. Similar to the medial surface, a view of the inferior surface shows some structures of the white matter. However, our discussion will be limited to cerebral cortex on this surface. Features of the cerebral cortex on this surface can easily be described by dividing the surface along the stem of lateral sulcus, to form two parts:
- an orbital part (or orbital surface)
- a tentorial part (tentorial surface)
Orbital part of the Inferior Surface:
Close to the medial border of the orbital surface, there is an anteroposterior sulcus called the olfactory sulcus. This sulcus has this name because the olfactory bulb and tract lie superficial to it (hence it is not also seen on surface view). The area medial to this sulcus is called the gyrus rectus. The rest of the orbital surface is divided by an H-shaped orbital sulcus into anterior, posterior, medial and lateral orbital gyri.
Tentorial part of the Inferior Surface:
The tentorial part of the inferior surface is marked by two major sulci that run in an anteroposterior direction. Those are the collateral sulcus medially, and the occipito-temporal sulcus laterally. The posterior part of the collateral sulcus runs parallel to the calcarine sulcus; the area between them is the lingual gyrus. Anteriorly, the lingual gyrus becomes continuous with the parahippocampal gyrus which is related medially to the midbrain and to the interpeduncular fossa. The anterior end of the parahippocampal gyrus is cut off from the curved temporal pole of the cerebral hemisphere by a curved rhinal sulcus. This part of the parahippocampal gyrus forms a hook-like structure called the uncus. Posteriorly this gyrus becomes continuous with the cingulate gyrus through the isthmus. The area between the collateral sulcus and the rhinal sulcus medially, and the occipitotemporal sulcus laterally, is the medial occipitotemporal gyrus. Lateral to the occipitotemporal sulcus is an area called the lateral occipitotemporal gyrus, which is continuous (around the inferolateral margin or inferolateral border) with the inferior temporal gyrus.
Important Functional Areas
Certain areas of the cerebral cortex have long been identified with specific functions. Those areas can be defined in terms of the gyri and sulci described above. However, many of those who have studied the microscopic structure of the cerebral cortex have found that there is a considerable difference from region to region and that definition of these functional areas is not confined to the boundaries of gyri and sulci, but often cross them. Most of these authors have also worked out “maps” of the cerebral cortex indicating areas of differing structure. The best known scheme is that by Brodmann, who represented different areas of the cortex by numbers. The numbers and areas most commonly referred to are represented in the following diagrams, and described as follows:
Motor Area (Primary Motor Cortex)
This is the area corresponding to area 4 of Brodmann and possibly to the part of area 6 which lies in the precentral gyrus. Motor area is located in the precentral gyrus on the superolateral surface, and in the anterior part of the paracentral lobule on the medial surface. That area of the cortex is responsible for initiation of voluntary movement. However, specific regions within the area are responsible for movements in specific parts of the body. Stimulation of the paracentral lobule produces movement in the lower limbs. The trunk and upper limbs are represented in the upper part of the precentral gyrus, while the face and head are represented in the lower part of the gyrus. This concept is referred to as “Homunculus” and basically, it shows how the brain sees the body.
Another feature of interest is that the area of the cortex representing a part of the body is not proportional to the actual size of the part, but rather to the intricacy of movements in the region. Thus relatively large areas of cortex are responsible for movements in the hands or in the lips.
Sensory Area (Primary Somatosensory Cortex)
This functional area is located in the postcentral gyrus. It corresponds to area 1, 3 and 2 of Brodmann. It extends into the medial surface, from the lateral surface, where it lies in the posterior part of the paracentral lobule. Just like the motor area, the concept of Homunculus can be applied to the sensory area. Responses can be recorded from the sensory area when individual parts of the body are stimulated. Mapping of the representation of various parts of the body in the sensory area shows the body is represented upside down. The area of the cortex that receives sensation from a particular part of the body is not proportional to the size of that part, but rather to the complexity of sensation received from it. Thus the digits, lips and the tongue have a disproportionately large representation.
Lying in the inferior part of the postcentral gyrus (also called frontoparietal operculum) is another important functional area responsible for the conscious appreciation of taste. That area is called the gustatory area, and it corresponds to area 43 of Brodmann.
The cerebral cortex is supplied by cortical branches of the anterior, middle and posterior cerebral arteries. These cortical branches have their origin from a network of anastomosing arteries that constitute a circle of arteries supplying blood to the brain. The network of arteries is called the Circle of Willis .
Neurodegenerative diseases such as Alzheimer's disease and Lafora disease, show as a marker an atrophy of the gray matter of the cerebral cortex.
This refers to disorder of brain formation in which the surface of the cerebral cortex are devoid of gyri and sulci, and appears smooth. The two most common forms of lissencephaly are: isolated lissencephaly and Miller-Dieker syndrome (MDS). In addition to the CNS abnomalities seen in both forms of lissencephaly above, patients with MDS have congenital abnormalities affecting the heart, kidneys, and other organ systems.
Another characteristics of lissencephaly is that the cellular layers (laminae) of the cerebral cortex are only four, as against six in normally developed cerebral cortex. Seizure disorders are also experienced in this condition.
This is the inability to speak even though the muscles concerned are not paralysed. It results from injury to the motor speech area (of Broca) which lies in the inferior frontal gyrus – areas 44 & 45 of Brodmann. This defect occur only if the damage is on the left cerebral cortex in right handed people, and in the right cortex in left handed persons. Hence, motor control of speech is confined to one hemisphere; the hemisphere that controls the dominant upper limb.
- Contralateral monoplegia & Hemiplegia: A localised lesion of the primary motor area normally produces contralateral monoplegia. An extensive lesion can cause hemiplegia. Lesions of the premotor area leads to adverse effects on skilled movements.
- Deviation of eyes and Personality changes: Lesions of the frontal eye field results in deviation of both eyes to the side of lesion. Hence if the lesion is on the right hemisphere, both eyes will deviate to the right. Lesions of the prefrontal areas lead to personality changes.
- Abnormalities resulting from damage to the Somatosensory Areas: Damage to the first somatosensory area, area 1, leads to loss of sensation from the opposite side of the body. If the damage is on the second somatosensory area, area 2, it may cause an inability to feel pain and temperature. Damage to some areas behind the main sensory area (that is areas 5 and 7) interferes with the ability to identify objects by touch. Damage to the area of Wernicke leads to failure to understand speech.
- Word deafness: This results from lesions in the secondary auditory area (area 22 of Brodmann), and it is characterized by interference with interpretation of speech. However, because the auditory areas receive impulses from both sides, a lesion on one side produces only partial loss of hearing.