General Organization of the Cerebrum
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. 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.
White Matter of Cerebral Hemispheres
The white matter is nerve fibres (myelinated axons) 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 white matter to terminate in other white matter or a gray matter.
- 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 fibres is the corpus callosum.
Gray Matter of Cerebral Hemispheres
- Basal ganglia (Basal nuclei) – these are gray matter nuclei located deep within the white matter of the cerebral hemispheres.
- Cerebral cortex – this is the largest gray matter and forms the most superficial layer of the cerebral hemisphere.
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 saggital 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 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 an apical and 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.
Other cells are:
- The cells of Martinotti
- Fusiform cells
- Horizontal cells of cajal
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 variable shaped cells with projection and long association output.
Recent investigations have brought to light some interesting features of cortical connections. The chat below summarizes one of such features.
The above chat illustrates the relationship of cortical laminae (or cortical cell layers) to incoming and outgoing fibres. This relationship can be described as follows:
a. fibres projecting from one area of cortex to another are referred to as feed forward fibres. They arise mainly from lamina 3 (pyramidal cell layer) with some from lamina 2 (external granule cell layer). Their termination is mainly in lamina 4 (internal granule cell layer) but also in laminae 1 and 6 (plexiform and polymorphous layers) of the receiving area of cortex (fig. 3),
b. the receiving area sends a feedback to the area of cortex from where it received a feed forward projection. These feedback fibres arise in lamina 5 (ganglionic layer) of the receiving cortex and end in lamina 1 of the cortex from which the feed forward projection was received,
c. the projections to the striatum (corticostriate fibres), to the spinal cord (corticospinal fibres), to pontine nuclei (corticopontine fibres), and to the medulla (corticobulber fibres) all arise mainly from lamina 5 of the cerebral cortex,
NOTE: please kindly refer to Ascending and Descending Tracts of the CNS for detailed description of these fibres connecting the cerebral cortex
d. corticothalamic projections arise from lamina 6, while
e. major afferent fibres entering the cortex (for example, from the thalamus, or feed forward fibres from other cortical areas) end mainly in lamina 4. Some get as far as laminae 1 and 6, while a few reach the remaining laminae.
The cerebral cortex is made up of three large surfaces, namely:
- the superolateral surface
- the medial surface
- the inferior surface
These surfaces are clearly defined by three edges called borders of the cerebral hemisphere. They are the:
- superomedial border
- inferomedial border
- inferolateral border
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 the 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, which is defined by the lateral fissure, has a stem – stem of the lateral sulcus from which an anterior ramus (anterior branch) and an ascending ramus extend into the inferior frontal gyrus to divide it into three 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.
The inferior parietal lobule is further divided into three parts by the posterior ends of three sulci: the upturned posterior end of the posterior ramus of lateral sulcus, the upturned posterior ends of the superior and inferior temporal sulci. These three parts are the:
- Supramarginal gyrus – which is the part that arches over the upturned posterior end of the posterior ramus of the lateral sulcus
- Angular gyrus – this part arches over the superior temporal sulcus
- Arcus temporooccipitalis – which arches over the posterior end of the inferior temporal sulcus
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 hidden. As its name implies, it is a part of the cerebral cortex that does not belong to any of the four lobes described above. 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, some white matter are seen on this surface. When the cerebral hemispheres are separated from each other by a cut in the midline. The corpus callosum (mentioned above under “Commissural fibres”) 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). Details of most deep structures seen on this surface are studied in the “Olfactory and Limbic Regions”.
NOTE: that the thalamus, hypothalamus and the basal ganglia are three different masses of gray matter like the cerebral cortex. They are located deep to the cerebral cortex and within the white matter of the cerebrum, and also that they are part of the small portion the cerebrum’s gray matter, while the cerebral cortex forms the largest portion of the cerebrum’s gray matter.
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 tentoral 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 three 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.
Premotor Area (Motor Association Cortex)
This is the area corresponding to areas 6,8,44 &45 of Brodmann. It is located just anterior to the motor area, occupying the posterior parts of the superior, middle and inferior frontal gyri. The part of the premotor area located in the superior and middle frontal gyri corresponds to area 6 and 8 of Brodmann. The part in the inferior frontal gyrus corresponds to areas 44 and 45, and constitutes the motor speech area (of Broca) or what is also called the anterior speech area (of Broca).
This Broca's area is usually situated in the inferior frontal gyrus on the left side (in right handed and in most left handed people) below and in front of the face area and centred on the pars triangularis between the anterior and ascending rami of the lateral fissure (see description of frontal lobe). Damage to this area produces motor aphasia – difficulty in finding the right words, but not paralysis of laryngeal musculature. That is because the Broca's area is responsible for speech production and articulation, while the entire premotor area functions to coordinate complex movement.
Closely related to the motor area are two specific areas, one is the Broca's area described above, and the other is the frontal eye field. The frontal eye field lies in the middle frontal gyrus just anterior to the precentral gyrus. It corresponds to parts of areas 6, 8 and 9 of Brodmann. Stimulation of that area causes both eyes to move to the opposite side. Those are called conjugate movements. Movements of the head and dilation of the pupil may also occur. The frontal eye field is connected to the visual cortex (areas 17, 18, 19 of Brodmann) and also to the thalamus (particularly to the medial dorsal nucleus). The frontal eye field is involved in voluntary eye movements and the accommodation pathway.
Other than the anterior speech area, there is also a posterior speech area (of Wernicke). That area is in the posterior parts of the superior and middle temporal gyri and extend into the lower part of the parietal lobe. Its integrity is necessary for understanding of speech (language comprehension). The posterior speech area (of Wernicke) corresponds to areas 22, 39 and 40 of Brodmann.
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.
Immediately posterior to the somatosensory area is the Sensory Association Cortex. This cortex is responsible for processing of multisensory information.
Auditory (Acoustic) Area
This is the cortex for hearing, situated in the temporal lobe. It is mostly hidden in the lateral sulcus, in the anterior transverse temporal gyrus. It corresponds to areas 41 and 42 of Brodmann. It extends into the superior temporal gyrus below the sulcus, and is here surrounded by the auditory association area (area 22). Those regions receive fibres from the medial geniculate body through the auditory radiation. The cochleae are found bilaterally, so a lesion of one cortex does not cause deafness. The acoustic areas are responsible for processing auditory information and detection of sound quality like volume and tone.
This area is in the uncus (described under “Tentorial part of Inferior surface”), at the front of the parahippocampal gyrus and adjacent parts of the cortex.
The areas concerned with vision are located in the occipital lobe, mainly on the medial surface, both above and below the calcarine sulcus (area 17). Area 17 extends into the cuneus, and into the lingual gyrus. Posteriorly, it may extend onto the superolateral surface where it is limited anteriorly by the lunate sulcus. Area 17 is continuous, both above and below with area 18 and beyond this area with area 19. Areas 18 and 19 are described as psychovisual areas (or visual association areas) and are responsible for the interprepation of visual impulses reaching area 17.
Occipital eye field
As stated above under frontal eye field, fibres from the visual areas reach the frontal eye field which is concerned with eye movements. The visual areas are therefore regarded as partly motor in function. This view is substantiated by the fact that movements of the eyeballs and head can be produced by stimulation of areas 17 and 18 which constitutes an occipital eye field. Efferents from the visual areas also reach the superior colliculus, the pretectal region, and the nuclei of cranial nerves supplying muscles that move the eyeballs.
Therefore, to summarize the information above, the visual areas can be categorized as follows:
- visual area – area 17 – sensory
- occipital eye field – area 17 & 18 – motor
- psychovisual area – area 18 & 19 – sensory
Prefrontal Areas (Prefrontal Cortex)
The part of the frontal lobe excluding the motor, premotor and motor association cortex is referred to as the prefrontal area. It includes the parts of the frontal gyri anterior to the motor association area, most of the anterior parts of the orbital gyri, most of the medial frontal gyrus, and the anterior part of the gyrus cinguli. This area is concerned with normal expression of emotions, the ability to predict consequences of actions, and complex thoughts. The medial part of the prefrontal area is associated with auditory and visual functions.
The cerebral cortex is supplied by cortical branches of the anterior, middle and posterior cerebral arteries as indicated in the diagrams below. 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.