Cerebrum

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Cerebrum


The cerebrum is the largest part of the human brain that fills most of the cranial cavity. Its large size is the result of a progressive (telencephalization) centralization of the various higher sensory and motor centres of the brain during evolution.


The cerebrum is a heavily, convoluted bilobed structure (Fig. 12.1). The two lateral halves are called cerebral hemispheres. When the two cerebral, hemispheres are viewed together from above, they assume the shape of an ovoid mass, which is broader behind than in front. The widest transverse diameter corresponds with a line connecting the two parietal tuberosities.



A deep median cleft, the longitudinal cerebral fissure, incompletely separates the two cerebral hemispheres. Both in front and behind, the cleft is complete, but in the central part the cleft extends downwards up to the corpus callosum which is a large mass of white fibres joining the two cerebral hemispheres across the median plane.


The longitudinal cerebral fissure is occupied by the following structures:



Each cerebral hemisphere consists of: (a) an outer layer of grey matter called cerebral cortex, (b) an inner mass of white matter, (c) large masses of grey matter embedded in the basal part of the white matter called basal ganglia/basal nuclei, and (d) a cavity within it called lateral ventricle (Fig. 12.2).




External Features of the Cerebral Hemisphere


The external features of the cerebral hemisphere include poles, surfaces, borders, sulci, and gyri.



Poles (Figs 12.2 and 12.3)


Each cerebral hemisphere presents three poles – frontal, occipital, and temporal. The anterior end of the hemisphere is the frontal pole and the posterior end the occipital pole. The temporal pole is below and in front at the junction of orbital and tentorial surfaces.







Borders


Each cerebral hemisphere presents six borders (Figs 12.2 and 12.3), viz. superomedial, superciliary, inferolateral, medial orbital, medial occipital and inferomedial.





1. The superomedial border separates the superolateral surface from the medial surface.


2. The superciliary border is at the junction of superolateral and orbital surfaces. It lies just behind the superciliary arch hence its name strictly speaking, it is the orbital part of the inferolateral border.


3. The inferolateral border separates the superolateral surface from the tentorial surface. Posteriorly this border exhibits a notch, the preoccipital notch about 3 cm in front of the occipital pole. This notch is used as a useful surface landmark.


4. The medial orbital border (Fig. 12.3) separates the medial surface from the orbital surface.


5. The inferomedial/hippocampal border (Fig. 12.3) surrounds the cerebral peduncle. It is formed by the medial aspect of the uncus and parahippocampal gyrus.


6. The medial occipital border (Fig. 12.3) separates the medial surface from the tentorial surface.



Sulci and Gyri


The cerebral cortex (the surface layer of grey matter) is highly extensive in man. To accommodate it in the limited space available within the rigid cranial cavity, the surface of cerebral hemisphere becomes folded, producing numerous convolutions separated by fissures. These convolutions and fissures are termed gyri and sulci respectively. In human brain the total surface area of cerebral hemisphere is about 2000 cm but approximately two-third of this is hidden from the surface view within the walls of the sulci.


A brain with convoluted cerebral cortex is termed gyrencephalic while the one with smooth cortex, lissen-cephalic (Gk. lissos = smooth).


In general, man and other higher mammals have gyren-cephalic brain while the reptiles, birds and lower mammals have lissencephalic brain.


The sulci vary in depth from slight grooves to deep fissures and some of them are sufficiently deep to indent the wall of the lateral ventricle in the depth of the hemisphere.


The gyri consist of a central core of white matter (nerve fibres running to and from the overlying cortex) covered by a layer of grey matter, the cerebral cortex.


There is a great deal of individual variations in the details of sulci and gyri. Therefore, the following account deals with only some important sulci and gyri.



Main cerebral sulci


Main cerebral sulci are fairly constant in position and shape and include lateral, central, parieto-occipital and calcarine sulci.



Lateral sulcus (of Sylvius) (Figs 12.4, 12.6)

Lateral sulcus is the most conspicuous of all the cerebral sulci and has a stem and three rami. The stem of the sulcus begins as a deep cleft on the inferior surface of the cerebral hemisphere at the anterior perforated substance and extends laterally between the temporal pole and the posterior part of the orbital surface of the hemisphere. On reaching the superolateral surface it divides into three rami: (a) anterior horizontal, (b) anterior ascending, and (c) posterior.


The anterior horizontal ramus is about 2.5 cm long and passes forwards into the inferior frontal gyrus. The anterior ascending ramus runs upwards for about 2.5 cm in the same gyrus. The posterior ramus (the main part of the sulcus) is about 7.5 cm long and runs posteriorly and slightly upwards across the lateral surface and ends in the inferior parietal lobule by an upturned posterior end.








Lobes of Cerebral Hemisphere (Fig. 12.4)


To discuss further about sulci and gyri and other aspects of the cerebral hemisphere, the superolateral surface of the hemisphere is arbitrarily divided into four lobes – frontal, parietal, temporal and occipital with the help of: (a) three main sulci, central, lateral and parieto-occipital, and (b) two imaginary lines. The first imaginary line is a vertical line joining the parieto-occipital sulcus to the preoccipital notch, and the second line is a backward continuation of the horizontal part of the posterior ramus of the lateral sulcus till it joins the first line (Fig. 12.4).



The frontal lobe lies anterior to the central sulcus, and above the posterior ramus of the lateral sulcus.


The parietal lobe lies behind the central sulcus and in front of the upper part of the first imaginary line. Below it is bounded by the posterior ramus of lateral sulcus and the second imaginary line.


The temporal lobe lies below the posterior ramus of lateral sulcus and second imaginary line. It is separated from the occipital lobe by the lower part of the first imaginary line.


The occipital lobe lies behind the vertical line joining the parieto-occipital sulcus and preoccipital notch.




Insula/island of Reil (also called central lobe)

It is customary to consider the insula separately from the four main lobes (vide supra) of the cerebral hemisphere.


The insula is the submerged (hidden) portion of the cerebral cortex in the floor of the lateral sulcus (Fig. 12.5). It has been submerged from the surface during development of brain due to the overgrowth of the surrounding cortical areas and can be seen only when the lips of the lateral sulcus are widely pulled apart. It is triangular in shape and surrounded all around by a sulcus, the circular sulcus except anteroinferiorly at its apex called limen insulae which is continuous with the anterior perforated substance.




The insula is divided into two regions – anterior and posterior by a central sulcus. The anterior region presents 3 or 4 short gyri called gyri brevia and the posterior region presents 1 or 2 long gyri called gyri longa.


The insula is hidden from the surface view by the overgrown cortical areas of frontal, parietal and temporal lobes. These areas are termed frontal, frontoparietal and temporal opercula (operculum = lid). The superior surface of the temporal operculum presents anterior and posterior transverse temporal gyri.


The middle cerebral artery and deep middle cerebral vein lie on the surface of the insula.



Sulci and Gyri on the Superolateral Surface of the Cerebral Hemisphere (Fig. 12.6)




In the frontal lobe




In the parietal lobe



• The postcentral sulcus runs downwards and forwards, a little behind and parallel to the central sulcus. The area between these two sulci is called the postcentral gyrus.


• The rest of the parietal lobe is divided into a superior and inferior parietal lobules by an intraparietal sulcus which runs horizontally backwards from the postcentral sulcus.


• The upturned posterior end of the posterior ramus of lateral sulcus, and the posterior ends of superior and inferior temporal sulci extends into the inferior parietal lobule to divide it into three parts: (a) the part that surrounds the posterior ramus of lateral sulcus is called supra marginal gyrus, (b) the part surrounding the superior temporal sulcus, the angular gyrus, and (c) the part surrounding the inferior temporal sulcus, the arcus temporo-occipitalis.





Sulci and Gyri on the Medial Surface of the Cerebral Hemisphere (Fig. 12.7)


The presence of corpus callosum is the most conspicuous feature seen on the medial surface of the cerebral hemisphere. It is C-shaped thick bundle of commissural fibres. It consists of a central part, the trunk, a thick posterior end, the splenium and curved anterior end, the genu.



The sulci and gyri on the medial surface are located above, in front and behind the corpus callosum.


These are as follows:



The area between the cingulate sulcus and the corpus callosum is termed cingulate gyrus.




The boundaries of paracentral lobule needs to be elaborated, as it is the cortical (highest) centre of micturition and defecation. The paracentral lobule is bounded above, by the superomedial border of the hemisphere, below by the cingulate sulcus, and posteriorly by the upturned posterior end of the cingulate sulcus. The paracentral lobule is invaded by the downturned upper end of the central sulcus.




• The posterior part of medial surface behind the para-central lobule has two main sulci: the calcarine sulcus, and the parieto-occipital sulcus.


    


Jan 2, 2017 | Posted by in NEUROLOGY | Comments Off on Cerebrum
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