White matter of the cerebrum and lateral ventricles 14


White Matter of the Cerebrum and Lateral Ventricles

White Matter Of The Cerebrum

The cerebral white matter is a compact mass of a vast number of nerve fibres and associated neuroglia. It lies deep to the cerebral cortex and forms the large volume of each cerebral hemisphere.

Types of Fibres in the White Matter

The fibres of white matter connect the various parts of cerebral cortex with each other and to the other parts of the central nervous system. They are classified into following three types, on the basis of the types of connections they provide (Fig. 14.1):

Association fibres

The association fibres interconnect the different regions of the cerebral cortex in the same hemisphere (intrahemi-spheric fibres). These are of two types (Fig. 14.2):

• Short association fibres (arcuate or ‘U’ fibres) which interconnect the adjacent gyri by hooking around the sulcus, hence they are also called arcuate fibres.

• Long association fibres travel for long distances and interconnect the widely separated gyri, viz. gyri of different lobes. The long association fibres are grouped into bundles. The examples of bundles of long association fibres are as follows:

1. Uncinate fasciculus which connects the motor speech area and orbital cortex of frontal lobe with the cortex of temporal pole by hooking around the stem of lateral sulcus. It is narrow in the middle and fanned out at both ends.

2. Cingulum (also called limbic association bundle): It is thick bundle of fibres occupying the cingulate and parahippocampal gyri. It extends from the parater-minal gyrus to the uncus forming almost a circle like a girdle (cingulum = girdle) hence its name.

3. Superior longitudinal bundle: It is the longest association bundle which connects the frontal lobe to the occipital and temporal lobes.

4. Inferior longitudinal bundle: It connects the visual association area of occipital lobe to the temporal lobe.

5. Fronto-occipital bundle: It commences in the frontal pole, runs backwards to radiate into the occipital and temporal lobes. The fronto-occipital bundle pursues a similar course to that of superior longitudinal fasciculus. However, it lies deep to the superior longitudinal bundle and is separated from it by the fibres of the corona radiata.

Commissural fibres

The commissural fibres interconnect the identical/corresponding areas of the two cerebral hemispheres (interhemispheric fibres). The bundles of such fibres are termed commissures.

The important commissures of the brain are as follows:

N.B. The commissural fibres are essential for interhemi-spheric transfer of information for bilateral responses and learning processes.

The primary visual area and hand region of primary somato-sensory area of cortex have no known commissural fibres.

Corpus callosum (Figs 14.3-14.5)

The corpus callosum is the largest commissure of the brain connecting the cerebral cortex of the two cerebral hemispheres. Since it connects the neocortex (neopallium) of the two sides, it attains enormous size in man. It is 10 cm long, nearly half of the anteroposterior length of the hemispheres, and consists of about 300 million fibres.

The corpus callosum connects all the parts of neocortex of two hemispheres except for the lower and anterior parts of temporal lobes which are connected by the anterior commissure.

External features and relations of corpus callosum

• Corpus callosum forms a massive arched interhemispheric bridge in the floor of the median longitudinal cerebral fissure connecting the medial surfaces of the two cerebral hemispheres.

• It forms massive, arched, interhemispheric bridge flooring the midline longitudinal fissure and roofing both the lateral ventricles.

• It lies nearer the anterior end (4 cm behind the frontal pole) of the hemisphere than the posterior end (6 cm in front of the occipital pole).

• In sagittal section of cerebrum it is seen as C-shaped mass of white fibres on the medial surface of the hemisphere forming the roof of the lateral ventricle (Fig. 14.4).

• The concave inferior aspect of corpus callosum is attached with the convex superior aspect of the fornix by the septum pellucidum and its convex superior aspect is covered by a thin layer of grey matter, the indusium griseum, embedded in which are the fibre bundles of bilateral medial and lateral longitudinal striae.

• The anterior cerebral vessels often lie on the pia mater covering its superior aspect of corpus callosum.

• The superior aspect of corpus callosum is covered on each side by cingulate gyrus from which it is separated by a callosal sulcus.

Parts of the corpus callosum

The corpus callosum is divided from before backwards into four parts: (a) rostrum, (b) genu, (c) trunk/body, and (d) splenium (Fig. 14.3).

Genu: It is thick curved anterior extremity of corpus callosum which lies 4 cm behind the frontal pole. Genu forms the anterior boundary of the anterior horn of the lateral ventricle. The fibres of genu sweep (curve) forwards on either side into the anterior parts of the frontal lobes, forming a fork-like structure, the forceps minor.

Rostrum: The genu extends downwards and backwards as a thin prolongation to join the lamina terminalis forming, rostrum of corpus callosum. The rostrum forms the floor of the anterior horn of lateral ventricle and its fibres extends inferiorly to connect the orbital surfaces of the two frontal lobes.

Trunk: The trunk is main (middle) part of the corpus callosum between its thick anterior (genu) and posterior (splenium) extremities. Its fibres connect most of the frontal and anterior parts of the parietal lobes of the two cerebral hemispheres. The central part of the corpus callosum forms the roof of the central part of the lateral ventricle.

Splenium: The splenium is the massive posterior extremity of the corpus callosum, lying 6 cm in front of the occipital pole. It overhangs the thalamic pulvinars, pineal gland and tectum of the midbrain. The transverse fissure containing tela choroidea of third ventricle, posterior choroidal arteries and great cerebral vein of Galen is located inferior to the splenium.

The fibres of the splenium connect the posterior parts of the parietal lobes, and temporal and occipital lobes of the two hemispheres. The fibres connecting the occipital lobes sweep backwards on either side above the calcarine sulcus forming a large fork-like structure, the forceps major (Fig. 14.5). The forceps major forms a swelling in the upper part of the medial wall of the posterior horn of lateral ventricle, the bulb of the posterior horn.

N.B. The tapetum is the thin lamina of white fibres (2 cm thick) which forms the roof and lateral wall of the posterior horn; and lateral wall of the inferior horn of the lateral ventricle.

The tapetum is formed by those fibres of the trunk and splenium of corpus callosum which are not intersected by the fibres of corona radiata. In the coronal section, the tapetum looks whiter than the surrounding white matter because section passes parallel to the fibres of the tapetum.

Functions of the corpus callosum

The corpus callosum is largely responsible for interhemi-spheric transfer of information which is essential for bilateral responses and in learning processes. However, its congenital absence or surgical division does not produce any change in personality or intelligence and patients remain completely unnoticed of any definite neurological disorder in day-to-day life. Only special tests of tactile and visual systems will reveal any abnormality.

N.B. The section of corpus callosum in past has been attempted surgically to prevent the spread of severe epileptic seizures from one hemisphere to the other.

Anterior commissure

The anterior commissure is a small round bundle of white fibres which crosses the midline in the upper part of the lamina terminalis, immediately in front of the anterior column of the fornix and the interventricular foramen.

Anterior commissure consists of two components (Fig. 14.6):

Seen from below the full extent of anterior commissure has the shape of a cupid’s bow.

Jan 2, 2017 | Posted by in NEUROLOGY | Comments Off on White matter of the cerebrum and lateral ventricles 14
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