Central nervous system: an overview

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Central Nervous System: an Overview


The central nervous system (CNS) consists of the brain and spinal cord, which are located in the cranial cavity and vertebral canal respectively (Fig. 6.1).



The delicate CNS is well protected because it is enclosed by the skull and vertebral canal and meninges, and is bathed in cerebrospinal fluid (CSF).


Structurally, the brain and spinal cord consist of grey and white matter. Therefore, sections through the CNS present greyish and whitish regions.


The grey matter consists of nerve cell bodies and dendrites, and the white matter is made up predominantly of myelinated nerve fibres.




In the brain (except the brainstem), the grey matter is present at the periphery and the white matter in the centre. Contrary to it, in the spinal cord the grey matter is present in the centre and white matter at the periphery.



Brain


The brain is that part of the CNS which lies within the cranial cavity.


The functions of the brain are as follows:





The adult brain constitutes about one-fiftieth of body weight and weighs about 1400 g in males and 1200 g in females. It consists of six major parts: (a) the cerebrum, (b) the diencephalon, (c) the midbrain, (d) the pons, (e) the medulla oblongata, and (f) the cerebellum (Fig. 6.2).



The midbrain, pons and medulla oblongata collectively form the brainstem.


The Figure 6.3 shows the superolateral aspect of the brain, as seen in dissection.




Cerebrum


The cerebrum is the largest part of the brain. It consists of two large, hemispheres (the left and the right cerebral hemispheres), which occupy the anterior and middle cranial fossae, and the supratentorial region of posterior cranial fossa.


Each cerebral hemisphere consists of a surface layer of grey matter, called cerebral cortex and a central core of white matter. In the basal part of the latter are located large masses of grey matter, known as basal nuclei/ganglia (Fig. 6.4).



The two hemispheres are partly separated from each other by a deep median longitudinal fissure, called longitudinal cerebral fissure. A massive commissure, the corpus callosum, whose fibres interconnect the corresponding cortical areas of the two cerebral hemispheres, lies in the floor of the fissure.


The surface of cerebral hemisphere is convoluted, i.e. it has a series of elevations, the gyri, separated by shallow depressions, the sulci or deep grooves called fissures (Fig. 6.3).


A central sulcus which runs downwards and forwards from superomedial border to the lateral sulcus is located about midway along the length of the hemisphere.


There are individual differences in the appearance of the sulci and gyri, but some sulci are constant in their position and appearance, and serve as an important landmark.


The superolateral surface of each cerebral hemisphere is divided into four lobes, which are named after the overlying skull bones (Fig. 6.5):




The frontal lobe is important for voluntary motor functions, motivation, aggression, emotions, effect, drive and awareness of self.


The parietal lobe is the major centre for reception and evaluation of all sensory informations except for smell, hearing, and vision.


The occipital lobe is responsible for reception and integration of visual input.


The temporal lobe receives and evaluates input for smell and hearing and plays an important role in memory.




The so-called limbic lobe is a composite bordering zone (limbus = border) between the telencephalon and diencephalon. It is somewhat ring-shaped. It is associated with basic survival instincts, viz. the acquisition of food and water and reproduction. It provides ability to store and retrieve information and is particularly important for short-term memory.


The medial surface of the cerebral hemisphere is visualized in the sagittal section of brain and presents a number of features as shown in Figure 6.6.



The inferior surface of the cerebral hemisphere is uneven and presents orbital and tentorial surfaces.



Basal ganglia/nuclei


The basal ganglia are subcortical masses of grey matter which are situated in the white core of each cerebral hemisphere.


The basal ganglia include the lentiform nucleus, caudate nucleus, claustrum, and amygdaloid body (Fig. 6.4).


During development of connections between the cerebral cortex and the brainstem, the bundles of fibres, converging as the internal capsule, partly divide the corpus striatum into a medial caudate nucleus and a lateral lentiform nucleus. Between the internal capsule and the cerebral cortex, the nerve fibres diverge as the corona radiata.


Functionally, the basal ganglia also include the subthalamic nucleus of diencephalon, and the substantia nigra and red nucleus of midbrain.


The basal ganglia influence the quality of motor functions and are sometimes termed extrapyramidal nuclei.


The major effect of basal ganglia is to decrease the muscle tone and inhibit the unwanted muscular activity.



Diencephalon


The diencephalon is the part of brain between the cerebrum and the brainstem. Its main components are: (a) two thalami, (b) hypothalamus, (c) metathalamus, (d) epithalamus, and (e) subthalamus.






Jan 2, 2017 | Posted by in NEUROLOGY | Comments Off on Central nervous system: an overview
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