This article throws light upon the two main parts of nervous system. The parts are: 1. The Cerebro-Spinal Axis 2. The Nerves and Their Functions.
Nervous System: Part # 1.
The Cerebro-Spinal Axis:
The Cerebro-Spinal Axis consists of the following parts:
(i) Spinal Cord:
It is a white soft cord-like Substance running through the spine or back-bone. It is composed of a grey central core of nerve-cells, and a thick outer layer of white matter which consists of nerve-threads. It is the centre of reflex actions.
It converts sensory impulses directly into motor impulses, because in the spinal cord, sensory and motor neurons, connected through their nerves-fibres with the sense-organs and the muscles The Cerebro-Spinal Axis. respectively, are in very close proximity to one another.
Therefore, an incoming sensory impulse easily finds it way out over motor nerves and produces reflex actions in immediate response to sensory stimulations, without passing to the brain and involving the guidance of the brain.
The spinal nerves are given off from the spinal cord on either side at regular intervals. There are thirty one pairs of these nerves. Each nerve has two roots, an anterior and a posterior root, which unite with each other.
The spinal cord is composed of the grey matter and the white matter. The grey matter consisting of nerve cells lies inside and the white matter consisting of nerve-fibres, lies outside completely covering the grey matter. The spinal cord is the channel of communication from the brain and to the brain.
(ii) The Brain:
The brain within the skull consists of the following parts:
(a) Medulla Oblongata:
It is prolongation of the spinal cord, called also the ‘Bulb’ or the ‘Stem’ which connects the brain with the spinal cord. It acts as a conductor between the spinal cord and the brain. All the nerve-fibres passing from the spinal cord to the brain pass through it.
It controls heart rate, respiration, circulation, etc. It lies between the two hemispheres of the cerebrum. It is composed of the grey matter and the white matter. The grey matter lies inside the white matter which cover it. In this respect, the medulla resembles the spinal cord.
(b) Pons Varolli:
It is a bridge-like band of nervous substance above the medulla oblongata. It connects the two lobes of the cerebellum. Nerve-fibres from the cerebrum pass through it to the lower parts of the body. As they pass they cross one another.
The nerve-fibres from the right cerebral hemisphere pass through the left side of the pons to the muscles of the left side of the body. The nerve-fibres from the left cerebral hemisphere pass through the light side of the pons to the muscles of the right side of the body.
Thus an injury to the right cerebral hemisphere brings about paralysis of the muscles of the left side of the body, and an injury to the left cerebral hemisphere causes paralysis of the muscles of the right side of the body.
(c) Cerebellum:
It is a wedge-shaped body situated below the hinder part of the cerebrum. It consists of two hemispheres composed of white nerve-fibres covered by a thin layer of grey cells. It is a furrowed structure. The furrows are much deeper than in the cerebrum. Its functions are the co-ordination of muscular movements and the maintenance of body.
It controls co-ordinated and balanced movements such as walking, sitting, standing, etc., and preserves the equilibrium of the body. It is the centre of integration. In it the grey matter encloses the white matter. The two hemispheres are connected by a broad band of fibres called the pons.
If there is impairment of cells and nerves in the cerebellum abnormality in muscular movement is produced. This pathological condition creates loss of co-ordination in muscle movements known as ‘dysmetria.’
(d) Mid-Brain or Inter-Brain:
It is above the cerebellum and forms the base of the brain. It contains the Thalamus, which controls and co-ordinates the bodily functions involved in metabolism, and the Hypothalamus, which controls the expressions and emotions. It contains the lower centres which are connected with the sense- organs by nerve-fibres.
Other nerve-fibres lead from them and to the sensory areas of the cortex. There are different lower centres for the different sense-organs. The cortex of the cerebrum is not directly connected with the sense-organs.
(e) Cerebrum:
It is the upper brain, and consists of two hemispheres called cerebral hemispheres separated by a fissure. The right hemisphere is connected with the left side of the body and left hemisphere with the right side of the body. There are folds and furrows in the cerebrum, called the convolutions or gyri and fissures of the brain. The greater is the number of convolutions in the cerebrum, the greater is the intelligence.
The fissures of Rolando and Sylvius divide each cerebral hemisphere into four lobes. The brain consists of the grey and white matter. In the cerebrum, the grey lies outside and encloses the white matter which lies inside. The grey matter is called the cortex which consists of collections of neurons. The white matter- consists of nerve-fibres. The grey matter consist of nerve cells.
There are approximately 14,000,000,000 nerve cells or neurons in the brain. The nerve cells are connected with one another by the synapses or the junctions between neurons.
The neurons transmit energy in the form of electrical currents through nerve-fibres. At the nerve endings there are synapses across which energy is transmitted by chemical process to other cell bodies. In this way nerve impulse passes from one neuron to another through the conduction of electrical potentials.
The cerebrum controls the higher form of actions, viz., voluntary actions. It regulates and inhibits the actions of the lower centres. It is the sovereign of the whole nervous system and the organism. It is the seat of sensation, perception, memory, thought, emotion, and volition. It contains the different sensory centres which are the seats of different kinds of sensations.
It contains the different motor centres through which it controls muscular movements or voluntary actions. It enables us to learn, remember, think, judge, and understand things. It is the seat of the intellect. Different portions of the cerebrum are connected with different kinds of consciousness. Some of them are sensory areas which receive sensory impulses from the sense-organs.
Others are motor areas which send out motor impulses to the muscles. Others are association areas which are coordinating centres. The cerebrum controls, arrests, separates and combines the activities of the lower centres in the spinal cord, the medulla oblongata, and the cerebellum. It receives sensory impulses from the sense-organs through the lower centres, and sends out motor impulses to the muscles through them.
Brain and Mind:
There are ample evidences which indicate that there is correlation between activities of the brain and the mental processes. W. Penfield and L. Roberts found through the technique of stimulation that superior and lateral surfaces of the temporal cortex are mainly associated with the ‘psychic responses’.
They could not detect the presence of ‘psychic responses’ in any other regions of the cerebral cortex. Hence, they hold that the interpretative cortex is located in the temporal lobe.
There is recording of the stream of consciousness in the neuronal pattern of each individual. The recorded counterpart of consciousness in the brain in the form of neuronal patterning’s is complete in its minute details.
The analysis of the components of the cortex show that the contents of consciousness are in cortex though it may be said that the notion of the little man residing inside the brain and witnessing everything may be given up. H. Kluver and P. C. Bucy found that there are complex connections between cerebral organization and psychic experiences.
W. R. Hess has found through experiments that brain is the seat of consciousness. According to his findings, the subjective experience of an individual is intimately related to the structure of the brain, and that only those contents of consciousness may develop which correspond to the organization of cells in the cerebral cortex.
There seems to be no doubt about the fact that there is close correlation between psychical experiences and brain events. However, the details of brain-consciousness integration has not been worked out and systematized into a theory.
It seems, however, that at the core of each cell there is the telic principle, and at the core of entire cerebral cortex there is a governing or regulating telic principle which is responsible for integration of experience as well as behaviour.
Nervous System: Part # 2.
The Nerves and Their Functions:
The nerves appear like fine white threads. They are bundles of extremely thin nerve-fibres. Some of them issue directly from the brain and are called cranial nerves or cerebral nerves; and some of them issue from the spinal cord in the back-bone and are called spinal nerves.
The cranial nerves run into the sense-organs in the head viz., the eyes, the ears, the nose, and the tongue. The spinal nerves run into the skin, viscera, and muscles.
The Nerves are of two kinds:
(1) Sensory or Afferent or In-carrying Nerves and
(2) Motor or Efferent or Out-carrying Nerves.
The former carry nerve currents from the sense-organs inward to the sensory centres in the spinal cord and the brain. The latter carry nerve currents from the motor centres in the spinal cord and the brain outward to muscles and glands.
The sensory nerves are the axons of sensory neurons situated in the sense-organs and other internal organs. The motor nerves are the axons of motor neurons situated in the motor centres of the spinal cord and the brain.
The former carry nerve-currents inward from the sense-organs and the internal organs to the sensory centres and produce sensations. The latter carry nerve-currents outward from the motor centres to the muscles and glands, and produce muscular movements and glandular secretions. The nerves are conductors of nerve-currents.