The nervous system of man consists of two parts:
(a) The central nervous system.
(b) The peripheral nervous system.
(a) The Central Nervous System: It consists of the brain and spinal cord. It is responsible for the coordination and control of the activity of the nervous system. CNS enables a person to give a more appropriate and more intelligent response to various situations. The work of the central nervous system is to collect all the information from all the receptors in our body. In complicated responses the brain and spinal cord, both are involved.
1. Brain: Human brain is shaped like a large mushroom. It is located in the bony cavity of the skull, called cranium. The brain is covered by three membranes, called meninges. The fluid (cerebrospinal) is filled between these membranes. This fluid protects the brain from mechanical shocks.
The brain is divided into three regions:
(a) Forebrain,
(b) Midbrain,
(c) Hindbrain.
The forebrain is the largest region, covering 2/3 portion of the brain. It mainly consists of the cerebrum.
Midbrain is quite small and inconspicuous. It connects cerebrum to the other parts of the brain and spinal cord.
Hindbrain consists of three main parts
(a) medulla oblongata,
(b) pons and
(c) cerebellum.
2. Spinal cord: It consists of a number of nerve cells, both fibres and cell bodies, grouped into a cylindrical mass that extends from the brain to the bottom edge of the first lumbar vertebra. It is covered by the same meninges as the brain and lies within the neural canal of the vertebral column. A cross section of the spinal cord reveals an outer region of white matter and an inner H-shaped region of grey matter. The grey matter consists of cell bodies of thousands of neurons.
The white matter is composed of myelinated nerve fibres, running up and down the cord or passing out to the spinal nerves. Running through the centre of the spinal cord is the central canal. Dorsal and ventral fissures (grooves), divides spinal cord into right and left halves. The spinal cord has two-fold
functions: It conducts impulses to and from the brain and acts as a reflex centre.
(b) Peripheral nervous system:
This consists of three parts: Cranial nerves; Spinal nerves and Autonomic nervous system.
Cranial nerves: The cranial nerves are those that emerge from the brain e.g. in man, there are twelve pairs of cranial nerves, which are mainly supplied by the organs in the head region.
Spinal nerves: These are supplied by the spinal cord. There are 31 pairs of spinal nerves, all of which are mixed nerves. These include 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.
Autonomic nervous system: This lies outside and parallel to the central nervous system and is organised into two distinct regions, sympathetic system and parasympathetic system. While the former consists of thoracic and lumbar ganglia and using only spinal nerves, the latter uses three spinal nerves of the lower region of the cord, i.e., sacral part and four cranial nerves. The autonomic nervous system consists of two sets of nerves with opposite actions- If one speeds up activity, the other decreases it. The autonomic nervous system controls the functions of the internal organs of the body automatically and unconsciously.
It sends impulses via the efferent or motor neurons to the effector structures, other than the skeletal muscles. These structures include smooth muscles, lining the wall of digestive, respiratory, urinary and reproductive tracts; the smooth muscles of blood vessels and eye, the cardiac muscles of heart and both, exocrine and endocrine glands. Thus, the autonomic nervous system regulates
1. the contraction of the involuntary muscle,
2. the rate and force of the heartbeat,
3. the secretion of the glands of the alimentary tract, and
4. the size of the pupil of the eye.
Functions of Brain:
1. The brain receives impulses from all sensory organs, such as eyes, ear, etc.
2. After receiving signals or impulses from sensory organs, the brain sends off instructions to muscles and glands, to respond through motor neurons.
3. Brain correlates various stimuli from different sense organs.
4. Brain efficiently coordinates the activities of our body.
5. Brain stores information, in the form of our knowledge.
6. The brain is responsible for thinking and intelligence.
