Two types of autonomic nervous system are:
1. Sympathetic Nervous System 2. Parasympathetic Nervous System.
The autonomic nervous system is responsible for the maintenance and regulation of internal environment by controlling automatic activities of the internal body organs.
It regulates the functions of those organs which are not under the control of will (involuntary functions) such as circulatory, respiratory systems, involuntary muscles and visceral organs. However, it is not fully autonomous as its centre of control lies in the central nervous system.
Longley (1921) first classified the autonomic nervous system into sympathetic and para sympathetic nervous system. In mammals these two divisions of the system are distinct anatomically and physiologically. While the sympathetic nervous system originates from the thoracic and lumbar part of the spinal cord, the parasympathetic nerves arise from the brain and sacral region of the spinal cord. All the internal organs receive nerve fibres from the parasympathetic as well as from sympathetic nervous system.
1. Sympathetic Nervous System (Fig. 1.18):
The sympathetic nervous system (also called thoraco-lumbar outflow as its preganglionic fibres arise from thoracic and lumbar region of spinal cord) consists of two sympathetic nerves or chains running from head to the end of sacral region, one on each side of the vertebral column.
Each sympathetic chain bears several ganglia called lateral or chain ganglia. Some chain ganglia fuse to form three ganglia, in the neck, after which there is a linear series of ganglia in the thoracic and lumbar regions. Other sympathetic ganglia (coeliac, superior and inferior mesenteric) lie in viscera and are collectively known as paravertebral ganglia.
The sympathetic system has two types of fibres:
(i) Preganglionic fibre which leave the spinal cord through the ventral root of the spinal nerve and finally joins the sympathetic chain, (a) post ganglionic fibre arising from the sympathetic ganglia and passes to the various organs, such as eye muscles, salivary glands, heart, stomach, small intestine, colon, adrenal gland etc. The post ganglionic fibres secrete sympathin which generally stimulate the above mentioned organs and regulate their functions. Excitation of sympathetic system during emotion causes acceleration of heart beat, dialation of pupil, dryness of the mouth etc.
2. Parasympathetic Nervous System:
This system is also known as carinosacral outflow as parasympathetic fibres arise from two different parts, cranial and sacral regions. This system consists of ganglia, preganglionic and post ganglionic fibres. The preganglionic fibres are joined to cranial nerves III, VII, IX, X, and to the second, third, and fourth sacral spinal nerves. The preganglionic fibres arise from the CNS and end in the individual ganglia lying near the peripheral tissues.
Thus, the ganglia of parasympathetic system are not interlinked to form a chain (unlike the sympathetic chain). From these ganglia arise very short post ganglionic fibres which are supplied to various organs of the body. The preganglionic fibres secrete acetylcholine; it has an inhibitory effect on organs which is antagonistic to that of the sympathetic system.
Parasympathetic nervous system regulates the physiological processes and maintains a constant internal environment. It is responsible for protective reflex reaction such as blinking of the eye, emptying of hollow organs like urinary bladder, gall bladder, rectum etc.
The peculiar feature of the autonomic nervous system is a double innervations (double supply of nerves) of organs from the sympathetic and parasympathetic system, the two systems work antagonistically in order to control the functions of all involuntary mechanisms of the body. A sympathetic fibre is generally stimulatory and it starts an action in an organ, then the parasympathetic fibre supplied to the same organ stops that action after sometime, hence the sympathetic and parasympathetic systems are antagonistic.