AUTONOMOUS NERVOUS SYSTEM, SYMPATHETIC NERVOUS SYSTEM AND PARASYMPATHETIC NERVOUS SYSTEM

The name autonomous (Gr. autos = self, nomos = governing) indicates that it is a self-governing nervous system. It is a partly independent system. It is not under the voluntary control of nervous system. The activities of some of the organs which are not under the control of the will of an organism are controlled by this system. The autonomous nervous system controls activities like heartbeat, peristalsis in alimentary canal, contraction of urinary bladder, secretion of digestive juices from digestive glands, etc. The autonomous nervous system is also known as "visceral nervous system".

 

The central nervous system generally, regulates various visceral organs and movements of involuntary muscles. Though it is involuntarily controlled by the nerve centres located in the central nervous system, it has connection with spinal nerves and some cranial nerves.

 

The autonomous nervous system consists of visceral sensory and visceral motor fibres. It has two divisions:

 

They ensure smooth functioning of the various organs.

 

It consists of a pair of slender longitudinal chains of ganglia, which lie on either side of the dorsal aorta. These chains are called sympathetic nerve chains or cords. They continue forward along the outer sides of systemic arches. Anteriorly, each sympathetic nerve cord enters the skull through jugular foramen and finally ends in vagus or gasserian ganglion. The sympathetic nerve chain is connected with corresponding spinal nerve by a narrow branch called ramus communicans.

 

Several nerves arise from sympathetic ganglia, which innervate various visceral organs like stomach, intestine, spleen, kidneys, urinary bladder, gonads etc. The nerve endings of the sympathetic nervous system secrete a chemical substance called sympathin which is functionally similar to adrenalin. This sympathin has an accelerating effect on the activities of various visceral organs.

 

 

This is a functionally different component of the autonomous nervous system. The nerve fibres of parasympathetic nerves are found in third, seventh, ninth and tenth cranial nerves and the spinal nerves of the sacral region. The ganglia of the parasympathetic system are located in the wall of heart, stomach, lungs, urinary bladder etc. The nerve endings of parasympathetic system secrete acetylcholine. This substance has inhibiting effect which retards the over activity of the organs. Thus the normal working of the body is ensured by opposing functions of two components (sympathetic and parasympathetic) of autonomous nervous system.

 

The opposing functions of sympathetic and parasympathetic sys­tems are given below:

 

Organ & Activities	Sympathetic control	Parasympathetic control
1. Rate of heart beat	Increases	Decreases
2. Stomach a) Tonus and motility b) Secretions	a) Decrease b) Decrease	a) Increase b) Increase
3. Intestine Tonus and motility	Decrease	Increase
4. Bronchial muscles	Dilate	Constrict
5. Blood vessels	Constrict	Dialate
6. Urinary bladder	Relaxation of muscles occur	Contraction of the muscles occur
7. Peristalsis of alimentary canal	Decreases	Increases
8. Secretions of salivary and gastric glands	Decrease	Increase