The autonomic nervous system is the division of nervous system which controls involuntary organs. As it is not under the control of will power so it is named as autonomic nervous system.
Organization of Autonomic Nervous System
The Autonomic Nervous System (ANS) has two divisions—parasympathetic and sympathetic. The parasympathetic nervous system is also called cranio-sacral division of the ANS which includes four pairs of cranial nerves namely oculomotor (Illrd), facial (Vllth), glossopharyngeal (IXth) and vagus (Xth) and three (2nd to 4th) pairs of sacral nerves.
On the other hand, the sympathetic division on the other hand, is called thoraco-lumbar outflow of spinal cord which includes 1st thoracic to 2nd or 3rd lumbar (T1 – L2 or L3) spinal nerves. The autonomic motor neurons present in the spinal nerves originate from the lateral horn cells and then pass out through the anterior root along with the somatic motor neurons originating from anterior horn cells.
Characteristic of Autonomic Nervous System
Autonomic nerves are characteristic in that they make synapses on their way to the effector organs after emerging from the Central Nervous System (CNS). These synapses are called ganglia. Thus, in contrast to a somatic motor pathway from CNS to a voluntary muscle consisting of a single neuron, an autonomic motor pathway from CNS to effectors organ consists of two neurons. The first neuron which are extending from the CNS to the ganglion is called preganglionic neuron while the second one extending from the ganglion to the effectors organ is called postganglionic neuron. The axon of the pre-ganglionic neurons are myelinated but that of the postganglionic neurons are unmyelinated. In the parasympathetic system, the ganglia are formed very close to the effector organs and are referred to as terminal ganglia. Hence, their preganglionic neurons are long and postganglionic neurons are very short.
On the other hand, greater part of the sympathetic preganglionic neurons form ganglia right away after emerging from the vertebral column. Such ganglia are called paravertebral ganglia as they lie just beside the vertebral column. They fuse to form a sympathetic chain (or sympathetic trunk) on either side of the vertebral column. A few sympathetic preganglionic fibres entering into the sympathetic chain pass out without making synapse within it; they form ganglia on the midway between the spinal cord and the effector organ. Such ganglia are termed collateral, ganglia. As same, just opposite to the parasympathetic system, the sympathetic preganglionic neurons are shorter than their postganglionic neurons.
A sympathetic pre-ganglionic fiber emerges out from spinal cord via the ventral root and then passes out of the vertebral column as the spinal nerve. Immediately after this, it leaves the spinal nerve and enters into the sympathetic chain through a linking nerve branch called white ramus. It is so named because it looks white due to the presence of myelinated fibers in it. The postganglionic fiber emerging from the sympathetic chain returns back to the spinal nerve through another route called grey ramus.
It should be noted that only the motor neurons of the cranio-sacral and thoraco-lumbar nerves supplying the involuntary muscles and glands are included in Autonomic Nervous System. The sensory neurons present in these nerves do not differ in arrangement from the somatic afferents. Thus, Autonomic Nervous System is considered as an efferent system.
Functions of Autonomic Nervous System
The main function of Autonomic Nervous System is to control the involuntary organs. It maintains homeostasis, i.e., steadiness of the internal organs in the body. Greater part of visceral organs are innervated by both sympathetic and parasympathetic nerves which produce opposing effects so that an organ may be properly synchronized according to the need of the body. In general, the sympathetic nerves are catabolic in action.
The parasympathietic nerves are cholinergic, that means they produce their actions by releasing acetylcholine from their terminals. On the other hand, sympathetic nerves except those supplying the sweat glands are noradrenergic or adrenergic, i.e., they act by energizing noradrenalin or adrenalin at the neuro-effector junction. The sympathetic supply to sweat glands is cholinergic. Effects of parasympathetic stimulation are usually localized to discrete areas of the body and are of short duration. Conversely, sympathetic stimulations generally affect prevalent areas of the body and the effects are relatively prolonged. Such isolated and widespread actions of parasympathetic and sympathetic nerves are due to the peculiarity of their distribution. The sympathetic actions are of longer duration because its stimulation results in secretion of adrenalin from adrenal medulla. This adrenalin circulates in blood for a significant period of time to maintain the actions of the sympathetic nerves. For this reason, thesympathetic nerves and adrenal medulla are collectively called sympatho-adrenal system.