Autonomic Nervous System: Meaning and Functions

In this article we will discuss about the meaning and functions of autonomic nervous system.

Meaning of Autonomic Nervous System:

It is the part of nervous system, which regulates functioning of visceral organs. Two different limbs of autonomic nervous system are sympathetic and parasympathetic nervous system components.

Sympathetic takes origin from lateral horn cells of thoracolumbar segments of spinal cord whereas parasympathetic takes origin from cranial (in brain) and sacral (in spinal cord) regions. In parasympathetic component, nerves included are cranial nerves 3, 7, 9 and 10 and also pelvic nerve from sacral segments of spinal cord (Fig. 9. 56).

i. Centers for autonomic nervous system are present in hypothalamus.

ii. The anterior hypothalamus controls activity of parasympathetic and posterior hypothalamus that of sympathetic nerves.

iii. In any autonomic nervous system pathway, there are two neurons along efferent pathway. The preganglionic and postganglionic. The neurotrans­mitter at pre- and post-ganglionic regions in para­sympathetic part is acetylcholine.

In sympathetic, at preganglionic region, it is acetylcholine whereas in post-ganglionic region in most of parts of body it is noradrenaline. In some parts, at the post­ganglionic region of sympathetic nervous system, the neurotransmitter liberated is ACh.

iv. Acetylcholine can act through nicotinic or muscarinic receptors present in pre- and post­ganglionic regions respectively.

v. Noradrenaline can act through alpha or beta receptors.

vi. Alpha receptors are of two types namely alpha 1 and alpha 2 and likewise even beta receptors are of two types namely beta 1 and beta 2.

Blockers for Various Receptors:

Nicotinic receptors can be blocked by:

i. Hexamethonium

ii. Pentolinium

Muscarinic receptors can be blocked by:

i. Atropine

ii. Scopolamine

Alpha receptors blockers are:

i. Phenoxybenzamine

ii. Phentolamine

Beta receptors can be blocked by:

i. Propranolol

ii. Atenelol

All involuntary organs/structures in the body have dual innervations.

But some exceptions are:

1. Adrenal medulla

2. Eccrine sweat glands

3. Vascular smooth muscles

4. Juxtaglomerular apparatus.

Functions of Autonomic Nervous System:

1. Regulation of functions of visceral organs, like heart, gastrointestinal tract, etc., and thereby help to regulate heart rate, blood pressure and gastro­intestinal secretion and motility.

2. In lungs, bring about bronchodilation when acts through sympathetic nerve and vice versa effect, when parasympathetic nerve is acting.

3. Sweat glands are supplied by sympathetic cholinergic fibers and are involved in regulation of body temperature. In addition to this, during thermoregulation, goose pimples are formed due to piloerrection which is brought about by contraction of erector pilorum muscle.

4. Secretions from adrenal medulla are regulated by activity of sympathetic nerves (pelvic splanchnic nerve).

5. In eyes, pupillary dilation is by sympathetic nerve stimulation and constriction by parasympathetic nerve stimulation.

6. Defecation and micturition are possible due to activity of parasympathetic nerves stimulation.

7. In male reproductive system, errection of penis is due to parasympathetic stimulation and ejaculation of semen is because of sympathetic nerve activity.