Autonomic nervous system (ANS)

The ANS is a visceral motor nervous system. According to definition it involves no sensory components. Though, the activities of the ANS are modified by sensory input which travels by way of somatic nervous system, and by CNS. The target tissues of an ANS are cardiac muscle, smooth muscle, endocrine & exocrine glands, kidney, liver, and adipose tissue. Synapses of the autonomic neurons with target cells are known as neuroeffector junctions.

The preganglionic neurons of the ANS have their cell bodies in the motor nuclei of cranial nerves, or the intermediolateral horn of the thoracic and lumbar spinal cord. Their axons are myelinated B fibers that secrete acetylcholine. Preganglionic axons synapse with postganglionic neurons within autonomic ganglia. The axons of postganglionic neurons are unmyelinated C fibers. The ANS has two partitions, the sympathetic and parasympathetic, the major distinguishing features of which are summarized in the table as shown.

In common the preganglionic axons of the sympathetic division are short and the postganglionic axons are long as the sympathetic ganglia lie close to the spinal cord in one of the two locations:

- In paired paravertebral chains which run parallel to the vertebral column in the neck and down the posterior wall of the abdomen and thorax.

                                                              Table: Divisions of the autonomic nervous system

- In subsidiary ganglia of the autonomic plexuses adjacent to main blood vessels. The path taken by sympathetic axons is described in Figure as shown.

The Preganglionic axons may synapse in the nearby ganglion; traverse the paravertebral chain to synapse in subsidiary ganglia, or rise or low the chain to synapse in a ganglion at a variety of level. The Preganglionic sympathetic axons can change the actions of up to 100 postganglionic cells. Mainly, but not all, postganglionic sympathetic axons secrete noradrenaline. The adrenal medulla secretes adrenaline (epinephrine) directly into the circulation in response to activity in the preganglionic sympathetic fibers that supply it. The adrenal medulla is hence regarded as part of the sympathetic system.The Parasympathetic autonomic ganglia are all subsidiary ganglia placwd close to the target organ. This is the reason; the parasympathetic partition the preganglionic axons are long,and the postganglionic ones are small. All the postganglionic parasympathetic axons secrete acetylcholine. Though all the major organs, except the liver, have a parasympathetic supply, this separation is far less extensive than the sympathetic. This is partly as only a few specialized blood vessels have a parasympathetic innervation, while all blood vessel smooth muscle receives a sympathetic supply.

and the postganglionic ones are small. All the postganglionic parasympathetic axons secrete acetylcholine. Though all the major organs, except the liver, have a parasympathetic supply, this separation is far less extensive than the sympathetic. This is partly as only a few specialized blood vessels have a parasympathetic innervation, while all blood vessel smooth muscle receives a sympathetic supply.