The nervous system is divided into central and peripheral nervous systems. The peripheral nervous system consists of autonomic and somatic nervous systems. The autonomic nervous system (ANS) is not under voluntary control and therefore was so named by Langley (Autos = self, nomos = governing in Greek). The ANS innervates the heart, the Smooth muscles, the glands and the viscera and controls the functions of these organs

The centres for autonomic reflexes are present in the hypothalamus, medulla and spinal cord. Hypothalamus coordinates the autonomic activity.

The ANS consists of two major divisions– the sympathetic and the parasympathetic. Most of the viscera have both sympathetic and parasympathetic innervation The two divisions have opposing effects and normally their effects are in a state of equilibrium. The prime function of the sympathetic system is to help a person to adjust to stress and prepare the body for fight or flight reactions, while the parasympathetic mainly participates in tissue building reactions. Man can survive and remain alive without sympathetic system (if maintained stress-free) but not without parasympathetic.

AUTONOMIC INNERVATION

Like the somatic nervous system, autonomic innervation also has an affarent, a center and an efferent.

Autonomic afferents:

The autonomic afferents (Fig. 6.3) are carried in visceral nerves through nonmyelinated fibres. For example, the para- sympathetic afferents are carried by the 9^th and 10^th cranial nerves. The autonomic efferent innervation consists of a myelinated preganglionic fiber which synapses with the postganglionic fiber. The postganglionic fiber in turn forms a junction with the receptors of the organs supplied by it. The junction between the pre- and postganglionic fibers is called a ganglion and that between the post- ganglionic fiber and the receptors is the neuro effector junction. The travelling of an impulse along the nerve fiber is known as conduction while its passage across a synapse is known as transmission.

Autonomic efferents:

The autonomic efferents are divided into sympathetic and para- s sympathetic divisions. The parasympathetic efferents are carried through the craniosacral = outflow. The parasympathetic ganglia are located close to the innervated structures and therefore their postganglionic fibers are short. The preganglionic fibers of the parasympathetic system are long and postganglionic fibers are short while in sympathetic system, the preganglionic fibers are short and post- ganglionic fibers are long. The sympathetic efferents extend from 1^st thoracic to 2^nd or 3^rd lumbar segments (T1,-L3) of the spinal cord. The sympathetic ganglia are found a three sites-paravertebral, prevertebral and terminal. Postganglionic fibers arising from sympathetic ganglia innervate the head, neck and the viscera of the thorax and abdomen Adrenal medulla is also considered a sympathetic ganglion and differs from other sympathetic ganglia in that the principle catecholamine that is released is adrenaline.

Neurotransmitters: For the transmission of a impulse across a synapse, a neurohumoral transmitter substance is released into the synaptic cleft. In the ANS, the neurotransmitters released are acetylcholine, noradrenaline, dopamine and in adrenal medulla, it is adrenaline and noradrenaline.

Enteric Nervous System

an Enteric nervous system (ENS) of nerve plexuses in the wall of the intestines includes myenteric plexus and submucosal plexus. The ENS receives inputs from the ANS -from both parasympathetic and sympathetic systems. Parasympathetic stimulation in ENS is excitatory. Parasympathetic stimulation increases gut motility and relaxes the sphincters while sympathetic activity has the opposite effect.

Cotransmission

 It has been noted that there are certain other substances apart from the principle neuro. Transmitter in the vesicles of the autonomic nerve terminals of some tissues. These include substances like dopamine, nitric oxide, ATP serotonin, vasoactive intestinal peptide (VIP) cholecystokinin (CCK) and y-aminobutyric acid (GABA). These are called cotransmitters Different sets of neurons contain different neurotransmitters. For example: CCK, VIF and GABA are present in the excitatory neurons of the gut, while ATP and nitric oxide are cotransmitters at inhibitory neurons in the gut or enteric nervous system. The function of these noradrenergic and noncholinergic cotransmitters are not exactly known. Might modulate or influence the effects of the principle neurotransmitter.