Autonomic Nervous system

Autonomic Nervous System - 119 - AUTONOMIC NERVOUS SYSTEM AUTONOMIC NERVOUS SYSTEM (ANS) • James Langley (1921) discovered this visceral motor system which constitutes as the part of PNS that innervate visceral organs like heart, lungs, digestive tract, liver spleen, kidneys and coordinate their involuntary activities. • It consists of chain of two neurons between CNS and effecter. The first of the two has its cyton located in the different areas of CNS and its axon is called as preganglionic motor fibers, it is medullated. The second is excitor neuron and its axon is called as postganglionic motor fiber, it is non-medullated. • The synapse between the two neurons lies somewhere outside the CNS in cell clusters called as autonomic ganglia. • The preganglionic neuron releases acehylcholine at the synapse while postganglionic neuron may be either cholinergic or adrenergic. • Higher nerve centers of the brain i.e.e, medulla oblongata and hypothalamus are the center of the autonomic nervous system. TYPES OF AUTONOMIC NERVOUS SYSTEM There are two components of ANS which operate as antagonists to each other hence described as 2 types: 1. SYMPATHETIC NERVOUS SYSTEM (THORACO-LUMBAR OUTFLOW) • It is excitor or activator system meant to meet the emergency need of the body. • The sympathetic ganglia are situated close to CNS. • The preganglionic fibres are short and cholinergic; the postganglionic fibres are much longer, and adrenergic.
Anteriorly each trunk enters cranium along with vagus nerve and traversing through vagus ganglion ends in Gasserian ganglion.
Posteriorly this trunk ends up by joining with a branch of 10th spinal nerve.
In mammals only thoracic and lumbar (anterior, 3 pairs) spinal nerves are its part hence, also referred as thoraco-lumbar outflow. Includes a chain of about 22 small lateral sympathetic ganglla interconnected by sympathetic trunk extending from below the head to tail. These ganglia are also called as paravertebral ganglia. Due to fusion the number and segment characters of these ganglia become obscured in adult. • Some preganglionic fibers pass through lateral ganglia, do not end there, but after emerging from here they enter some other isolated ganglia called as collateral ganglia. These ganglia are away from the lateral ganglia and near to the tissue of organs. • The postganglionic fibers from collateral ganglia innervate the tissue or organs. - 120 - Autonomic Nervous System 2. PARASYMPATHETIC NERVOUS SYSTEM (CRANIO-SACRAL OUTFLOW) • Nerves originate from the brain stem and sacral segments of the spinal cord hence is also termed as craniosacral outflow, it includes the IIIrd, VIIth, Ixth and Xth cranial nerves. Sacral component include II, III and IV sacral spinal nerves. • Preganglionic fiber is much longer, after emerging from CNS it terminates into the ganglia situated close to or within the respective organs. - 121 • Postganglionic fiber is too short and terminates into the smooth muscle of the same organ and cardiac muscle in heart, it secretes acetylcholine. Autonomic Nervous System EFFECTS OF AUTONOMIC NERVOUS SYSTEM Sympathetic and parasympathetic nervous system are antagonistic in their actions as represented in the following table. Autonomic Nervous System - 122 -