. Explain Autonomic Nervous System


Best Answer

The autonomic system governs involuntary, internal functions of the body that do not ordinarily affect consciousness, such as movements of alimen¬tary canal and heart, contraction of smooth muscle of blood ves¬sels, urinary bladder, iris of the eye, and others, plus secretions of various glands.

Autonomic nerves originate in the brain or spinal cord as do nerves of the somatic nervous system, but unlike the latter, auto¬nomic fibers consist of not one but two motor neurons They synapse once after leaving the cord and before arriving at the effector organ. These synapses are located outside the spinal cord in ganglia. Fibers passing from the cord to the ganglia are called preganglionic autonomic fibers; those passing from the ganglia to the effector organs are called postganglionic fibers. These rela¬tionships are illustrated.

Subdivisions of the autonomic system are the parasympa¬thetic and sympathetic systems. Most organs in the body are innervated by both sympathetic and parasympathetic fibers whose actions are antagonistic. If one fiber stimu¬lates an activity, the other inhibits it. However, neither kind of nerve is exclusively excitatory or inhibitory. For example, parasympathetic fibers inhibit heartbeat but excite peristaltic movements of the intestine; sympathetic fibers increase heart-beat but inhibit intestinal peristaltic movement.

The parasympathetic system consists of motor neurons, some of which emerge from the brain stem by certain cranial nerves and others of which emerge from the sacral (pelvic) region of the spinal cord. In the sym¬pathetic division nerve cell bodies of all preganglionic fibers are located in thoracic and upper lumbar areas of the spinal cord. Their fibers exit through the ventral roots of spinal nerves, sepa¬rate from these, and go to sympathetic ganglia, which are paired and form a chain on each side of the spinal column. 

All preganglionic fibers, whether sympathetic or parasym¬pathetic, release acetylcholine at their synapse with postgan¬glionic cells. However, parasympathetic postganglionic fibers release acetylcholine at their endings, whereas sympathetic post¬ganglionic fibers with few exceptions release norepinephrine (also called noradrenaline). This difference is another important characteristic distinguishing the two parts of the autonomic ner¬vous system.

A general rule the parasympathetic division is associated with nonstressful activities, such as resting, eating, digestion, and urination. The sympathetic division is active under conditions of physical or emotional stress. Under such conditions heart rate increases, blood vessels to the skeletal muscles dilate, blood ves¬sels in the viscera constricts, activity of the intestinal tract decreases, and metabolic rate increases. The importance of these responses in emergency reactions (sometimes called the fright, fight or flight response). It should be noted, however, that the sympathetic division is active also during resting conditions in maintaining normal blood pressure and body temperature.

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