autonomic nervous system

autonomic nervous system - The part of the nervous system that controls the involuntary activities of a variety of body tissues. The autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems.

The sympathetic nervous system comprises 2 chains of nerves that pass from the spinal cord throughout the body tissues. Into these tissues, the nerve endings release the neurotransmitters adrenaline (epinephrine) and noradrenaline (norepinephrine).

The system also stimulates adrenaline release from the adrenal glands. In general, the actions of the sympathetic nervous system heighten activity in the body, quickening the heartbeat and breathing rate, widening blood vessels, and inducing sweating.

The parasympathetic nervous system is composed of a chain of nerves that passes from the brain and another that leaves the lower spinal cord. The nerves are distributed to the same tissues that are supplied by the sympathetic nerves.

The parasympathetic nerves release the neurotransmitter acetylcholine, which has the opposite effect to adrenaline and noradrenaline. The parasympathetic system is mainly concerned with everyday functions such as digestion and excretion.

The 2 systems act in conjunction and normally balance each other. During exercise or at times of stress, the sympathetic system predominates, however, while during sleep the parasympathetic system exerts more control.