A division of the (vertebrate) autonomic nervous system whose physiological effects are mostly in opposition to those of the sympathetic nervous system; e.g. it is involved in the slowing of the heart beat and relaxing the muscles.
The parasympathetic neurons lie between the brain and the spinal cord. Most of them have cholinergic nerve endings (i.e. involving acetylcholine as neurotransmitter) and their responses are mediated by muscarinic receptors. They are known to produce responses that inhibit or oppose the actions of the sympathetic nervous system. Whereas the sympathetic nervous system is responsible for the fight-or-flight responses the parasympathetic nervous system is mainly involved in responses or effects found during rest, sleep and digestion. The system innervates the salivary glands, thoracic and abdominal viscera, bladder and genitalia where the physiological effects produced are involuntary, such as stimulating digestive secretions, slowing heart rate, constricting the pupils, dilating the blood vessels, etc.
Compare: sympathetic nervous system
See also: autonomic nervous system