The parasympathetic nervous system (PSNS, or occasionally PNS) is one of the two main divisions of the autonomic nervous system (ANS) (shown in Figure 1). The autonomic nervous system (ANS, or visceral nervous system, or involuntary nervous system) is the part of the peripheral nervous system that acts as a control system, functioning largely below the level of consciousness and controlling visceral functions. The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously. Its roles include stimulation of "rest and digest" activities that occur when the body is at rest, including sexual arousal, salivation, lacrimation (tears), urination, digestion, and defecation. Its action is described as being complementary to that of one of the other main branches of the ANS, the sympathetic nervous system, which is responsible for stimulating activities associated with the "fight or flight" response.
Sympathetic and parasympathetic divisions typically function in opposition to each other. This natural opposition is better understood as complementary in nature rather than antagonistic. The sympathetic nervous system can be considered a quick response, mobilizing system; and the parasympathetic a more slowly activated dampening system. The sympathetic division typically facilitates quick responses. The parasympathetic division functions in a slower, stabler, and often inhibitory manner. A useful acronym to summarize the functions of the parasympathetic nervous system is SLUDD (salivation, lacrimation, urination, digestion, and defecation). The parasympathetic nervous system may also be known as the parasympathetic division.
The parasympathetic nervous system uses chiefly acetylcholine (ACh) as its neurotransmitter, although peptides (such as cholecystokinin) may act on the PSNS as neurotransmitters. The ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors. Most transmission occurs in two stages. When stimulated, the preganglionic nerve releases ACh at the ganglion, which acts on nicotinic receptors (Figure 2) of postganglionic neurons. The postganglionic nerve then releases ACh to stimulate the muscarinic receptors of the target organ.