Parasympathetic nervous system

The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system.

The autonomic nervous system is responsible for regulating the body's unconscious actions. The parasympathetic system is responsible for stimulation of "rest-and-digest" or "feed-and-breed" activities that occur when the body is at rest, especially after eating, including sexual arousal, salivation, lacrimation (tears), urination, digestion, and defecation. Its action is described as being complementary to that of the sympathetic nervous system, which is responsible for stimulating activities associated with the fight-or-flight response. In contrast to the sympathetic division, which predominates during acute stress responses, parasympathetic activity promotes energy conservation, metabolic storage (for example by enhancing glycogen synthesis), and the maintenance of internal homeostasis over longer time scales.

Beyond its role in visceral motor control, parasympathetic activity—particularly via the vagus nerve—forms part of an "inflammatory reflex" in which efferent vagal fibers, acting through cholinergic signaling, dampen the release of pro-inflammatory cytokines and thereby modulate innate immune responses. Non-invasive indices such as heart rate variability (HRV), especially measures that reflect vagal modulation of the sinoatrial node, are widely used as surrogate markers of cardiac parasympathetic tone in research and clinical settings.

Nerve fibres of the parasympathetic nervous system arise from the central nervous system. Specific nerves include several cranial nerves, specifically the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve. Three spinal nerves in the sacrum (S2–4), commonly referred to as the pelvic splanchnic nerves, also act as parasympathetic nerves.

Owing to its location, the parasympathetic system is commonly referred to as having "craniosacral outflow", which stands in contrast to the sympathetic nervous system, which is said to have "thoracolumbar outflow".