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( Parasympathetic nervous system) The parasympathetic nervous system (PSNS) is a division of the autonomic nervous system (ANS), along with the sympathetic nervous system (SNS) and enteric nervous system (ENS or "bowels NS"). The ANS is a subdivision of the peripheral nervous system (PNS). ANS sends fibers to three tissues cardiac muscle, smooth muscle, or glandular tissue. This stimulation, sympathetic or parasympathetic, is to control smooth muscle contraction, regulate cardiac muscle, or stimulate or inhibit glandular secretion.

The parasympathetic nerves (PSN) are visceral, autonomic branches of the peripheral nervous system (PNS). The autonomic nervous system (ANS), which includes sympathetic and parasympathetic divisions, regulates the body's visceral organs via the innervation of three kinds of tissues smooth muscle, cardiac muscle, and glands. The sympathetic and parasympathetic system work in tandem to create a synergistic stimulation that is not merely on or off, but can be described as a continuum depending upon how vigorously each division is attempting to carry out its actions. The regions of the body associated with the parasympathetic division of the ANS are in the cranial and sacral regions of the spinal cord. Because of its location the parasympathetic system is commonly referred to as having craniosacral outflow whereas the sympathetic system is referred to as thoracolumbar outflow (T1-L2 spinal nerves). In the cranium the PSN originate from cranial nerves CN III (oculomotor nerve), CN VII (facial nerve), CN IX (glossopharyngeal nerve) and CN X (vagus n.) In the sacral region of the body the PSN is derived from spinal nerves S2, S3 and S4, commonly referred to as the pelvic splanchnics.

Similar to the SN, the PSN follows a two-neuron efferent (motor signals leaving CNS) system that has pre- and post- ganglionic neurons. In the cranium, preganglionic PSN (CN III, CN VII, and CN IX) arise from specific nuclei in the CNS and synapse at one of four parasympathetic ganglion ciliary, pterygopalatine, otic, or submandibular. From these four ganglion the PSN complete their journey to target tissues via CN V (trigeminal) branches (ophthalmic nerve CN V₁, maxillary nerve CN V₂, mandibular nerve CN V₃). The vagus nerve does not participate in these cranial ganglion as most of its PSN fibers are destined for a broad array of ganglion on or near organs including the thoracic viscera (esophagus, trachea, esophagus, heart, lungs) and abdominal viscera (stomach, pancreas, liver, kidneys) traveling all the way down to the midgut/hindgut junction just before the splenic flexure of the transverse colon. The pelvic splanchnic preganglionic nerve cell bodies arise in the lateral horn of the spinal cord and continue away from the CNS to synapse at a autonomic ganglion. The PSN ganglion, where the preganglionic neurons synapse, will be close to the organ of innervation (unlike the SN where the ganglion is typically farther away from the target organ). The two neuron system is only for efferent innervation. Afferent, unconscious sensations sent from the viscera to the CNS are done so in a one neuron tract.

The afferent parasympathetic sensations are mostly unconscious visceral motor reflex sensations from hollow organs and glands that are transmitted to the CNS. Like regular somatic sensory neurons, parasympathetic afferent cell bodies are located in the dorsal root ganglion. While the unconscious reflex arcs normally are undetectable, in certain instances they may send pain sensations to the CNS masked as referred pain. If the peritoneal cavity becomes inflamed or if the bowel is suddenly distended your body will interpret the afferent pain stimulus as somatic in origin. This pain is usually non-localized. The pain is also usually referred to dermatomes that are at the same spinal nerve level as the visceral afferent synapse.

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