There are two motors that are part of the oculomotor nerve known as the somatic motor and visceral motor. The short ciliary nerves innervate the orbit to control the ciliary muscle (responsible for accommodation) and the iris sphincter muscle, which is responsible for miosis or constriction of the pupil (in response to light or accommodation). From the ciliary ganglion the postganglionic parasympathetic fibers leave via short ciliary nerve fibers, a continuation of the nasociliary nerve (a branch of ophthalmic division of the trigeminal nerve (CN V 1)). The oculomotor PNS fibers originate in the Edinger-Westphal nucleus in the central nervous system and travel through the superior orbital fissure to synapse in the ciliary ganglion located just behind the orbit (eye). The oculomotor nerve is responsible for a number of parasympathetic functions related to the eye. As a result, the postsynaptic parasympathetic nerve fibers are very short. The axons of presynaptic parasympathetic neurons are usually long, extending from the CNS into a ganglion that is either very close to or embedded in their target organ. Its cell body sits in the central nervous system and its axon usually extends to synapse with the dendrites of a postganglionic neuron somewhere else in the body. The first neuron in this pathway is referred to as the preganglionic or presynaptic neuron. This differs from the sympathetic nervous system, where synapses between pre- and post-ganglionic efferent nerves in general occur at ganglia that are farther away from the target organ.Īs in the sympathetic nervous system, efferent parasympathetic nerve signals are carried from the central nervous system to their targets by a system of two neurons. The parasympathetic ganglion where these preganglionic neurons synapse will be close to the organ of innervation. Their axons continue away from the CNS to synapse at an autonomic ganglion. The pelvic splanchnic efferent preganglionic nerve cell bodies reside in the lateral gray horn of the spinal cord at the T12–L1 vertebral levels (the spinal cord terminates at the L1–L2 vertebrae with the conus medullaris), and their axons exit the vertebral column as S2–S4 spinal nerves through the sacral foramina.The vagus innervation ends at the junction between the midgut and hindgut, just before the splenic flexure of the transverse colon. The vagus nerve does not participate in these cranial ganglia as most of its parasympathetic fibers are destined for a broad array of ganglia on or near thoracic viscera ( esophagus, trachea, heart, lungs) and abdominal viscera ( stomach, pancreas, liver, kidneys, small intestine, and about half of the large intestine).From these four ganglia the parasympathetic nerves complete their journey to target tissues via trigeminal branches ( ophthalmic nerve, maxillary nerve, mandibular nerve). Certain cranial nerves in the cranium, namely the preganglionic parasympathetic nerves (CN III, CN VII, CN IX and CN X) usually arise from specific nuclei in the central nervous system (CNS) and synapse at one of four parasympathetic ganglia: ciliary, pterygopalatine, otic, or submandibular.Parasympathetic nerve supply arises through three primary areas: The parasympathetic nerves are autonomic or visceral branches of the peripheral nervous system (PNS). 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".
Three spinal nerves in the sacrum (S2–4), commonly referred to as the pelvic splanchnic nerves, also act as parasympathetic nerves. Specific nerves include several cranial nerves, specifically the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve. Nerve fibres of the parasympathetic nervous system arise from the central nervous system. 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. 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. The autonomic nervous system is responsible for regulating the body's unconscious actions.
The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent 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.
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