Parasympathetic nerves



The parasympathetic nerves (PSN) are autonomic (aka "visceral" ) branches of the peripheral nervous system (PNS). Parasympathetic nerve fibers arise from the central nervous system with the S2, S3, and S4 spinal nerves and from the third, seventh, ninth, and tenth cranial nerves. Because of 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".

The parasympathetic nerves that arise from the S2, S3, and S4 spinal nerves are commonly referred to as the pelvic splanchnic nerves or the "nervi erigentes".

Pathways
As is true 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 first neuron in this pathway is referred to as the preganglionic or presynaptic neuron. Its cell body sits in the central nervous system and its axon usually extends to a ganglion somewhere else in the body where it synapses with the dendrites of the second neuron in the chain. This second neuron is referred to as the postganglionic or postsynaptic neuron.

The axons of presynaptic parasympathetic neurons are usually long: They extend from the CNS into a ganglion that is either very close to or embedded in their target organ. As a result, the postsynaptic parasympathetic nerve fibers are very short.

In the cranium, preganglionic PSN (CN III, CN VII, and CN IX) usually arise from specific nuclei in the Central Nervous System (CNS) and synapse at one of four parasympathetic ganglia: ciliary, pterygopalatine, otic, or submandibular. From these four ganglia 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 (CN X) does not participate in these cranial ganglia as most of its PSN 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). It travels all the way down to the midgut/hindgut junction, which occurs just before the splenic flexure of the transverse colon.

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. Their axons continue away from the CNS to synapse at an autonomic ganglion. The PSN ganglion where these preganglionic neurons synapse will be close to the organ of innervation. 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.