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The parasympathetic nervous system (usually abbreviated PSNS, not PNS, to avoid confusion with the peripheral nervous system ()) is one of the two divisions of the autonomic nervous system, the other being the sympathetic 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. 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) also act as parasympathetic nerves. These are commonly referred to as the pelvic splanchnic 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". ==Structure== The parasympathetic nerves are autonomic or ''visceral''〔(【引用サイトリンク】title=visceral nerve fibers - definition of visceral nerve fibers in the Medical dictionary - by the Free Online Medical Dictionary, Thesaurus and Encyclopedia )〕 branches of the peripheral nervous system (PNS). Parasympathetic nervous supply arises through three primary areas: # Some of the cranial nerves in the cranium, namely the preganglionic parasympathetic nerves (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 parasympathetic nerves complete their journey to target tissues via trigeminal branches (ophthalmic nerve, maxillary nerve, mandibular nerve). # 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). It travels all the way down to the junction between the midgut and hindgut, 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 parasympathetic 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. As 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 synapse with the dendrites of a postganglionic neuron somewhere else in the body. 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. As a result, the postsynaptic parasympathetic nerve fibers are very short.〔Moore, K.L., & Agur, A.M. (2007). ''Essential Clinical Anatomy: Third Edition.'' Baltimore: Lippincott Williams & Wilkins. 42. ISBN 978-0-7817-6274-8〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Parasympathetic nervous system」の詳細全文を読む スポンサード リンク
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