Peripheral nervous system

A nervous system other than the central nervous system (brain and spinal cord), which is a pathway that transmits excitation between the central organs and the center in the body. The peripheral nervous system is mainly made up of bundles of axon processes, that is, peripheral nerves (commonly known as nerves). In addition, the peripheral nervous system also includes groups of nerve cells in the body called ganglia. The peripheral nervous system is generally divided into the somatic nervous system (animal nervous system) and the autonomic nervous system (plant nervous system), and the former is further divided into the motor nervous system and the sensory nervous system. The motor nervous system transmits excitation from the center to the extremities (skeletal muscles), causing muscle movement (this function is called efferent). The sensory nervous system transmits excitation from the sensory receptors of the peripheral organs to the central organ, causing sensation (this function is called centripetal). The autonomic nervous system is distributed to autonomic (involuntary) organs such as internal organs, blood vessels, and glands, and is opposed to the somatic nervous system. The autonomic nervous system consists of the sympathetic nervous system and the parasympathetic nervous system, which work in harmony while performing opposing actions. This is called antagonistic action.

The peripheral nerves that emerge from the central nervous system gradually branch out and spread throughout the body, reaching the extremities, but depending on where they transition from the central nervous system to the peripheral nerves, they are divided into cranial nerves (12 pairs) and spinal nerves (31 pairs). A disease in which a specific peripheral nerve loses its function and the muscles it controls are unable to contract is called nerve paralysis, and pain that occurs along the pathways of a peripheral nerve is called neuralgia.

[Kazuyo Shimai]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

中枢神経系（脳と脊髄(せきずい)）以外の神経系で、体内の末端器官と中枢との間で興奮を伝達する経路。末梢神経系の主体をなすものは軸索突起の束、すなわち末梢神経（いわゆる神経）とよばれるものである。このほか、神経節とよばれる体内の神経細胞集団も末梢神経系に属する。末梢神経系は、一般に体性神経系（動物神経系）と自律神経系（植物神経系）とに分けられ、前者はさらに運動神経系と知覚神経系に分けられる。運動神経系は興奮を中枢から末端（骨格筋）へ伝え、筋の運動をおこさせる（この働きを遠心性とよぶ）。知覚神経系は興奮を末端器官の感覚受容装置から中枢へ伝え、感覚をおこさせる（この働きを求心性とよぶ）。自律神経系は内臓器官、血管、腺(せん)などの自律性（不随意性）器官に分布していて、体性神経系と対立するものである。自律神経系には交感神経系と副交感神経系とがあり、両者は相反する作用をしながら協調的に働いている。これを拮抗(きっこう)的作用という。

　中枢神経から出た末梢神経はしだいに枝分れしながら全身に広がり、末端に達するが、中枢神経から末梢神経に移行する場所によって、末梢神経を脳神経（12対）と脊髄神経（31対）に分ける。特定の末梢神経がその働きを失い、支配する筋肉が収縮しなくなる疾患を神経麻痺(まひ)といい、末梢神経の経路に沿っておこる痛みを神経痛とよんでいる。

［嶋井和世］

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