中枢自律神経系の神経伝達物質

Neurotransmitters in the central autonomic nervous system.

はじめに

　自律神経系は末梢神経系が基本である。その伝達物質も明解に論ずることができる。その上に乗る中枢となると，構造的には大脳皮質から脊髄に至るまで，機能的には高度な精神活動からすべてにわたって関係している。したがって，伝達物質を論ずるために，中枢自律神経系とはどのようなものであるかを単純化しておきたい。さらにそれに関わる伝達関連物質についても，中枢自律神経伝導路ニューロンに含まれるそれではなく，調節系ニューロンにならざるをえない理由も述べなくてはならない。そのうえで，いわゆる中枢自律神経系の伝達物質を論ずることにする。

The modulatory neurons that may control the activity of the central autonomic nervous system were described with special reference to their transmitters. The reason is that transmitters are still poorly defined in the central autonomic impulse-conducting neuronal chains, and that the modulatory neuronal mechanisms in the central nervous system seem to underlie the autonomic function to main-tain the homeostatic state of the organism.

The so-called central autonomic nervous system could be roughly divided into two main parts: the lower executive system situated in the lower brain system and spinal cord and the higher control system of the hypothalamus. The lower system includes the regional information handling system adding to the preganglionic output neurons. The hypothalamic system, whose function is significantly influenced by the communication with the limbic system and the cerebral cortex as well as the cerebellum, may be composed of neuronal circuits related to the instinct behavior, to the emotional display and to the general autonomic function, each of which may be overlapped by using common neurons. From this higher control system the information descends via the mesencephalon to the lower executive system. The predominance of final output, sympathetic or parasympathetic, would be dependent on the balanc-ing of the descending influences. Although little is known of neurotransmitters contained in the higher and lower neuronal circuits as well as in the descending pathways except for acetylcholine in the final executive neurons, it is easily supposed that, even if they were known, there should be no transmitter specific for either sympathetic or parasympathetic function in the central nervous system. Meanwhile, most of the neurons that have been documented to contain various amines, peptides or acetylcholine and to be situated in the regions of the higher or lower autonomic center or in the lower brain stem, may not be constituents of the main neuronal chains of the central autonomic nervous system, but they may be of modulatory function to the activity of these circuits.