autonomic nervous system
autonomic nervous system defined in 1951 yearautonomic nervous system - autonomic nervous system;
autonomic nervous system - (1) Of vertebrates, motor nerve supply to smooth muscles (e.g. in gut, blood-vessels, etc.) and glands. A characteristic feature is that it does not involve single nerve fibres running all the way from central nervous system to effector (as with supply to striped muscles); but the nerve fibres (preganglionic) which leave C.N.S. stop short of effectors, and form synapses with a second group of nerve cells whose nerve-fibres (postganglionic), which are mainly unmyelinated, then continue to effectors. The cell-bodies of the postganglionic fibres, and the synapses, occur in ganglia or in the organ where the effectors are. The system is subdivided into sympathetic (or orthosympathetic) system, and parasympathetic system. In mammals and birds (other vertebrates are less well known) preganglionic fibres of sympathetic system leave the spinal cord (in thoracic and lumbar region) through ventral roots, and go to chains of sympathetic ganglia, one on each side just ventral to vertebral column, where they meet postganglionic nerve-cells, or pass through the paired ganglia and meet them in another, median, row of ganglia (e.g. the solar plexus). From all these ganglia postganglionic fibres go to effectors. Preganglionic fibres of parasympathetic system on the other hand leave C.N.S. (a) through cranial nerves, especially vagus, (b) through ventral roots of hind end of spinal cord (sacral region), and the postganglionic cell-bodies are scattered amongst the effectors or are in ganglia very close to the effectors. The sympathetic system is the larger of the two; it alone supplies skin and limbs. Many internal organs receive nerve-fibres from both systems, and in such cases they may act antagonistically; e.g. smooth musculature of gut is stimulated to peristalsis by parasympathetic supply (mostly from vagus), inhibited by sympathetic supply. Parasympathetic and preganglionic sympathetic fibres are cholinergic; postganglionic sympathetic fibres often adrenergic. sometimes (e.g. those to sweat glands) cholinergic. Much of the co-ordination of activities of the auto-nomic system occurs in spinal cord, medulla, and hypothalamus, (2) The motor system as described under (1), together with the sensory fibres from internal receptors which are concerned in reflex activities of this motor system.
near autonomic nervous system in Knolik
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