Значение термина nervous system в knolik
nervous system - nervous system
nervous system - A mechanism which co-ordinates the various activities of an animal with each other and with events in the external world, by means of messages rapidly conducted from part to part. Present in all multicellular animals except sponges. It consists of numerous cells of special kind (nerve-cells), with branching thread-like processes. Every nerve-cell is in contact with others by means of these processes, the points of contact being called synapses. The nerve-cells and their processes, linked by synapses, form a system which permeates the whole body. Some of the processes of the nerve-cells terminate at sense-organs, some at muscles or other effectors, while many end at synapses with other nerve-cells. The nervous system functions by 'messages' (impulses), which run along the nerve-cell processes and are able to cross synapses. Impulses are started by sense-organs, and they can set a muscle or other effector in operation when they reach it. But the nervous system does not simply provide a means by which the stimulation of a certain sense-organ automatically activates a certain effector. That would limit the number of possible activities to the number of direct receptor-effector connections. There usually are many fairly direct connections of this type. See also: Reflex. But the work of the nervous system co-ordinates activities in a far more subtle and complicated way, by means of the synapses. A synapse discriminates which impulses it passes through to the succeeding nerve-cell according to what impulses are arriving at other synapses involving that nerve cell. See also: Summation and Inhibition. These other synapses link up with other parts of the nervous system. Consequently, an impulse started by a sense-organ is not limited to the two possibilities of activating or failing to activate a given receptor. It can, by means of connecting synapses, promote or inhibit numerous other activities, according to the state of other parts of the nervous system. Very complicated coordination is, therefore, possible with suitably complex connections. In most animals, the elaborate co-ordination for which the synapses are an essential basis occurs in the central nervous system. Here most of the synapses are located, and the rest of the nervous system (peripheral nervous system) consists mainly of nerve-cell processes running directly to sense-organs or effectors. The nervous system is not the only co-ordinating system. There is a much more slowly acting and less flexible one based on hormones.
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