impulse defined in 1951 yearimpulse - impulse;
impulse - The 'message' which is conducted along a nerve-fibre. It is fundamentally the same in all nerve-fibres. It is a travelling wave of chemical and physical events involving particularly the surface membrane of the fibre. It moves at between i and 100 metres per second (speed depending on species of animal and on the diameter of, and amount of myelin around, the nerve-fibre; and on temperature and other conditions at the time). The energy for the impulse is provided locally along the course of the nerve-fibre, and not by the stimulus which sets the impulse going. Consequently the characteristics of the impulse (action potential, speed, etc.) are unaffected by the nature or intensity of the stimulus. See also: All-or-None Law. The impulse runs without loss of vigour; and wherever in the nerve cell it starts, it travels right through all the branches of that nerve-cell (See also: Nerve-Net). Each impulse occupies up to an inch or two of the length of the nerve-fibre at any instant. When it has passed a given place in the nerve, that place is, for a few thousandths of a second, refractory, i.e. it will not transmit another impulse. A succession of impulses in one nerve-fibre must therefore be spaced out, never being closer than the refractory zone. Most stimuli, including stimulation via a receptor, set off a train of successive impulses in a nerve-cell, rather than a single impulse; and different stimuli can therefore produce different conduction in a nerve-fibre by affecting not the individual impulse, but the number and frequency of successive impulses in a train of impulses.
near impulse in Knolik
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