summation



summation defined in 1951 year

summation - summation;
summation - Generally, additive effect of separate stimuli. In neurophysiology, additive effect of separate impulses arriving at a nerve-cell or effector-cell. As a very simplified example, suppose that nerve-cell A has synapses with the nerve-fibres of nerve-cells B and G. Impulses arriving in B alone or in G alone fail to stimulate the production of an impulse in A. But if impulses arrive, simultaneously or very shortly after each other in both B and G, their joint action produces an impulse in A. The separation of the summating impulses may be spatial, that is, the impulses arrive at different synapses on the same nerve-cell; or temporal, that is, they arrive successively at the same cell; or both. Many impulses may be involved in any one process of summation. Temporal summation at a single synapse probably does not occur in vertebrates owing to the short duration of effect of each impulse which fades before the refractory period (See also: Impulse) will allow the next impulse to arrive. Besides summation which produces impulses (i.e. is excitatory), summation of inhibition also occurs. In general, summation is one of the main ways in which impulses can interact, so that nerve-cells in co-operation can produce effects quite different from the sum of those due to the nerve-cells singly. The result is the exquisite gradation of behaviour corresponding to the immense variety of the stimulus situations to which an animal reacts.

near summation in Knolik


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letter "S"
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superior ovary

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