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in physiology, the cumulative effect of several stimuli following one another in rapid succession (temporal summation) or occurring simultaneously (spatial summation) and arising in such excitable structures as receptors, neurons, and muscles.
Summation was first described in 1868 by I. M. Sechenov, who observed that under certain conditions of rhythmic stimulation there was a delay in the appearance and subsequent intensification of reflexes. Temporal summation occurs in the intervals between stimuli. These intervals are limited by the period of subthreshold or trace shifts in membrane potential toward depolarization, with development of excitation, or toward hyper-polarization, with development of inhibition. Temporal summation ensures that the duration of the reaction will be sufficient and that it can be maintained by the circular relationship between the neurons.
Spatial summation changes continuously and is manifested by the simultaneous excitation or inhibition of many neurons in different parts of the brain as well as of numerous synapses in a single neuron. Summation helps to intensify individual reactions and to coordinate the body’s reactions. In muscles, spatial summation intensifies contractions by increasing the number of excited motor units, that is, groups of fibers innervated by a single neuron. Temporal summation in muscles induces tetanus by uniting successive individual contractions.
A. N. KABANOV