summation [L, summa, total] , 1. a cumulative effect or action; a total aggregate; totality. 2. (in neurology) the concentration of a neurotransmitter at a synapse, either by increasing the frequency of nerve impulses in each fiber (temporal summation) or by increasing the number of fibers stimulated (spatial summation), so that the threshold of the postsynaptic neuron is overcome and an impulse is transmitted. See also facilitation, def. 2.
