Spatial Summation:
The summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time. Example: the buildup of a NT released simultaneously by several presynaptic end bulbs.
Temporal Summation:
The summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times. Example: the buildup of a NT released by a single presynaptic end bulb two or more times in rapid succession. Most of the time, both spatial and temporal summation are working together to ensure that a neuron fires an AP.
A single postsynaptic neuron receive input from many presynaptic neurons, some releasing excitatory NTs and some releasing inhibitory NTs. The sum of all excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neuron which may respond in the following ways: EPSP – If the total excitatory effects are greater than the total inhibitory effects but less than threshold, the result is an EPSP that does not reach threshold. Following an EPSP, a subsequent stimulus can more easily generate a nerve impulse through summation because the neuron is already partially depolarized. Action Potential – If the total excitatory effects are greater than the total inhibitory and threshold is reached, one or more APs are triggered. APs will continue as long as the EPSPs are at or above threshold.
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