1. (in neurophysiology) The combined effect of the changes in electric potential elicited in one or more postsynaptic membranes by the transmission of impulses at synapses or neuromuscular junctions. If the excitatory postsynaptic potential is sufficiently large, it will trigger an action potential in the postsynaptic neuron or muscle fibre. Summation occurs when one or a few postsynaptic potentials alone are insufficient to elicit a response in the postsynaptic cell; it may consist of the effect of two or more potentials evoked simultaneously at different synapses on the same postsynaptic cell (spatial summation) or in rapid succession at the same synapse (temporal summation). Inhibitory postsynaptic potentials also exhibit summation, causing hyperpolarization of the recipient cell and making it less likely to produce an action potential in response to excitatory input.