Figure 5.17  EPSP Summation.  (a) A presynaptic action potential triggers a small EPSP in a postsynaptic neuron.  (b) Spatial summation of EPSPs: when two or more presynaptic inputs are active at the same time, their individual EPSPs add together. (c)  Temporal summation of EPSPs: when the same presynaptic fiber fires action potentials in quick succession, the individual EPSPs add together.