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George E. Carvell, PhD, PT   University of Pittsburgh
Fig 5.17
 p. 121
LEGEND: 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.” BC&P p 121
EPSPs are small depolarizing potentials that individually are inadequate to depolarize the CNS neuron’s membrane to threshold for generation of an AP (Fig 5.17a); many EPSPs must sum to create an AP. Summation may occur within both a spatial domain (Fig 5.17b) and a temporal domain (Fig 5.17c). In reality summation is almost always a combination: a spatiotemporal summation of EPSPs arriving at the cell at about the same time. In fact, the most effective excitatory drive is one where there is a synchronous convergent input from one or more input sources. Therefore, network properties that tend to synchronize the activity of groups of neurons will enhance the integrative nature of the neurons participating in the neural processing (integrate & fire) and provide additional information (correlated discharge).
a = Unitary EPSP (rarely occurs in intact brain)
c = Temporal Summation of EPSPs
b = Spatial Summation of EPSPs