Intro | Spatial Summation | Temporal Summation
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Spatial summation occurs when subthreshold EPSPs, occurring simultaneously at different points along the postsynaptic membrane, combine to cause a depolarization that reaches the threshold of excitation. For example, two EPSPs that depolarize the membrane close to the axon hillock by -2.5 millivolts may summate to bring the resting membrane potential of -70 millivolts to the neuron's threshold of excitation at -65 millivolts. Conversely, EPSPs and IPSPs may cancel each other out resulting in no triggered action potential. One EPSP depolarizes the membrane by -2.5 millivolts, while one IPSP hyperpolarizes the membrane by +2.5 millivolts. The threshold of excitation is not reached and no action potential is triggered.