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The sequence of events from axon terminal depolarization to integration of information by summation of separate synaptic effects is described. Following depolarization of the terminal, transmitter substance is released probabilistically in integral multiples of a basic quantity, the transmitter unit. The average rate of unit release depends upon membrane potential, ion concentrations (particularly calcium) in the bathing medium, the quantity of transmitter available for release, and the history of synaptic use. After diffusing across the synaptic cleft, transmitter molecules interact with receptor sites on the postsynaptic membrane and cause the conductance for certain ions to increase; meanwhile, the transmitter is destroyed enzymatically or lost from the vicinity of the postsynaptic membrane by diffusion. The effects from the simultaneous action of many synapses add--often linearly--to permit information from many sources to be combined in a single neuron.