Changes in membrane potentials of the presynaptic terminal affect the amount of transmitter released as assayed by measuring theamplitude of the postsynatpic potential.

When the membrane potential of the presynaptic terminal is at the normal resting potential, an action potential produces a postsynaptic potential of about 8 mV.

When it is hyperpolarized by 10 mV, the steady-state (leakage) Ca++ influx is decreased and the presynaptic spike produces apostsynaptic potential of only 5 mV.

When the presynaptic neuron is returned to the resting level and then depolarized by 10 mV, the steady-state Ca++ influx is increased and the resulting presunaptic action potential produces a synaptic potential of 15 mV, which triggers an action potential in the postsynaptic cell.