End plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions blocked with high Mg and/or low Ca to characterize the processes underlying increased transmitter release during repetitive stimulation. There was a progressive increase in the amplitude of successive e.p.p.s during repetitive stimulation. Increasing the frequency or duration of stimulation increased this facilitation of e.p.p. amplitudes. Facilitation is defined as the fractional increase in amplitude of a test e.p.p. over a control.
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