End plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions blocked with high Mg and/or low Ca. Estimates of f(t), the facilitation contributed by each impulse, were obtained during and following repetitive stimulation from the incremental change in e.p.p. amplitudes following step changes in the stimulation rate during the conditioning and testing trains. Estimates of f(t) increased during the conditioning stimulation and returned to control in the posttetanic period. This increase in f(t) was proportional to the magnitude of tetanic or posttetanic potentiation (PTP) present. PTP was present immediately following the conditioning train. However, if depression occurred during the conditioning train, PTP developed after a delay. It is suggested that facilitation and potentiation represent increases in two independent factors which act jointly to increase the probability of transmitter release.
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