Facilitation of transmitter secretion from toad motor nerve terminals during brief trains of action potentials.

1. End-plate potentials produced by brief trains of action potentials (5-7 at 50-100 Hz) were recorded at toad sciatic-sartorius neuromuscular junctions. When transmitter secretion was depressed in solutions containing magnesium, the increase in amplitude (growth pattern) of successive end-plate-potentials was greater than could be accounted for by arithmetic summation of facilitation (arithmetic model) as proposed by Mallart & Martin (1967). 3. With e.p.p.s of normal quantral content or in solutions in which the calcium concentration was lowered, growth patterns were occasionally reasonably close to those predicted by the arithmetic model but there was always some degree of disparity. 4. A simple, two-step, kinetic model is described which is more consistent with the varied growth patterns of end-plate potentials that have been recorded. The model can predict growth patterns of e.p.p.s with high or low quantal content.