The effect of a preceding stimulus on temporal facilitation at corticomotoneuronal synapses

1. Intracellular recordings were made of minimal corticomotoneuronal e.p.s.p.s in lumbar motoneurones of anaesthetized monkeys. For intervals of 2 msec and greater between paired cortical shocks, the average time course of facilitation of the second e.p.s.p. with respect to the first could be fitted closely by a negative exponential with a time constant of 10 msec.2. In the same motoneurones, ;triplets' of corticomotoneuronal e.p.s.p.s were generated by delivering three identical stimuli to the motor cortex. Considering the triplet as a conditioning e.p.s.p. followed by a test pair, the facilitation of the third e.p.s.p. with respect to the second was measured for various combinations of test and conditioning intervals. In each case the amplitude of the third e.p.s.p. was also compared with that of the first (conditioning) e.p.s.p.3. The effect of a brief conditioning interval was to reduce considerably the facilitation of the third e.p.s.p. with respect to the second at all test intervals from 2 to 50 msec. Combinations of brief conditioning intervals (e.g. 2 or 5 msec) and long test intervals (e.g. 20 or 50 msec) caused the third e.p.s.p. to be smaller than the second. As the conditioning interval lengthened, facilitation in the test pair increased towards the unconditioned values at all test intervals.4. Facilitation of the third e.p.s.p. with respect to the first could be described approximately as the linear addition of two facilitation components, one due to the conditioning input and one due to the first stimulus of the test pair. Each component followed the same negative exponential time course as found for an isolated pair of e.p.s.p.s and each of the first two inputs contributed to the facilitation of the third e.p.s.p. as if the other of these two inputs had not occurred.