FACILITATION AND INHIBITION OF SPINAL MOTONEURONS

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Abstract
Facilitation and inhibition, by the direct actions of primary afferent fibers, of two-neuron-arc reflexes have been examined. Facilitation is maximal with synchronous convergence of conditioning and test volleys; decays exponentially along a curve falling to 1/e in 4.0 msec. Inhibition displays an incremental phase of 0.5 msec. duration, thereafter decaying in the same manner as facilitation. The facilitation period here described is regarded as a process additional to detonator action and is called "residual facilitation". The assumption of 2 excitatory events at synapses accords with known elementary processes in peripheral nerve. Residual facilitation and inhibition are regarded as functional opposites being similar in known characteristics excepting direction. There is no known inhibitory counterpart of detonator action. Reason is given for supposing that the brief facilitation periods evident in earlier experiments do indeed measure the effective duration of the detonator process.