Change in short-latency response to limb displacement in primates.

When a muscle is stretched in an awake, behaving primate the earliest electromyographic (EMG) responses of the muscle consists of several reflex components. The earliest, M1, is mediated entirely at the segmental level and is largely monosynaptic. The next M2, is at least in part the result of an oligosynaptic transcortical loop to control the M1 response remains to be established. If primates can learn to control M1 amplitude, the M1 loop, which is the simplest and most accessible stimulus-response pathway in the primate central nervous system, may serve as a model for the study of mechanisms of memory. Preliminary evidence presented here strongly suggests that primates can control M1 gain without change in prestimulus EMG activity. The mechanisms of such change remain to be determined. Possibilities include change in gamma motor neuron tone, in Ia synaptic function, and in alpha motor neuron recruitment. The usefulness of learned M1 change as a model for memory will depend to a large extent on the nature of the responsible mechanism and on demonstration that the change can eventually become resident at the segmental level, without need for continued descending input.