Modulation of the functional stretch reflex by the segmental reflex pathway.

Electromyographic (EMG) reflex responses were examined in the biceps muscle of awake Cebus monkeys trained to resist perturbations of a handle with their forearm. In particular responses at latencies of 15-20 msec (M1) and 40-55 msec (M2), thought to correspond to segmental and suprasegmental reflex pathways respectively, were studied. The experiments demonstrated that the magnitude of the M1 response was large, as compared to M2, only when the muscle was tonically active and small perturbations were applied. For larger perturbations the magnitude of M1 saturated and the M2 response became functionally significant, its magnitude being directly related to the magnitude of the perturbation. By means of delayed reductions in torque, the magnitude of this M2 response was also shown to be very sensitive to changes in facilitatory drive provided by segmental pathways.