Firing of motor units upon flexion reflex in dogs

A study is made of the changes in the transient processes of the instantaneous frequency of the discharges of preactive motor units (MU) in dog m. gastrocnemius and m. semitendinosus under conditions of flexion reflex caused by electrical stimulation. Surface electrodes are used to record the activity of the single MUs. Five types of transient processes are described mainly depending on the intensity of stimulation. A stimulus with weaker intensity leads to an increase in the instantaneous frequency of the discharges 60 ms after its application, followed by a decrease and later by a new increase. Inhibition phenomena are observed in cases of higher intensity. The interpretation of these data takes into consideration the mechanisms of recurrent inhibition and the role of some of the afferent inputs of the flexion reflex.