Motor unit responses in muscles stretched by imposed displacements of the monkey wrist

Imposed angular displacements of monkey's wrist produce three major peaks of activity (terms M1, M2, and M3 peaks) in the averaged gross EMG activity ot the stretched muscles. The displacements were imposed on monkey's wrist by computer controlled step loads (range 60--540 g). Gross EMG was recorded simultaneously with the activity of single motor units (SMUs) in the stretched muscles. SMUs were identified and separated using a computer "shape-fitting" program. Average response histograms (ARHs) were constructed for each SMU's responses to randomly presented repetitions of up to five different step loads. Eighty-three percent of the SMUs showed a single excitatory response peak that was limited to a time interval corresponding to that of only one of the gross EMG peaks. Despite marked differences in the time courses of the imposed displacements, the time courses of the SMU excitatory responses were maintained. By increasing the background load and hence the tonic firing of the individual SMUs, the histograms show that the long latency of the M2 peak is not due to an inhibitory-excitatory sequence. The SMUs responding at longer latencies (M2 and M3 SMUs) show firing characteristics in keeping with those reported for fast twitch motor units while those of M1 SMUs correlate with those found for slow twitch units. These results establish that the M1, M2, and M3 peaks do not merely represent synchronized oscillatory activity of the motoneurons but largely result from the reflex excitation of separately responding motoneuron "subpopulations", each of which contributes to the generation of only one of the gross EMG peaks.