Motoneuron activation by low intensity tetanic stimulation of muscle afferents in man

The tibial nerve was stimulated repetitively in normal man with electrical pulses of subliminal intensity for motor responses to single shocks. This stimulus elicited a sustained reflex activity in the soleus muscle. The activity was smoothly graded with the stimulus parameters. The latency was long and afterdischarge was regularly seen. The activity was inhibited by slight voluntary activation of the antagonistic muscles. Recordings from single motor units showed that the motoneurons did not fire with a regular time interval after the single stimulating pulses but there was a continuous discharge of about 10 impulse/sec. The results suggest that the structures and mechanisms involved in the tonic stretch reflex are essential for the motor activity described.