RESPONSES OF MOTOR CORTICAL-CELLS TO SHORT TRAINS OF VIBRATION

The response discharges of precentral motor cortical cells to brief tr ains of vibration applied to the tendon of biceps brachii were analyze d in two alert but passive monkeys. The activity of 20 phasic-tonic an d 6 tonic cells was analyzed. All had functional linkages with flexor muscles during a preceding flexion task and responded to passive movem ent of the elbow. Taking as a reference the stereotyped reflex respons e in the stretched muscle, the effect of changes in the amplitude of a constant frequency vibration (4 vibrations at 58 Hz) was quantified s tatistically in peristimulus histograms of the cortical cell discharge s. All cells were transiently influenced by low vibration amplitudes. Most responses (71%) were excitatory and occurred at a mean latency of 24 ms. which is consistent with cells activated by input from stretch receptors. Excitatory, reproducible responses to the lowest vibration amplitudes were more frequent in phasic-tonic than in pure tonic cell s. Large-amplitude vibrations always excited the motor cortical cells. The sign of the responses to vibration matched that to passive elbow movements for most cells. These findings show that elbow-related motor cortical cells are very sensitive to proprioceptive input from primar y spindle afferents.