RESPONSES OF HUMAN MUSCLE-SPINDLE ENDINGS TO VIBRATION DURING ISOMETRIC CONTRACTION

In human subjects, vibration of amplitude 1.5 mm and frequency 20-220 Hz was applied to the tendons of muscles in the leg to examine the effects on the discharge of primary and secondary endings during maneuvers designed to alter the level of fusimotor drive. In 4 experiments, the peroneal nerve was completely blocked with lidocaine proximal to the recording site in order to de-efferent spindle endings temporarily. The responses to muscle stretch and vibration, as seen in multi-unit recordings and in single unit recordings, were similar during the block as in the relaxed prior to the block. Thus, these experiments provided no evidence of a functionally effective resting fusimotor drive. The responses to vibration of 9 primary endings and 4 secondary endings were examined during isometric voluntary contractions of the receptor-bearing muscles. Provided that the endings were responding submaximally in the relaxed state, voluntary contraction enhanced the response to vibration, suggesting co-activation of the fusimotor system sufficient to compensate for mechanical unloading. Unloading effects were observed during contractions of neighboring synergistic muscles, indicating a close spatial relationship between the co-activated skeletomotor and fusimotor outflows. Recordings were obtained from 10 primary endings and 7 secondary endings during isometric reflex contractions resulting from the vibratory stimulus (TVR contractions). For 12 endings, the appearance of the tonic vibration reflex in the receptor-bearing muscle resulted in a significant decrease in the response to vibration, suggesting that the endings were unloaded by the extrafusal contraction. On voluntary suppression of the reflex contraction, spindle responses reverted to their previous levels. The tonic vibration reflex, like the tendon jerk reflex, may operate predominantly or exclusively on .alpha. motoneurons, and it may not utilize the same cortically originating efferent pathways as are used in the performance of voluntary contractions.