Modulation of cutaneous flexor responses induced in man by vibration-elicited proprioceptive or exteroceptive inputs

The effects of muscle tendon or skin vibration on the early and late components of polyphasic cutaneous responses elicited in the flexor carpi radialis by electrical stimulation of the radial nerve at the wrist were studied in the human, with all muscles at rest. Both early and late flexor responses were enhanced by flexor vibration and depressed by extensor vibration; facilitation was accompanied by a reduction of latency. Furthermore, when an "antagonist vibration response" was present, inhibition of the flexor reflexes was replaced by a facilitation. Palm skin vibration depressed both components of the flexor reflex, while dorsal or "back-hand" skin vibration induced either a facilitation or an inhibition. In addition, back-hand vibration modified the location of the sensations evoked by electrical stimulation of the nerve. In all cases, vibratory stimulus attenuated the perceived intensity of the electrical stimulus. These observations indicate that proprioceptive or exteroceptive information can modulate the gain of the cutaneous reflex loops in a flexible way, under supraspinal control. These data also suggest a possible impairment of the protective withdrawal reflex under vibratory environmental conditions at rest and eventually in active muscles.