A silent period in orbicularis oculi muscles of humans.

Surface electromyographic activity was recorded bilaterally from orbicularis oculi muscles when subjects relaxed and contracted eyelid muscles. Cutaneous reflex responses were evoked during both the relaxed and contraction states. Following reflex elicitation periods of muscle silence in orbicularis oculi were observed for about 10 to 15 ms after the ipsilateral R1 response and for up to 100 ms after the bilateral R2 responses. Reflex responses appeared to be enhanced when elicited during contractions. Possible physiological mechanisms are discussed regarding the presence of silent periods in a motor system that is presumably devoid of spindles, Golgi tendon organs, and Renshaw-like interneurons.