Modulation of the Electrically Evoked Blink Reflex by Different Levels of Tonic Preinnervation of the Orbicularis Oculi Muscle

Abstract

Eleven subjects were instructed to maintain different levels of tonic pre-innervation of the two orbicularis oculi muscles: relaxation, 25%, 50%, 75% and 100% of maximum voluntary contraction. Surface electromyographic activity was recorded unilaterally from the left orbicularis oculi muscle while cutaneous reflex responses were evoked electrically during different levels of pre-innervation. Latencies of the early (R1) and late (R2) blink reflex responses were significantly shorter during moderate pre-innervation level (25%) compared to relaxation, with no further latency decrease under stronger tonic pre-innervation. The amplitude of the R1 component increased significantly until a pre-innervation level of 50% of maximum voluntary contraction was reached. There was no further amplitude enhancement with stronger pre-innervation. R2 amplitudes linearly increased with the increasing pre-innervation levels, possibly reflecting recruitment of a greater number of motor units for the late component or a postponement of motoneuron synchronization of the late component.

Key Words:

• eye blink reflex
• pre-innervation
• reflex components
• M. orbicularis oculi
• EMG