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

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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.

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