Silent periods in single orbicularis oculi motoneurones

The activity of single motor units (MUs) of the orbicularis oculi muscle was recorded in normal volunteers. While the subject maintained a constant firing rate, electrical stimuli were applied to the ipsilateral supraorbital nerve. Peristimulus histograms were constructed showing 2 periods of discharge accumulation at times corresponding to the R1 and R2 component of the blink reflex, each followed by a silent period, called SP1 and SP2, respectively. Whereas SP1 may merely result from the synchronisation of discharges in the preceding R1, SP2 can only be explained by an inhibition of the motoneurone activity. These findings confirm the existence of silent periods in the orbicularis oculi muscle in spite of the absence of mechanisms probably responsible for silent periods in load-bearing muscles.