Single motor unit and surface electromyogram analysis of human jaw-closing muscle reflexes after tapping an upper tooth.

The mechanical stimulation of an upper tooth elicited reflex responses in masseter and temporalis motor units, which were recorded with both surface and needle electromyograms (EMGs) simultaneously. The subjects maintained one of the recorded motor units, which was the latest recruited unit, at a constant firing frequency, varying from 12 to 25 Hz. After a latency of 10 ms, all 19 motor units were inhibited for a period, the duration of which depended on the prestimulus firing frequency. A motor unit with a low firing frequency was inhibited for a longer time than a faster firing one. At the end of this inhibition there was an increased probability of firing recorded, in the form of a time-locked clustering of action potentials. Furthermore, in three motor units, all firing at a frequency of about 25 Hz, the first interspike interval after the inhibition was regularly half the duration of the mean prestimulus interspike interval. The timing of the last action potential before the stimulus influenced significantly the reflex responses in all motor units.