Neural conduction of the myo-monitor stimulus: a quantitative analysis.

With the introduction of the Myo-Monitor to dentistry, the question has arisen whether the stimulus is neurally mediated or results from direct depolarization of only the fibers of the masseter muscle. Intensity-duration curves recorded for 10 subjects quantified the relationship between stimulus intensity and the duration of the stimulus required to elicit a consistent contraction response to transcutaneous stimulation via the Myo-Monitor. Individual chronaxies ranged from 0.125 to 0.180 msec., with a mean calculated at 0.158 msec. Stimulating the muscle fibers directly, without transmission of the signal across the neuromuscular junction, would have produced chronaxy values at least 50 to 100 times greater. The distinction is clear-cut. The chronaxy values unequivocally establish transmission of the stimulus across the neuromuscular junction. In all 10 subjects, contraction of muscles remote from the site of stimulation was evident by inspection and palpation. These data lend support to the conclusion of Choi and Mitani that the Myo-Monitor stimulates the fifth and seventh cranial nerves. The data derived here correlate with those of other investigations and clearly establish that the transmission of the Myo-Monitor stimulus is accomplished by transcutaneous neural stimulation.