EVOKED EMG of THE MASTICATORY MUSCLES by TRANSCRANIAL MAGNETIC STIMULATION IN MAN

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Using a transcranial magnetic stimulation, peripherally or centrally induced responses of the masseter muscle were electromyographically analized in man.By positioning the center of a stimulating coil at 9cm far inferolaterally from the vertex along a contour of the skull, four responses were induced from the ipsilateral masseter muscle. Other two responses were recorded from the contralatera masseter muscle by positioning the coil center at 3-5.5cm laterally and 2.5cm anteroirly from the vertex.latencies of the ipsilateral responses were 6msec, 10-13msec and 2-3msec in the order of threshold level. These responses were thought to be M-wave, a wave corresponding to S-wave in the extremity muscles, one drived from the cortex, and H-wave, respectively.Latencies of the contralateral responses were 11-13msec and 23msec. The former is the most likely to be I-wave (Amassian : 1987) drived from the cortex, or D-wave (Edyly : 1990).When the stimulus was applied during a gum-chewing without any expectations, a cycle time of the stroke was remarkably prolonged, and the rhythm of following 2-3 strokes became unstable. This is thought to e an effect of the disturbance on a rhythm generator due to cortical stimulation.By magnetic stimulation, an incomplete depression phase appeared on the masseter EMG with a latency of approximately 80msec. This long-latency depression might related to the cortex in the orign.