Magnetic motor evoked potentials (MEPs) in masseter muscles.

Electromyographic responses of the masseter muscles and orbicularis oris muscles following transcranial magnetic stimulations were recorded with surface and needle electrodes. MEPs in masseter muscles (latency 6.9 +/- 0.71 ms, mean +/- SD) due to activation of controlateral cortico-nuclear connections were evoked by magnetic stimulations at 4 cm laterally to the vertex on the biauricular line. These MEPs were followed bilaterally by a silent period lasting about 20 ms and, less constantly, by a later silent period lasting up to 80 ms. The ipsilateral responses to the same stimuli presented shorter latencies and higher amplitudes and they were ascribed to direct stimulation of the trigeminal nerve, probably its intracisternal portion. Ipsilateral masseter "cortical" MEPs could be elicited only by a lower output setting (70% of the maximum output) of the stimulator. Orbicularis oris MEPs were polyphasic and dispersed with latencies ranging from 7 to 11 ms. In patients with hemispheric or capsular ischemic lesions "cortical" MEPs were absent when stimulating the affected hemisphere and present when stimulating the unaffected one. We suggest that the direct corticomotoneuronal projections for the masseter are mainly crossed.