Motor-evoked potentials in masseter muscle by electrical and magnetic stimulation in intact alert man.

The electromyographic responses of the masseter after different types of transcranial stimulation were recorded with surface and needle electrodes. Magnetic stimulation at 4 cm lateral to the vertex on the biauricular line elicited MEPs in the contralateral masseter (latency 6.9 ms) due to activation of motor cortex or adjacent elements along the cortico-nuclear pathway. The ipsilateral responses to the same stimuli and to more lateral ones had shorter latencies and were ascribed to direct stimulation of the trigeminal nerve, probably its intracisternal portion. This was also the probable origin of the ipsilateral MEPs after both anodic and cathodic bipolar electrical stimulation at 7 and 11 cm lateral to the vertex on the biauricular line.