Magnetic stimulation of corticonuclear systems and of cranial nerves in man: physiological basis and clinical application.

After transcranial magnetic brain stimulation, two types of responses in muscles supplied by cranial nerves (trigeminal, facial, accessory, and hypoglosseal nerves) can be observed. With a placement of the coil center approximately 6 cm lateral of the vertex on the interaural line, purely ipsilateral responses can be evoked in the cranial muscles which are induced by excitation of the nerves at or near their intracisternal course ('short-latency' responses). With the coil center 4 cm lateral of the vertex bilaterally 'long-latency' responses can be evoked which are the result of an excitation of that part of the motor cortex which mediates impulses to the motoneurons of cranial nerves via the corticonuclear tract. Evaluations of latencies and amplitudes of the responses in cranial muscles, and of the excitability of their supplying nerves at a proximal site considerably improve the electrophysiological assessment of the site, severity, and prognosis of cranial motor disturbances, especially in facial nerve palsies.