FACILITATION OF MUSCLE EVOKED-RESPONSES AFTER REPETITIVE CORTICAL STIMULATION IN MAN

The technique of repetitive transcranial magnetic stimulation (rTMS) a llows cortical motor areas to be activated by trains of magnetic stimu li at different frequencies and intensities. In this paper, we studied longterm neurophysiological effects of rTMS delivered to the motor co rtex at 5 Hz with an intensity of 120% of motor threshold. Each stimul us of the train produced muscle-evoked potentials (MEPs) in hand and f orearm muscles, which gradually increased in size from the first to th e last shock. After the end of the train, the response to a single-tes t stimulus remained enhanced for 600-900 ms. In contrast, the train ha d no effect on the size of the MEPs evoked by transcranial electrical stimulation, while it suppressed ii-reflexes in forearm muscles for 90 0 ms. We conclude that rTMS of these parameters increases the excitabi lity of the motor cortex and that this effect outlasts the train for a lmost 1 s. At the spinal level, rTMS may increase presynaptic inhibiti on of Ia afferent fibers responsible for the H-reflex.