Modulating spinal cord excitability with repetitive transcranial magnetic stimulation on the primary motor cortex of human subjects

In the present study we explored the effects of high and low frequency Repetitive Transcranial Magnetic Stimulation (rTMS) on H-reflexes, a marker of spinal excitability. In two different groups of right handed subjects, 20 Hz or 1Hz rTMS was applied to the optimal motor cortex position for activation of the flexor carpi radialis (FCR) muscle at an intensity 10% below motor threshold. Before and after rTMS a complete recruitment curve of the compound motor action potential M (direct response) and H (monosynaptic reflex) responses were determined by stimulating the median nerve with increasing intensities. After 20 Hz rTMS, H wave minimal and optimal threshold were both increased to 115 and 126% of their pre rTMS values. Maximal H wave and H/M amplitude ratio were both significantly decreased to 63% of its (b)aseline, whereas M responses remained again unchanged. The area under the H response recruitment curve was significantly decreased by 55% whereas the M wave area showed no significant change (99%). After 1 Hz rTMS, H wave minimal and optimal thresholds were decreased to 78 and 86% of their pre rTMS values. Maximal H wave and H/M amplitude ratio were both significantly increased to 152-154% of its baseline, whereas M responses remained unchanged (98%). Similarly, the area under the H response recruitment curve was significantly increased to 186%, whereas the M wave area remained unchanged (100%). We conclude that 20 Hz rTMS applied to primary motor areas decreases the monosynaptic spinal cord H response excitability, whereas 1 Hz induces the opposite effect, presumably by modulating inhibitory descending cortico-spinal projections onto spinal, motoneurons.