Impact of TMS on the primary motor cortex and associated spinal systems

Repetitive transcranial magnetic stimulation (rTMS) is able to modulate corticospinal and intracortical motor cortex excitability, depending on the stimulation parameters. In the present study, we explored the effects of highand low-frequency rTMS on spinal reflex monosynaptic responses (H waves or H reflexes) in humans recorded from the flexor carpi radialis (FCR) muscle. After 1-Hz rTMS, H-wave minimal and optimal thresholds were decreased by 14-22% of their pre-rTMS values. The maximal H-wave and H/M amplitude ratio were both significantly increased by 52-54%, whereas direct muscle responses (M waves) remained unchanged. The area under the H-wave recruitment curve was also significantly increased by 86 ± 21% of its pre-rTMS baseline values. Conversely, after 20-Hz rTMS, H-wave minimal and optimal thresholds were both increased by 15-26%. The maximal H wave and H/M amplitude ratio were significantly decreased by -37 ± 11%, whereas M responses remained unchanged. The area under the H-response recruitment curve was also significantly decreased by -45±5%. Neither 20-Hz nor 1-Hz sham rTMS induced noticeable changes in M or H responses. We conclude that 1-Hz rTMS applied to primary motor areas increases the monosynaptic spinal cord H reflex, whereas 20-Hz rTMS induces the opposite effect, presumably by modulating inhibitory descending conicospinal projections onto spinal motoneurons. These results offer an opportunity to influence the level of spinal excitability and modify descending corticospinal influences in a controlled manner using noninvasive approaches that could prove useful in the study of the pathophysiology and eventual treatment of spasticity.