| Abstrakt: |
We studied the effect of the inhibitory phenomenon following transcranial magnetic stimulation (TMS) on brainstem motor neuron excitability and on the cortical control of brainstem reflexes in 5 normal subjects. The R1 component of the blink reflex assessed excitability of the facial nucleus during the period of post-TMS inhibition (experiment 1). TMS effect on the cortical control of brainstem reflexes was assessed by delivery of a stimulus that was less than or equal to the threshold for the R1 with the subject relaxed, yet elicited a potentiated reflex during maneuvers requiring cortical input (i.e., voluntary eye-closure and anticipation, experiment 2). In experiment 1, a suprathreshold electric stimulus applied during post-TMS inhibition elicited an R1 response that was preserved in amplitude (mean 476 +/- 44 microV; 123%) compared to control responses not preceded by TMS (mean 386 +/- 31 microV). Conversely, in experiment 2, when the R1 was dependent on cortical drive, the mean R1 amplitude during post-TMS inhibition was only 15% (P < 0.01) and 8% (P < 0.01) of control values obtained during eye-closure and anticipation, respectively. These findings imply that the inhibition following TMS originates rostral to the facial nucleus, at a cortical level. |
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