MEDIAN NERVE STIMULATION PO TENTIATES THE MU SCLE RESPONSES TO TRANS C RANIAL MAGNETIC STIMULATION

Motor responses evoked by transcranial magnetic stimulation OMS) or transcranial electrical stimulation (TCS) can be facilitated by a prior conditioning stimulus to an afferent nerve. Two facilitation periods are described short (SI), when the nerve stimulus is given near 0 to 10 ms after cranial stimulation, and long (LI), when nerve stimulation is given 25-60 ms before the cranial stimulation. I These facilitation periods were examined in more detail in 10 normal consenting subjects. The study has ethical committee approval. Focal cortical TMS was applied contralaterally by a figure-of-eight coil over the "hot spot" for the right hand muscles and the strength adjusted to be just above twitch threshold for the relaxed muscle. Conditioning electrical stimuli were applied to the right median nerve at the wrist, again at a strength just suprathreshold for a twitch in APE. The conditioning-test (CT) interval was varied from -80 to + 1 0 with respect to the magnetic stimulus and 5 magnetic stimuli were tried at each interval. The results confirm the short facilitation period when the C-T interval was -6 to +3 ms. Consideration of the timing indicates that this must occur at the spinal segmental level. The long period of facilitation lasted from 27-70 ms, but it was divided into two periods (27-35 and 55-70 ms) in all subjects, separated by an interval of about 20 ms during which the test response fell to control levels. The long late facilitations may be cortical as the earliest facilitation began at 27 ms having the afferent volley reached the sensory cortex at 20 ms. The long interval facilitation consists of two temporally separate processes, implying separate cortical mechanisms creating a bimodal excitability cycle at the level of motor cortex.