TRANSCRANIAL STIMULATION EXCITES VIRTUALLY ALL MOTOR-NEURONS SUPPLYING THE TARGET MUSCLE - A DEMONSTRATION AND A METHOD IMPROVING THE STUDYOF MOTOR EVOKED-POTENTIALS

Transcranial stimulation has become an established method in the evalu ation of corticospinal tract function. Clinical studies mainly address slowing of conduction through measurement of increased central conduc tion time (CCT) and failures' of conduction through observation of mar ked reductions in the size of the motor evoked potential (MEP). While CCT is of great interest in detecting subclinical slowing of conductio n, the method discloses only gross failures of conduction, since the s ize of the MEP varies markedly between normal subjects and from one st imulus to another leading to a broad range of normal values. Furthermo re, transcranial stimulation does not appear to achieve depolarization of all spinal motor neurons leading to the target muscles, since in m ost normal subjects MEPs are smaller in amplitude than the responses e voked by peripheral nerve stimulation. We have developed a triple stim ulation technique (TST) which, through two collisions, links central t o peripheral conduction and suppresses desynchronization of MEPs. This technique shows that transcranial stimulation does achieve depolariza tion of all, or nearly all, spinal motor neurons supplying the target muscle in healthy subjects. Our dam thus demonstrate that the amplitud es of MEPs are (i) smaller than those of peripheral responses, mostly due to phase cancellation of the action potentials caused by the desyn chronization occurring within the corticospinal tract or at spinal cel l level and (ii) variable between normal subjects and from one stimulu s to another mostly due to variability of this desynchronization. This technique provides new insights into normal corticospinal tract condu ction. It will improve detection and quantification of central motor c onduction failures.