Brain, Vol 121, Issue 3 437-450, Copyright © 1998 by Oxford University Press
MR Magistris, KM Rosler, A Truffert and JP Myers
Transcranial stimulation has become an established method in the evaluation
of corticospinal tract function. Clinical studies mainly address slowing of
conduction through measurement of increased central conduction time (CCT)
and 'failures' of conduction through observation of marked reductions in
the size of the motor evoked potential (MEP). While CCT is of great
interest in detecting subclinical slowing of conduction, the method
discloses only gross failures of conduction, since the size of the MEP
varies markedly between normal subjects and from one stimulus to another,
leading to a broad range of normal values. Furthermore, transcranial
stimulation does not appear to achieve depolarization of all spinal motor
neurons leading to the target muscles, since in most normal subjects MEPs
are smaller in amplitude than the responses evoked by peripheral nerve
stimulation. We have developed a triple stimulation technique (TST) which,
through two collisions, links central to peripheral conduction and
suppresses desynchronization of MEPs. This technique shows that
transcranial stimulation does achieve depolarization of all, or nearly all,
spinal motor neurons supplying the target muscle in healthy subjects. Our
data thus demonstrate that the amplitudes of MEPs are (i) smaller than
those of peripheral responses, mostly due to phase cancellation of the
action potentials caused by the desynchronization occurring within the
corticospinal tract or at spinal cell level and (ii) variable between
normal subjects and from one stimulus to another, mostly due to variability
of this desynchronization. This technique provides new insights into normal
corticospinal tract conduction. It will improve detection and
quantification of central motor conduction failures.
ARTICLES
Transcranial stimulation excites virtually all motor neurons supplying the target muscle. A demonstration and a method improving the study of motor evoked potentials
Department of Clinical Neurology, Geneva University Hospital, Switzerland.
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