title: Activation of high-threshold motor units in man by transcranial magnetic stimulation.
abstract: Motor units (MUs) with low voluntary recruitment thresholds are the first to be activated by transcranial magnetic stimulation. It is not clear, however, if high-threshold MUs can also be activated and if they contribute to motor evoked potentials (MEPs). We therefore studied 11 high-threshold motor units in the first dorsal interosseous muscle of 11 healthy subjects. Voluntary recruitment thresholds ranged from 22 to 41% (29.5 +/- 5.6%; mean +/- S.D.) of maximal muscle force. When MUs were driven at their recruitment thresholds, transcranial magnetic stimuli were applied to the vertex. Peri-stimulus time histograms of MU discharges were constructed. All MUs studied revealed a period of increased firing probability at 19-27 ms after the stimulus (primary peak). Stimulus intensities were lower by 10-57% of the maximal stimulator output than required to produce near maximal MEPs in conventional surface EMG recordings in the same subjects. We conclude that high-threshold MUs can be activated by transcranial magnetic stimulation and that they contribute to conventionally recorded MEPs.
authors: Department of Neurology, University of Bonn, FRG.
authors: M
authors: M
authors: Schubert
authors: R
authors: R
authors: Dengler
authors: K
authors: K
authors: Wohlfarth
authors: J
authors: J
authors: Elek
authors: A
authors: A
authors: Stallkamp
mesh_terms: D000328:Adult; D001921:Brain / Q000502:physiology*; D005071:Evoked Potentials; D005260:Female; D006801:Humans; D008297:Male; D009043:Motor Activity; D009068:Movement; D009132:Muscles / Q000294:innervation / Q000502:physiology; D050781:Transcranial Magnetic Stimulation