Inhibitory conditioning stimulus in transcranial magnetic stimulation reduces the number of excited spinal motor neurons.
OBJECTIVE
To study the mechanisms of amplitude attenuation caused by a transcranial magnetic conditioning stimulus. Both conventional MEPs and the recently described triple stimulation technique (TST) were applied; the latter to improve the quantification of the response size decrease.


METHODS
TST uses a peripheral collision method to eliminate the effects of desynchronization of the transcranial magnetic stimulation (TMS) induced spinal motor neuron discharges. The attenuation of motor evoked potentials (MEPs) and responses to TST was studied in 10 healthy volunteers using the conditioning-test paradigm with 2 ms interstimulus intervals.


RESULTS
Conventional MEPs and responses to TST demonstrated a marked attenuation by the preceding conditioning stimulus in all subjects. The ratio of MEP to TST amplitudes was the same in conditioned and unconditioned responses.


CONCLUSIONS
Our findings suggest that the transcranial conditioning stimulus does not change the degrees of desynchronization of spinal motor neuron discharges, but results in a reduced number of excited alpha motor neurons. This reduction can be estimated by both MEPs and TST.
Mall
V
V
Department of Neuropediatrics and Muscle Diseases, Children's Hospital, University of Freiburg, Mathildenstrasse 1, D-79106, Freiburg, Germany. mall@kkl200.ukl.uni-freiburg.de
Glocker
F X
FX
Fietzek
U
U
Heinen
F
F
Berweck
S
S
Korinthenberg
R
R
Rösler
K M
KM
D000328:Adult; D003213:Conditioning, Psychological / Q000502:physiology*; D005260:Female; D006801:Humans; D007258:Informed Consent; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D012016:Reference Values; D013116:Spinal Cord / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation; D014955:Wrist Joint / Q000502:physiology*