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Type: Journal article
Title: Responses of single motor units in human masseter to transcranial magnetic stimulation of either hemisphere
Author: Pearce, S.
Miles, T.
Thompson, P.
Nordstrom, M.
Citation: The Journal of Physiology, 2003; 549(2):583-596
Publisher: Cambridge Univ Press
Issue Date: 2003
ISSN: 0022-3751
Statement of
Sophie L. Pearce, Timothy S. Miles, Philip D. Thompson and Michael A. Nordstrom
Abstract: The corticobulbar inputs to single masseter motoneurons from the contra- and ipsilateral motor cortex were examined using focal transcranial magnetic stimulation (TMS) with a figure-of-eight stimulating coil. Fine-wire electrodes were inserted into the masseter muscle of six subjects, and the responses of 30 motor units were examined. All were tested with contralateral TMS, and 87 % showed a short-latency excitation in the peristimulus time histogram at 7.0 ± 0.3 ms. The response was a single peak of 1.5 ± 0.2 ms duration, consistent with monosynaptic excitation via a single D- or I1-wave volley elicited by the stimulus. Increased TMS intensity produced a higher response probability (n = 13, paired t test, P< 0.05) but did not affect response latency. Of the remaining motor units tested with contralateral TMS, 7 % did not respond at intensities tested, and 7 % had reduced firing probability without any preceding excitation. Sixteen of these motor units were also tested with ipsilateral TMS and four (25 %) showed short-latency excitation at 6.7 ± 0.6 ms, with a duration of 1.5 ± 0.3 ms. Latency and duration of excitatory peaks for these four motor units did not differ significantly with ipsilateral vs. contralateral TMS (paired t tests, P> 0.05). Of the motor units tested with ipsilateral TMS, 56 % responded with a reduced firing probability without a preceding excitation, and 19 % did not respond. These data suggest that masseter motoneurons receive monosynaptic input from the motor cortex that is asymmetrical from each hemisphere, with most low threshold motoneurons receiving short-latency excitatory input from the contralateral hemisphere only.
Keywords: Masseter Muscle
Motor Cortex
Motor Neurons
Electric Stimulation
Differential Threshold
Dominance, Cerebral
Reaction Time
Transcranial Magnetic Stimulation
Description: Copyright © 2003 The Physiological Society The definitive version is available at
DOI: 10.1113/jphysiol.2002.035352
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