title: Responses of single motor units in human masseter to transcranial magnetic stimulation of either hemisphere.
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.
authors.affiliation: Department of Physiology, The University of Adelaide, Adelaide, South Australia 5005, Australia.
authors.forename: Sophie L
authors.initials: SL
authors.lastname: Pearce
authors.forename: Timothy S
authors.initials: TS
authors.lastname: Miles
authors.forename: Philip D
authors.initials: PD
authors.lastname: Thompson
authors.forename: Michael A
authors.initials: MA
authors.lastname: Nordstrom
mesh_terms: D000328:Adult; D004056:Differential Threshold; D004292:Dominance, Cerebral; D004558:Electric Stimulation / Q000379:methods; D004594:Electrophysiology; D005260:Female; D006801:Humans; D008297:Male; D008406:Masseter Muscle / Q000294:innervation*; D009044:Motor Cortex / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D011336:Probability; D011930:Reaction Time; D050781:Transcranial Magnetic Stimulation