Responses of single motor units in human masseter to transcranial magnetic stimulation of either hemisphere

• Published in: The Journal of physiology 2003-06, Vol.549 (2), p.583-596
• Main Authors: Pearce, Sophie L., Miles, Timothy S., Thompson, Philip D., Nordstrom, Michael A.
• Format: Article
• Language: English
• Subjects: Adult; Differential Threshold; Dominance, Cerebral; Electric Stimulation - methods; Electrophysiology; Female; Humans; Male; Masseter Muscle - innervation; Motor Cortex - physiology; Motor Neurons - physiology; Original; Probability; Reaction Time; Transcranial Magnetic Stimulation
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.2002.035352

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 I 1 -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.