Focal transcranial magnetic stimulation of motor cortex evokes bilateral and symmetrical silent periods in human masseter muscles.
OBJECTIVE
To determine whether a single hemisphere exerts distinct inhibitory influences over masseter muscles on each side, and to compare features of the masseter cortical silent period (CSP) evoked by transcranial magnetic stimulation (TMS) with previous reports from limb and other cranial muscles.


METHODS
Focal TMS was applied over the motor cortex jaw area in 14 normal subjects. In one experiment, TMS intensity was constant (1.1 or 1.3x active motor threshold, T) and masseter muscle activation varied from 10% to 100% of maximal. In another experiment, muscle activation was constant (20% maximal) and TMS intensity varied from 0.7 to 1.3T.


RESULTS
In all subjects, TMS evoked a silent period of similar duration in masseter muscles on both sides. Masseter CSP duration increased at higher TMS intensities, but was not affected by muscle activation level or the size of the excitatory response evoked by TMS. Weak TMS produced a bilateral CSP without short-latency excitation. The masseter CSP was short ( approximately 100ms at 1.3T), yet this was not due to maintenance of excitatory drive from the unstimulated hemisphere, as the masseter CSP was not prolonged with dual-hemisphere TMS.


CONCLUSIONS
Intracortical inhibitory circuits activated by TMS have a relatively weak effect on corticotrigeminal neurons supplying masseter, and effects are equivalent for corticobulbar efferents directed to contralateral and ipsilateral masseter motoneuron pools.


CONCLUSIONS
Trigeminally innervated masseter muscles exhibit weak, bilaterally symmetric inhibition following focal TMS. This method can be used to investigate abnormalities of intracortical inhibition in movement disorders or focal lesions affecting the masticatory muscles in humans.
Jaberzadeh
Shapour
S
Discipline of Physiology, School of Molecular & Biomedical Sciences, The University of Adelaide, Adelaide, SA 5005, Australia; Research Centre for Human Movement Control, The University of Adelaide, Adelaide, SA 5005, Australia.
Sakuma
Shigemitsu
S
Department of Fixed Prosthodontics, School of Dentistry Aichi-Gakuin University, Nagoya, Japan.
Zoghi
Maryam
M
Discipline of Physiology, School of Molecular & Biomedical Sciences, The University of Adelaide, Adelaide, SA 5005, Australia; Research Centre for Human Movement Control, The University of Adelaide, Adelaide, SA 5005, Australia.
Miles
Timothy S
TS
Discipline of Physiology, School of Molecular & Biomedical Sciences, The University of Adelaide, Adelaide, SA 5005, Australia; Research Centre for Human Movement Control, The University of Adelaide, Adelaide, SA 5005, Australia.
Nordstrom
Michael A
MA
Discipline of Physiology, School of Molecular & Biomedical Sciences, The University of Adelaide, Adelaide, SA 5005, Australia; Research Centre for Human Movement Control, The University of Adelaide, Adelaide, SA 5005, Australia. Electronic address: michael.nordstrom@adelaide.edu.au.
D000328:Adult; D013181:Cortical Spreading Depression / Q000502:physiology / Q000528:radiation effects*; D004056:Differential Threshold; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology / Q000528:radiation effects*; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008297:Male; D008406:Masseter Muscle / Q000502:physiology / Q000528:radiation effects*; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology* / Q000528:radiation effects; D009119:Muscle Contraction / Q000502:physiology / Q000528:radiation effects; D011930:Reaction Time / Q000502:physiology / Q000528:radiation effects; D050781:Transcranial Magnetic Stimulation / Q000379:methods*