Focal transcranial magnetic stimulation of motor cortex evokes bilateral and symmetrical silent periods in human masseter muscles

• Published in: Clinical neurophysiology 2008-03, Vol.119 (3), p.693-703
• Main Authors: Jaberzadeh, Shapour, Sakuma, Shigemitsu, Zoghi, Maryam, Miles, Timothy S, Nordstrom, Michael A
• Format: Article
• Language: English
• Subjects: Adult; Biological and medical sciences; Cortical Spreading Depression - physiology; Cortical Spreading Depression - radiation effects; Corticobulbar; Differential Threshold; Electric Stimulation - methods; Electrodiagnosis. Electric activity recording; Electromyography - methods; Evoked Potentials, Motor - physiology; Evoked Potentials, Motor - radiation effects; Female; Functional Laterality - physiology; Fundamental and applied biological sciences. Psychology; GABA; Humans; Investigative techniques, diagnostic techniques (general aspects); Male; Masseter Muscle - physiology; Masseter Muscle - radiation effects; Mastication; Medical sciences; Middle Aged; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor Cortex - physiology; Motor Cortex - radiation effects; Muscle Contraction - physiology; Muscle Contraction - radiation effects; Nervous system; Neurology; Reaction Time - physiology; Reaction Time - radiation effects; Silent period; TMS; Transcranial Magnetic Stimulation - methods; Trigeminal; Vertebrates: nervous system and sense organs
• ISSN: 1388-2457; 1872-8952
• DOI: 10.1016/j.clinph.2007.11.005

Abstract 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 1.3× 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 1.3 T. 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 (∼100 ms at 1.3 T), 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. Significance 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.