Excitability of the masseter inhibitory reflex after high frequency rTMS over the motor cortex: A study in healthy humans

Abstract

ObjectiveWe aimed to investigate the influence of high frequency repetitive transcranial magnetic stimulation (rTMS) of the motor cortex on excitability of the masseter inhibitory reflex (MIR) in healthy subjects. Our goal in conducting this study was to obtain a better understanding of the subcortical and cortical networks related to the excitability of the central masticatory pathway. MethodsTwenty healthy volunteers participated in this study. MIRs were evaluated both before and after high frequency rTMS to elicit motor evoked potentials (MEPs) using single- and double-shock techniques over the motor cortex. During measurements, the latency and duration of silent periods (SPs) with single magnetic stimulation, and the recovery of SP2 with the double-shock technique, were recorded in basal conditions and immediately after high frequency rTMS. The “real” rTMS condition consisted of 20 stimulus trains delivered at 10Hz with a 5-s stimulation duration and an intensity of 70% of the active motor threshold (AMT) over the right optimal motor spot for the masseter muscle. Resultswere also compared to those obtained after “sham” rTMS 1 week later ResultsUsing the double-shock technique, SP2 recovery time was significantly shorter after high frequency rTMS compared to the basal condition (from 100 to 400ms; p<0.001), while at ISIs of 500 and 600ms, no significant effect was observed (p=0.084, p=0.948). There was also no significant change in the SP latency or duration between groups (p>0.05). ConclusionsHigh frequency, sub-threshold rTMS of the motor cortex had a facilitative after-effect on the excitability of the MIR. This effect was likely mediated through increased cortical drive to brainstem reflex pathways, which would ultimately accelerate MIR recovery.