Aim: 

The present work was aimed at evaluating short interval intracortical inhibition (SICI), intracortical facilitation (ICF) and cortical silent period (SP) in masseter muscles (MM).

Methods: 

Motor evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) were recorded from contralateral (cMM) and ipsilateral (iMM) masseters, activated at 10% of maximal voluntary contraction (MVC), in 12 subjects. Interstimulus intervals (ISIs) were 2 and 3 ms for SICI, 10 and 15 ms for ICF.

Results: 

TMS of the masseteric motor cortex induced MEPs that were larger in the cMM (0.525 0.05 mV) than in the iMM (0.326 0.04 mV). SICI was only observed using a CS intensity of 70% AMT and was equal in both cMM and iMM. SICI was stronger at higher intensities of the test stimulus (TS), it was abolished by muscle activation greater than 10% MVC and it was unaffected by coil orientation changes. Control experiments confirmed that SICI was not contaminated by any inhibitory peripheral reflexes. However, ICF could not be obtained because it was masked by bilateral reflex depression of masseter EMG caused by auditory input from the coil discharge. The SP was bilateral and symmetric; its duration ranged 35–70 ms depending on TS intensity and coil orientation.

Conclusion: 

SICI operates in the cortical representation of masseter muscles. ICF was not detectable. The corticotrigeminal projection seems to be weakly influenced by inhibitory interneurons mediating the cortical silent period.