The role of intracortical circuits in the modulation of facial muscle (FM) activity is controversial and neither sensorimotor integration nor LTP-like plasticity have been investigated in these muscles. We proposed to investigate short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), short-afferent inhibition (SAI) and effects of paired associative stimulation (PAS) in relaxed and active depressor angulis oris (DAO).

SICI and ICF were tested in 16 subjects at interstimulus intervals (ISI) of 2–3ms and 10–15ms, respectively. SAI was tested in 9 subjects by pairing facial nerve electrical stimulation (ES) with TMS of the facial motor cortex. Resting and active MEPs were acquired before as well as after 0, 10, 20 and 30 min from PAS delivery (200 pairs of ES and TMS; 20ms ISI, 0.25Hz, at rest). ANOVA and post hoc t-test were performed.

SICI and ICF were significant (p<0.001) in both relaxed and active DAO. SAI paradigm did not affect either resting or active MEP amplitude. By contrast, PAS protocol significantly (p=0.035) facilitated DAO MEPs in the resting but not in the active condition. Resting MEPs were significantly facilitated at all post-PAS times tested: T0 (p=0.017), T10 (p=0.022), T20 (p=0.003) and T30 (p=0.011).

Our data shows that SICI and ICF work in the facial motor cortex in both resting and active conditions. In addition, despite SAI appearing to be ineffective in FM, evidence was provided for LTP-like plasticity in the facial area.