Abstract

Physiology of corticofacial motor system and the role of facial intracortical circuits are controversial and neither sensorimotor integration nor LTP-like plasticity has been investigated in these muscles. We investigated properties of motor responses (MEP) evoked by trasncranial magnetic stimulation (TMS) of the facial motor cortex, 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 muscle (DAO). SICI and ICF were tested using paired-pulse TMS. SAI was tested by pairing facial nerve electrical stimulation with TMS of the facial motor cortex. Resting and active MEPs were acquired before and after 0-30 min from PAS delivery. SICI and ICF were significant (p < 0.001) in both relaxed and active DAO. SAI was absent in both resting and active condition, but PAS protocol significantly (p < 0.01) facilitated the resting DAO MEPs at all post-PAS times tested. Data showed that cortico-facial projection is bilateral and symmetric, with a contralateral predominance. 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.

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