P 20. Corticobulbar projection and intracortical circuits in the facial motor cortex innervating lower facial muscles in healthy humans

Abstract

Despite the importance of facial muscles in social life and their frequent and disabling involvement in stroke or movement disorders, conclusive data concerning bilaterality, symmetry and hemisphere dominance in the control of lower facial muscles are not available yet. In addition, the role of intracortical circuits in the modulation of resting and active facial muscles is still debated. (1) To reassess controversy about the symmetry of cortical output to lower facial muscles, arising from the left and right hemisphere. (2) To clarify whether intracortical excitability modulates the ipsi-and contralateral corticobulbar projection, depending on the muscle state and on the stimulated hemisphere. In nine healthy subjects, left and right hemisphere were tested using single and paired pulse TMS methods to probe hot spot, motor evoked potentials (MEPs) latencies and amplitudes as well as short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) in the ipsi-and contralateral depressor anguli oris (DAO) muscles both at rest and during activation at 10% of maximal voluntary contraction. Resting and active motor thresholds (RMT and AMT, respectively) were determined following TMS of both hemispheres. Test and conditioning stimuli were 120% and 70% of motor threshold, respectively, and interstimulus intervals (ISIs) were 2 ms for SICI and 10 ms for ICF. Student’s paired t -test, ANOVA and post hoc t -test have been performed for single paired pulse TMS on the left and right hemispheres. Resting and active MEPs were evoked bilaterally in the relaxed and active DAO following left and right hemisphere stimulations. Student’s paired t test showed no differences between left and right hemisphere as for hot spot, RMT, AMT, MEP amplitudes and onset latencies. In both muscle states, the contralateral MEP had a significantly shorter latency ( p = 0.005) and larger amplitude ( p = 0.02) than the ipsilateral MEP, with no significant differences according to the side of the stimulated hemisphere. Repeated measures ANOVA showed a significant effect of the paired pulse protocol ( p < 0.001) at rest and during voluntary contraction, regardless the side of the stimulated hemisphere. Post hoc analysis showed, in both muscle states and for both hemispheres, significant SICI ( p < 0.01) at 2 ms and ICF ( p < 0.05) at 10 ms. SICI effects were smaller in the ipsilateral respect to the contralateral DAO at rest ( p = 0.003) and during voluntary contraction (p = 0.032); ICF was reduced ipsilaterally only in the resting state ( p = 0.037). The amount of SICI and ICF was significantly smaller during voluntary contraction in the ipsi-and contralateral DAO ( p = 0.026 and p = 0.005, respectively). TMS applied to the left and right facial motor cortex evokes, in the DAO muscle, bilateral and asymmetric MEPs, with contralateral predominance regardless of the stimulated hemisphere. SICI and ICF are present bilaterally at rest and during voluntary activation and show similar extent following stimulation of both hemispheres. In conclusion, corticobulbar responses as well as intracortical excitability of the left and right facial motor cortex innervating lower facial muscles show similar physiological properties. These data may provide further insight into studies of pathologies affecting the facial motor system. This research was founded by Fondazione Italiana Sclerosi Multipla (FISM 2011/R/17).