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Intracortical circuits, sensorimotor integration and plasticity in human motor cortical projections to muscles of the lower face

Authors


F. Deriu: Department of Biomedical Sciences, University of Sassari, Viale San Pietro 43/b, 07100–Sassari, Italy.   Email: deriuf@uniss.it

Key points

  • • Previous studies documented features of corticobulbar projections and of intracortical circuits in the motor cortex innervating lower facial muscles (face M1). However, there have been no studies of the afferent modulation of corticobulbar excitability or of the plasticity of synaptic connections in face M1.
  • • Intracortical circuits, sensorimotor integration and plasticity in face M1 were investigated in healthy volunteers using standard protocols of the transcranial magnetic stimulation technique.
  • • This study showed, for the first time, that face M1 is prone to plastic changes following paired associative stimulation and that its excitability is modulated by afferent stimulation at long latency (200 ms) but not at short latency (20 ms). Furthermore, contralateral predominance of cortical projection to lower facial muscles was confirmed, and the presence of bilateral intracortical inhibitory and facilitatory mechanisms at rest and during voluntary muscle activation was clarified.
  • • These data provide further physiological insight into pathologies affecting the facial motor system.

Abstract  Previous studies of the cortical control of human facial muscles documented the distribution of corticobulbar projections and the presence of intracortical inhibitory and facilitatory mechanisms. Yet surprisingly, given the importance and precision in control of facial expression, there have been no studies of the afferent modulation of corticobulbar excitability or of the plasticity of synaptic connections in the facial primary motor cortex (face M1). In 25 healthy volunteers, we used standard single- and paired-pulse transcranial magnetic stimulation (TMS) methods to probe motor-evoked potentials (MEPs), short-intracortical inhibition, intracortical facilitation, short-afferent and long-afferent inhibition and paired associative stimulation in relaxed and active depressor anguli oris muscles. Single-pulse TMS evoked bilateral MEPs at rest and during activity that were larger in contralateral muscles, confirming that corticobulbar projection to lower facial muscles is bilateral and asymmetric, with contralateral predominance. Both short-intracortical inhibition and intracortical facilitation were present bilaterally in resting and active conditions. Electrical stimulation of the facial nerve paired with a TMS pulse 5–200 ms later showed no short-afferent inhibition, but long-afferent inhibition was present. Paired associative stimulation tested with an electrical stimulation–TMS interval of 20 ms significantly facilitated MEPs for up to 30 min. The long-term potentiation, evoked for the first time in face M1, demonstrates that excitability of the facial motor cortex is prone to plastic changes after paired associative stimulation. Evaluation of intracortical circuits in both relaxed and active lower facial muscles as well as of plasticity in the facial motor cortex may provide further physiological insight into pathologies affecting the facial motor system.

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