Influence of the blink reflex to motor evoked responses of facial muscles in man

During magnetic brain stimulation (TMS) the round coil covers a large area of the skull and can possibly excite also the trigeminal sensory afferents which can in turn trigger the blink reflex. The ipsilateral R1 component of the blink reflex may sum up with the ipsilateral responses of the facial muscles that arise from motor cortex stimulation. Both of them namely appear with rather similar latencies. To investigate this possibility we examined 11 healthy subjects. Electromyographic responses of the orbicularis oris (OR) and orbicularis oculi (OC) muscles were studied after the supraorbital an TMS stimulations before and after lidocain blocks of the before mentioned nerves. Following TMS the ipsilateral OR muscle responses occurred in 5 subjects and persisted after the lidocaine blocks. They may therefore represent activity in the corticobulbar projection to this muscle and can be taken as an evidence of its bilateral cortical representation. Ipsilateral responses of the OC muscles could, on the other hand, be diminshed or obstructed by the supraorbital nerve blocks. They may at least partially be due also to the reflex activation of the lower facial muscles.