FUNCTIONAL-ORGANIZATION OF CORTICONUCLEAR PATHWAYS TO MOTONEURONS OF LOWER FACIAL-MUSCLES IN MAN

EMG responses were recorded from lower facial muscles (depressor labii inferioris or depressor anguli oris) of 12 normal subjects after magnetic stimulation of the motor cortex. Using a figure-of-eight stimulating coil, the largest responses were obtained from points around 8-10 cm lateral to the vertex. Usually they were bilateral and had the same latency (11-12 ms) on both sides of the face. Patients with complete Bell's palsy had no response in muscles on the same side as the lesion, indicating that the ipsilateral component to cortical stimulation was not the result of recrossing in the periphery of nerve fibres from the contralateral side. Single-unit studies showed that cortical stimulation produced two phases of motoneuronal facilitation: a short-latency (central motor delay from contralateral cortex to the intracranial portion of the facial nerve, 7.6 ms), short-duration (1- to 2-ms duration peak in the post-stimulus time histogram) input, which was more commonly evoked by contralateral than ipsilateral stimulation; and a longer latency (central delay >15 ms), long-duration input evoked equally well from either hemisphere. The former may represent activity in a predominantly contralateral oligosynaptic corticobulbar pathway; the latter, a polysynaptic indirect (e.g. cortico-tegmento-nuclear) bilateral pathway to lower facial muscles.