Changes in motor representation related to facial nerve damage and regeneration in adult rats

The present study examined how facial nerve regeneration shapes movement re presentation patterns in previously disconnected motor cortices. Electrical microstimulation was used to bilaterally map the motor cortices of adult r ats subjected to unilateral facial nerve lesion and reanastomosis stamps at the stylomastoid foramen level. The motor cortex output patterns of two gr oups of experimental hemispheres (contralateral to lesioned side) were comp ared before and after facial nerve reinnervation. The motor cortex output p atterns in the control hemispheres (ipsilateral to the lesioned side) were used as reference. Before facial nerve reinnervation, the motor cortex fore limb and eye output area extended into the vibrissa area; such enlargement did not occupy the medial part of the former vibrissa area where ipsilatera l vibrissa or neck movements were represented. After facial nerve reinnerva tion, the contralateral vibrissa movement reorganized into a shrunken corti cal area corresponding to the medial portion of the former vibrissa represe ntation, i.e., where the ipsilateral vibrissa and neck movements were mainl y represented prior to facial nerve reinnervation. The enlargement of the f orelimb and eye representation remained unchanged, even after the vibrissa motor innervation was reactivated. Before facial nerve reinnervation into e xpanded forelimb and eye representations, a minimal current was required to evoke these movements, which did not vary from the normal range. A higher current was necessary to evoke the ipsilateral vibrissa and neck movements in the medial part of the vibrissa representation than the threshold needed to elicit the vibrissa movement normally represented in this cortical regi on. After facial nerve reinnervation, the overall current required to evoke movement remained the same as that which evoked movement before the vibris sa motor innervation was reactivated.