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

The present study examined how facial nerve regeneration shapes movement representation patterns in previously disconnected motor cortices. Electrical microstimulation was used to bilaterally map the motor cortices of adult rats subjected to unilateral facial nerve lesion and reanastomosis stamps at the stylomastoid foramen level. The motor cortex output patterns of two groups of experimental hemispheres (contralateral to lesioned side) were compared before and after facial nerve reinnervation. The motor cortex output patterns in the control hemispheres (ipsilateral to the lesioned side) were used as reference. Before facial nerve reinnervation, the motor cortex forelimb and eye output area extended into the vibrissa area; such enlargement did not occupy the medial part of the former vibrissa area where ipsilateral vibrissa or neck movements were represented. After facial nerve reinnervation, the contralateral vibrissa movement reorganized into a shrunken cortical area corresponding to the medial portion of the former vibrissa representation, i.e., where the ipsilateral vibrissa and neck movements were mainly represented prior to facial nerve reinnervation. The enlargement of the forelimb and eye representation remained unchanged, even after the vibrissa motor innervation was reactivated. Before facial nerve reinnervation into expanded 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 region. After facial nerve reinnervation, the overall current required to evoke movement remained the same as that which evoked movement before the vibrissa motor innervation was reactivated.