Reorganization of vibrissal motor representation following severing and repair of the facial nerve in adult rats

Abstract

This study examined the ability of adult rat motor cortex to reorganize its relationship with the somatic musculature following the severing and regeneration of a motor nerve. For this purpose experiments were performed on ten male albino rats where the facial nerve on one side was severed, sutured and allowed to regenerate for 6 months. Cortical motor output organization was assessed by mapping the vibrissal movement area extension and thresholds evoked by intracortical electrical stimulation in anesthetized rats. In all ten animals, the cortical output pattern of the motor cortex contralateral to the normal side was compared with that contralateral to the reinnervated side. After facial nerve reinnervation, the most notable differences in primary motor cortex (M1) output organization in the hemispheres contralateral to the reinnervated side were: (a) the area from which vibrissa movements could be evoked at low thresholds was smaller (mean 1.2+/-0.38 mm, range 0.75-1.75 mm), decreasing to 64.2% below those in hemispheres contralateral to the normal side (mean 3.4+/-0.52 mm, range 2.5-4 mm). The reorganized vibrissa area consisted of contiguous or discontinuous points shrunken to the medialmost portion of normal M1 vibrissal representation. (b) There was a clear medial extension of the forelimb representation, and a more modest lateral expansion of eye representation, into the vibrissa territory. The mean threshold required to evoke vibrissa movements was significantly higher in the hemispheres contralateral to the reinnervated side than in the other hemispheres (normal 23.9+/-9.7 microA vs reinnervated 37.8+/-11.9 microA; P< or =0.0001; t-test). The stimulation currents required to evoke other types of body movements were similar in the normal and reinnervated sides. Similar results were observed in all rats. In conclusion, these results indicate that motor nerve reinnervation is sufficient to produce long-lasting changes at a higher motor cortical level. This supports the notion that central supranuclear mechanisms may also be involved in the disorder of facial movements observed after facial nerve reinnervation.