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

This study examined the ability of adult rat motor cortex to reorganize its relationship with the somatic musculature following the severing and regen eration of a motor nerve. For this pul pose experiments were performed on t en 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 threshol ds: evoked by intracortical electrical stimulation in anesthetized rats. In all ten animals, the cortical output pattern of the motor cortex contralat eral to the normal side was compared with that contralateral to the reinner vated side. After facial nerve reinnervation, the most notable differences in primary motor cortex (M1) output organization in the hemispheres contral ateral to the reinnervated side were: (a) the area from which vibrissa move ments could he evoked at low thresholds was smaller (mean 1.2+/-0.38 mm, ra nge 0.75-1.75 mm), decreasing to 64.2% below those in hemispheres contralat eral to the normal side (mean 3.4+/-0.52 mm, range 2.5-4 mm). The reorganiz ed vibrissa area consisted of contiguous or discontinuous points shrunken t o the medialmost portion of normal M1 vibrissal representation. (b) There w as a clear medial extension of the forelimb representation, and a more mode st lateral expansion of eye representation, into the vibrissa territory. Th e mean threshold required to evoke vibrissa movements was significantly hig her in the hemispheres contralateral to the reinnervated side than ill the other hemispheres (normal 23.9+/-9.7 mu A vs reinnervated 37.8+/-11.9 mu A: P less than or equal to 0.0001; t-test). The stimulation currents required to evoke other types of body movements were similar in the normal and rein nervated sides. Similar results were observed in all rats. In conclusion, t hese results indicate that motor nerve reinnervation is sufficient to produ ce long-lasting changes at a higher motor cortical level. This supports the notion that central supranuclear mechanisms may also be involved in the di sorder of facial movements observed after facial nerve reinnervation.