The minimum number of facial motor neurons essential for minimal whisker movement in neonatally nerve-transected young adult rats.

Object: This study was performed to provide quantitative data on the number of surviving facial motor neurons that extend regenerated nerve fibers through the nerve-injured site and to reveal the relationship between facial function and the number of those motor neurons in which the facial nerve has been transected or resected in neonatal rats.

Methods: After transection of facial nerves in 1-day-old rat pups, facial function was estimated on postoperative Day 56 and a retrograde neuronal tracer was applied to the specific facial nerve branch responsible for the whisker movement. The mean number of the tracer-labeled neurons in the control rats was 2623+/-31 (mean+/-standard error of the mean) and that of the nerve-transected rats was 74+/-11 (range 0-221). Based on whisker movement, the nerve-transected rats were divided into two groups: clear spontaneous whisker movement and no whisker movement. The mean number of the tracer-labeled neurons in the nerve-transected rats with mobile whiskers was (106+/-12, range 44 [2% of the control value]-221 [8%]), whereas that in the nerve-transected rats with nonmobile whiskers was 24+/-6 (range 0-54 [2% of the control value]). The nerve-resected rats produced no labeled neurons.

Conclusions: It was concluded that axotomized neonatal facial motor neurons extended regenerated nerve fibers through the nerve-transected site with the maximum value of 8% of the control value and that minimal whisker movement was preserved with a very small population of motor neurons (2%).