Altered sensory input improves the accuracy of muscle reinnervation

To improve functional recovery after peripheral nerve suture, we characterized the quality of target reinnervation in rats in which the afferent trigeminal connection to facial motoneurons had been altered.

Employing an improved lesion model and a refined mode of retrograde tracer application, we studied the accuracy of reinnervation in rats which underwent buccal-buccal nerve anastomosis (BBA) alone (group 1), BBA plus excision of the ipsilateral infraorbital nerve (ION; group 2), and BBA plus excision of the contralateral ION (group 3). This was done by comparison between the number of double-labeled motoneurons after pre-operative injection of Fluoro-Gold (FG) and post-operative injection of Fast Blue (FB) into the whisker pad muscles.

In the first group we counted 398 +/- 80 FG+FB double labeled cells (mean +/- SD; n = 9 rats), i.e., only 27% of all motoneurons that grew axons into the whisker pad had projected to these muscles before surgery. In group 2, this value was increased marginally to 436 +/- 68 (32%). In group 3,. we counted 580 +/- 63 double-labeled neurons. This is the first morphological report demonstrating significantly improved specificity of reinnervation. Indeed, 41% of the motoneurons innervating the target in group 3 belonged to the original neuron pool. These morphological findings are supported by evidence obtained from electrophysiological recordings and behavioural studies.

The principle finding of the present study is that a peripheral lesion to the contralateral trigeminal nerve improves the quality of reinnervation of the whisker pad musculature by its original nerve. The contralateral trigeminal lesion may trigger behavioural demand and forced overuse of the axotomized facial nerve, which may be a key issue for recovery of vibrissae rhythmical whisking after facial nerve surgery.