Experimental Studies on Facial Nerve Regeneration

Abstract

Insufficient recovery after injury of a peripheral motor nerve is due to (1) inappropriate pathfinding as a result of axonal regrowth to inappropriate targets, (2) excessive collateral axonal branching at the lesion site, and (3) polyinnervation of the neuromuscular junctions (NMJs). The rat facial nerve model is often used because of its simple and reliable readout to measure recovery of function (vibrissal whisking). Over the last decades scientists have concentrated their efforts to combat mostly NMJ polyinnervation, because it turned out to be very difficult to reduce collateral axonal branching and impossible to navigate thousands of axons toward the original fascicles. In the past, several groups of scientists concentrated their efforts to reduce the activity-dependent polyinnervation of NMJs by electrical stimulation of the muscles (square 0.1 msec pulses at 5 Hz). The results showed no recovery of functions and a severe reduction in the number of innervated NMJs to approximately one fifth of those observed in intact animals. More recent experiments, however, have shown that motor recovery improved significantly following mechanical stimulation of the denervated facial muscles (vibrissal and orbicularis oculi) and that restored functions could invariably be linked to reduced polyinnervation at the NMJ while the number of innervated NMJ remained the same. These results suggest that clinically feasible and effective therapies could be developed and tested in the near future.

Events Occurring in the Axotomized Facial Nucleus

“…This animal model has made it possible to investigate the reactions between neurons and glial cells in the parenchyma without involving the effects of blood-derived cells and blood constituents. This model has also been used to evaluate the effects of survival factors on injured motoneurons [11,12] and to study the regeneration of injured motor nerves [13].…”

Anatomical description of the extratemporal facial nerve under high-definition system: a microsurgical study in rats

“…In the field of experimentation, the rat stands out as an important model used in studies involving nerves due to its easy acquisition and handling, not requiring large physical spaces, and few complications secondary to the surgical procedure are reported. It is also noteworthy that this model has great anatomical similarities with humans, allowing various comparisons 17,18 , such as the techniques of end-to-end neurorrhaphy in cases of neural gaps or performing autografts 19 .…”

The Regenerative Potential of Facial Nerve Motoneurons following Chronic Axotomy in Rats

“…Although, some scholars believed the facial nerve had a greater capacity for regeneration than any other neuron in the central nervous system; in this regard, the facial nerve was very similar to peripheral motor nerves [ 5 – 8 ]. For those patients with facial paralysis for a long time, the curative effects after operation were often not ideal [ 1 – 4 ]; the most important reason was the loss of the most facial nerve motor neurons, which led to the ability decline of facial nerve regeneration [ 9 ].…”

“…No matter which way to promote the regeneration of facial nerve after injury, how to protect or reduce the nonapoptosis of motor neurons of facial nerve after injury is indeed the most critical step to improve the repair of facial nerve regeneration [ 9 ]. Although many molecules involved in facial nerve repair have been characterized, the precise mechanisms of nerve regeneration remain unclear.…”