Axonal Regeneration in Severed Peripheral Facial Nerve of the Rabbit: Relation of the Number of Axonal Regenerates to Behavioral and Evoked Muscle Activity

Abstract

The minimum number of regenerating facial nerve myelinated motor axons that are required to innervate and activate the mimetic musculature is not known. We compare rabbit facial nerve regeneration following complete transectional injuries of the buccal division to the evoked and behavioral muscle activities of the quadratus labii superioris muscle of the rabbit in three experimental models: end-to-end direct anastomosis (N = 4), 8-mm autologous nerve grafts (N = 8), and 10-mm silicone chamber implants (N = 40). Data are presented as total numbers of regenerating myelinated axons that traverse the surgical repair and innervate the fascicles of the transected distal nerve stump, as well as the percentage of regenerating neurites, as compared to the preoperative normal controls. Five weeks after neural repair, direct end-to-end anastomosis regained more myelinated axons across the reconstructed defect (2,632 +/- 1,232; 67%) than silicone tube implants (2,006 +/- 445; 51%) or autologous cable graft repairs (1,660 +/- 1,169; 42%). However, only a small percentage of myelinated fibers innervated the intrafascicular region of the distal transected neural stump in direct anastomosis (948 +/- 168; 24%), silicone tube implants (670 +/- 275; 17%), or autologous nerve grafts (445 +/- 120; 12%) in rabbits that regained evoked and behavioral mimetic muscle activity. All rabbits with direct anastomosis and neural cable grafts regained motor activity, despite the fact that 66% of regenerating motor neurites in cable graft repairs and 54% in direct anastomosis were collateral sprouts that did not contribute to effective muscle activity. In 17 rabbits with neural regenerates within the silicone tube implants that did not regain mimetic activity, the mean number of regenerating myelinated motor axons across the defect was 504 +/- 419 (13%), and the mean number of axons that innervated the distal transected nerve stump fascicles was 277 +/- 128 (7%). Therefore, the minimal number of motor axons that is required to activate the quadratus labii superioris muscle is 12% of the original motor axon population of the normal buccal nerve division.

Delayed repair of facial nerve trauma: an experimental study in guinea pigs

“…Ma et al reported that regenerated fibers passing through the injury clearance reach the target organ 21 days after trauma [10]. Spector and Lee reported that the muscle cannot effectively contract unless 12% of motor fibers under normal conditions regenerate and enter the target muscle [11]. In the present study, we observed that the passing rates of myelinated fibers in the groups with immediate FFA and FFA delayed for 7 days were the highest.…”

Inhibition of Motor Nerve Regeneration in a Rabbit Facial Nerve Model

“…These branches are easily dissected and identified through small incisions over the zygomatic arch, making the rabbit model ideal for these experiments. 9 Twenty healthy female rabbits weighing 2.5 to 3.0 kg were anesthetized with 10 mg/kg xylazine hydrochloride and 50 mg/kg ketamine hydrochloride by intramuscular injection. The skin over the proximal facial nerve on both sides was shaved, prepped with 70% ethanol and povidone‐iodine, and air dried.…”

Agmatine treatment and vein graft reconstruction enhance recovery after experimental facial nerve injury

“…The question therefore of the significance of the number of axons or the number of myelinated axons (over a given unknown minimum), and of the axon diameter, still remains. In a later study by Spector and Lee (1998), it was found that only a small number of axons are needed to activate distal musculature. It is possible therefore, as mentioned above, that perhaps by the time measurements were performed in the present study, not all regenerating axons have formed proper functional neuromuscular synapses.…”