Somatotopic organization of the facial nucleus is disrupted after lesioning and regeneration of the facial nerve: the histological representation of synkinesis.

OBJECTIVE: After facial nerve repair, involuntary movement of part of the face during voluntary movement of another part of the face is common. We describe an animal model of facial nerve lesion, repair, and regeneration that demonstrates abnormal organization of the facial nucleus; this model may be used to study synkinesis.
METHODS: In 18 rats, the facial nerve was cut completely, proximal to the parotid gland, and immediately sutured end-to-end. After a period of regeneration of 1 to 10 months, retrograde fluorescence labeling of the distal branches of the facial nerve was performed. The distribution of the tracers in the facial nucleus was assessed in both the lesioned animals and in a nonlesioned group (n = 20).
RESULTS: In the control animals, muscle groups were somatotopically represented in the facial nucleus. After lesioning, repair, and regeneration, the somatotopy of the facial nucleus was disrupted. Axons projected from the facial nucleus to incorrect peripheral muscle groups, and aberrant branches were observed to simultaneously innervate different subdivisions of the facial nerve. The numbers of aberrant axons and branches did not change significantly during periods of regeneration ranging from 1 to 10 months.
CONCLUSION: Our model provides a clear demonstration of the failure of adult facial nerve axons to make correct connections with their distal targets during regeneration. This model may be used to assess strategies aimed at minimizing synkinesis and, by assessing histology together with behavior, provides a more robust model than those previously described.