The Dilator Naris Muscle as a Reporter of Facial Nerve Regeneration in a Rat Model

Abstract

Objective: Many investigators study facial nerve regeneration using the rat whisker pad model, although widely standardized outcomes measures of facial nerve regeneration in the rodent have not yet been developed. The intrinsic whisker pad "sling" muscles producing whisker protraction, situated at the base of each individual whisker, are extremely small and difficult to study en bloc. Here, we compare the functional innervation of 2 potential reporter muscles for whisker pad innervation: the dilator naris (DN) and the levator labii superioris (LLS), to characterize facial nerve regeneration.

Methods: Motor supply of the DN and LLS was elucidated by measuring contraction force and compound muscle action potentials during stimulation of individual facial nerve branches, and by measuring whisking amplitude before and after DN distal tendon release.

Results: The pattern of DN innervation matched that of the intrinsic whisker pad musculature (ie, via the buccal and marginal mandibular branches of the facial nerve), whereas the LLS seemed to be innervated almost entirely by the zygomatic branch, whose primary target is the orbicularis oculi muscle.

Conclusions: Although the LLS has been commonly used as a reporter muscle of whisker pad innervation, the present data show that its innervation pattern does not overlap substantially with the muscles producing whisker protraction. The DN muscle may serve as a more appropriate reporter for whisker pad innervation because it is innervated by the same facial nerve branches as the intrinsic whisker pad musculature, making structure/function correlations more accurate, and more relevant to investigators studying facial nerve regeneration.

Figure 1

A, Rat facial nerve anatomy, showing the arrays ofwhiskers, organized in 5 distinct rows (gray dots) (Ref 22). B, Invivo CN VII exposure revealing the main trunk and zygomatic, buccal, and marginal mandibular branches. C, Rat whisker padextrinsic muscles, exposing the DN muscle: (1) LLS, (2) DN, (3) zygomatic arch, (4) mandibular branch of VII, (5) buccalbranch of VII, (6) masseter, (7) zygomatic branch of VII, and (8) temporal branch of VII. Drawing is from Muscle Nerve. 2001;4:1202–1212 with permission. Adaptations are themselves works protected by copyright. So in order topublish this adaptation, authorization must be obtained bothfrom the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.Photograph insert shows DN under slight tension.

Figure 2

A, Example of force tension in DN during facialnerve main trunk stimulation with a 100-Hz biphasic pulsetrain. B, Example of CMAP in LLS during zygomatic branch stimulation of 0.40 milliseconds biphasic pulses. Bothtechniques used increasing stimulation amplitudes untilresponses plateaued.

Figure 3

DN (in blue) and LLS (in red) contraction force as apercentage of the force and CMAP generated during facialnerve main trunk stimulation (n = 4). Bars are ±SE.

Figure 4

DN (in blue) and LLS (in red) CMAP peak-to-peak amplitude (B) as a percentage of the force and CMAPgenerated during facial nerve main trunk stimulation (n = 4). Bars are ±SE.

Figure 5

DN (A) and LLS (B) average contractile force and CMAP amplitude correlation at each stimulation site withPearson correlation coefficient R2.

Figure 6

Whisking amplitude pre- and post-DN distal tendon release.