An electrophysiological evaluation method for the ovine facial nerve

Abstract

Introduction: Large-animal models such as sheep for facial nerve regeneration research have not yet been established because of the lack of methods for assessing the electrophysiological function of regenerated nerves. In this study, we developed a percutaneous measurement method for the evoked compound muscle action potential (CMAP) of the facial nerve in sheep.

Methods: Six 3-year-old castrated male Corriedale sheep were used in this study. Under general anesthesia, an anatomical exploration was performed to identify the course of the buccal branch of the facial nerve and its innervating muscles on one side, followed by the application of surface stimulating electrodes to the contralateral side of the face along the course of the buccal branch of the facial nerve to obtain CMAP measurements of the nasolabial levator muscle.

Results: Percutaneous CMAP measurements of the nasolabial levator muscle could be obtained in all animals by placing stimulating electrodes 1 cm apart on the line coinciding with the course of the buccal branch of the facial nerve revealed by the preceding anatomical exploration. Mean values for electrophysiological parameters were amplitude 4.7 ± 0.7 mV, duration 2.1 ± 0.6 ms, and latency 3.6 ± 0.4 ms.

Conclusion: We have established a percutaneous measurement method for CMAP of the buccal branch of the facial nerve in sheep. This method is expected to be very useful in future studies of facial nerve regeneration for long nerve defects in sheep.

Keywords: ADSC, adipose-derived stem cell; CFNG, cross-facial nerve grafting; CMAP, compound muscle action potential; Compound muscle action potential; Facial nerve; Nasolabial levator muscle; Paralysis.

Fig. 1

Localization of the facial nerve and mimetic muscles in sheep as determined by facial anatomical exploration. When electric stimulation was applied via the stimulating electrodes applied directly to the buccal branch of the facial nerve at the site where it emerged from the anterior margin of the parotid gland and coursed toward the nasolabial levator muscle, the movement of the exposed nasolabial levator muscle upon stimulation was visually confirmed. ∗: buccal branch; ■: marginal mandibular branch of facial nerve; †: parotid gland; ▲: nasolabial levator muscle; ★: zygomatic muscle.

Fig. 2

Sites of percutaneous measurement of CMAP of the buccal branch of the facial nerve in sheep and CMAP wave patterns. A. Stimulating electrodes were applied to two sites on the line connecting (a) the midpoint of the line connecting the outer edge of the orbit and the mandibular angle and (b) the most posterior molar. The recording microelectrode was inserted to a depth of about 1 cm at (c) the midpoint on the line connecting the nostril and the angle of the mouth, where the nasolabial levator muscle is located. CMAP, compound muscle action potential.

Fig. 3

Schematic diagram of the course of the buccal branch of the facial nerve and the localization of the nasolabial levator muscle in sheep. The buccal branch of the facial nerve emerged superficially from the parotid tissue at the midpoint of the blue dotted line connecting the outer edge of the orbit and the mandibular angle, coursed behind the zygomatic muscle in the posterior molar region, and entered into the nasolabial levator muscle. Red and blue squares indicate surface stimulating electrodes arranged to measure CMAP of the buccal branch of facial nerve. The recording microelectrode was inserted at the midpoint of the red dotted line connecting the nostril and the angle of the mouth. TB, temporal branch; ZB, zygomatic branch; BB, buccal branch; MMB, marginal mandibular branch of facial nerve; PG, parotid gland; SG, submandibular gland; DM, digastric posterior belly; MM, masseter muscle; FA & V, facial artery and vein; ZM: zygomatic muscle; NLM, nasolabial levator muscle.