The occurrence of inappropriate co-contraction (synkinesis) of facially innervated muscles in humans is a common sequela of facial nerve injury and recovery. We have developed a system for studying facial nerve function and synkinesis in restrained rats using non-contact opto-electronic techniques that enable simultaneous bilateral monitoring of eyelid and whisker movements. Whisking is monitored in high spatio-temporal resolution using laser micrometers, and eyelid movements are detected using infrared diode and phototransistor pairs that respond to the increased reflection when the eyelids cover the cornea. To validate the system, 8 rats were tested with multiple 5-min sessions that included corneal air puffs to elicit blink and scented air flows to elicit robust whisking. Four rats then received unilateral facial nerve section and were tested at weeks 3-6. Whisking and eye blink behavior occurred both spontaneously and under stimulus control, with no detectable difference from published whisking data. Proximal facial nerve section caused an immediate ipsilateral loss of whisking and eye blink response, but some ocular closures emerged due to retractor bulbi muscle function. The independence observed between whisker and eyelid control indicates that this system may provide a powerful tool for identifying abnormal co-activation of facial zones resulting from aberrant axonal regeneration.

This study establishes normative data for assessing cranial nerve VII-controlled facial movement in four separate facial regions. We demonstrate the capability and tendency of animals to move their orbicularis oculi muscles independently of and simultaneously with their midfacial muscles. This model provides an excellent tool for the study of aberrant regeneration after facial nerve injury in the rodent.

Background Measuring rodent facial movements is a reliable method for studying recovery from facial nerve manipulation, and for examining the behavioral correlates of aberrant regeneration. We quantitatively compared recovery of vibrissal and ocular function following three types of clinically relevant nerve injury. Methods 178 adult rats underwent facial nerve manipulation and testing. In the experimental groups, the left facial nerve was either crushed, transected and repaired epineurially, or transected and the stumps suture-secured into a tube with a 2 mm gap between them. Facial recovery was measured for the ensuing 1–4 months. Data were analyzed for whisking recovery. Previously developed markers of co-contraction of the upper and midfacial zones (possible synkinesis markers) were also examined. Results Animals in the crush groups recovered nearly normal whisking parameters within 25 days. The distal branch crush group showed improved recovery over the main trunk crush group for several days during early recovery. By week 9, the transection/repair groups showed evidence of recovery that trended further upward throughout the study period. The entubulation groups followed a similar recovery pattern, though they did not maintain significant recovery levels by the study conclusion. Markers of potential synkinesis increased in selected groups following facial nerve injury. Conclusions Rodent vibrissial function recovers in a predictable fashion following manipulation. Generalized co-contraction of the upper and midfacial zones emerges following facial nerve manipulation, possibly related to aberrant regeneration, polyterminal axons, or hypersensitivity of the rodent to sensory stimuli following nerve manipulation.