Blink reflex in facial-hypoglossal anastomosis

Motoneuron adaptability to new motor tasks following two types of facial-facial anastomosis in cats

The ability of the facial motor system to adapt to a new motor function was studied in alert cats after unilateral transection, 180 degrees rotation and suture of the zygomatic nerve, or transection and cross-anastomosis of the proximal stump of the buccal nerve to the distal stump of the zygomatic nerve. These procedures induced reinnervation of the orbicularis oculi (OO) muscle by different OO- or mouth-related facial motoneurons. Eyelid movements and the electromyographic activity of the OO muscle were recorded up to 1 year following the two types of anastomosis. Animals with a zygomatic nerve rotation recovered spontaneous and reflex responses, but with evident deficits in eyelid kinematics, i.e. the proper regional distribution of OO motor units was disorganized by zygomatic nerve rotation and resuture, producing a permanent defect in eyelid motor performance. Following buccal-zygomatic anastomosis, the electrical activity of the OO muscle was recovered after 6-7 weeks, but air puff-, flash- and tone-evoked reflex blinks never reached the control values on the operated side. Electromyographic OO activities and lid movements corresponding to licking and deglutition activities were observed on the operated side in buccal-zygomatic anastomosed animals up to 1 year following surgery. Mouth-related facial motoneurons did not readapt their discharges to the kinetic, timing and oscillatory properties of OO muscle fibres. A significant hyper-reflexia was observed following both types of nerve repair in response to air puffs, but not to light flashes or tones. In conclusion, adult mammal facial premotor circuits maintain their motor programmes when motoneurons are induced to reinnervate a foreign muscle, or even a new set of muscle fibres.

Axon reflexes or ephaptic responses simulating blink reflex R1 after XII-VII nerve anastomosis

It has been claimed that functional recovery of the blink reflex occurs after hypoglossal-facial nerve anastomosis. This has been explained through central nervous system plasticity and reorganization of neuronal connections. In 5 patients with reinnervated facial muscles after hypoglossal-facial nerve anastomosis we observed "R1-like" responses that fulfilled criteria for facial nerve axon reflexes or ephapses. First, displacement of the stimulating electrode from the supraorbital to zygomatic area shortened the latency of the evoked response. Second, these responses were stable (jitter mean consecutive difference < 25 microsec) and they had complex potential shapes unmodified by high-frequency stimulation. Finally, collision techniques demonstrated antidromic conduction of impulses in the facial nerve from supraorbital to zygomatic points. Therefore, these "R1-like" responses are not the early component of a functionally recovered blink reflex but motor axon reflexes or ephaptic responses similar to the short latency responses observed following facial nerve regeneration or from sutured nerves in human forearms.

Changes in eye blink responses following hypoglossal-facial anastomosis in the cat: evidence of adult mammal motoneuron unadaptability to new motor tasks

Hypoglossal-facial anastomosis is used in humans to restore the activity of the mimic musculature following irrecoverable facial nerve lesions. As eyelid movement kinetics is very well known, we have used this experimental model in cats to follow the evolution of blink responses and the adaptability of hypoglossal motor pools to new motor tasks. Although the electromyographic activity of the orbicularis oculi muscle in response to corneal air puffs, flashes of light or electrical stimulation of the supraorbital nerve was not recovered in the seven months following this crossed anastomosis, reflex blinks were got back by the increased activity of the retractor bulbi and extraocular recti muscles. The lid of the anastomosed side oscillated in perfect synchronization with tongue movements during licking, while it was severely affected in its motor function during optokinetic stimulation because of the spontaneous appearance of tongue-related hypoglossal activity. Present results suggest that adult mammal motoneurons are unable to readapt their motor programs to the kinetic needs of new motor targets and that most of the functional recovery observed in the cat was achieved by the compensatory hyperactivity of motor systems not directly affected by the surgery.