The localization of the motorneurons innervating facial muscles and the masseter muscle was examined by the retrograde horseradish peroxidase (HRP) technique. Ten microliters of 50% HRP was injected into the orbicularis oculi muscle, the orbicularis oris muscle, the zygomaticus muscle and the masseter muscle of rabbits. At 48 hours after the HRP injection, perfusion fixation was done through the left ventricule of the heart under general anesthesia. The brain stem was removed and reacted with diaminobenzidine.
The nucleus of the orbicularis oculi muscle was distributed in the medial-dorsal portion of the facial nerve nucleus. On the other hand, the nucleus of the orbicularis oris muscle and the nucleus of the zygomaticus muscle were located in the lateral-ventral, and medial-ventral portions, respectively. The nucleus of the masseter muscle (trigeminal motornucleus) was located about 1.1mm proximal to the facial nucleus.
Experimental facial nerve palsy was produced by the following procedures.
The intratemporal facial nerve was exposed under local anesthesia and crushed for 20sec. using a hemostatic forceps.
Recovery from the facial nerve palsy was evaluated by evoked electromyography 3 months later. After another 3 months, HRP was injected into the zygomaticus muscle.
In animals that had recovered from the facial palsy, motorneurons labeled with HRP were found not only in the medial-ventral portion but also partially in the medial-dorsal portion, which was the position of the labeled nucleus of the orbicularis oculi muscle in control animals. However, there was no significant difference in the size or number labeled motorneurons between the control and recovered animals.
In 5 animals with synkinesis, hypolabeled premotorneurons of various size were observed bilaterally in the reticular formation form the pons through the medulla oblongata.
Synkinesis seems to result in premotorneurons which are newly promoted.
These findings suggest that the reticular formation is involved in the recovery from facial palsy.