Facial Nerve Innervating Pinnae Muscles of the Gerbil: Three-Dimensional Construction with Respect to Neighboring Structures.

Abstract

The Mongolian gerbil (Meriones unguiculatus) is an animal model of epilepsy in which the epileptic behaviors develop along with chronological development. In this study, to clarify the mechanism of epileptogenesis in the gerbil, the developmental change of the behaviors elicited by posture change was followed in the previously established animals of a seizure-sensitive strain and a-resistant one. Posture change is a strong inducer of seizure in the adult animals of the sensitive strain. The results show that a bilateral and synchronized rhythmical movement of the pinnae was induced in the young animals of the sensitive strain, which seemed to correspond to an earlier stage, if not the first, in the establishment of epileptogenesis. In contrast, posture change did not induce either the adult-type seizure or the characteristic movement of the pinnae in the animals of the resistant strain. As the first step in clarifying the reason for this difference between these two strains, we studied the structure of the facial nerve which innervates the muscle of the pinnae, using consecutive paraffin sections of decalcified heads of both strains.In both strains, the facial nerve left the brainstem, ran laterally along the vestibular nerve bundle and at the geniculate ganglion, it changed its direction posteriorly and laterally. When it left the skull, it changed its direction again and descended ventrally, extending a branch to the ear on its way. In the seizure-resistant gerbils, this branch entered the muscle in a similar way to the seizure-sensitive animals. Therefore, these two strains probably have difference (s) in other components although the possibility of a functional difference in the facial nerve remains.