CENTRIPETAL REGENERATION OF THE 8TH CRANIAL NERVE ROOT WITH SYSTEMATIC RESTORATION OF VESTIBULAR REFLEXES

Abstract

After intracranial transection of the 7th and 8th cranial nerve root complex in 17 adult and 11 larval anuran amphibians, the severed nerves, which in the periphery supply the various end organs of the labyrinth, regenerated centripetally into the vestibular centers of the medulla where they succeeded in reestablishing functional reflex connections. Although there were at least 11 different types of afferent fibers intermingled at the region of severance with ample opportunity among them for a completely chaotic interspersion into abnormal pathways and with only negligible opportunity for particular fiber types to be directed to their proper terminations by mechanical factors alone, the central reflex relations were nevertheless restored consistently in an orderly systematic manner. There were no functional indications of the formation of any abnormal central synaptic connections. Very probably the regulatory factors involved are similar to those which operate in the pre-functional development of reflex associations in ontogeny. Differential contact affinities between the different types of ingrowing axons and the neurons of the vestibular centers is suggested as a possible explanation. The required constitutional specification of the vestibular nerve fibers could conceivably arise embryologically through induction effects of end organ differentiation in the labyrinth.