Vestibular nerve regeneration in the bullfrog, Rana catesbeiana: A morphologic and physiologic description of the peripheral dendrites

Experiments utilizing American bullfrogs were designed to provide a better understanding of the potential for injury-induced regeneration of the vestibular nerve. Afferent morphology and physiology were characterized and the specificity of reinnervation were determined following complete peripheral vestibular nerve transection.;In Chapter I morphometric analyses were utilized to evaluate the axon diameter distribution of Regenerated afferents following peripheral vestibular nerve transection. Total fiber count and axon diameter measurements were obtained at weekly intervals up to 16 weeks. ANOVA revealed that the total fiber count returned to Control values by 4 weeks. However, the Regenerated nerve (4-16 weeks) exhibited a reduction in axon diameter. Thus, the diversity in axon diameter was not preserved following centrifugal vestibular nerve regeneration.;In Chapter II electrophysiological characteristics of 221 Regenerated canal afferents (16 weeks post-transection) were compared to 175 afferents in Sham-operated Controls. Afferent discharge CV and response phase to 0.05 Hz sinusoid stimulation in the Regenerated condition did not differ from Control. However, response gain was reduced (ANOVA, p ;In Chapter III the issue of reinnervation specificity was addressed. Tract tracing experiments revealed that Regenerated fibers nonspecifically reinnervated the various peripheral receptors denervated by nerve transection. Thus, the connections between the afferent fibers and the epithelium are dissociated in the Regenerated condition. However, despite the dissociation between afferent fibers and epithelial topography, morphophysiologic evaluation revealed that the association between epithelial topography and afferent physiology remained preserved.;The data not only provide a greater understanding of the centrifugal regenerative potential of the bullfrog vestibular nerve but also contribute to the understanding of afferent physiology. By 16 weeks the parallel reduction in afferent gain and axon diameter is suggestive that the diversity in axon diameter influences the diversity in gain. However, the preservation of afferent CV and response phase indicates that diameter-independent factors contribute to their diversity. For example, the preserved association between afferent phase and epithelial topography despite nonspecific reinnervation argues in favor of a greater emphasis on epithelial contributions to phase.