Vestibular primary afferent projection to the cerebellum of the rabbit.

The vestibular primary afferent projection to the cerebellum of the rabbit was studied with retrograde and orthograde tracers. We injected individual lobules of the cerebellum with horseradish peroxidase (HRP) or wheat germ agglutinin-HRP (WGA-HRP). Following these injections the numbers of labeled and unlabeled cells in Scarpa's ganglion were counted. Approximately 64-89% of the cells in Scarpa's ganglion were labeled retrogradely following uvula-nodular injections. About 2% of the cells in the ipsilateral Scarpa's ganglion were labeled after injections of the flocculus. Virtually no cells were labeled following injections of the ventral paraflocculus. The vestibular primary afferent projection to the uvula-nodulus is so extensive that it must be part of a collateral system that also innervates the vestibular nuclei. This collateral projection pattern was confirmed by using fluorescent tracers injected into the uvula-nodulus and vestibular complex. Fluorogold was injected into the uvula-nodulus and peroxidase-rhodamine isothiocyanate was injected into the vestibular complex. More than 50% of the neurons in Scarpa's ganglion were double labeled by these subtotal injections. The dense vestibular primary afferent projection to the uvula-nodulus was confirmed by using the C fragment of tetanus toxin (TTC) injected into the labyrinth as an orthograde tracer. With the TTC technique, the vestibular primary afferent projection to the uvula-nodulus terminated exclusively in the ipsilateral granule cell layer of lobules 9d and 10. Much sparser vestibular primary afferent projections were found in the banks of major cerebellar sulci. A barely detectable projection was found to the flocculus and ventral paraflocculus.