Central vestibular projections of primary cervical fibers in the frog.

The origin of cervicovestibular inputs was documented in frogs, as well as the number of fibers, site of projection, and their distribution within the nuclei. The first spinal nerve in 15 frogs was labeled with extracellular injections of horseradish peroxidase. The brain stem and the posterior root were sectioned serially. The trajectories of the fibers in the central nervous system were reconstructed, and the number and diameters of the fibers in the posterior root were determined. The average number of fibers in the first posterior root was 143 +/- 6.2, their number decreasing exponentially with increased diameter. After entering the spinal cord the fibers were located in the dorsal funiculus. The thick and medium-sized fibers coursed medially in relation to the thin ones, giving collaterals to the spinal cord and to the obex region. The thinnest fibers projected to the reticular formation and nucleus of the solitary tract. Only collaterals from fibers of medium and thick caliber reached the vestibular area in their trajectory to the cerebellum (spinocerebellar fibers). All the vestibular nuclei received collaterals and endings from the spinocerebellar fibers, the ventral nucleus being the most innervated. The total number of branches for the vestibular area, however, was very small. The results of this experiment are correlated with physiological and anatomical findings described in the literature.