Secondary vestibulocerebellar projections to the flocculus and uvulo-nodular lobule of the rabbit: a study using HRP and double fluorescent tracer techniques.

The distribution of vestibular neurons projecting to the flocculus and the nodulus and uvula of the caudal vermis (Larsell's lobules X and IX) was investigated with retrograde axonal transport of horseradish peroxidase and the fluorescent tracers Fast Blue, Nuclear Yellow and Diamindino Yellow. The presence of collateral axons innervating the flocculus on one hand and the nodulus and uvula on the other was studied with simultaneous injection of the different fluorescent racers. The distribution of vestibular neurons projecting to either flocculus or caudal vermis is rather similar and has a bilateral symmetry. The projection from the magnocellular medial vestibular nucleus is very sparse, while that from the lateral vestibular nucleus is absent. The majority of labeled neurons was found in the medial, superior, and descending vestibular nuclei, in that order. Double labeled neurons were distributed in a similar way as the single labeled ones. Labeled neurons project to the nodulus and uvula, the flocculus, and to both parts of the cerebellum simultaneously in a ratio of 12:4:1. Five different populations of vestibulocerebellar neurons can be distinguished on the basis of their projection to the: (1) ipsilateral flocculus, (2) contralateral flocculus, (3) ipsilateral flocculus and nodulus/uvula, (4) contralateral flocculus and nodulus/uvula, and (5) nodulus/uvula.