The primary vestibulocerebellar projection in the rabbit: absence of primary afferents in the flocculus.

The central projection of vestibular nerve fibers was investigated with anterograde axonal transport of wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) and tritiated leucine following injection in the vestibular ganglion. Labeled fibers and terminal ramifications were observed throughout the vestibular complex, but absent from the lateral vestibular nucleus. Termination in the cortex was restricted to the vermis. Small numbers of mossy fiber terminals were present bilaterally, close to the midline in lobules I and II, and in the depth of the main fissures separating lobules II-VI. In the posterior vermis labeled mossy fiber terminals were found in lobule X and the ventral aspect of lobule IXd. Here, the entire ipsilateral hemivermis contained a large number of terminals, while contralaterally the medial one-third hemivermis contained fewer terminals. Labeled mossy fibers and terminals were absent in the flocculus and adjacent ventral paraflocculus.