Mossy fiber projections from the brain stem to the nodulus in the cat. An experimental study comparing the nodulus, the uvula and the flocculus.

Mossy fiber projections from the brainstem to the nodulus were studied by means of the retrograde transport of horseradish peroxidase (HRP) in the cat. The findings indicate exclusive secondary vestibular projections to the nodulus (96.5% of the total number of labeled cells in cat 1). Major sources of such projections include the caudal half of the medial and inferior vestibular nuclei, and the dorsal half of the superior vestibular nucleus. Groups-x and -z of the vestibular nuclei give rise to minor projections. The projections from groups-f and -y, and the interstitial nucleus of the eighth nerve are very few, if any. Minor extravestibular projections originate from the prepositus hypoglossal nucleus and the infratrigeminal nucleus. All these projections are bilateral. No other nuclei in the brainstem were labeled following HRP injection in the nodulus. No indications of mediolateral and dorsoventral differences in mossy fiber projections were found. There are quantitative differences in the sources of projections to the nodulus, the ventral uvula and the flocculus, all of which belong to the vestibulocerebellum. The largest sources for projections to the nodulus and the ventral uvula are from the vestibular nuclei. Among the vestibular nuclei, group-x provides the major projections to the ventral uvula. For the flocculus, in contrast, the major sources of input are the reticular formation and raphe nuclei. These quantitative differences may play an important role for differential functions of the nodulus, the ventral uvula and the flocculus.