Differential mossy fiber projections to the dorsal and ventral uvula in the cat.

The brainstem afferents to the uvula were studied by using retrograde axonal transport of horseradish peroxidase in the cat. Findings indicate differential afferent projections to the ventral and dorsal uvula. Major sources projecting to the ventral uvula include the caudal parts of the medial and inferior vestibular nuclei, the x- and f-groups of the vestibular nuclei, the dorsal and central parts of the superior vestibular nucleus, the rostral dorsomedial part of the paramedian nucleus of the pontine nuclei, the caudal part of the prepositus hypoglossal nucleus, and the infratrigeminal nucleus. Labeled cells in the vestibular nuclei were 74.7% of the total number of labeled cells in cat 40. On the other hand, the major sources projecting to the dorsal uvula are the peduncular, paramedian, and lateral nuclei of the pontine nuclei at the rostral and intermediate levels. Labeled cells in the pontine nuclei comprised 82.1% of the total number of labeled cells in cat 1. Findings also indicate that the lateral part of the ventral uvula receives input mainly from the pontine nuclei, whereas the medial part of the ventral uvula receives input mainly from the vestibular nuclei. Mediolateral differences were not found for the dorsal uvula. These mossy fiber zones are mediolaterally wide, with a dorsoventral partition in the uvula, in contrast to the climbing fiber zones, which are narrow (about 0.4 mm) and extend longitudinally throughout the uvula. There are quantitative differences in afferent sources to the ventral uvula and flocculus, both of which belong to the vestibulocerebellum. The largest afferent sources for the ventral uvula are the vestibular nerve and nuclei, whereas the largest sources for the flocculus are the reticular formation and raphe nuclei. These quantitative differences may have an important role for differential functions between the ventral uvula and flocculus. It has been suggested that the ventral uvula controls the velocity storage integrator of the vestibuloocular and optokinetic reflexes, whereas the flocculus is responsible for rapid changes of eye velocity in these reflexes.