Differential projections from the vestibular nuclei to the flocculus and uvula-nodulus in pigeons (Columba livia)

The pigeon vestibulocerebellum is divided into two regions based on the responses of Purkinje cells to optic flow stimuli: the uvula-nodulus responds best to self-translation, and the flocculus responds best to self-rotation. We used retrograde tracing to determine whether the flocculus and uvula-nodulus receive differential mossy fiber input from the vestibular and cerebellar nuclei. From retrograde injections into the both the flocculus and uvula-nodulus, numerous cells were found in the superior vestibular nucleus (VeS), the cerebellovestibular process (pcv), the descending vestibular nucleus (VeD), and the medial vestibular nucleus (VeM). Less labeling was found in the prepositus hypoglossi, the cerebellar nuclei, the dorsolateral vestibular nucleus, and the lateral vestibular nucleus, pars ventralis. In the VeS, the differential input to the flocculus and uvula-nodulus was distinct: cells were localized to the medial and lateral regions, respectively. The same pattern was observed in the VeD, although there was considerable overlap. In the VeM, the majority of cells labeled from the flocculus were in rostral margins on the ipsilateral side, whereas labeling from uvula-nodulus injections was distributed bilaterally throughout the VeM. Finally, from injections in the flocculus but not the uvula-nodulus, moderate labeling was observed in a paramedian area, adjacent to the medial longitudinal fasciculus. In summary, there were clear differences with respect to the projections from the vestibular nuclei to functionally distinct parts of the vestibulocerebellum. Generally speaking, the mossy fibers to the flocculus and uvula-nodulus arise from regions of the vestibular nuclei that receive input from the semicircular canals and otolith organs, respectively.