Projections from the cochlear nucleus to the cerebellum.

The connections between the brainstem auditory system and the cerebellar auditory area in the posterior vermis (VI, VII, and VIIIA) were studied using the horseradish peroxidase technique. Injections also were made in the cerebellar hemispheres, the paramedian and the vestibulocerebellum (VIIIB, IX). Labeled cells were found in the cochlear nuclei bilaterally, but not in the superior olivary complex or the inferior colliculus. As the injection sites moved more laterally from the vermis area to the paramedian and the cerebellar hemispheres, the number of labeled cochlear nucleus neurons decreased progressively. This observation suggests that the terminals of the cochlear-nucleus-cerebellum projection tend to concentrate in the vermis auditory area. In the cochlear nucleus, labeled cells did not appear to belong to a single, homogenous cell type, nor did the cells group into a single area in the cochlear nucleus. The total number of labeled neurons in one cochlear nucleus did not exceed 200 in our largest injection.