Projections from the superior olivary complex to the cochlear nucleus in the tree shrew

The origins and targets of projections from the superior olivary complex to the cochlear nuclei were studied in the tree shrew by placing small injections of horseradish peroxidase (HRP) in the cochlear nucleus and small injections of 3H-leucine in the superior olivary complex. The results show that the descending pathways originate in periolivary cell groups surrounding the medial and lateral superior olives and that periolivary nuclei differ from one another in their patterns of projections to the cochlear nucleus. For example, cell groups may project either ipsilaterally or bilaterally. Cells in the lateral nucleus of the trapezoid body project only to the ipsilateral cochlear nucleus. Other periolivary cell groups project bilaterally, although some of these may project more heavily to one side than the other. Some pathways have widespread targets in the cochlear nucleus whereas others have relatively specific targets. Diffuse projections to all divisions of the cochlear nucleus arise from the lateral nucleus of the trapezoid body ipsilaterally and from the medial perioliviary nucleus bilaterally. The targets of other descending pathways are more restricted. The anterolateral, dorsal, and dorsolateral periolivary nuclei project mainly to the anteroventral cochlear nucleus; the ventral nucleus of the trapezoid body and the posterior periolivary nucleus project mainly to the dorsal and posteroventral cochlear nuclei. All of these specific projections are bilateral. These results suggest that projections from the periolivary cell groups to the cochlear nucleus consist of multiple components with different degrees of specificity.