Ascending and descending projections from the superior olivary complex in guinea pigs: different cells project to the cochlear nucleus and the inferior colliculus.

Abstract

The superior olivary complex is a source of ascending projections to the inferior colliculus and descending projections to the cochlear nucleus. We used multiple-labeling techniques with fluorescent retrograde tracers to determine whether individual superior olivary cells project to the inferior colliculus and the cochlear nucleus. Almost all labeled cells contained one tracer, suggesting that they projected to only one of the injected targets. A small number of cells sent collateral projections to the ipsilateral cochlear nucleus and ipsilateral inferior colliculus. The double-labeled cells constituted fewer than 2% of the cells that projected to the cochlear nucleus or to the inferior colliculus. There was no evidence for cells projecting to both contralateral targets or to one ipsilateral target and one contralateral target. We conclude that the ascending projections to the inferior colliculus and the descending projections to the cochlear nucleus arise almost exclusively from separate populations of cells in the superior olivary complex. Their separate origins suggest that these projections are sending different information to higher and lower centers of the auditory pathways.