Distribution of cochlear efferents and olivo-collicular neurons in the brainstem of rat and guinea pig. A double labeling study with fluorescent tracers.

In rat and guinea pig, cochlear efferents to the two ears were labeled simultaneously with different fluorescent tracers. It was found that in both species only few (1-3%) olivo-cochlear neurons were double-labeled and project to both cochleae. In most periolivary regions large olivocochlear neurons (OCN) projecting to the ipsilateral and contralateral side are intermingled and form a continuous cell column between the facial nucleus and lateral lemniscus. In a second series of experiments in rat, cochlear efferents and ascending olivo-collicular neurons were labeled. Olivo-cochlear and olivo-collicular neurons are intermingled in the lateral superior olive (LSO) and in the ventromedial periolivary region. No double-labeled neurons were found that project to the cochlea and the inferior colliculus.