The dual origins of the olivocochlear bundle in the albino rat

Abstract

Recent studies of the origins and terminations of the olivocochlear bundle (OCB) in the cat provide evidence that separate efferent systems differentially innervate the 2 types of hair cells in the organ of Corti. To begin to test the generality of these separate olivocochlear systems, the cells of origin of the OCB were determined in the albino rat by using axonal transport of horseradish peroxidase. Findings are that, as in the cat, 2 classes of olivocochlear (OC) neurons project to the cochlea. These neurons could be dichotomized according to their location in the superior olivary complex lateral or medial, their size small or large, and their preferred side of projection (ipsilateral or contralateral). All labeled OC neurons also exhibited a positive reaction for acetylcholinesterase. In the rat lateral and medial OC neurons are each restricted to a single nucleus, and the lateral OC neurons project only ipsilaterally. The rat also has significantly fewer mean totals of both lateral (240 vs. 710) and medial (240 vs. 520) OC neurons than the cat. These methods also demonstrated the course of axons of the OCB and axon collaterals entering the cochlear nuclear complex. Vestibular efferent neurons were labeled bilaterally medial and lateral to the facial genu. Results suggest that the general organizational plan of OC neurons in the rat offer advantages over the situation in the cat for studies of connectional neuroanatomy, neurophysiology and behavioral functions of the 2 olivocochlear systems.