Origins of the Crossed Olivocochlear Bundle Shown by an Acid Phosphatase Method in the Cat

Abstract

This study uses acid phosphatase to show that more than one cell type in the superior olive contributes to the crossed olivocochlear bundle. Rasmussen's anterograde degeneration findings point to the dorsomedial periolivary nucleus as a site of origin, but retrograde chromatolysis is not recognized there after cochlear ablations in cats. In other regions of the nervous system large increases of lysosomal enzyme levels have been demonstrated in the parent perikarya a few days after peripheral nerve section. Such changes occurred in the hindbrains of adult cats 4, 9, and 19 days after unilateral labyrinthectomy. Enzyme in the dorsomedial periolivary nucleus was related to a distinct red precipitate in the perikarya of the small elongate and large radiate neurons. In an unoperated control, precipitate appeared in 30% of both kinds of cells on each side. In the operated cats, precipitate also occurred in 30% of the cells (n=1000+) on each of the control sides, compared to 60–70% (n=1000+) on the opposite side. This result corresponds to 200% and 250% increases for elongate and radiate cells. The results did not vary from cat to cat or with the survival times used or the rostro-caudal level of the nucleus sampled. Other origins of the crossed olivocochlear bundle appear to be stellate cells in the ventral and medial trapezoid nuclei. Inasmuch as two distinct types of neurons contribute to the crossed olivocochlear bundle, their respective endings in the cochlea might express physiological differences. In the dorsomedial periolivary nucleus there are more olivocochlear neurons with small axons than with large axons. The small axons may have been overlooked in previous light microscopic observations of the olivocochlear bundles in the cochlea. Moreover, these two populations of olivocochlear neurons might have different innervation patterns in the organ of Corti.