The Evolution of Auditory Pathways Underlying Human Hearing

Abstract

The projections from the superior olivary complex to the inferior colliculus (IC) were studied in five diverse mammalian species, including monodelphis and didelphis opossum, rat, ferret, and bushbaby, and compared to the same projections in cat. In each animal, injections of tritiated-glycine or horseradish peroxidase (HRP) were made in IC, and retrogradely-labeled neurons in the lateral superior olive (LSO), medial superior olive (MSO), and the dorsomedial periolivary nucleus (DMPO) or its homologue the superior paraolivary nucleus (SPN) were examined. While HRP was used to demonstrate the totality of projections from these nuclei, tritiated-glycine was used to demonstrate the subset of these projections which are glycinergic, and hence, inhibitory. The results confirmed earlier work showing that in each animal, LSO sends both uncrossed and crossed projections to IC. Although the relative proportion of projections from LSO to each IC varied among species, in each, both sets of projections are substantial. This is the first across-species comparison showing that the uncrossed projections from LSO to IC in all these species are largely glycinergic, while the crossed projections are not. In addition, this study shows that these projections arise from different areas in LSO in each species. Similarly, in each species, MSO sends both uncrossed and crossed projections to IC. In contrast to the LSO projections, however, the relative proportion of projections from MSO to the ipsilateral and contralateral IC varies widely. Consistent with the reports of others, in the placentals, the majority of projections from MSO were found to terminate in the ipsilateral IC. However, the marsupials differed from placentals in having a strong bilateral projection from the MSO. That is, in the marsupials, MSO projections terminate more or less equally in the ipsilateral and contralateral IC. SPN/DMPO was labeled ipsilateral but rarely contralateral to the IC injection and was found to have a glycinergic projection to IC.