The anatomy of the cochlear nuclei and superior olivary complex of arboreal Australian marsupials.

Cytoarchitectural and morphometric analyses were carried out on the cochlear nuclear and superior olivary complexes of nine representative possums and gliders, members of a large group of nocturnal, arboreal Australian marsupials, many of which have well developed vocalizations. The cochlear nuclear complex was displaced medial to the restiform body in all species; this has previously been reported in other marsupials. The dorsal cochlear nucleus was generally very much larger than any other nucleus in this complex. A small cochlear nerve root nucleus was present in all species, a feature shared with rodents. The anteroventral cochlear nucleus was present throughout almost the entire rostrocaudal extent of the complex. The component nuclei of the superior olivary complex had similar positional relationships to those in eutherians. The lateral superior olive was the largest nucleus, having a volume usually greater than the sum of the volumes of the medial superior olive and medial nucleus of the trapezoid body. The smaller species had a very much larger number of neurons in the superior olive relative to brain size than did the larger species. A similar disproportion was demonstrated between cochlear nucleus volume and brain weight.