Anatomy of the Auditory Brainstem

Abstract

Although specialized anatomical features are present in the auditory systems of many echolocating bats, the basic structure of their central auditory system is the same as that of other mammals. In all species a well-defined tonotopic organization is preserved throughout successive levels. The bat differs from other mammals mainly in that its range of hearing extends to unusually high frequencies and that certain frequencies that are important for echolocation are overrepresented. The cochlear nucleus conforms to the general mammalian plan and consists of three major divisions that give rise to divergent projections to the superior olivary complex, to the nuclei of the lateral lemniscus and directly to the inferior colliculus. However, in echolocating bats, certain cell groups within these pathways are unusually large and well differentiated. In the introduction we pointed out that some pathways are designed to provide convergence of signals from the two ears whereas others provide separation of the inputs from the two ears; we refer to these two types of pathways as binaural and monaural, respectively. Virtually all of the brainstem auditory pathways, whether binaural or monaural, converge at a common destination in the inferior colliculus. Thus, in every species, a major challenge in the study of auditory pathways is the question of how signals, relayed via many different parallel routes, interact in the mid-brain to produce an output that synthesizes the information from all these sources.