Physiology of the Auditory Brainstem

Abstract

As described in the chapters in Volume I of this series, a striking feature of the auditory brainstem pathways is the highly divergent and convergent nature of the projections from the auditory nerve to the inferior colliculus (IC). The fibers of the auditory nerve (AN) bifurcate on entering the cochlear nucleus (CN) and make synapses on a number of morphologically different types of neuron in different divisions of the CN. Each of the major neuronal types that sends its axon out of the CN does so with a distinctive projection pattern that gives rise to further divergence. Some CN neurons project directly to the IC, whereas others project to various divisions of the superior olivary complex (SOC) and nuclei of the lateral lemniscus (NLL), which in turn project to the IC. The central nucleus of the IC (ICC; see definition below) consequently receives a highly convergent projection from more than 20 identified neuron types in approximately 10 major brainstem nuclei. This highly divergent and convergent pattern of projections is illustrated schematically in Figure 4.1 for the projections deriving from a restricted region of the basilar membrane (cf. Merzenich, Jenkins, and Middlebrooks 1984).