The Auditory Nerve: Peripheral Innervation, Cell Body Morphology, and Central Projections

Abstract

In mammals, all known auditory information enters the brain by way of the cochlear division of the vestibulocochlear nerve, hereafter referred to as the auditory nerve. Primary neurons, whose cell bodies reside in the spiral ganglion of the cochlea, send peripheral processes out to the organ of Corti to contact the acoustic receptor cells; the central processes or axons bundle together to form the auditory nerve. The terminus of the auditory nerve is the cochlear nucleus. In this way, primary neurons convey the output of the receptors to neurons of the cochlear nucleus. There arc two types of receptors, inner hair cells and outer hair cells (Retzius 1884; Ramón y Cajal 1909), two populations of primary neurons (Munzer 1931; Spoendlin 1973), and many neuron classes in the cochlear nucleus (Lorente de Nó 1933; Osen 1969; Brawer, Morest, and Kane 1974). In turn, the cells of the cochlear nucleus give rise to all central auditory pathways. In a general way, the role of the cochlear nucleus is to receive incoming auditory nerve discharges, to preserve or transform the signals, and to distribute outgoing activity to higher brain centers. In order to understand the earliest stages of stimulus coding in the auditory system, we need to know (1) the nature of the signals conveyed by auditory nerve fibers, (2) their source in the periphery, and (3) their destination in the brain. This report shall review the progress that has been made along these lines of investigation.