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Structure and function of cochlear afferent innervation

Purpose of reviewFor the perception of sound, acoustic signals need to be encoded into a neuronal code. This takes place at the inner hair cells of the organ of Corti and the afferent fibres of the auditory nerve. We will review the current knowledge of the anatomy and function of these elements as well as their connection – formed by the afferent inner hair cell synapse. Recent findingsDepending on their tonotopic location, inner hair cells are innervated by 5–30 dendrites of spiral ganglion neurons. Electrophysiological recordings from single fibres demonstrate – apart from a high-frequency selectivity – a pronounced heterogeneity in their response to sound of varying intensity. The source as well as the function of this heterogeneity is not well understood, but recent publications have suggested several mechanisms, including variations in the presynaptic Ca2+ influx and subsequent transmitter release, the postsynaptic sensitivity to neurotransmitter and electrical as well as anatomical variability of single fibres. These mechanisms might act together to expand the dynamic range of sound that can be encoded. SummaryClassical studies as well as recent publications demonstrate that sound encoding at the inner hair cell afferent synapse involves mechanisms leading to tonotopic frequency separation and distribution of intensity coding over many neuronal channels.