The role of the brain in the functional development of the ear

Abstract

The olivocochlear bundle (OCB) invades and organizes within the organ of Corti around the time of birth in cats, roughly coincident with the first detectable sound-elicited responses from the cochlea or auditory nerve. A model of innervation is proposed, in which the medial OCB (MOC) regulates the development of both inner (IHC) and outer hair cell (OHC) systems. In stage 1, early arriving fibers of the MOC make transient axosomatic synaptic contact with IHCs, rhythmically modulating their transduction currents. Operating through an excitatory synapse, the MOC contributes to an activity-dependent system through which exuberant auditory nerve fiber contacts with IHCs are refined. In stage 2, MOC fibers detach from IHCs, migrate across the tunnel of Corti, and contact OHCs forming their well recognized calyces with the basolateral wall of the cell, and in the process ‘‘energize’’ the cochlear amplifier. By removing the OCBs influence at birth, prior to the completion of its innervation of the cochlea, its role during development can be evaluated. The otopathology resulting from neonatal cochlear differentiation is consistent with the overall predictions that would be made based upon the proposed model. [Work supported by NIH/NIDCD.]