Determining the identity of the cochlear amplifier: electrical stimulation of the tecta mouse cochlea

The sensitivity, large dynamic range and narrow frequency tuning of the mammalian cochlea is determined by the passive mechanical properties of the basilar membrane (BM) and active feedback from the outer hair cells (OHCs). Two mechanisms have been proposed to provide amplification: Hair bundle motility, and OHC somatic-motility. Acoustically- and electrically-elicited mechanical responses were measured from the BMs of the cochleae of wild type and genetically modified mice where the hair bundles are freestanding and cannot react against the tectorial membrane (TM) to contribute to amplification. We found the electrically elicited responses in mutant mice, where only somatic motility can provide amplification, to be remarkably similar to acoustical and electrical responses in the wild type animals. We, therefore, conclude that somatic, not stereocilia motility is the basis of the cochlear amplifier.