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Nestled inside the inner ear is the cochlea, the snail-like organ that houses the sensory hair cells responsible for transducing sound waves into electrical impulses. Hairlike stereocilia on the upper surface of these sensory cells project into a cavity filled with a fluid called endolymph (see the figure). On reaching the inner ear, sound waves deflect the stereocilia causing the transducer channels near their tips to open. Potassium ions flood from the endolymph into the hair cells, depolarizing their cell membranes and initiating the electrical signal that is carried along the auditory nerve to the brain. The canon balance in the endolymph is such that the K+ concentration is high and the Na+ concentration is low. Cochlear endolymph is maintained at a high resting potential (the endocochlear potential). The voltage gradient-from the positive endocochlear potential to the negative potential inside the hair cell-drives the K- flow, but must be continuously maintained by rapid recycling of K+ back into the endolymph. In the latest of a series of studies showing the importance of K+ recycling for normal hearing, Minowa and colleagues (1) report on page 1408 that an abnormality in cochlear fibrocyte cells that recycle K+ contributes to hearing impairment in a mouse model of one form of human deafness.
Recycling of K+ in the cochlear duct has long been thought to be important for hearing (2). At least six of the many proteins associated with deafness in humans and mice are probably directly involved in K+ recycling (see the figure). A potassium channel (encoded by the KCNQ4 gene, which is mutated in a form of dominant, progressive hearing loss) in outer hair cells is thought to transport K+ ions out of the cell (3). The K+ ions are then taken up by the supporting hair cells below. From here they pass through a network of gap junctions that extends from the epithelial supporting cells to the mesenchymal fibrocytes that form the spiral ligament and then to the epithelial marginal cells of the stria vascularis, which secrete endolymph. Connexin 26 (GJB2) and connexin 31 (GJB3) are components of these gap junctions that when...