Weak lateral coupling between stereocilia of mammalian cochlear hair cells requires new stimulus methods to study the biomechanics of hearing

Abstract

The forces felt by different transduction channels in a bundle depend critically on how well stereocilia remain cohesive during deflection. In the bullfrog saccule, sliding adhesion mediated by horizontal top connectors (HTC) confers coherent motion to hair cell stereocilia and parallel gating to all transduction channels. In cochlear inner and outer hair cells (IHCs and OHCs), the mature complement of HTC is established by postnatal day 12; they extend between adjacent stereocilia of both rows and columns. Contrary to our expectation that bundle cohesion should be robust in all directions, our experiments on gerbil cochleas show that lateral coupling among stereocilia of the tallest row is weak. These findings suggest that the function and molecular composition of the HTC in the cochlea are different from those in bullfrog hair cells. They also raise concern for current stimulus methods, which involve glass probes that are often small compared to the bundle width. Our data suggest that only stereocilia in contact with the probe are stimulated, and delivery of the stimulus to the remaining stereocilia is weak and inhomogeneous. To mimic the OHC stimulus delivered by the overlying tectorial membrane in vivo, we are developing new stimulation technologies.