The Functional Contributions of Links in Mammalian Cochlear Hair Bundles

In the mammalian cochlea, hair bundles of the sensory outer and inner hair cells detect mechanical signals. A hair bundle comprises a set of rod-like stereocilia that pivot around their insertion points in the hair-cell's apex. Stereocilia are linked by gating springs and connectors, also known as top or shaft connectors, side, lateral, or ankle links. Gating springs link neighboring stereocilia of differing height, while connectors link all neighboring stereocilia. Sound-induced gating-spring oscillations open and close mechanoelectrical transduction channels attached to the gating springs, causing oscillations in the hair cell's receptor current. In contrast to gating springs, connectors are not attached to channels and their functional role is unclear. To determine how the specific properties of gating springs and connectors contribute to outer-hair-cell bundle function, we use a computational model of an outer-hair-cell bundle, which accounts for nonlinear hair-bundle splaying at rest, nonlinear fluid forces on stereocilia, and nonlinear channel gating. The model is validated by reproducing many experimental observations, including stereocilium splaying at rest and hair-bundle stiffness decreases caused by breaking gating springs or connectors. We discuss how varying the gating-spring and connector stiffnesses affects the receptor current in response to stimulation at the characteristic frequency of the hair cell.