Linking fast adaptation, slow adaptation, and mechanical creep in mammalian hair cell mechano-electric transducer channels

The ear relies on the transduction of sound input to mechano-electric transducer (MET) current by hair bundles (HBs) for normal hearing. These HBs, composed of hair-like stereocilia, have ion channels at the tip of all shorter stereocilia that enable transduction. Experiments have shown that HBs stimulated by a step-like fluid-jet force display a displacement creep post initial ascent, termed mechanical rise, and the MET current adapts with a single slow time constant. Under a step-like probe-actuated displacement stimulus, the creep disappears, and the current decays with two (fast and slow) time constants. Since adaptation plays a possible role in precluding cell damage and restoring HB sensitivity upon exposure to loud sounds, it is important to understand the underlying mechanisms for these seemingly disparate responses. We developed a single nonlinear model of an isolated mammalian HB capable of representing the mechanical and electrical response under both the slow fluid jet and much faster stiff probe stimulus. Linearizing the model enabled us to identify the three underlying system time constants (mechanical rise, fast, and slow adaptation), along with a mechanistic explanation of how these three different behaviors arise.