Stimulation of wild-type and double mutant hair bundles. (A) Wild-type hair bundle. Stimulation of the bundle in the positive direction (right; “Normal polarity stimulus”) puts tension on tip links, which tug open transduction channels. SC, stereocilium; KC, kinocilium. (B) Tmc1dn/dn;Tmc2Δ/Δ hair bundle after stimulus-induced bundle “destruction.” Although not described in detail in Kim et al. (2013), we presume that means a bundle that has lost all connections between its cilia. Stimulation of the bundle in the negative direction with a fluid jet stimulator (right; “reverse polarity stimulus”) leads to stretching on the positive side of each stereocilium and the kinocilium and compression on the negative side. Here, channels are depicted as localizing only to the kinocilial base, although there is no direct evidence for this model. Channels could be located in the stereocilia bases as well. In addition, this diagram illustrates channel activation as occurring through membrane stretch, although it is plausible that compression could open channels as well.
