[Are tip links the basis for mechanosensitivity of hair cells?]

The singular sensitivity of cochlear outer hair cells suggests an extremely efficient exploitation of the energy supplied by the mechanical stimulus. If, however, the transduction channels were located at one end of the hair bundle's tip links, not more than 1/250 of the stimulus energy could be used to change the open probability of the channels. Furthermore, the mechanosensitive nematocytes of Hydra vulgaris possess a sensory hair bundle with horizontal links and have a transduction mechanism with functional properties similar to those of hair cells, even though tip links are absent between stereocilia. Therefore, I propose that the transduction channels of hair cells are connected to the short row-to-row horizontal links at the distal ends of neighboring stereocilia. These links, as well as the tip links, are oriented in accordance with the directionality of hair cell mechanosensitivity. The elastic elements connected with the horizontal links receive a larger part of the stimulus energy than the tip links. Since the short horizontal links resemble rigid rods rather than "spare springs", the points of insertion of these links in the stereocilia (i.e., the transduction channels) must move a distance upsilon parallel to the longitudinal axis of the stereocilium when the hair bundle is deflected by an angle phi. Computation shows that upsilon is proportional to phi. In the stereocilium, elastic elements ("gating springs") connect the cytoskeleton and transduction channel. The force in the gating springs that is counteracting the movement of the transduction channels may be varied in the process of adaptation by an active motor, as has been proposed in other investigations.