AHG: tip links

The singular sensitivity of cochlear outer hair cells suggests an extremely efficient exploitation of the energy supplied by the mechanical stimulus [Gitter & Klinke, 1989, Naturwissenschaften 76: 160-164]. If, however, the transduction channels were located at the 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 [Gitter, 1994, HNO 42: 327-333].

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 [Gitter et al., 1993, Naturwissenschaften 80: 273-276], even though tip links are absent between stereocilia [Gitter & Golz, 1993, Eur. Arch. Oto-Rhino-Laryngol., Suppl. 1993/II: 348-349].

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 d parallel to the longitudinal axis of the stereocilium when the hair bundle is deflected by an angle a.

Computation shows that d is proportional to a [Gitter, 1996, ORL 58: 1-3]. Thus, 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.

Figure 1 Different elastic elements (depicted as coil springs) connect the sterecilia with the cuticular plate and with neighboring stereocilia. It is generally assumed that, as shown here, the transduction channels (tc) are in series to the "vertical links", also called "tip links" (tl). External force (F) deflects the stereocilia. The mechanical work distributes to i) the reversable bending of the basis (sb), which anchors the stereocilum in the cuticular plate (cp), ii) the stretching of the "tip links", iii) the strain of the elastic elements that connect the horizontal links (hl) with the cytoskeleton (microfilaments) in the stereocilium, and iv) the conformational changes which bestowe elastic properties to the transduction channels.