Static length changes of cochlear outer hair cells can tune low-frequency hearing
Fig 3
Direction of motion of the Hensen cells.
(A) Confocal microscopy shows the motion of the reticular lamina when a negative externally-applied current is switched to a positive current of equal magnitude, causing contraction of the outer hair cells. The green arrows show the displacement for the first and third row of outer hair cells (the displacement of the second row was similar to the first row). A pivot point emerges between the second and third row of outer hair cells: the first and second row move towards the basilar membrane whereas the third row moves away from it, following the displacement of the Hensen cells [20]. (B) Direction of displacement of the third row of outer hair cells. In this angle histogram, 0° corresponds to motion directed to the right in the image shown in panel A. According to morphometric measurements by Kelly, the basilar membrane is inclined by 37.26° on average with respect to the reticular lamina (dashed line) [42]. Our own measurements from anatomical 3D-reconstructions indicate that this inclination is slightly, but significantly, larger in the undamaged organ of Corti of our in vitro cochlear preparation (42.77° ± 6.43°, continuous black line; N = 13, p = 0.009 by two-tailed t-test, t = 3.09, d.f. = 12.). (C) The first row of outer hair cells (squares) moves only little. The larger displacement of third-row outer hair cells (circles) mirrors the large displacement of the Hensen cells. Error bars indicate the standard error of the mean from the different measurements. Data in (A-C) are from 683 measurements from 15 preparations for the first row of outer hair cells, and from 905 measurements from 18 preparations for the third row of outer hair cells. (D) The radial component of the Hensen-cell displacements was measured directly by tilting the preparation with respect to the interferometer beam. Representative data from one preparation show that the largest motion occurs in a direction with a small component towards the modiolus (red) for positive current injections, consistent with the reticular-lamina data shown in (A, B). Consistent results were obtained from four additional preparations.