Nanomechanics of the subtectorial space caused by electromechanics of cochlear outer hair cells

Abstract

The stereocilia of the cochlear inner hair cells (IHCs) transduce vibrations into the sensory receptor current. Until now, mechanisms for deflecting these stereocilia have not been identified experimentally. Here, we identify a mechanism by using the electromechanical properties of the soma of the outer hair cell to produce an intracochlear, mechanical force stimulus. It is known that the soma of this cell generates mechanical force in response to a change of its transmembrane potential. In the present experiments, the force was induced by intracochlear electrical stimulation at frequencies that covered the entire functionally relevant range of 50 kHz. Vibration responses were measured in the transverse direction with a laser Doppler vibrometer. For frequencies up to approximately 3 kHz in the first three turns of the guinea-pig cochlea, the apical surface of the IHC and the opposing surface of the tectorial membrane were found to vibrate with similar amplitudes but opposite phases. At high frequencies, there was little relative motion between these surfaces in the transverse direction. The counterphasic motion up to approximately 3 kHz results in a pulsatile motion of the fluid surrounding the stereocilia of the IHCs. Based on physical principles of fluid flow between narrowly spaced elastic plates, we show that radial fluid motion is amplified relative to transverse membrane motion and that the radial motion is capable of bending the stereocilia. In conclusion, for frequencies up to at least 3 kHz, there appears to be direct fluid coupling between outer hair cells and IHCs.

Fig. 1.

Recording locations in the organ of Corti. (A) The organ vibrates as a result of electromechanical force produced by the OHCs, for which the stimulus is an extracellular electric field produced between the two upper electrodes placed in scala vestibuli (SV) and the lower electrode in scala tympani (ST). The lower electrode also served as a mirror for illumination. Velocity was measured with a laser Doppler vibrometer (not shown); its laser beam (red cone) is shown focused on the lower surface of the TM. Velocity was measured at different radial positions along this surface and at the opposing locations on the RL of the OHCs, IHC, and pillar cells (PCs), and also on the Hensen’s cells (HeCs) and BM. The yellow line shown diagonally below SV depicts Reissner’s membrane, which was left (visually) intact to enable the apical surface of the cells to be bathed in the endolymphatic fluid of scala media (SM). HS, Hensen’s stripe. (B) Measurement regions (red) along the cochlea, with distances indicated from the basal end of the BM.

Fig. 2.

Displacement amplitudes (A, C, and E) and phases (B, D, and F) of the lower surface of the TM (colored symbols) and the opposing RL (black symbols) for three preparations at 13 mm (A and B), 9 mm (C and D), and 2.5 mm (E and F) from the basal end of the BM. Displacement is referred to the voltage in scala vestibuli relative to that in scala tympani; phase is positive for motion toward scala vestibuli. Arrows denote CFs calculated from the neuronal tonotopic map in Tsuji and Liberman (32): 0.8, 3, and 24 kHz. Data symbols, circles for second-row OHCs (closed black, RL; opened purple, TM) and triangles for IHCs (closed black, RL; opened red, TM). The measurement order for each experiment was random. Notice that up to at least 3 kHz the TM at both radial positions vibrates in phase with the RL of the OHC, which in turn vibrates 180° out of phase with the RL of the IHC. This counterphasic motion of TM and RL at the IHCs implies pulsating fluid motion in the subtectorial space. Also notice that for frequencies near in vivo CF in the first cochlear turn (F), the TM and RL vibrate in phase at the IHC; that is, there is no evidence here of pulsating fluid motion. MS identifies the preparation.

Fig. 3.

Dynamics of the subtectorial space in response to the somatic electromechanical action of the OHCs for stimulus frequencies up to at least 3 kHz. (A) Phase relations of the RL and TM. Blue dashed-dotted lines: OHC contraction causes anticlockwise rotation of the RL about the apex of the pillar cells (PC), reducing the depth of the subtectorial space at the IHC. Red dashed lines: OHC elongation increases the depth of the subtectorial space at the IHC. (B) Elliptical fluid–particle trajectories resulting from counterphasic sinusoidal motion of the RL and TM at the IHC. The numerals 1–4 track the phases of the trajectories. The radial component of the displacement (x direction) is a parabolic function of vertical position in the space, whereas the transversal component (z direction) decreases cubically from the value (η_m) at the RL and TM, to zero at the midline (Supporting Text). The radial amplitude is at least an order of magnitude greater than the transverse amplitude, indicative of a pulsating fluid mode. The cartoon is not scaled: the measured value of η_m was 1–10 nm, whereas the depth of the subtectorial space is 4–8 μm for the first to third cochlear turns, respectively. The three gray vertical rectangles depict the IHC stereocilia.

Fig. 4.

Effect of 9-AC on electrically evoked displacement at 0.8 kHz in the third turn of the cochlea. 9-AC was applied at the time indicated by the arrow marked 1; wash out began at the arrow marked 2. RL: black circles (second-row OHC) and triangles (IHC). TM: colored circles (second-row OHC) and triangles (IHC). BM: black squares. BM recording location was at the intersection with the BM of a normal vector from the RL of the IHC. Because such pharmacological experiments required longer time than normal, the uppermost data points (open black circles) represent control data from a cochlea (at the RL of a second-row OHC), which was not perfused with 9-AC. This compound reduced the amplitude within 7 min (including ≈5-min perfusion time); there was no effect on the phase response (data not illustrated). Amplitudes returned asymptotically to the values expected for an experiment of this long duration.