Reticular lamina and basilar membrane vibrations in living mouse cochleae

Abstract

It is commonly believed that the exceptional sensitivity of mammalian hearing depends on outer hair cells which generate forces for amplifying sound-induced basilar membrane vibrations, yet how cellular forces amplify vibrations is poorly understood. In this study, by measuring subnanometer vibrations directly from the reticular lamina at the apical ends of outer hair cells and from the basilar membrane using a custom-built heterodyne low-coherence interferometer, we demonstrate in living mouse cochleae that the sound-induced reticular lamina vibration is substantially larger than the basilar membrane vibration not only at the best frequency but surprisingly also at low frequencies. The phase relation of reticular lamina to basilar membrane vibration changes with frequency by up to 180 degrees from ∼135 degrees at low frequencies to ∼-45 degrees at the best frequency. The magnitude and phase differences between reticular lamina and basilar membrane vibrations are absent in postmortem cochleae. These results indicate that outer hair cells do not amplify the basilar membrane vibration directly through a local feedback as commonly expected; instead, they actively vibrate the reticular lamina over a broad frequency range. The outer hair cell-driven reticular lamina vibration collaboratively interacts with the basilar membrane traveling wave primarily through the cochlear fluid, which boosts peak responses at the best-frequency location and consequently enhances hearing sensitivity and frequency selectivity.

Keywords: cochlea; cochlear amplifier; hearing; interferometry; outer hair cells.

Fig. 1.

Diagram of the organ of Corti and vibrations of the reticular lamina and basilar membrane in a sensitive mouse cochlea. (A) Intracochlear image through the intact round window membrane in a living mouse. BM, basilar membrane; OSL, osseous spiral lamina. (B) Diagram of the organ of Corti. RL, reticular lamina; IHC, inner hair cell; OHCs, outer hair cells; TM, tectorial membrane; DCs, Deiters' cells. (C and D) Displacements of the BM (red) and RL (blue) as a function of frequency. The noise floor (black dotted line in C) is <0.01 nm. BF, best frequency. (E and F) Displacement ratios of the BM and RL to the malleus. (G and H) BM and RL phase referred to the malleus. (I) The ratio of the RL to BM displacement. (J) Phase difference between the RL and BM.

Fig. 2.

Postmortem changes in reticular lamina and basilar membrane vibrations. Postmortem data were collected immediately after death (blue) and are compared with those measured under sensitive conditions (red) in the same cochlea. (A) Displacements of basilar membrane (BM) as a function of frequency at different sound levels. (B) Reticular lamina (RL) displacements. (C) Displacement ratios of the BM to malleus. (D) Displacement ratios of the RL to malleus. (E and F) BM and RL phase referred to the malleus. (G) Magnitude ratios of the RL to BM. (H) Phase difference between the BM and RL. In addition to poor sensitivity, broad tuning, and linear growth, the magnitude and phase differences between the RL and BM were absent under postmortem conditions.

Fig. 3.

Differences between the reticular lamina and basilar membrane vibrations. (A) Reticular lamina (RL) and basilar membrane (BM) displacements at different sound levels in sensitive cochleae. (B) RL and BM phase. Phase was normalized to that at 80 dB SPL. (C) RL and BM displacements in postmortem cochleae. (D) Insensitive RL and BM phase. (E) Displacement ratios of the RL to BM show that the RL vibrated >fivefold more than the BM at low frequencies. This difference decreased with frequency and approached one near the best frequency. (F) RL phase led the BM phase by ∼135 degree at low frequencies and lagged the BM phase by ∼-45 degrees at the best frequency (48.08 ± 0.25 kHz, n = 5) in sensitive cochleae. (G and H) The magnitude and phase differences were absent in postmortem cochleae.

Fig. 4.

Longitudinal patterns and interaction of the cochlear partition vibrations. The active component of the reticular lamina vibration (green lines in panels C–F) was obtained by vector subtraction of the basilar membrane vibration from the measured reticular lamina vibration. (A) Diagram of the conventional traveling wave at ∼48 kHz shows that the vibration was measured at the peak of the traveling wave (red arrow). (B) At a frequency lower than the BF, the vibration was measured at a basal part of the traveling wave. (C) At 40 dB SPL, the reticular lamina (RL) and basilar membrane (BM) vibrations were at a small region centered at the BF location. (E) At 70 dB SPL, both RL and BM displacements extended from the BF location to the base. (D and F) Near the base, RL phase led BM phase by >120 degrees; they were approximately in-phase at the BF location. (G) Diagram of a single tone-induced cochlear traveling wave. D_OHC, the outer hair cell force-induced displacement; D_TW, the traveling wave-induced displacement. (H) At the base of the traveling wave, RL and BM move in approximately opposite directions. The active RL movement creates a negative fluid pressure in the scala vestibuli and a positive fluid pressure inside the organ of Corti. (I) At the peak of the traveling wave, the RL and BM move in the same direction, which results in the maximal vibration at the apical ends of outer hair cells through constructive interference. Dotted lines show resting positions.