The reticular lamina and basilar membrane vibrations in the transverse direction in the basal turn of the living gerbil cochlea

Abstract

The prevailing theory of cochlear function states that outer hair cells amplify sound-induced vibration to improve hearing sensitivity and frequency specificity. Recent micromechanical measurements in the basal turn of gerbil cochleae through the round window have demonstrated that the reticular lamina vibration lags the basilar membrane vibration, and it is physiologically vulnerable not only at the best frequency but also at the low frequencies. These results suggest that outer hair cells from a broad cochlear region enhance hearing sensitivity through a global hydromechanical mechanism. However, the time difference between the reticular lamina and basilar membrane vibration has been thought to result from a systematic measurement error caused by the optical axis non-perpendicular to the cochlear partition. To address this concern, we measured the reticular lamina and basilar membrane vibrations in the transverse direction through an opening in the cochlear lateral wall in this study. Present results show that the phase difference between the reticular lamina and basilar membrane vibration decreases with frequency by ~ 180 degrees from low frequencies to the best frequency, consistent with those measured through the round window. Together with the round-window measurement, the low-coherence interferometry through the cochlear lateral wall demonstrates that the time difference between the reticular lamina and basilar membrane vibration results from the cochlear active processing rather than a measurement error.

A frame and a hotspot in cochlear mechanics

“…Conversely, fluid moving to accommodate the hotspot motion may be critical to the longitudinal flow of energy towards the best place. Several recent cochlear models have explored the role of active fluid flow in frequency tuning (Altoe et al, 2022; Guinan, 2022; He et al, 2022). The findings here add to the experimental data guiding the models.…”

“…Sub-BF responses were linear; all positions were within the frame. He et al, 2022 made measurements at this BF location through a cochleostomy after surgically displacing the stapedial artery to afford a more complete view of the OCC. Their system did not include B-scan imaging, and the RL and BM were identified as reflective surfaces with a reasonable separation distance.…”

Asymmetric vibrations in the organ of Corti by outer hair cells measured from excised gerbil cochlea

“…Reticular lamina motion has drawn increasing attention recently 12,17,24 . Our results demonstrate the significance of presenting the accurate locus and the orientation of measurement, especially in sensitive cochleae, considering the complex vibration pattern with a small characteristic length.…”

“…First, in live cochlear experiments, it is tricky to isolate active OoC motion due to outer hair cell motility from passive motion due to acoustic pressures. Second, it is difficult to resolve the motions of OoC sub-components unless imaged through the round window or a surgical opening 12 , 17 . Third, it is difficult to choose or determine the incidence angle of vibrometry 11 , 18 .…”