Abstract
The micromechanical mechanisms that underpin tuning and dynamic range compression in the mammalian inner ear are fundamental to hearing, but poorly understood. Here, we present new, high-resolution optical measurements that directly map sound-evoked vibrations on to anatomical structures in the intact, living gerbil cochlea. The largest vibrations occur in a tightly delineated hotspot centering near the interface between the Deiters’ and outer hair cells. Hotspot vibrations are less sharply tuned, but more nonlinear, than basilar membrane vibrations, and behave non-monotonically (exhibiting hyper-compression) near their characteristic frequency. Amplitude and phase differences between hotspot and basilar membrane responses depend on both frequency and measurement angle, and indicate that hotspot vibrations involve longitudinal motion. We hypothesize that structural coupling between the Deiters’ and outer hair cells funnels sound-evoked motion into the hotspot region, under the control of the outer hair cells, to optimize cochlear tuning and compression.
Highlights
The micromechanical mechanisms that underpin tuning and dynamic range compression in the mammalian inner ear are fundamental to hearing, but poorly understood
Before sound-evoked vibrations are converted into the neural signals that underlie our sense of hearing, the inner ear separates them by frequency and compresses them nonlinearly into a physiologically manageable dynamic range[1,2,3,4,5]
Optical measurements of the cochlear partition’s structure and mechanical function were made in the basal turns of living gerbil cochleae (Fig. 1; see Methods)
Summary
The micromechanical mechanisms that underpin tuning and dynamic range compression in the mammalian inner ear are fundamental to hearing, but poorly understood. We present new, high-resolution optical measurements that directly map sound-evoked vibrations on to anatomical structures in the intact, living gerbil cochlea. Each location along the length of the spiraling cochlear partition is tuned, in a level-dependent manner, to its own characteristic range of frequencies: high frequencies stimulate the cochlear base, and low frequencies, the apex[1] This place-based spectral analysis, or mechanical tonotopy, underlies the brain’s ability to distinguish and identify sounds, even when multiple sound sources are present simultaneously. These maps are inconsistent with the fundamental predictions made by many active models of cochlear mechanics They reveal a sharply delineated vibration hotspot in the vicinity of the OHC and Deiters’ cell bodies that stands out from the surrounding structures in several respects: it has much larger vibration amplitudes, broader frequency tuning, and a hyper-compressive dependence on sound intensity. Abstracts of this work have been presented at recent scientific meetings[30,31]
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