Elsevier

Current Opinion in Physiology

Volume 18, December 2020, Pages 56-62
Current Opinion in Physiology

Cochlear mechanics: new insights from vibrometry and optical coherence tomography

https://doi.org/10.1016/j.cophys.2020.08.022Get rights and content

The cochlea is a complex biological machine that transduces sound-induced mechanical vibrations to neural signals. Hair cells within the sensory tissue of the cochlea transduce vibrations into electrical signals, and exert electromechanical feedback that enhances the passive frequency separation provided by the cochlea's traveling wave mechanics; this enhancement is termed cochlear amplification. The vibration of the sensory tissue has been studied with many techniques, and the current state of the art is optical coherence tomography (OCT). The OCT technique allows for motion of intra-organ structures to be measured in vivo at many layers within the sensory tissue, at several angles and in previously under-explored species. OCT-based observations are already impacting our understanding of hair cell excitation and cochlear amplification.

Section snippets

Early and recent techniques of cochlear vibrometry

The cellular component of the cochlea's sensory tissue, termed the organ of Corti (OoC), is a long narrow strip, bounded by two acellular structures, the basilar and tectorial membranes (BM and TM) and surrounded by fluid chambers. A sound stimulus enters the cochlea at the basal end, and makes its way to frequency-dependent locations by means of a fluid-mechanical traveling wave. The leading actors of hearing are the hair cells (HC), whose stereocilia ‘hair’ extend from the apical (top)

OCT vibrometry in the cochlea

OCT imaging uses a low-coherence infrared light source that can penetrate biological tissue, allowing imaging and interferometric motion measurements at depths of several millimeters. The axial imaging resolution is determined by the bandwidth of the light source (larger bandwidth = better resolution), with resolution values in the micrometer range. Lateral resolution is determined by the lens numerical aperture, as in standard microscopy [26]. In the first applications of OCT to cochlear

Physiological advances

There are two significant benefits of OCT-vibrometry over classic heterodyne vibrometry: the ability to measure different layers within the OoC and the ability to measure through the bone of the cochlear capsule.

The ability to measure within the OoC has exposed significant and unanticipated OoC motions. Motions at the RL and in the OHC/Deiters cell regions exhibit higher amplitudes than at the BM, and enhanced and nonlinear responses in these regions extend to sub-CF frequencies [37, 38, 39, 40

Future

The development and application of more intense and broader bandwidth light sources will improve axial resolution and vibrometry signal:noise [56]. Axial resolution is also improved by employing a shorter wavelength light source [26]. Lateral resolution can be improved by using a higher numerical aperture objective lens [57,28]. However, this comes at the expense of reduced axial working range and distance and will not be practical for some physiological measurements. OCT systems have been

Funding

The authors are supported by an NIH/NIDCD grant to EO and the Emil Capita Foundation.

Conflict of interest statement

Nothing declared.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • •• of outstanding interest

Acknowledgements

We thank Marcel van der Heijden and coauthors for the data shown in Figure 2 of [37] and John Oghalai and coauthors for the use of Figure 3 in [45••].

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