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Cochlear Mechanics

The long-term goal for this research is to understand the cochlear mechanisms that support the high sensitivity, high frequency resolution, and nonlinear properties of normal hearing. This will allow functional characterization of structural changes arising from a variety of cochlear pathologies, genetic alterations, and future interventions such as the regeneration of cochlear substructures using stem cells.

Diagram of Y-shaped structures in the organ of Corti

Our current efforts are centered on understanding the function of the prominent cytoarchitectural feature within the cochlea’s organ of Corti that consists of over 12,000 overlapping and repeating Y-shaped structures between the reticular lamina and basilar membrane, in which the base of each angled outer hair cell (OHC) connects with the base of an angled phalangeal process at the top of each supporting Deiters’ cell (Figure 1).

The presence of these Y-shaped structures leads to a central hypothesis that cochlear amplification within the “best-frequency” region of the basilar membrane (i.e., the region that is best tuned to a given input frequency) results from the actuation of the motile and tilted OHCs such that they provide an accumulating “feed-forward” force directed towards the helicotrema at the cochlear apex and from the passive phalangeal processes that in turn provide a “feed-backward” force directed towards the stapes at the base of the cochlea.

We have so far developed two-photon imaging methods to derive 3D cellular morphometry for 1-mm-long sections of the organ of Corti, using “membrane-tomato” (mT) fluorescent mice. Our results (Figure 2) demonstrate our ability to distinguish cellular features as small as 1 µm, which is sufficient for the purposes of building finite-element models.

Figure 1: The inner ear based on a micro-CT reconstruction (top), the repeating Y-shaped cellular arrangement of the organ of Corti (Raphael et al, 1991) from SEM imaging (middle), and a corresponding model representation (bottom).



We have so far developed two-photon imaging methods to derive 3D cellular morphometry for 1-mm-long sections of the organ of Corti, using “membrane-tomato” (mT) fluorescent mice. Our results (Figure 2) demonstrate our ability to distinguish cellular features as small as 1 µm, which is sufficient for the purposes of building finite-element models.

The results of our studies are expected to be scientifically significant because there is not yet a single unifying framework that can explain amplification in the cochlea in a manner consistent with the cytoarchitecture of the organ of Corti, physiological measurements of basilar membrane mechanics, and measurements of ear-canal otoacoustic emissions.

Two-photon imaging of the mouse organ of Corti

Figure 2: 3D reconstructions illustrating the anatomy of a section of the mouse organ of Corti (OoC), as viewed from (A) radial, (B) longitudinal, and (C) tranverse planes. Structures, indicated in the color legend, include the outer hair cells (OHCs), phalangeal processes (PhPs), Deiters’ cells (DCs), stereocilia (Cilia), basilar membrane (BM), outer pillar cells (OPs), inner pillar cells (IPs), inner hair cells (IHCs), tectorial membrane (TM), and reticular lamina (RL). The tunnel of Corti (ToC) extends along the length of the OoC, and is defined by the triangular space formed between the BM, OPs, and IPs as seen in (A).

Despite the discovery of the active mechanism of the OHCs, there remains some controversy as to whether the active cochlea provides net mechanical power gain; and despite significant developments in experimental techniques, direct measurements of energy remain difficult because they require simultaneous measurements of velocity and pressure. An essential criterion for cochlear models is a quantitative treatment of energy flow, which has typically been treated mostly qualitatively in the past. To address this need, we have formulated a 3D cochlear model, solved using the WKB asymptotic method formulations for energy flow, which quantitatively indicates the conversion between electrical power and the mechanical amplification in the cochlea that gives mammals their exquisite sensitivity of hearing.