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Section on Auditory Mechanics
Richard Chadwick, Ph.D., Chief
The aim of the section on Auditory Mechanics is to increase knowledge of the hearing process via an interweaving of mathematical, physical, and biological approaches and techniques. Advanced modeling techniques are used interactively with biophysical and structural data obtained from tissues in the organ of Corti to study cochlear physical phenomena related to the transduction of sound to auditory nerve impulses. Special emphasis is directed toward developing techniques to integrate subcellular, cellular, and macroscopic interactions.
Our efforts to study the multiscale nature of the hearing process run parallel to the multidisciplinary theme. Modeling wave propagation in a spiral shape cochlea begins as macroscopic phenomena, which cascades down to the molecular scale and ultimately affects how well mammals hear low frequencies. Nature has found a way to improve on the whispering gallery effect, where sound clings to a concave wall in architectural acoustics. In a spiral cochlea, with ever changing curvature, wave energy is focused at the apex, where low frequencies are processed, affecting the excitatory displacements of hair cell bundles. In a comparative study of morphology and audiometry across mammalian species, we have robustly validated our theory. We suggest the spiral form offers a selective advantage for hearing low frequencies, thus aligning the ideas of Darwin with “form follows function,” which can be traced back to Aristotle.
At the nanometer scale our lab specializes in probing cochlear tissues using the Atomic Force Microscope (AFM). We modified ours to impress controlled indentations in all three directions. We have coupled this capability with optical tracking of fluorescent microspheres at a resolution of tens of nanometers (Fig. 1) This technology, combined with our mathematical modeling ability, poises us to be highly competitive with the best labs in nanoscale measurement and analysis of cochlear tissue. While we have focused primarily on determining the elastic moduli of normal tectorial membrane fibers and matrix (Fig. 2), we also study the disrupted tectorial membranes of mutant mice.
Figure 1. Our experimental approach to study the mechanical properties of a biological material combines Atomic Force Microscopy (AFM) to exert point-like forces and optical tracking of microspheres to measure surface displacements. View Larger Image
Figure 2. A set of measurements on the tectorial membrane consists of several indentation measurements performed at increasing tip-bead distances in directions parallel or perpendicular to the collagen bundle. View Larger Image
- Chadwick, RS. A dual probe and two tones reveal dual waves in the cochlea (commentary). Biophys J 110(11), 2528-39, 2016.
- Logue JS, Cartagena-Rivera AX, Baird MA, Davidson MW, Chadwick RS, Waterman CM. Erk regulation of actin capping and bundling by Eps8 promotes cortex tension and leader bleb-based migration. eLife 4, e08314, 2015.
- Manoussaki D, Shin WD, Waterman CM, Chadwick RS. Cytosolic pressure provides a propulsive force comparable to actin polymerization during lamellipod protrusion. Sci. Rep. 5, 12314, 2015.
- Lamb JS, Chadwick RS. Phase of shear vibrations within cochlear partition leads to activation of the cochlear amplifier. PLoS One 14;9(2):e85969, 2014.
- Gavara N, Chadwick RS. Determination of the elastic moduli of thin samples and adherent cells using conical atomic force microscope tips. Nature Nanotechnology 7, 733-736, 2012.
- Petrie RJ, Gavara N, Chadwick RS, Yamada KM. Nonpolarized signaling reveals two distinct modes of 3D cell migration. J Cell Biol. 30;197(3):439-55, 2012.
- Lamb JS, Chadwick RS. Dual traveling waves in an inner ear model with two degrees of freedom. Physical Reviews Letters 107, 088101, 2011.
- Smith ST, Chadwick RS. Simulation of the response of the inner hair cell stereocilia bundle to an acoustical stimulus. PLoS One 6(3):e18161, 2011.
- Gavara N, Chadwick RS. Noncontact microrheology at acoustic frequencies using frequency-modulated atomic force microscopy. Nature Methods 7, 650-654, 2010.
- Gavara N, Chadwick RS. Collagen-based mechanical anisotropy of the tectorial membrane: Implications for inter-row coupling of outer hair cell bundles. PLoS One 4:e4887, 2009.
- Manoussaki D, Chadwick RS, Ketten DR, Arruda J, Dimitriadis EK, O’Malley JT. The influence of cochlear shape on low-frequency hearing. Proceedings of the National Academy of Sciences of the USA 105: 6162-6166, 2008.
- Chadwick RS, Liao Z. High frequency oscillations of a sphere in a viscous fluid near a rigid plane. SIAM Review 50, 313-322, 2008.
- Manoussaki D, Dimitriadis EK, Chadwick RS. Cochlea's graded curvature effect on low frequency waves. Physical Review Letters, 96: 088701, 2006.
- Cai H, Manoussaki D, Chadwick R. Effects of coiling on the micromechanics of the mammalian cochlea. Journal of the Royal Society Interface 2, 341-348, 2005.
- Chadwick RS, Shoelson B, Cai H. Surface green's functions for an incompressible, transversely isotropic half space. SIAM Journal on Applied Mathematics 4, 1186-1197, 2004.
- Shoelson B, Dimitriadis EK, Cai H, Kachar B, Chadwick RS. Evidence and implications of inhomogeneity in tectorial membrane elasticity. Biophysical Journal 87(4):2768–77, 2004.
- Cai H, Shoelson B, Chadwick RS. Evidence of tectorial membrane radial motion in a propagating mode of a complex cochlear model. Proceedings of the National Academy of Sciences of the USA 101(16):6243–8, 2004.
- Dimitriadis EK, Horkay F, Maresca J, Kachar B, Chadwick RS. Determination of elastic moduli of thin layers of soft material using the atomic force microscope. Biophysical Journal 82, 2798-2810, 2002.