Kathryn D. Breneman1, William E. Brownell2, Richard D. Rabbitt1,3*
1 Department of Bioengineering, University of Utah, Salt Lake City, Utah, United States of America,
2 Department of Otolaryngology - H&NS, Baylor College of Medicine, Houston, Texas, United States of America,
3 Marine Biological Laboratory, Woods Hole, Massachusetts, United States of America
Abstract Top
Microvilli (stereocilia) projecting from the apex of hair cells in the inner ear are actively motile structures that feed energy into the vibration of the inner ear and enhance sensitivity to sound. The biophysical mechanism underlying the hair bundle motor is unknown. In this study, we examined a membrane flexoelectric origin for active movements in stereocilia and conclude that it is likely to be an important contributor to mechanical power output by hair bundles. We formulated a realistic biophysical model of stereocilia incorporating stereocilia dimensions, the known flexoelectric coefficient of lipid membranes, mechanical compliance, and fluid drag. Electrical power enters the stereocilia through displacement sensitive ion channels and, due to the small diameter of stereocilia, is converted to useful mechanical power output by flexoelectricity. This motor augments molecular motors associated with the mechanosensitive apparatus itself that have been described previously. The model reveals stereocilia to be highly efficient and fast flexoelectric motors that capture the energy in the extracellular electro-chemical potential of the inner ear to generate mechanical power output. The power analysis provides an explanation for the correlation between stereocilia height and the tonotopic organization of hearing organs. Further, results suggest that flexoelectricity may be essential to the exquisite sensitivity and frequency selectivity of non-mammalian hearing organs at high auditory frequencies, and may contribute to the “cochlear amplifier” in mammals.
Citation: Breneman KD, Brownell WE, Rabbitt RD (2009) Hair Cell Bundles: Flexoelectric Motors of the Inner Ear. PLoS ONE 4(4): e5201. doi:10.1371/journal.pone.0005201
Editor: Vladimir Brezina, Mount Sinai School of Medicine, United States of America
Received: January 16, 2009; Accepted: March 16, 2009; Published: April 22, 2009
Copyright: © 2009 Breneman et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Financial support was provided by the NIDCD R01 DC04928 & R01 DC06685 (Rabbitt), R01 DC00384 (Brownell), NSF IGERT DGE9987616 and NASA GSRP 56000135 (Breneman). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: r.rabbitt@utah.edu
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Figure 1. Stereocilium flexoelectric biophysics.
a) As an excitatory force is applied the bundle deflects towards the tallest stereocilia and the tip link tension increases. Tip displacement causes the MET to open, current (IT) to enter the stereocilia, thus leading to cable-like membrane depolarization. b–c) Through the membrane flexoelectric effect, depolarization compels a decrease in radius () and increase in height () under constant volume. Changes in length are accompanied by transverse motion due to the staircase gradient in stereocilia lengths and diagonal tip links. Deflections are resisted by actin stiffness and polymerization at the tip, the angular stiffness at the base, and fluid drag in the axial and transverse directions.
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