Liu Yanju, Gracewski Sheryl M, Nam Jong-Hoon
Department of Mechanical Engineering, University of Rochester, Rochester, New York, United States of America.
Department of Biomedical Engineering, University of Rochester, Rochester, New York, United States of America.
PLoS Comput Biol. 2017 Sep 7;13(9):e1005701. doi: 10.1371/journal.pcbi.1005701. eCollection 2017 Sep.
In the mammalian cochlea, small vibrations of the sensory epithelium are amplified due to active electro-mechanical feedback of the outer hair cells. The level of amplification is greater in the base than in the apex of the cochlea. Theoretical studies have used longitudinally varying active feedback properties to reproduce the location-dependent amplification. The active feedback force has been considered to be proportional to the basilar membrane displacement or velocity. An underlying assumption was that organ of Corti mechanics are governed by rigid body kinematics. However, recent progress in vibration measurement techniques reveals that organ of Corti mechanics are too complicated to be fully represented with rigid body kinematics. In this study, two components of the active feedback are considered explicitly-organ of Corti mechanics, and outer hair cell electro-mechanics. Physiological properties for the outer hair cells were incorporated, such as the active force gain, mechano-transduction properties, and membrane RC time constant. Instead of a kinematical model, a fully deformable 3D finite element model was used. We show that the organ of Corti mechanics dictate the longitudinal trend of cochlear amplification. Specifically, our results suggest that two mechanical conditions are responsible for location-dependent cochlear amplification. First, the phase of the outer hair cell's somatic force with respect to its elongation rate varies along the cochlear length. Second, the local stiffness of the organ of Corti complex felt by individual outer hair cells varies along the cochlear length. We describe how these two mechanical conditions result in greater amplification toward the base of the cochlea.
在哺乳动物的耳蜗中，由于外毛细胞的主动机电反馈，感觉上皮的微小振动会被放大。放大水平在耳蜗底部比顶部更高。理论研究利用纵向变化的主动反馈特性来重现位置依赖性放大。主动反馈力被认为与基底膜位移或速度成正比。一个潜在的假设是，柯蒂氏器力学受刚体运动学支配。然而，振动测量技术的最新进展表明，柯蒂氏器力学过于复杂，无法用刚体运动学完全描述。在本研究中，明确考虑了主动反馈的两个组成部分——柯蒂氏器力学和外毛细胞机电学。纳入了外毛细胞的生理特性，如主动力增益、机械转导特性和膜RC时间常数。使用了一个完全可变形的三维有限元模型，而不是运动学模型。我们表明，柯蒂氏器力学决定了耳蜗放大的纵向趋势。具体而言，我们的结果表明，两种机械条件导致了位置依赖性耳蜗放大。首先，外毛细胞体元力相对于其伸长率的相位沿耳蜗长度变化。其次，单个外毛细胞感受到的柯蒂氏器复合体的局部刚度沿耳蜗长度变化。我们描述了这两种机械条件如何导致向耳蜗底部的放大增强。
