Liu Yanju, Gracewski Sheryl M, Nam Jong-Hoon
Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627, United States of America.
Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627, United States of America; Department of Biomedical Engineering, University of Rochester, Rochester, NY 14627, United States of America.
PLoS One. 2015 Aug 28;10(8):e0133284. doi: 10.1371/journal.pone.0133284. eCollection 2015.
The cochlea performs frequency analysis and amplification of sounds. The graded stiffness of the basilar membrane along the cochlear length underlies the frequency-location relationship of the mammalian cochlea. The somatic motility of outer hair cell is central for cochlear amplification. Despite two to three orders of magnitude change in the basilar membrane stiffness, the force capacity of the outer hair cell's somatic motility, is nearly invariant over the cochlear length. It is puzzling how actuators with a constant force capacity can operate under such a wide stiffness range. We hypothesize that the organ of Corti sets the mechanical conditions so that the outer hair cell's somatic motility effectively interacts with the media of traveling waves-the basilar membrane and the tectorial membrane. To test this hypothesis, a computational model of the gerbil cochlea was developed that incorporates organ of Corti structural mechanics, cochlear fluid dynamics, and hair cell electro-physiology. The model simulations showed that the micro-mechanical responses of the organ of Corti are different along the cochlear length. For example, the top surface of the organ of Corti vibrated more than the bottom surface at the basal (high frequency) location, but the amplitude ratio was reversed at the apical (low frequency) location. Unlike the basilar membrane stiffness varying by a factor of 1700 along the cochlear length, the stiffness of the organ of Corti complex felt by the outer hair cell remained between 1.5 and 0.4 times the outer hair cell stiffness. The Y-shaped structure in the organ of Corti formed by outer hair cell, Deiters cell and its phalange was the primary determinant of the elastic reactance imposed on the outer hair cells. The stiffness and geometry of the Deiters cell and its phalange affected cochlear amplification differently depending on the location.
耳蜗执行声音的频率分析和放大功能。基底膜沿耳蜗长度方向的渐变刚度是哺乳动物耳蜗频率-位置关系的基础。外毛细胞的体细胞运动性是耳蜗放大的核心。尽管基底膜刚度变化了两到三个数量级，但外毛细胞体细胞运动性的力容量在耳蜗长度范围内几乎不变。具有恒定力容量的致动器如何能在如此宽的刚度范围内运行，这令人费解。我们假设柯蒂氏器设定了机械条件，使得外毛细胞的体细胞运动性能够有效地与行波介质——基底膜和盖膜相互作用。为了验证这一假设，开发了一种沙鼠耳蜗的计算模型，该模型纳入了柯蒂氏器的结构力学、耳蜗流体动力学和毛细胞电生理学。模型模拟表明，柯蒂氏器的微机械响应沿耳蜗长度方向是不同的。例如，在基底（高频）位置，柯蒂氏器的顶面振动比底面更剧烈，但在顶端（低频）位置，振幅比则相反。与基底膜刚度沿耳蜗长度变化1700倍不同，外毛细胞所感受到的柯蒂氏器复合体的刚度保持在外毛细胞刚度的1.5至0.4倍之间。由外毛细胞、Dieters细胞及其指状突在柯蒂氏器中形成的Y形结构是施加在外毛细胞上的弹性电抗的主要决定因素。Dieters细胞及其指状突的刚度和几何形状对耳蜗放大的影响因位置而异。
