Consequences of Location-Dependent Organ of Corti Micro-Mechanics

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（cochlea）负责对声音进行频率分析与放大。基底膜（basilar membrane）沿耳蜗长度呈现的梯度刚度，是哺乳动物耳蜗频率-位置对应关系的基础。外毛细胞（outer hair cell）的躯体运动是耳蜗放大功能的核心。尽管基底膜的刚度存在2至3个数量级的变化，外毛细胞躯体运动的力输出能力在整个耳蜗长度上几乎保持恒定。这一现象令人困惑：力输出能力恒定的执行机构，如何能在如此宽泛的刚度范围内正常工作？我们提出假说：柯蒂氏器（organ of Corti）会设定机械环境，使外毛细胞的躯体运动能够与行波（traveling waves）的介质——基底膜与盖膜（tectorial membrane）——实现有效相互作用。为验证该假说，我们构建了沙鼠（gerbil）耳蜗的计算模型，该模型整合了柯蒂氏器的结构力学、耳蜗流体动力学以及毛细胞的电生理学特性。模型仿真结果显示，柯蒂氏器的微观力学响应沿耳蜗长度存在差异。例如，在基底圈（高频区），柯蒂氏器的顶面振动幅度大于底面；而在顶圈（低频区），这一幅度比则完全相反。与沿耳蜗长度刚度变化达1700倍的基底膜不同，外毛细胞所感知到的柯蒂氏器复合体刚度，始终维持在外毛细胞自身刚度的0.4至1.5倍之间。柯蒂氏器中由外毛细胞、戴特细胞（Deiters cell）及其指突（phalange）构成的Y形结构，是施加于外毛细胞的弹性电抗的主要决定因素。戴特细胞及其指突的刚度与几何形态，对耳蜗放大功能的影响随耳蜗位置不同而存在差异。