The mammalian cochlea amplifies sounds selectively to improve frequency resolution. However, vibrations around the outer hair cells (OHCs) are amplified non-selectively. The mechanism of the selective or non-selective amplification is unknown. This study demonstrates that active force transmission through the extracellular fluid in the organ of Corti (Corti fluid) can explain how the cochlea achieves selective sound amplification despite the non-frequency-selective action of OHCs. Computational model simulations and experiments with excised cochleae from young gerbils of both sexes were exploited. OHC motility resulted in characteristic off-axis motion of the joint between the OHC and Deiters cell (ODJ). Incorporating the Corti fluid dynamics was critical to account for the ODJ motion due to OHC motility. The incorporation of pressure transmission through the Corti fluid resulted in three distinct frequency tuning patterns depending on sites in the organ of Corti. In the basilar membrane, the responses were amplified near the best-responding frequency (BF). In the ODJ region, the responses were amplified non-selectively. In the reticular lamina, the responses were amplified near the BF but suppressed in lower frequencies. The suppressive effect of OHCs was further examined by observing the changes in tuning curves due to local inhibition of OHC motility. The frequency response of the reticular lamina resembled neural tuning, such as the hypersensitivity of tuning-curve tails after hair cell damage. Our results demonstrate how active OHCs exploit the elastic frame and viscous fluid in the organ of Corti to amplify and suppress cochlear vibrations for better frequency selectivity.Significance Statement Active outer hair cells have been considered to selectively amplify the basilar membrane vibrations near the sound's tonotopic location. However, recent observations from different labs showed that outer hair cells' action is non-selective-it spreads over the broad span of traveling waves. These observations challenge the existing theory pegged to basilar-membrane mechanics. The motion at the joint between the outer hair cell and the Deiters (ODJ) cell holds the key to account for the non-selective action of outer hair cells. We show that the characteristic motions at the ODJ are explained coherently when Corti fluid acts as the medium for outer hair cell force transmission. Our results demonstrate how non-selective outer hair cell action produces selective neural responses.

中文翻译：

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Corti 液是外层毛细胞力传递的介质。


哺乳动物耳蜗选择性地放大声音以提高频率分辨率。然而，外毛细胞 （OHC） 周围的振动被非选择性放大。选择性或非选择性扩增的机制尚不清楚。这项研究表明，通过 Corti 器官（Corti 液）中的细胞外液的主动力传递可以解释尽管 OHC 具有非频率选择性作用，但耳蜗如何实现选择性声音放大。利用了从两性幼年沙鼠中切除的耳蜗的计算模型模拟和实验。OHC 运动导致 OHC 和 Deiters 细胞 （ODJ） 之间关节的特征性离轴运动。结合 Corti 流体动力学对于解释由于 OHC 运动引起的 ODJ 运动至关重要。通过 Corti 流体的压力传递的结合导致了三种不同的频率调谐模式，具体取决于 Corti 器官中的部位。在基底膜中，反应在最佳响应频率 （BF） 附近被放大。在 ODJ 区域，反应被非选择性放大。在网状板中，反应在 BF 附近被放大，但在较低频率下被抑制。通过观察由于局部抑制 OHC 运动而导致的调谐曲线的变化，进一步研究了 OHCs 的抑制作用。网状板的频率响应类似于神经调谐，例如毛细胞损伤后调谐曲线尾部的超敏反应。我们的结果表明，活性 OHC 如何利用 Corti 器官中的弹性框架和粘性流体来放大和抑制耳蜗振动，从而获得更好的频率选择性。意义陈述 活性外毛细胞被认为选择性地放大声音音调位置附近的基底膜振动。然而，最近来自不同实验室的观察表明，外层毛细胞的作用是非选择性的——它会扩散到很宽的行波范围内。这些观察结果挑战了现有的基底膜力学理论。外毛细胞和 Deiters （ODJ） 细胞之间关节处的运动是解释外毛细胞非选择性作用的关键。我们表明，当 Corti 流体充当外毛细胞力传递的介质时，ODJ 的特征运动可以连贯地解释。我们的结果证明了非选择性外毛细胞作用如何产生选择性神经反应。