In mammals, outer-hair-cell hair bundles (OHBs) transduce sound-induced forces into receptor currents and are required for the wide dynamic range and high sensitivity of hearing. OHBs differ conspicuously in morphology from other types of bundles. Here, we show that the 3D morphology of an OHB greatly impacts its mechanics and transduction. An OHB comprises rod-like stereocilia, which pivot on the surface of its sensory outer hair cell. Stereocilium pivot positions are arranged in columns and form a V shape. We measure the pivot positions and determine that OHB columns are far from parallel. To calculate the consequences of an OHB’s V shape and far-from-parallel columns, we develop a mathematical model of an OHB that relates its pivot positions, 3D morphology, mechanics, and receptor current. We find that the 3D morphology of the OHB can halve its stiffness, can double its damping coefficient, and causes stereocilium displacements driven by stimulus forces to differ substantially across the OHB. Stereocilium displacements drive the opening and closing of ion channels through which the receptor current flows. Owing to the stereocilium-displacement differences, the currents passing through the ion channels can peak versus the stimulus frequency and vary considerably across the OHB. Consequently, the receptor current peaks versus the stimulus frequency. Ultimately, the OHB’s 3D morphology can increase its receptor-current dynamic range more than twofold. Our findings imply that potential pivot-position changes owing to development, mutations, or location within the mammalian auditory organ might greatly alter OHB function.

中文翻译：

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外毛细胞发束的 3D 形态增加了其位移和动态范围


在哺乳动物中，外毛细胞毛束 （OHB） 将声音诱导的力转化为受体电流，是宽动态范围和高灵敏度听力所必需的。OHB 在形态上与其他类型的束明显不同。在这里，我们表明 OHB 的 3D 形态极大地影响了它的力学和转导。OHB 由杆状立体纤毛组成，其在其感觉外毛细胞的表面旋转。立体纤毛枢轴位置呈列状排列并形成 V 形。我们测量枢轴位置并确定 OHB 柱远非平行。为了计算 OHB 的 V 形和远非平行的柱子的后果，我们开发了一个 OHB 的数学模型，该模型将其枢轴位置、3D 形态、力学和受体电流联系起来。我们发现 OHB 的 3D 形态可以使其刚度减半，可以使其阻尼系数增加一倍，并导致由刺激力驱动的立体纤毛位移在整个 OHB 上发生很大差异。立体纤毛位移驱动受体电流流经的离子通道的打开和关闭。由于立体纤毛-位移的差异，通过离子通道的电流可以相对于刺激频率达到峰值，并且在整个 OHB 上变化很大。因此，受体电流相对于刺激频率达到峰值。最终，OHB 的 3D 形态可以将其受体电流动态范围增加两倍以上。我们的研究结果表明，由于哺乳动物听觉器官内的发育、突变或位置而导致的潜在枢轴位置变化可能会极大地改变 OHB 功能。