Department of Mechanical Engineering, University of Rochester, 203 Hopeman Engineering Bldg, Rochester, NY, 14627, USA.
Department of Biomedical Engineering, University of Rochester, Rochester, NY, USA.
Sci Rep. 2020 Sep 9;10(1):14847. doi: 10.1038/s41598-020-71380-5.
The cochlea is filled with two lymphatic fluids. Homeostasis of the cochlear fluids is essential for healthy hearing. The sensory epithelium called the organ of Corti separates the two fluids. Corti fluid space, extracellular fluid space within the organ of Corti, looks like a slender micro-tube. Substantial potassium ions are constantly released into the Corti fluid by sensory receptor cells. Excess potassium ions in the Corti fluid are resorbed by supporting cells to maintain fluid homeostasis. Through computational simulations, we investigated fluid mixing within the Corti fluid space. Two assumptions were made: first, there exists a longitudinal gradient of potassium ion concentration; second, outer hair cell motility causes organ of Corti deformations that alter the cross-sectional area of the Corti fluid space. We hypothesized that mechanical agitations can accelerate longitudinal mixing of Corti fluid. Corti fluid motion was determined by solving the Navier-Stokes equations incorporating nonlinear advection term. Advection-diffusion equation determined the mixing dynamics. Simulating traveling boundary waves, we found that advection and diffusion caused comparable mixing when the wave amplitude and speed were 25 nm and 7 m/s, respectively. Higher-amplitude and faster waves caused stronger advection. When physiological traveling waves corresponding to 70 dB sound pressure level at 9 kHz were simulated, advection speed was as large as 1 mm/s in the region basal to the peak responding location. Such physiological agitation accelerated longitudinal mixing by more than an order of magnitude, compared to pure diffusion. Our results suggest that fluid motion due to outer hair cell motility can help maintain longitudinal homeostasis of the Corti fluid.
耳蜗充满了两种淋巴液。耳蜗液的动态平衡对于听力健康至关重要。被称为柯蒂氏器的感觉上皮将两种液体隔开。柯蒂氏器内的细胞外液空间，即柯蒂氏器的细胞外液空间，看起来像一个细长的微管。感觉受体细胞不断将大量钾离子释放到柯蒂氏器的细胞外液中。多余的钾离子由支持细胞吸收，以维持液体的动态平衡。通过计算模拟，我们研究了柯蒂氏器细胞外液空间内的液体混合。做出了两个假设：第一，存在钾离子浓度的纵向梯度；第二，外毛细胞的运动导致柯蒂氏器变形，改变了柯蒂氏器细胞外液空间的横截面积。我们假设机械搅拌可以加速柯蒂氏器细胞外液的纵向混合。通过求解纳维-斯托克斯方程并纳入非线性对流项来确定柯蒂氏器细胞外液的运动。用对流扩散方程确定混合动力学。模拟行波时，我们发现当波幅和速度分别为 25nm 和 7m/s 时，对流和扩散会导致相当的混合。更高的波幅和更快的波会产生更强的对流。当模拟对应于 9kHz 处 70dB 声压级的生理行波时，在峰值响应位置下方的区域中，对流速度高达 1mm/s。与单纯的扩散相比，这种生理搅拌可使纵向混合速度提高一个数量级以上。我们的研究结果表明，外毛细胞运动引起的液体运动有助于维持柯蒂氏器细胞外液的纵向动态平衡。
