Abstract
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.
Highlights
The cochlea is filled with two lymphatic fluids
Mechanotransduction current through the hair cells are mostly carried by K+ ions, and the current is driven by an electric potential—the difference between endocochlear potential and hair cell membrane potential
The organ of Corti (OoC) has substantial extracellular fluid (Corti fluid), such as the fluid in the tunnel of Corti, Nuel’s space, and the outer tunnel (Fig. 1B). These extracellular fluid spaces in the OoC are inter-connected through the gaps between the outer hair cells, and pillar cells—the gaps are as large as a few m icrometers[4]
Summary
The cochlea is filled with two lymphatic fluids. Homeostasis of the cochlear fluids is essential for healthy hearing. 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. 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. We explored if mechanical advection can alleviate the burden of cochlear fluid homeostasis in the basal cochlea by longitudinal fluid mixing. This hypothesis was motivated by two previous observations. We investigated fluid dynamics and mass transport within a micro-tube with similar dimensions as the Corti fluid tube, to examine whether advective flows can enhance fluid mixing
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