Significance of the Microfluidic Flow Inside the Organ of Corti

Abstract

<jats:title>Abstract</jats:title> <jats:p>We study the vibration modes of a short section in the middle turn of the gerbil cochlea including both longitudinal and radial interstitial fluid spaces between the pillar cells (PC) and the sensory hair cells to determine the role of the interstitial fluid flow within the organ of corti (OoC). Three detailed finite element (FE) models of the cochlear short section (CSS) are studied. In model 1, the CSS is without fluids; model 2 includes the OoC fluid, but not the exterior scalae fluids; and model 3 is the CSS with both scalae and OoC fluids. We find that: (1) the fundamental mode shape of models 1 or 3 is similar to the classical basilar membrane (BM) bending mode that includes pivoting of the arch of corti, and hence determines the low frequency vibrational mode shape of the cochlea in the presence of the cochlear wave. (2) The fundamental mode shape of model 2 is characterized by a cross-sectional shape change similar to the passive response of the cochlea. This mode shape includes a tilting motion of the inner hair cell (IHC) region, a fluid motion within the tunnel of corti (ToC) in the radial direction and along the OoC, and a bulging motion of the reticular lamina (RL) above the outer hair cell (OHC). Each of these motions provides a plausible mode of excitation of the sensory hair cells. (3) The higher vibrational modes of model 1 are similar to the electrically evoked response within the OoC and suggests that the higher vibrational modes are responsible for the active response of the cochlea. We also observed that the fluid flow through the OoC interstitial space is significant, and the model comparison suggests that the OoC fluid contributes to the biphasic BM motion seen in electrical stimulation experiments. The effect of fluid viscosity on cilium deflection was assessed by performing a transient analysis to calculate the cilium shearing gain. The gain values are found to be within the range of experimentally measured values reported by Dallos et al. (1996, The Cochlea, Springer-Verlag, New York).</jats:p>

Watching Death in the Gerbil Cochlea Using Optical Coherence Tomography

“…It has been observed that electrical excitation of OHC motility causes a pumping of fluid through the SN and into the ToC, and that this possibly enhances cochlear amplification through yet-to-be discovered mechanisms (Karavitaki and Mountain, 2007). A recent FE modeling study features a representation of the gerbil mid-frequency OoC interstitial fluid spaces, and suggests that the role of the interconnected OoC fluid spaces is to redistribute pressure from OHC contractions so as to avoid excessive pressure buildup in the cortilymph space (Zagadou et al, 2020). Comparable FE studies of the high-frequency hook region are currently lacking.…”

“…In the middle turn of the gerbil cochlea, electrical stimulation of the OHCs has produced fluid waves in the ToC (Karavitaki and Mountain, 2007). Thus, knowledge of the OoC fluid spaces and surrounding structures is important for a full understanding of cochlear amplification (Zagadou et al, 2020). However, these fluid spaces are typically absent in histological images of the high-frequency hook region.…”

A Cochlea-Slice Model using Floquet Boundary Conditions shows Global Tuning

“…The boundary conditions on the basal and apical faces of the slice need to be specified. In real cochleae there is considerable motion of cochlear structures in the longitudinal direction (Karavitaki and Mountain, 2007;Wang et al, 2021), so clamping the slice faces, as was done by Zagadou et al (2020), is not realistic. Instead, we strived to capture the motion induced by the traveling wave by using a computationally-efficient "Floquet" boundary condition that mimicked how the traveling wave changed the phases of the motions at the edges of the slice.…”

“…The boundary conditions on the basal and apical faces of the slice need to be specified. In real cochleae there is considerable motion of cochlear structures in the longitudinal direction (Karavitaki and Mountain, 2007; Cooper et al 2018; Wang et al, 2021), so clamping the slice faces, as was done by Zagadou et al . (2020), is not realistic.…”

Microstructural interactions contribute to the hotspot in the living cochlea

“…Traditional two-dimensional models cannot simulate the longitudinal movement of the organ of Corti ( Cai and Chadwick, 2003 ; Cai et al, 2004 ; Ni et al, 2016 ; Steele and Puria, 2005 ). Beam models always need to simplify the geometry of the organ of Corti ( Nam, 2014 ; Nam and Fettiplace, 2010 ), but three-dimensional models could reflect more detailed physiological structures ( Zagadou et al, 2020 ). The “feedforward” models proposed by Steele et al (1993 , 1999 ) and Geisler and Sang (1995) reproduced many of the structural features but did not focus on producing the longitudinal vibration in the hotspot.…”