Abstract
The mammalian sense of hearing relies on two types of sensory cells: inner hair cells transmit the auditory stimulus to the brain, while outer hair cells mechanically modulate the stimulus through active feedback. Stimulation of a hair cell is mediated by displacements of its mechanosensitive hair bundle which protrudes from the apical surface of the cell into a narrow fluid-filled space between reticular lamina and tectorial membrane. While hair bundles of inner hair cells are of linear shape, those of outer hair cells exhibit a distinctive V-shape. The biophysical rationale behind this morphology, however, remains unknown. Here we use analytical and computational methods to study the fluid flow across rows of differently shaped hair bundles. We find that rows of V-shaped hair bundles have a considerably reduced resistance to crossflow, and that the biologically observed shapes of hair bundles of outer hair cells are near-optimal in this regard. This observation accords with the function of outer hair cells and lends support to the recent hypothesis that inner hair cells are stimulated by a net flow, in addition to the well-established shear flow that arises from shearing between the reticular lamina and the tectorial membrane.
Highlights
Fluid dynamics plays an important role in many biological systems and the resulting constraints on functioning and efficiency have been shown to be an important factor in evolution[1]
As the hair bundles of outer hair cells move with the shear between reticular lamina and tectorial membrane, they experience no drag from the resulting shear flow
Fluid flow around the hair bundles occurs at low Reynolds numbers, which allows us to linearize the flow equations
Summary
Fluid dynamics plays an important role in many biological systems and the resulting constraints on functioning and efficiency have been shown to be an important factor in evolution[1]. The hair bundles of outer hair cells connect the reticular lamina, in which the apical surfaces of the hair cells are embedded, to the tectorial membrane that lies in parallel above it (Fig. 1b,c). Hair bundles of inner hair cells are anchored only in the reticular lamina and not in the tectorial membrane, and are stimulated by radial fluid flow between the two structures. As the hair bundles of outer hair cells move with the shear between reticular lamina and tectorial membrane, they experience no drag from the resulting shear flow. They will, present an obstacle to any net flow and may reduce the stimulation of the hair bundles of the inner hair cells. We investigate systematically the influence of different shapes of hair bundles of outer hair cells on the bundles’ resistance to such crossflow
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