A cytoskeletal spring in cochlear outer hair cells

Abstract

Normal hearing in mammals depends on an active mechanical filter, within the cochlea, which separates different sound frequencies before neural encoding. Experiments on the intact cochlea^1—4 indicate that the critical cellular components underlying the process are probably the outer hair cells^5,6 which are strategically placed to influence movement of the basilar membrane. This idea is attractive because isolated cells can generate axial forces⁷ at acoustic frequencies⁸ when electrically stimulated. The mechanical properties of cells are largely determined by structures closely associated with the plasma membrane^9,10. We show here, using light and electron microscopy, that beneath this membrane lies a lattice of crosslinked circumferential filaments which are pitched at a mean angle of 15^° to the transverse axis of the cell. The lattice is sufficient to retain the shape of the cell following demembranation and mechanical deformation. The structure of the lattice allows it to be described as a coiled helical spring but with longitudinal stiffness primarily determined by the crosslinks. Direct measurements of longitudinal stiffness reported here indicate that the lattice contributes 5–10% of the stiffness. We propose that the 'circumferential lattice' ensures that outer hair cells can act as directed force generators within the organ of Corti, a prerequisite in current descriptions of cochlear micro-mechanics^11–13.