A two-layer outer hair cell model with orthotropic piezoelectric properties: correlation of cell resonant frequencies with tuning in the cochlea

Outer hair cell active force generation in the cochlear environment

Outer hair cells are critical to the amplification and frequency selectivity of the mammalian ear acting via a fine mechanism called the cochlear amplifier, which is especially effective in the high-frequency region of the cochlea. How this mechanism works under physiological conditions and how these cells overcome the viscous (mechanical) and electrical (membrane) filtering has yet to be fully understood. Outer hair cells are electromotile, and they are strategically located in the cochlea to generate an active force amplifying basilar membrane vibration. To investigate the mechanism of this cell's active force production under physiological conditions, a model that takes into account the mechanical, electrical, and mechanoelectrical properties of the cell wall (membrane) and cochlear environment is proposed. It is shown that, despite the mechanical and electrical filtering, the cell is capable of generating a frequency-tuned force with a maximal value of about 40 pN. It is also found that the force per unit basilar membrane displacement stays essentially the same (40 pNnm) for the entire linear range of the basilar membrane responses, including sound pressure levels close to hearing threshold. Our findings can provide a better understanding of the outer hair cell's role in the cochlear amplifier.

Contribution of outer hair cell bending to stereocilium deflection in the cochlea

The outer hair cell (OHC) in the cochlea is believed to actively enhance the cochlear sensitivity and frequency selectivity. Besides the well-known axial length change of the OHC, the bending mode of the OHC may also contribute to the stereocilium deflection. To investigate the contribution of the OHC bending to the stereocilium deflection, and the active process in the cochlea, we develop a simple kinematic model of the organ of Corti, consisting of the reticular lamina, the stereocilia and tectorial membrane. The electrically evoked axial length change and bending of the OHC are simulated, and their contributions to the stereocilium deflection are obtained. At the apical turn of the cochlea, the bending mode of the OHC results in stereocilium deflection comparable to that due to the axisymmetric length change of the OHC. At the basal turn, the contribution of the bending mode to the stereocilium deflection becomes insignificant compared to that of the axisymmetric mode.