The effects of cochlear loading on the motility of active outer hair cells
Outer hair cells power the amplification of sound-induced vibrations in the mammalian inner ear through an active process that involves hair-bundle motility and somatic motility. It is unclear, though, how either mechanism can be effective at high frequencies, especially when the cells are mechanically loaded by other structures in the cochlea. We addressed this issue by developing a model of an active outer hair cell on the basis of observations from isolated cells, then used the model to predict the response of an active cell in the intact cochlea. We found that active hair-bundle motility amplifies the receptor potential that drives somatic motility. Inertial loading of a hair bundle by the tectorial membrane counters the bundle's reactive load, allowing the outer hair cell's active motility to influence the motion of the cochlear partition. The system exhibits enhanced sensitivity and tuning only when it operates near a dynamical instability, a Hopf bifurcation. This analysis clarifies the roles of cochlear structures and shows how the two mechanisms of motility function synergistically to create the cochlear amplifier. The results suggest that somatic motility evolved to enhance a preexisting amplifier based on active hair-bundle motility, thus allowing mammals to hear high-frequency sounds.
The mechanical response of an outer hair cell loaded by the mass of the tectorial membrane (red) is much larger and more frequency-selective than that of a cell loaded by only the basilar membrane (blue).