Mechanical nonlinearity in the cochlea produces acuoustic distortion products that can be measured in the ear canal. These distortion products can be altered by changes in the endolymphatic potential as well as by stimulation of the crossed olivocochlear bundle, which provides efferent innervation to cochlear hair cells.

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Mammalian outer hair cells (OHC) are believed to increase cochlear sensitivity and frequency selectivity via electromechanical feedback. A simple piezoelectric model of outer hair cell function is presented which integrates existing data from isolated OHC experiments. The model predicts maximum OHC force production to equal 1.25 nN/mV. The model also predicts that the maximum velocity of OHC contraction in situ to be 800 microns/s. These predictions are compared to available experimental data and are found to be in good agreement. The good agreement between the predicted and experimental results suggests that, at the characteristic frequency of a given cochlear location, the OHC receptor current is very efficiently converted into basilar membrane motion.

The mechanical properties of the cochlear partition are fundamental to auditory transduction. We measured the point stiffness of the partition, in vivo, at up to 17 radial positions spanning its width, in the basal turn of the gerbil cochlea. We found the linear stiffness at the position that is most likely under the outer pillar cells to be 1.5 times greater than adjacent positions toward the ligament, in the pectinate zone, and five times greater than adjacent positions toward the lamina, in the arcuate zone. This radial variation seems to reflect the cellular geometry of the partition: The pillar cell is positioned as a structural element, and the basilar membrane supports a rich cellular structure in the pectinate zone, whereas it borders a fluid-filled space in the arcuate zone. The radial variation in partition stiffness we find will influence passive cochlear mechanics, and also bears on active cochlear mechanics, since it supports the plausibility of cells as effective force generators. Our results from measurements made in vivo extend the findings of previous measurements made in excised cochleae, in which the cellular contribution to stiffness was less evident.