Broad nonlinearity in reticular lamina vibrations requires compliant organ of Corti structures

In the mammalian cochlea, each longitudinal position of the basilar membrane (BM) has a nonlinear vibratory response in a limited frequency range around the location-dependent frequency of maximum response, known as the best fre-quency (BF). This nonlinear response arises from the electromechanical feedback from outer hair cells (OHCs). However, recent in vivo measurements have demonstrated that the mechanical response of other organ of Corti (OoC) structures, such as the reticular lamina (RL), and the electrical response of OHCs (measured in the local cochlear microphonic [LCM]) are nonlinear even at frequencies significantly below BF. In this work, a physiologically motivated model of the gerbil cochlea is used to demonstrate that the source of this discrepancy between the frequency range of the BM, RL, and LCM nonlinearities is greater compliance in the structures at the top of the OHCs. The predicted responses of the BM, RL, and LCM to pure tone and two-tone stimuli are shown to be in line with experimental evidence. Simulations then demonstrate that the sub-BF nonlinearity in the RL requires the structures at the top of the OHCs to be significantly more compliant than the BM. This same condition is also necessary for optimal'' gain near BF, i.e., high amplification that is in line with the experiment. This demonstrates that the con-ditions for OHCs to operate optimally at BF inevitably yield nonlinearity of the RL response over a broad frequency range.

Automated summary

In the mammalian cochlea, both the basilar membrane (BM) and the organ of Corti (OoC) structures exhibit nonlinearity in their responses, even at frequencies below the best frequency (BF). A physiological model of the gerbil cochlea suggests that this difference in nonlinearity is due to greater compliance in the structures at the top of the outer hair cells (OHCs). The model's predictions align with experimental evidence and demonstrate that optimal OHC function at BF leads to nonlinearity in the response of the OoC structures over a broad frequency range.