The sulcus connection. On a mode of participation of outer hair cells in cochlear mechanics

Motile outer hair cells (OHCs) can only participate well in cochlear mechanics when one end of the hair cells is more restrained in its movements than the other. On this thought a model of the organ of Corti (OoC) is developed in which (in every cross section) the tectorial membrane (TM) is considered to consist of two stiff segments connected by a hinge. Movements of the TM then induce movements of the fluid contained in the inner spiral sulcus (ISS) and the fluid dynamics of the sulcus will play an important part in restraining the top ends of the hair cells. Since the model is meant to reflect low-level phenomena, it is conceived as linear. For simplicity, it operates in the long-wave mode. The sulcus is considered as a narrow channel that is closed at both ends. Since the sulcus fluid is an independent energy-storage element of the system, the entire cochlear model, consisting of two main fluid channels, the organ of Corti (OoC), the three-element TM and the sulcus, and equipped with motile OHCs, can be described by a fourth-order differential equation, with four boundary conditions. It is shown that the model can ideally produce response curves that reflect more than 40 dB of cochlear gain. When the actual effect of the sulcus is taken into account, however, the maximum gain is reduced to approximately 30 dB. The main reason for the reduction is that the sulcus cannot produce a large enough pressure to restrain the upper ends of the OHCs.