Some electrical circuit properties of the organ of Corti. II. Analysis including reactive elements.

The simplified network model of the organ of Corti-hair cell complex that was studied in previous publications (Dallos, P. (1983): In: Hearing - Physiological Bases and Biophysics, pp. 32-37, editors: R. Klinke and R. Hartmann; Dallos, P. (1983): Hearing Res. 12, 89-119) is extended to include the capacitive reactances of hair cell 12, 89-119) is extended to include the capacitive reactances of hair cell membranes. With the assumption that the circuit is parametrically excited by resistance changes of the apical hair cell membrane, intracellular responses for outer (eO) and inner (eI) hair cells and the organ of Corti potential (eOC) are computed for the linear case. In order that frequency-dependent effects be primarily determined by the network properties of the system, the eO/eOC and eI/eOC ratios are considered. The amplitude and phase dependence of these computed functions is compared to corresponding experimental results from the third turn of the guinea pig's cochlea. The eO/eOC function is predicted to be a simple low-pass filter whose cutoff frequency (1250 Hz) is determined by the parallel resistance-capacitance circuit of the cell's baso-lateral membrane. This circuit prediction is in excellent agreement with the data. The eI/eOC function is again derived as a low-pass filter. The cutoff frequency is obtained as 472 Hz, which corresponds to an RC membrane time constant of 0.34 ms. Experimental data are widely divergent from this predicted pattern. In order to bring the theoretical and experimental results in agreement, two steps are necessary. First, a differentiation (i.e., multiplication by j omega) needs to be included in the input to inner hair cells to account for their velocity dependence [4,10]. Second, an underdamped complex pole, located above the cell's best frequency must be introduced. The velocity dependence and the resonance represented by the underdamped pole-pair indicate that the electrical response pattern of inner hair cells is more complex than that of outer hair cells.