Kinetic Membrane Model of Outer Hair Cells

Multiple Modes of Motion for the Effectiveness of Outer Hair Cells at High Frequencies

Outer hair cells (OHCs) are essential for the sensitivity and frequency specificity of the mammalian ear. To perform this function, OHCs need to amplify the motion of the basilar membrane (BM), which is much stiffer than themselves. If OHCs and the BM are components of a single oscillator, this impedance mismatch seriously limits the effectiveness of OHCs. However, the elaborated structure of the organ of Corti can support multiple modes of motion. Here, systems of two coupled oscillators are examined as the simplest models of the system with multiple modes of motion. It is found that some of these model systems have conditions, under which an OHC can function as an effective amplifier, overcoming the impedance mismatch. The present examination suggests that the presence of multiple modes of motion is a key to the exquisite performance of the mammalian ear.

The tonotopic cochlea puzzle: A resonant transmission line with a "non-resonant" response peak

The peaked cochlear tonotopic response does not show the typical phenomenology of a resonant system. Simulations of a 2 D viscous model show that the position of the peak is determined by the competition between a sharp pressure boost due to the increase in the real part of the wavenumber as the forward wave enters the short-wave region, and a sudden increase in the viscous losses, partly counteracted by the input power provided by the outer hair cells. This viewpoint also explains the peculiar experimental behavior of the cochlear admittance (broadly tuned and almost level-independent) in the peak region.

Of mice and chickens: Revisiting the RC time constant problem

Avian hair cells depend on electrical resonance for frequency selectivity. The upper bound of the frequency range is limited by the RC time constant of hair cells because the sharpness of tuning requires that the resonance frequency must be lower than the RC roll-off frequency. In contrast, tuned mechanical vibration of the inner ear is the basis of frequency selectivity of the mammalian ear. This mechanical vibration is supported by outer hair cells (OHC) with their electromotility (or piezoelectricity), which is driven by the receptor potential. Thus, it is also subjected to the RC time constant problem. Association of OHCs with a system with mechanical resonance leads to piezoelectric resonance. This resonance can nullify the membrane capacitance and solves the RC time constant problem for OHCs. Therefore, avian and mammalian ears solve the same problem in the opposite way.

Prestin derived OHC surface area reduction underlies age-related rescaling of frequency place coding

Outer hair cells (OHC) are key to the mammalian cochlear amplifier, powered by the lateral membrane protein Prestin. In this study, we explored age-related OHC changes and how the changes affected hearing in mouse. OHC nonlinear membrane capacitance measurements revealed that, starting upon completion of postnatal auditory development, a continuous reduction of total Prestin in OHCs accompanied by a significant reduction in their cell surface area. Prestin's density is unaffected by Prestin level drop over the whole age range tested, suggesting that the OHC size reduction is Prestin-dependent. Stereocilia length in aged OHCs remained unchanged but the first row stereocilia on the aged inner hair cells (IHCs) were elongated. Distortion product otoacoustic emission (DPOAE) group delays became longer with aging, suggesting an apical shift in vibration on basilar membrane. Acoustic lesion experiments revealed an apical shift in damage place in old cochleae accompanied by a shallower progression in synaptic damage over a wider frequency range that was indicative of a broader frequency filter. Overall, these findings suggest that in aging cochlea, a shift in frequency place coding could occur due to the changes in cochlear active and passive mechanics. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.