Outer hair cell active force generation in the cochlear environment

Abstract

Outer hair cells are critical to the amplification and frequency selectivity of the mammalian ear acting via a fine mechanism called the cochlear amplifier, which is especially effective in the high-frequency region of the cochlea. How this mechanism works under physiological conditions and how these cells overcome the viscous (mechanical) and electrical (membrane) filtering has yet to be fully understood. Outer hair cells are electromotile, and they are strategically located in the cochlea to generate an active force amplifying basilar membrane vibration. To investigate the mechanism of this cell's active force production under physiological conditions, a model that takes into account the mechanical, electrical, and mechanoelectrical properties of the cell wall (membrane) and cochlear environment is proposed. It is shown that, despite the mechanical and electrical filtering, the cell is capable of generating a frequency-tuned force with a maximal value of about 40 pN. It is also found that the force per unit basilar membrane displacement stays essentially the same (40 pNnm) for the entire linear range of the basilar membrane responses, including sound pressure levels close to hearing threshold. Our findings can provide a better understanding of the outer hair cell's role in the cochlear amplifier.

Cochlear amplification, outer hair cells and prestin

“…Using different approaches, investigators show that in the unique environment of OHCs and with prestin’s reciprocal behavior, the speed limitation may be overcome and should not constitute a “fatal flaw” in prestin’s putative role as the amplifier [72,73]. A brief review of the variety of schemes proposed to overcome membrane filtering is included in [74].…”

Multiple roles for the tectorial membrane in the active cochlea

“…The OHC length change/force production in this case would be in phase with maximum displacement towards the scala tympani and have no effect on the BM responses. The internal viscoelasticity of the OC (Scherer and Gummer, 2004a,b;Liao et al, 2007) could, potentially, provide the required 0.25-cycle phase lag between the OHC excitation due to the extracellular potentials and the forces delivered to the cochlear partition, but there has been no experimental confirmation of this mechanism.…”

“…Thus, how might extracellular OHC potentials control effective cochlear amplification? It has been mentioned in part 1.1 that the internal viscoelasticity of the OC (Scherer and Gummer, 2004a, b;Liao et al, 2007) can provide the required phase lag between voltage stimulation of the OHCs, caused by their extracellular potentials, and the application of voltage-dependent forces generated by the OHCs to the cochlear partition. Whatever the underlying mechanism, it is clear that the TM is a crucial structure involved in the synchronizing of the active cochlear mechanism.…”

Functional Stem Cell Biomechanics: Application of Biophysical Techniques and Multi-content 3D Image Analysis

“…These activities appear to mediate numerous cellular functions such as, to name just a few, locomotion, cell adhesion, differentiation, and proliferation [Hoffman, 2009]. The magnitude of physiological forces experienced by the cells in vivo has been estimated to span > 15 orders of magnitude from pN-range forces in cochlea hair cells [Liao, 2007] to kN-range tensile forces in tendons [Maffulli, 2003]. The mechanical behavior of a living cell cannot be described simply in terms of fixed mechanical parameters, however.…”