Electromechanical Models of the Outer Hair Cell Composite Membrane

Abstract

The outer hair cell (OHC) is an extremely specialized cell and its proper functioning is essential for normal mammalian hearing. This article reviews recent developments in theoretical modeling that have increased our knowledge of the operation of this fascinating cell. The earliest models aimed at capturing experimental observations on voltage-induced cellular length changes and capacitance were based on isotropic elasticity and a two-state Boltzmann function. Recent advances in modeling based on the thermodynamics of orthotropic electroelastic materials better capture the cell's voltage-dependent stiffness, capacitance, interaction with its environment and ability to generate force at high frequencies. While complete models are crucial, simpler continuum models can be derived that retain fidelity over small changes in transmembrane voltage and strains occurring in vivo. By its function in the cochlea, the OHC behaves like a piezoelectric-like actuator, and the main cellular features can be described by piezoelectric models. However, a finer characterization of the cell's composite wall requires understanding the local mechanical and electrical fields. One of the key questions is the relative contribution of the in-plane and bending modes of electromechanical strains and forces (moments). The latter mode is associated with the flexoelectric effect in curved membranes. New data, including a novel experiment with tethers pulled from the cell membrane, can help in estimating the role of different modes of electromechanical coupling. Despite considerable progress, many problems still confound modelers. Thus, this article will conclude with a discussion of unanswered questions and highlight directions for future research.

Actuation of flexoelectric membranes in viscoelastic fluids with applications to outer hair cells

“…The sensor effect is performed by bending induced electric polarization, whereas the converse actuation effect is performed by the membrane curvature induced by an imposed electric field. Membrane flexoelectricity is relevant to the biological functioning of the OHCs which act as amplifiers to counteract viscous dissipation through mechanic transduction and thus allowing hearing [11][12][13][20][21][22][23][24][25][26][27][28]. The key challenge is to understand the coupling of oscillatory flexoelectric actuation and the viscoelastic phenomena of the fluids that are in contact with the oscillating membrane [11][12][13].…”

“…Both the direct and converse membrane flexoelectric effects are sensor-actuator properties when membrane curvature and polarization are coupled as in nematic LCs. Membrane flexoelectricity due to its inherent sensoractuator capabilities is an area of current interest in soft matter materials [1,7,8,[14][15][16][17][18][19][20][21][22][23][24]. Over the years, much literature has dealt with the problem of measuring flexoelectric coefficients in various LCs [11,13].…”

Linear oscillatory dynamics of flexoelectric membranes embedded in viscoelastic media with applications to outer hair cells

“…(4)) where the membrane elasticity ( ) 5) close to the viscous/membrane elasticity material plane in which inertial coefficient is relatively negligible. The results are computed using eqns (31,32) for the in-out phase and out-in phase moduli and eqns. (35,38) for the resonance power and elastic energy membrane.…”

“…Hair Cells (OHCs) exhibit an actuator-based phenomenon called "electromotility" [28][29][30][31][32][33][34][35]. OHCs produce a gain in the cochlear amplifier of over 50 dB and the cochlear amplifier provides mammalian physiology with the capacity of "hearing" a wide range of frequencies.…”

Flexoelectricity and thermal fluctuations of lipid bilayer membranes: Renormalization of flexoelectric, dielectric, and elastic properties

“…[20] where its role in cell-membrane tether formation was also elucidated. Indeed, past research suggests that flexoelectricity underlies an important coupling between mechanical deformation and electric stimulus which has ramifications for ion transport [14], hearing mechanism [21,22,23,24,25], tether formation [26,20] among others. For example, Raphael et.…”