Theory of electrically driven shape changes of cochlear outer hair cells

Abstract

1. A theory of cochlear outer hair cell electromotility is developed and specifically applied to somatic shape changes elicited in a microchamber. The microchamber permits the arbitrary electrical and mechanical partitioning of the outer hair cell along its length. This means that the two partitioned segments are stimulated with different input voltages and undergo different shape changes. Consequently, by imposing more constraints than other methods, experiments in the microchamber are particularly suitable for testing different theories of outer hair cell motility. 2. The present model is based on simple hypotheses. They include a distributed motor associated with the cell membrane or cortex and the assumption that the displacement generated by the motor is related to the transmembrane voltage across the associated membrane element. It is expected that the force generated by the motor is counterbalanced by an elastic restoring force indigenous to the cell membrane and cortex, and a tensile force due to intracellular pressure. It is assumed that all changes take place while total cell volume is conserved. The above elements of the theory taken together permit the development of qualitative and quantitative predictions about the expected motile responses of both partitioned segments of the cell. Only a DC treatment is offered here. 3. Both a linear motor and an expanded treatment that incorporates a stochastic molecular motor model are considered. The latter is represented by a two-state Boltzmann process. We show that the linear motor treatment is an appropriate extrapolation of the stochastic motor theory for the case of small voltage driving signals. Comparison of experimental results with model responses permits the estimation of model parameters. Good match of data is obtained if it is assumed that the molecular motors undergo conformational length changes of 0.7-1.0 nm, that they have an effective displacement vector at approximately -20 degrees with the long axis of the cell, and that their linear density is approximately 80/microns. 4. An effort is made to parcel out motile response components that are a direct consequence of the motor action from those that are mediated by cytoplasmic pressure changes brought about by the concerted action of the motors. We show that pressure effects are of minor importance, and thus rule out models that rely on radial constriction/expansion-mediated internal pressure change as the primary cause of longitudinal motility. 5. As a consequence of the interaction between the Boltzmann process and the mechanical characteristics of the cell, the electromotile response is asymmetric.(ABSTRACT TRUNCATED AT 400 WORDS)

Altered Traveling Wave Propagation and Reduced Endocochlear Potential Associated with Cochlear Dysplasia in the BETA2/NeuroD1 Null Mouse

“…Future studies to quantify hair cell numbers along the length of the cochlear duct would be needed to assess this concept. Another possibility is that null mutant outer hair cells are shorter than wildtype or heterozygote outer hair cells, and the amplitude of electromotility depends on cell length (Dallos et al 1993). A third reason is that the biomechanics of the cochlear partition as a whole are likely to be different in the null mouse.…”

High-Frequency Force Generation in the Constrained Cochlear Outer Hair Cell: A Model Study

“…They pointed out that the set-point in their experiment was not located at the steepest part of the electromechanical transduction function of the OHC. To estimate such a set-point, we selected three points (0, À50, and À100 mV) along the electromechanical transduction curve (Dallos et al 1993). The slope (sl) at each point can be easily estimated from that curve.…”

“…An accurate prediction of the active force production in vivo will require a more complete model of the constraints imposed on the OHC, in which all mechanical properties of each of the cell- The slope is estimated according to Figure 6 of Dallos et al (1993). It is assumed that e x =e at two different points changes with corresponding slopes in the same proportion.…”

“…All of these force measurements were performed under low-frequency conditions. Frank et al (1999) applied the microchamber setup (Dallos et al 1993;Dallos and Evans 1995) and, for the first time, measured the OHC high-frequency active force generation. They have demonstrated (see also Scherer and Gummer 2004) that the force generated by the OHC can be constant up to tens of kHz.…”

Distortion in those good vibrations

“…Even if the electromotility in outer hair cells was fully linear, the non-linearity of forward transduction should cause the responses of the basilar membrane to tone pairs to contain distortion products. In fact, electromotility is non-linear, with a voltage-to-displacement relationship described by a Boltzman function [28,29]. Whether this non-linearity actually leads to significant two-tone distortion is controversial.…”