Outer Hair Cell Force Research Articles

Interactions between Passive and Active Vibrations in the Organ of Corti In Vitro

High sensitivity and selectivity of hearing require an active cochlea. The cochlear sensory epithelium, the organ of Corti, vibrates because of external and internal excitations. The external stimulation is acoustic pressures mediated by the scala fluids, whereas the internal excitation is generated by a type of sensory receptor cells (the outer hair cells) in response to the acoustic vibrations. The outer hair cells are cellular actuators that are responsible for cochlear amplification. The organ of Corti is highly structured for transmitting vibrations originating from acoustic pressure and active outer hair cell force to the inner hair cells that synapse on afferent nerves. Understanding how the organ of Corti vibrates because of acoustic pressure and outer hair cell force is critical for explaining cochlear function. In this study, cochleae were freshly isolated from young gerbils. The organ of Corti in the excised cochlea was subjected to mechanical and electrical stimulation that are analogous to acoustic and cellular stimulation in the natural cochlea. Organ of Corti vibrations, including those of individual outer hair cells, were measured using optical coherence tomography. Respective vibration patterns due to mechanical and electrical stimulation were characterized. Interactions between the two vibration patterns were investigated by applying the two forms of stimulation simultaneously. Our results show that the interactions could be either constructive or destructive, which implies that the outer hair cells can either amplify or reduce vibrations in the organ of Corti. We discuss a potential consequence of the two interaction modes for cochlear frequency tuning.

Nonlinearity and amplification in cochlear responses to single and multi-tone stimuli

Mechanical displacements of the basilar membrane (BM) and the electrophysiological responses of the auditory outer hair cells (OHCs) are key components of the frequency tuning and cochlear amplification in the mammalian cochlea. In the work presented here, we measured the responses of (1) the extracellular voltage generated by OHCs (VOHC) and (2) displacements within the organ of Corti complex (OCC) to a multi-tone stimulus, and to single tones. Using optical coherence tomography (OCT), we were able to measure displacements of different layers in the OCC simultaneously, in the base of the gerbil cochlea. We explored the effect of the two types of sound stimuli to the nonlinear behavior of voltage and displacement in two frequency regions: a frequency region below the BM nonlinearity (sub-BF region: f < ∼0.7 BF), and in the best frequency (BF) region. In the sub-BF region, BM motion (XBM) had linear growth for both stimulus types, and the motion in the OHC region (XOHC) was mildly nonlinear for single tones, and relatively strongly nonlinear for multi-tones. Sub-BF, the nonlinear character of VOHC was similar to that of XOHC. In the BF region XBM, VOHC and XOHC all possessed the now-classic nonlinearity of the BF peak. Coupling these observations with previous findings on phasing between OHC force and traveling wave motions, we propose the following framework for cochlear nonlinearity: The BF-region nonlinearity is an amplifying nonlinearity, in which OHC forces input power into the traveling wave, allowing it to travel further apical to the region where it peaks. The sub-BF nonlinearity is a non-amplifying nonlinearity; it represents OHC electromotility, and saturates due to OHC current saturation, but the OHC forces do not possess the proper phasing to feed power into the traveling wave.

Optogenetic Control of Mouse Outer Hair Cells

Normal hearing in mammals depends on sound amplification by outer hair cells (OHCs) presumably by their somatic motility and force production. However, the role of OHC force production in cochlear amplification and frequency tuning are not yet fully understood. Currently, available OHC manipulation techniques for physiological or clinical studies are limited by their invasive nature, lack of precision, and poor temporal-spatial resolution. To overcome these limitations, we explored an optogenetic approach based on channelrhodopsin 2 (ChR-2), a direct light-activated nonselective cation channel originally discovered in Chlamydomonas reinhardtii. Three approaches were compared: 1) adeno-associated virus-mediated in utero transfer of the ChR-2 gene into the developing murine otocyst, 2) expression of ChR-2(H134R) in an auditory cell line (HEI-OC1), and 3) expression of ChR-2 in the OHCs of a mouse line carrying a ChR-2 conditional allele. Whole cell recording showed that blue light (470 nm) elicited the typical nonselective cation current of ChR-2 with reversal potential around zero in both mouse OHCs and HEI-OC1 cells and generated depolarization in both cell types. In addition, pulsed light stimulation (10 Hz) elicited a 1:1 repetitive depolarization and ChR-2 currents in mouse OHCs and HEI-OC1 cells, respectively. The time constant of depolarization in OHCs, 1.45 ms, is 10 times faster than HEI-OC1 cells, which allowed light stimulation up to rates of 10/s to elicit corresponding membrane potential changes. Our study demonstrates that ChR-2 can successfully be expressed in mouse OHCs and HEI-OC1 cells and that these present a typical light-sensitive current and depolarization. However, the amount of ChR-2 current induced in our in vivo experiments was insufficient to result in measurable cochlear effects.

Vibration of the organ of Corti within the cochlear apex in mice

The tonotopic map of the mammalian cochlea is commonly thought to be determined by the passive mechanical properties of the basilar membrane. The other tissues and cells that make up the organ of Corti also have passive mechanical properties; however, their roles are less well understood. In addition, active forces produced by outer hair cells (OHCs) enhance the vibration of the basilar membrane, termed cochlear amplification. Here, we studied how these biomechanical components interact using optical coherence tomography, which permits vibratory measurements within tissue. We measured not only classical basilar membrane tuning curves, but also vibratory responses from the rest of the organ of Corti within the mouse cochlear apex in vivo. As expected, basilar membrane tuning was sharp in live mice and broad in dead mice. Interestingly, the vibratory response of the region lateral to the OHCs, the "lateral compartment," demonstrated frequency-dependent phase differences relative to the basilar membrane. This was sharply tuned in both live and dead mice. We then measured basilar membrane and lateral compartment vibration in transgenic mice with targeted alterations in cochlear mechanics. Prestin(499/499), Prestin(-/-), and Tecta(C1509G/C1509G) mice demonstrated no cochlear amplification but maintained the lateral compartment phase difference. In contrast, Sfswap(Tg/Tg) mice maintained cochlear amplification but did not demonstrate the lateral compartment phase difference. These data indicate that the organ of Corti has complex micromechanical vibratory characteristics, with passive, yet sharply tuned, vibratory characteristics associated with the supporting cells. These characteristics may tune OHC force generation to produce the sharp frequency selectivity of mammalian hearing.

Finite element model of feed-forward/feed-backward amplification in the mouse cochlea

Thousands of hair cells in the organ of Corti, situated along the basilar membrane (BM), detect displacements due to sound input. For low input sounds, these displacements are amplified by active outer hair cells (OHCs). A proposed theory is the feed-forward/feed-backward mechanism for the OHC amplification where an expanding hair cell gives a forward push through the Deiters Cells and a backward pull on the BM through the Phalangeal process. Previously this was implemented mathematically using WKB theory (Yoon et al. 2011, Biophys. J.). In the present work, we explicitly modeled this as a Y-shaped arrangement of the OHC-DeitersCell-PhallangealProcess to form a building block using beam elements in a finite element formulation. These Y-shaped blocks were chained together to construct a single-row organ-of-Corti model from the base to apex, coupled to the BM and scalae fluid, of a mouse cochlea. The OHC force is proportional to the shear on the BM. For a 10 kHz stapes input tone, the passive BM reaches a peak gain of about 26 dB. For the active case the BM gain increases to 58 dB and shifts apically by about 0.6 mm. These results are consistent with physiological measurements in several other living animals.

Lateral wall protein content mediates alterations in cochlear outer hair cell mechanics before and after hearing onset

Specialized outer hair cells (OHCs) housed within the mammalian cochlea exhibit active, nonlinear, mechanical responses to auditory stimulation termed electromotility. The extraordinary frequency resolution capacity of the cochlea requires an exquisitely equilibrated mechanical system of sensory and supporting cells. OHC electromotile length change, stiffness, and force generation are responsible for a 100-fold increase in hearing sensitivity by augmenting vibrational input to non-motile sensory inner hair cells. Characterization of OHC mechanics is crucial for understanding and ultimately preventing permanent functional deficits due to overstimulation or as a consequence of various cochlear pathologies. The OHCs' major structural assembly is a highly-specialized lateral wall. The lateral wall consists of three structures; a plasma membrane highly-enriched with the motor-protein prestin, an actin-spectrin cortical lattice, and one or more layers of subsurface cisternae. Technical difficulties in independently manipulating each lateral wall constituent have constrained previous attempts to analyze the determinants of OHCs' mechanical properties. Temporal separations in the accumulation of each lateral wall constituent during postnatal development permit associations between lateral wall structure and OHC mechanics. We compared developing and adult gerbil OHC axial stiffness using calibrated glass fibers. Alterations in each lateral wall component and OHC stiffness were correlated as a function of age. Reduced F-actin labeling was correlated with reduced OHC stiffness before hearing onset. Prestin incorporation into the PM was correlated with increased OHC stiffness at hearing onset. Our data indicate lateral wall F-actin and prestin are the primary determinants of OHC mechanical properties before and after hearing onset, respectively.

Mechanical Transduction in Outer Hair Cells

The outer hair cells are responsible for the exquisite sensitivity, frequency selectivity and dynamic range of the cochlea. These cells are part of a mechanical feedback system involving the basilar membrane and tectorial membrane. Transverse displacement of the basilar membrane results in relative motion between the tectorial membrane and the reticular lamina, causing deflection of the stereocilia and modulation of the open probability of their transduction channels. The resulting current causes a change of membrane potential, which in turn produces mechanical force, that is fed back into the motion of the basilar membrane. Experiments were conducted to address mechanical transduction mechanisms in both the stereocilia and the basolateral cell membrane, as well as modes of coupling of the outer hair cell force to the organ of Corti.

Analysis of Force Generation Mechanism of Outer Hair Cell Using Elastic Shell Theory.

Recent experiments have shown that the isolated outer hair cell (OHC) can elongate and contract in response to electrical stimulation. From the OHC's position relative to the basilar membrane, it is deduced that the OHC length change directly affects the basilar membrane motion. Thus, this electromotile property of the OHC may play a key role in cochlear tuning. In order to evaluate the OHC's ability to influence the basilar membrane motion and tuning, a few experiments and numerical analyses have already been reported. However, the OHC motility within the range of the human ear (20 Hz∼20 kHz) and the force generated by the OHC are unknown. In this study, an attempt is made to analyze the OHC motility by using the shell theory. The OHC is modelled as a cylindrical composite shell and the function of the motor protein is taken into consideration. Substituting the estimated mechanical properties of the plasma membrane and cortical lattice to the shell model, the force generated by the OHC is obtained numerically. Then these mechanical properties are determined by comparing the numerically obtained OHC force with the experimental data of the previous report. Also the dynamic equations of the shell model are derived, and the dynamic behavior of the OHC is examined. The results indicate that the axial and circumferential stiffnesses of the cortical lattice are 1.3×10-2N/m and 2.0×10-2N/m, respectively, and that the numerically obtained dynamic behavior of the OHC is consistent with the experimental data.

Modulation of outer hair cell compliance and force by agents that affect hearing

Force generated by outer hair cells is thought to be an essential source of mechanical input to the normal cochlea. Many disease processes in the inner ear act via outer hair cells. It is therefore plausible that such disease processes modulate outer hair cell force generation. The force generated by an isolated, electrically stimulated outer hair cell against a load may be represented by an intrinsic motor and a passive axial stiffness in series. Thus modulation of outer hair cell force generation may occur either by action on the motor or indirectly by an action on cell stiffness. In this study, the effects of agents that affect hearing on outer hair cell stiffness and force generation have been examined. Overstimulation and hypoosmotic challenge caused cells to decrease in length and increase in stiffness. The force generated by a constant voltage stimulus increased consequent to the stiffness increase. Hyperosmotic challenge elicited a stiffness decrease and a force decrease. In contrast, salicylate caused a decrease in force without stiffness change. The results suggest that outer hair cell force generation in vivo may be modulated in at least two ways.

A piezoelectric model of outer hair cell function.

Mammalian outer hair cells (OHC) are believed to increase cochlear sensitivity and frequency selectivity via electromechanical feedback. A simple piezoelectric model of outer hair cell function is presented which integrates existing data from isolated OHC experiments. The model predicts maximum OHC force production to equal 1.25 nN/mV. The model also predicts that the maximum velocity of OHC contraction in situ to be 800 microns/s. These predictions are compared to available experimental data and are found to be in good agreement. The good agreement between the predicted and experimental results suggests that, at the characteristic frequency of a given cochlear location, the OHC receptor current is very efficiently converted into basilar membrane motion.

A cochlear model using feedback from motile outer hair cells

A model of cochlear vibrations based upon motile outer hair cells (OHCs) has been developed using physiologically demonstrated phenomena. Rapid longitudinally directed OHC forces are connected in such a way as to form a negative- feedback system. The responses at the higher frequencies (> 1 kHZ) are quite realistic: they have properly shaped amplitude curves with large tip-to-tail ratios (30–50 dB), Q 10' s of 2–6, and ‘shoulders’ at frequencies an octave below the resonant frequency. The phases are also quite realistic, though asymptoting at somewhat lower values (about −6π radians) than observed physiologically. The responses in the apical section are not so realistic. The form of the OHC force is physically unrealizable, but realizable forms are discussed.