Local mechanical properties of mouse outer hair cells: atomic force microscopic study

OBJECTIVES: Outer hair cells (OHCs) are capable of altering their cell length in response to changes in membrane potential. Due to this electromotility, OHCs probably subject the basilar membrane to force, resulting in cochlear amplification. To understand the mechanism of such amplification, knowledge of the mechanical properties of OHCs is required since the force produced by OHC electromotility is thought to depend on such properties. Various studies have been conducted to investigate the mechanical properties of guinea pig OHCs. With regard to mice, however, although various kinds of transgenic and knockout mice possess great potential as research models, the mechanical properties of mouse OHCs have not as yet been reported since the cells and/or tissues in the mouse hearing organ are relatively small and vulnerable to external stimuli, rendering sample preparation difficult. In this study, therefore, to establish indicators of the mechanical properties of OHCs in mice, such properties were measured by atomic force microscopy (AFM). METHODS: CBA/JNCrj strain male mice aged 10-12 weeks (25-30 g) were used. Cochleae were dissected out from the animal and both the basilar membrane and the organ of Corti were simultaneously unwrapped from the modiolus with forceps. Dissected coiled tissue was then incubated with an enzymatic digestion medium for 15 min. After digestion, OHCs were isolated by gently triturating the coiled tissue. Local mechanical properties of the OHCs were then measured by an indentation test using an AFM. RESULTS: Young's modulus and stiffness of the OHC in the apical turn of the mouse cochlea were 2.1+/-0.5 kPa and 4.4+/-1.2 mN/m, respectively. CONCLUSIONS: Young's modulus of the OHC in the apical turn of the cochlea in mice was roughly the same as that in the apical turn of the cochlea in guinea pigs; however, the stiffness of the former was about two times greater than that of the latter because the cell length of the former was shorter than that of the latter.     

Quantitative Relations between Outer Hair Cell Electromotility and Nonlinear Capacitance

The electrically evoked somatic motility of outer hair cells (OHC), briefly termed OHC electromotility, plays a crucial role in cochlear amplification that underlies the remarkably high sensitivity and frequency selectivity of the mammalian hearing. Accompanying OHC electromotility is a voltage-dependent gating charge movement within the cell lateral membrane, manifested as a measurable nonlinear capacitance (NLC) in OHCs. The electromotility and NLC of OHCs are highly correlated by sharing a common molecular substrate, the motor protein prestin. In this study, we systematically characterized the quantitative relationship between OHC electromotility and NLC in their voltage dependences for the purpose of further understanding the electromechanical transduction in OHCs. The results demonstrated that the two possess differing voltage dependences with the V1/2 of electromotility consistently being ～20 mV depolarized in comparison with that of NLC although their slope factors α are statistically identical. Further investigations showed that the initial state of OHCs influences the voltage dependence of electromotility but not that of NLC, indicating that some biophysical factors other than the motor protein per se are involved in the process of OHC length changes. We proposed that the cytoskeletal spectrin-actin framework underneath the OHC plasma membrane and the cell’s turgor are the two most probable factors that cause the voltage-dependence discrepancy between OHC electromotility and NLC.     

The radial pattern of basilar membrane motion evoked by electric stimulation of the cochlea.

Electric current applied to the cochlea can evoke in situ electromotile responses of the organ of Corti. These nonsound-generated responses can give insight into the mechanics of the organ as the putative forces produced by outer hair cells (OHC) must couple to the modes of vibration of the basilar membrane (BM). In this study, platinum-iridium wire electrodes were positioned into the scala vestibuli and scala tympani of the first cochlear turn in the guinea pig. Current (1.5 ms rectangular-shaped pulses) was applied to these electrodes at levels to 500 microA peak. A laser Doppler velocimeter was used to record the velocity or displacement of the basilar membrane at the tonotopic 18 kHz place via an opening into the scala tympani of the first cochlear turn. Beads were positioned across the width of the BM so that the velocity or displacement of the BM could be studied in the radial direction. It was found that the current pulses evoked linear displacements of up to 2 nm for current levels of 500 microA (higher levels were damaging to the organ of Corti). The pattern of motion across the width of the BM was such that maximum displacement and velocity was located near the first row of OHCs and the position of the outer pillar cell footplate. The BM motion was biphasic in that the zona arcuata moved in the opposite direction to that of the zona pectinata. The results of this study demonstrate that the level of force produced by OHCs is effective in moving the BM and that the distribution of force within the organ of Corti leads to a multimodal motion pattern of the BM for this experimentally artificial means of evoking OHC motion.     

Cochlear compression: perceptual measures and implications for normal and impaired hearing

This article provides a review of recent developments in our understanding of how cochlear nonlinearity affects sound perception and how a loss of the nonlinearity associated with cochlear hearing impairment changes the way sounds are perceived. The response of the healthy mammalian basilar membrane (BM) to sound is sharply tuned, highly nonlinear, and compressive. Damage to the outer hair cells (OHCs) results in changes to all three attributes: in the case of total OHC loss, the response of the BM becomes broadly tuned and linear. Many of the differences in auditory perception and performance between normal-hearing and hearing-impaired listeners can be explained in terms of these changes in BM response. Effects that can be accounted for in this way include poorer audiometric thresholds, loudness recruitment, reduced frequency selectivity, and changes in apparent temporal processing. All these effects can influence the ability of hearing-impaired listeners to perceive speech, especially in complex acoustic backgrounds. A number of behavioral methods have been proposed to estimate cochlear nonlinearity in individual listeners. By separating the effects of cochlear nonlinearity from other aspects of hearing impairment, such methods may contribute towards identifying the different physiological mechanisms responsible for hearing loss in individual patients. This in turn may lead to more accurate diagnoses and more effective hearing-aid fitting for individual patients. A remaining challenge is to devise a behavioral measure that is sufficiently accurate and efficient to be used in a clinical setting.     

Effects of intense noise exposure on the outer hair cell plasma membrane fluidity

Outer hair cells (OHCs) play an important role in cochlear amplification via their length changes (electromotility). A noise-induced cochlear amplification loss leading to a permanent threshold shift (PTS) was observed without a significant hair cell loss in rats [Chen, G.D., Liu, Y., 2005. Mechanisms of noise-induced hearing loss potentiation by hypoxia. Hear. Res. 200, 1-9.]. Since motor proteins are inserted in the OHC lateral membrane, any change in the OHC plasma membrane may result in a loss of OHC electromotility, leading to a loss of cochlear amplification. In this study, the lateral diffusion in the OHC plasma membrane was determined in vitro in guinea pigs by fluorescent recovery after photobleaching (FRAP) after an in vivo noise exposure. The lateral diffusion in the OHC plasma membrane demonstrated a length-dependence, which increased as OHC length increased. A reduction in the lateral diffusion was observed in those OHCs with lengths of 50-70 microm after exposure to an 8-kHz octave band noise at 110 dB SPL for 3h. This membrane fluidity change was associated with the selective PTS at frequencies around 8 kHz. The reduction of the lateral diffusion in the OHC lateral wall indicated that noise could impair the micromechanics of the OHC lateral wall and might consequently impair OHC electromotility to induce threshold shift.     

Effects of salicylates on evoked otoacoustic emissions and remote masking in humans.

The aim of this study was to evaluate, in young volunteer subjects, the effects of salicylates on evoked otoacoustic emissions (EOAEs), which presumably reflect an active mechanical process in the cochlea due to outer hair cell (OHC) activity, and on remote masking (RM), which has been proposed as a useful tool in the study of the non-linear cochlear distortion products generated by high-frequency maskers. Data from the present research are consistent with the literature showing a reversible effect of salicylate leading to elevated hearing thresholds and reduced EOAE amplitudes. From the point of view of new findings, the results demonstrate a reversible effect of salicylates on RM magnitude, which decreases as serum salicylate concentration increases. As described previously by other authors, salicylate selectivity inhibits OHC motility and, in consequence, reduces the amplitude of the motion of the basilar membrane. According to these data it is very likely that the observed reduction in RM magnitude after salicylate administration is also the result of the decreased ability of the OHCs to contract and of the reduced basilar membrane motion. The results are consistent with the conclusion that the OHC system function plays a role in producing RM.     

Imaging by Atomic Force Microscopy of the Plasma Membrane of Prestin-Transfected Chinese Hamster Ovary Cells

The high sensitivity of mammalian hearing is achieved by amplification of the motion of the cochlear partition. This cochlear amplification is thought to be generated by the elongation and contraction of outer hair cells (OHCs) in response to acoustical stimulation. This motility is made possible by a membrane protein embedded in the lateral membrane of OHCs. Although a fructose transporter, GLUT-5, was initially proposed to be this protein, a later study identified the gene of the motor protein distributed throughout the OHC plasma membrane. This protein has been named “prestin.” However, although previous morphological studies by electron microscopy and atomic force microscopy (AFM) found the lateral wall of OHCs to be covered with 10-nm particles, believed to be motor proteins, it is unknown whether such particles consist only of prestin or are a complex of GLUT-5 and prestin molecules. To determine if the 10-nm particles are indeed constituted only of prestin, plasma membranes of prestin-transfected and untransfected Chinese hamster ovary (CHO) cells, which do not express GLUT-5, were observed by AFM. First, the cells attached to a substrate were sonicated so that only the plasma membrane remained on the substrate. The cytoplasmic face of the cell was observed by the tapping mode of the AFM in liquid. As a result, particle-like structures were recognized on the plasma membranes of both the prestin-transfected and untransfected CHO cells. Comparison of the difference in the frequency distribution of these structures between those two cells showed approximately 75% of the particle-like structures with a diameter of 8–12 nm in the prestin-transfected CHO cells to be possibly constituted only by prestin molecules. Our data suggest that the densely packed 10-nm particles observed on the OHC lateral wall are likely to be constituted only of prestin molecules.     

Unitary ototoxic gentamicin exposure may not disrupt the function of cochlear outer hair cells in mice

Background: Previous study showed that mild ototoxic exposure could induce a reversible hearing impairment, and the loss and secondary incomplete recovery of cochlear ribbon synapses could be responsible for the hearing loss. However, it remains unclear whether cochlear outer hair cells’ (OHCs) functions are affected.

Objective: To verify whether the function of OHCs are also affected significantly after the ototoxic exposure.

Methods: Mice were injected intraperitoneally with 100?mg/kg concentration of gentamicin daily for 14 days. Distortion Product of Oto-acoustic Emission (DPOAE) was detected at control (pre-treatment), Day 0, day 4, day 7, day 14 and day 28 after the ototoxic exposure, respectively. In addition, the morphology of OHCs was observed by electron microscopy, OHCs has been counted by light microscopy, and the hearing thresholds were detected by auditory brain response (ABR).

Results: No significant changes have been found in OHC and OHC stereocilia among the experimental groups (p?>?.05). Further, no significant changes or loss was found in the morphology of OHCs either. However, we found ABR threshold elevations occurred after ototoxic exposure.

Conclusions: Unitary ototoxic gentamicin exposure may not disrupt the function of cochlear OHCs in mice, regardless of hearing loss identified in this ototoxic exposure.     

Thyroid hormone is not necessary for the development of outer hair cell electromotility

The outer hair cell (OHC), one of two receptor cell types in the organ of Corti, plays a critical role in mammalian hearing. OHCs enhance basilar membrane motion through a local mechanical feedback process within the cochlea, termed the 'cochlear amplifier'. It is generally believed that the basis of cochlear amplification is a voltage-dependent electromotile response of OHCs. Measurements of electromotility in developing animals indicate that the onset of motility normally occurs around 7 days after birth in altricial rodents such as gerbils and rats. Thyroid hormone (TH) plays a crucial role in the development of the auditory system. Deficiency of the hormone between the late embryonic stage and the second postnatal week can cause severe hearing loss. Several studies suggest that TH deficiency might also affect the development of the cochlear amplifier. The goal of this study was therefore to examine whether TH was necessary for the development of OHC motility. The organ of Corti of gerbils was dissected out at birth and grown in culture with defined concentration of triiodothyronine (T3), the active ligand for the TH receptor. Motility was measured from OHCs isolated from 7-, 11- and 14-day-old cultures. Motility did indeed develop in OHCs deprived of normal concentration of T3. This suggests that the defective auditory function seen in TH-deficient animals is most likely due to morphological and physiological changes in the cochlea, rather than the motor function of the OHCs.     

Cisplatin-induced ototoxicity: morphological evidence of spontaneous outer hair cell recovery in albino guinea pigs?

Cisplatin is frequently used in the treatment of various forms of malignancies. Its therapeutic efficacy, however, is limited by the occurrence of sensorineural hearing loss. Little is known about the course of hearing loss over longer time intervals after cessation of cisplatin administration. Infrequently, recovery of hearing has been described in animals and humans. Stengs et al. (1997) treated guinea pigs with cisplatin at a daily dose of 1.5 mg/kg for 8 consecutive days and subsequently studied cochlear function after survival times varying from 1 day to 16 weeks. Spontaneous improvement of the hair cell-related potentials (cochlear microphonics and summating potentials) was observed starting 2 weeks after cessation of treatment. In the present study we examined light microscopically the cochleas used in the study of Stengs et al. (1997). One day after cessation of cisplatin administration outer hair cell (OHC) loss in the basal cochlear turn averaged 66%. In the 1-week survival group, OHC counts were similar to those of the 1-day survival group. In the 4-week survival group, however, a relatively small loss of OHCs was found in the basal cochlear turn; OHC loss averaged only 15%. A similar loss was found after 8 weeks. In the 16-week survival group, OHC loss in the basal turn increased to 48%, but this was not statistically significant. Our histological observations are in line with the electrophysiological data from the same animals. Our findings suggest that OHCs recover from cisplatin-induced damage 1-4 weeks after treatment. However, the results do not allow a conclusion as to whether the observed recovery is due to the formation of new OHCs or to (self-)repair of damaged OHCs.     

Biophysical Mechanisms Underlying Outer Hair Cell Loss Associated with a Shortened Tectorial Membrane

The tectorial membrane (TM) connects to the stereociliary bundles of outer hair cells (OHCs). Humans with an autosomal dominant C1509G mutation in alpha-tectorin, a protein constituent of the TM, are born with a partial hearing loss that worsens over time. The Tecta ^C1509/+ transgenic mouse with the same point mutation has partial hearing loss secondary to a shortened TM that only contacts the first row of OHCs. As well, Tecta ^C1509G/+ mice have increased expression of the OHC electromotility protein, prestin. We sought to determine whether these changes impact OHC survival. Distortion product otoacoustic emission thresholds in a quiet environment did not change to 6 months of age. However, noise exposure produced acute threshold shifts that fully recovered in Tecta ^+/+ mice but only partially recovered in Tecta ^C1509G/+ mice. While Tecta ^+/+ mice lost OHCs primarily at the base and within all three rows, Tecta ^C1509G/+ mice lost most of their OHCs in a more apical region of the cochlea and nearly completely within the first row. In order to estimate the impact of a shorter TM on the forces faced by the stereocilia within the first OHC row, both the wild type and the heterozygous conditions were simulated in a computational model. These analyses predicted that the shear force on the stereocilia is ~50% higher in the heterozygous condition. We then measured electrically induced movements of the reticular lamina in situ and found that while they decreased to the noise floor in prestin null mice, they were increased by 4.58 dB in Tecta ^C1509G/+ mice compared to Tecta ^+/+ mice. The increased movements were associated with a fourfold increase in OHC death as measured by vital dye staining. Together, these findings indicate that uncoupling the TM from some OHCs leads to partial hearing loss and places the remaining coupled OHCs at higher risk. Both the mechanics of the malformed TM and the increased prestin-related movements of the organ of Corti contribute to this higher risk profile.     

A new model for calculating auditory excitation patterns and loudness for cases of cochlear hearing loss

A model for calculating auditory excitation patterns and loudness for steady sounds for normal hearing is extended to deal with cochlear hearing loss. The filters used in the model have a double ROEX-shape, the gain of the narrow active filter being controlled by the output of the broad passive filter. It is assumed that the hearing loss at each audiometric frequency can be partitioned into a loss due to dysfunction of outer hair cells (OHCs) and a loss due to dysfunction of inner hair cells (IHCs). OHC loss is modeled by decreasing the maximum gain of the active filter, which results in increased absolute threshold, reduced compressive nonlinearity and reduced frequency selectivity. IHC loss is modeled by a level-dependent attenuation of excitation level, which results in elevated absolute threshold. The magnitude of OHC loss and IHC loss can be derived from measures of loudness recruitment and the measured absolute threshold, using an iterative procedure. The model accurately fits loudness recruitment data obtained using subjects with unilateral or highly asymmetric cochlear hearing loss who were required to make loudness matches between tones presented alternately to the two ears. With the same parameters, the model predicted loudness matches between narrowband and broadband sound reasonably well, reflecting loudness summation. The model can also predict when a dead region is present.     

Tectorial membrane. II: Stiffness measurements in vivo

The tectorial membrane is assumed to play a crucial role in the stimulation of the cochlear hair cells and was thought for decades to serve as a stiff anchor for the tips of the hair-cell stereocilia, particularly those belonging to the OHCs. Yet, its stiffness has never been measured under conditions approximating its normal environment in live animals. We have developed a method for doing this. The tectorial membrane is approached through the lateral wall of scala media. The bony cochlear capsule is removed along scala media over somewhat less than 1/4 turn, and the underlying spiral ligament and stria vascularis are carefully reflected. With the help of a three axial hydraulic manipulator, a flexible micropipette filled with isotonic KCl is inserted into the tectorial membrane at one of two different angles and moved either transversally, away from the basilar membrane, or radially, toward or away from the modiolus. This causes the tectorial membrane to be deformed and the micropipette to bend. The micropipette stiffness is calibrated on an instrument of a new kind, so as to convert the bend into force. The calibration allows us to determine the point stiffness of the tectorial membrane from the amount of micropipette bend. The stiffness of the tectorial membrane per unit length has been calculated from the point stiffness with the help of the deformation pattern. Transversal and radial stiffness magnitudes have been determined in the second cochlear turn in Mongolian gerbils. Both are smaller by almost an order of magnitude than the corresponding aggregate stiffness of the OHC stereocilia. As a consequence, the tectorial membrane cannot act as a stiff anchor for the stereocilia but only as a mass load, except at relatively low sound frequencies where mass effects are negligible. This means that the classical model of shear motion between the tectorial membrane and the reticular lamina must be replaced.     

The functional age of hearing loss in a mouse model of presbycusis. II. Neuroanatomical correlates

This report relates patterns of age-related outer hair cell (OHC) loss to auditory behavioral deficits in C57BL/6J mice. Hair cell counts were made from serial sections of the cochlear partition in three subject groups representing young (2-3 months), middle (8-9 months), and old ages (12-13 months). The cochlear location of OHC counts was determined from three-dimensional computerized reconstructions of the serial sections. Comparisons of the topographic distribution of surviving OHCs across the subject groups confirmed an orderly base-to-apex progression of cochlear degeneration that is well known in this mouse strain. All mice appeared to follow the same progression of OHC loss, although subjects showed considerable variation in the rate at which they advanced through a uniform sequence of structural changes. Behavioral implications of the magnitude and location of OHC loss were investigated by correlating the histological status of individual mice with sound detection thresholds from the same subjects [Hear. Res. 183 (2003) 44-56]. The analysis revealed regionalized patterns of OHC loss that were correlated with frequency-dependent changes in hearing thresholds, and validates the use of 'functional age' as an indicator of age-related cochlear degeneration and dysfunction. In the absence of physiologically defined cochlear frequency maps for C57BL/6J mice, these structure-function correlation techniques offer an alternative approach for linking anatomical results to hearing abilities.     

Mathematical model of outer hair cell regulation including ion transport and cell motility

To understand the regulatory processes within the cochlea, and outer hair cells (OHCs) in particular, we have developed a mathematical model of OHC regulation that takes into account their known electrical properties, and includes fast and slow somatic motility. We model how cytosolic Ca(2+) is involved in regulation of (i) the OHC membrane potential, (ii) the operating point of OHC mechano-electrical transduction (MET) channels via slow motility; (iii) basolateral wall K(+) permeability via Ca(2+)-sensitive K(+) channels; and (iv) cytosolic concentrations of Ca(2+) itself, via Ca(2+)-ATPase-mediated sequestration within the OHCs and Ca(2+)-induced Ca(2+)-release (CICR) from the same intracellular Ca(2+) storage organelles. To account for some aspects of the cochlea's transient response to experimental perturbations, we have included a putative intracellular second-messenger cascade based on cytosolic Ca(2+). Overall, the OHC basolateral permeability determines the resting membrane potential of the OHCs and their standing current, which influences the endocochlear potential, and also affects the AC receptor potential that drives the prestin-mediated somatic electromotility and active cochlear gain. The model we have developed provides a physiologically-plausible and internally-consistent explanation for the time-courses of the cochlear changes we have observed during a number of different experimental perturbations, including a slow oscillatory behaviour presumed due to oscillations in cytosolic Ca(2+) concentration. We also show how the known ionic mechanisms within OHCs act to regulate membrane potential and hair bundle angle over a very wide range of strial current and intracochlear hydrostatic pressure. Not included in the model are osmotic effects, the nonlinear aspects of prestin's electromotility, the intracellular role of Cl(-) in modifying this motility, nor adaptation of MET at the apex of OHCs. Only one Ca(2+) sequestration compartment has been included in this implementation of the model, with the two types of basolateral Ca(2+) cisternae combined into a single compartment. Despite these limitations, the model as presented offers insights into the regulation of OHC membrane potential and MET at the hair cell apex, and is our first step in understanding in a quantitative way the integrated function of the molecular components of ion transport and motility in these cells.     

Reciprocal Synapses Between Outer Hair Cells and their Afferent Terminals: Evidence for a Local Neural Network in the Mammalian Cochlea

Cochlear outer hair cells (OHCs) serve both as sensory receptors and biological motors. Their sensory function is poorly understood because their afferent innervation, the type-II spiral ganglion cell, has small unmyelinated axons and constitutes only 5% of the cochlear nerve. Reciprocal synapses between OHCs and their type-II terminals, consisting of paired afferent and efferent specialization, have been described in the primate cochlea. Here, we use serial and semi-serial-section transmission electron microscopy to quantify the nature and number of synaptic interactions in the OHC area of adult cats. Reciprocal synapses were found in all OHC rows and all cochlear frequency regions. They were more common among third-row OHCs and in the apical half of the cochlea, where 86% of synapses were reciprocal. The relative frequency of reciprocal synapses was unchanged following surgical transection of the olivocochlear bundle in one cat, confirming that reciprocal synapses were not formed by efferent fibers. In the normal ear, axo-dendritic synapses between olivocochlear terminals and type-II terminals and/or dendrites were as common as synapses between olivocochlear terminals and OHCs, especially in the first row, where, on average, almost 30 such synapses were seen in the region under a single OHC. The results suggest that a complex local neuronal circuitry in the OHC area, formed by the dendrites of type-II neurons and modulated by the olivocochlear system, may be a fundamental property of the mammalian cochlea, rather than a curiosity of the primate ear. This network may mediate local feedback control of, and bidirectional communication among, OHCs throughout the cochlear spiral.     

Relation between outer hair cell loss and hearing loss in rats exposed to styrene

The relationship between outer hair cell (OHC) loss and cochlear sensitivity is still unclear, because in many animal models there exist surviving but dysfunctional OHCs and also injured/dead inner hair cells (IHC). Styrene is an ototoxic agent, which targets and destroys OHCs starting from the third row to the second and first rows depending on the exposure level. The remaining cells may be less affected. In this experiment, rats were exposed to styrene by gavage at different doses (200-800mg/kg/day) for varying periods (5 days/week for 3-12 weeks). An interesting finding was that the cochlear sensitivity was not affected in a few rats with all OHCs in the third row being destroyed by styrene. A further loss of OHCs was usually accompanied with a linear input/output (I/O) function of cochlear compound action potentials (CAP), indicating the loss of cochlear amplification. However, normal CAP amplitudes at the highest stimulation level of 90dB SPL were often observed when all OHCs were destroyed, indicating normal function of the remaining IHCs. The OHC-loss/hearing-loss relation appeared to be a sigmoid-type function. Initially, styrene-induced OHC losses (<33%) did not result in a significant threshold shift. Then CAP threshold shift increased dramatically with OHC loss from 33% to 66%. Then, CAP threshold changed less with OHC loss. The data suggest a tri-modal relationship between OHC loss and cochlear amplification. That is, under the condition that all surviving OHCs are ideally functioning, the cochlear amplifier is not affected until 33% of OHCs are absent, then the gain of the amplifier decreases proportionally with the OHC loss, and at last the amplifier may fail completely when more than 67% of OHCs are lost.     

Reduced Electromotility of Outer Hair Cells Associated with Connexin-Related Forms of Deafness: An In silico Study of a Cochlear Network Mechanism

Mutations in the GJB2 gene encoding for the connexin 26 (Cx26) protein are the most common source of nonsyndromic forms of deafness. Cx26 is a building block of gap junctions (GJs) which establish electrical connectivity in distinct cochlear compartments by allowing intercellular ionic (and metabolic) exchange. Animal models of the Cx26 deficiency in the organ of Corti seem to suggest that the hearing loss and the degeneration of outer hair cells (OHCs) and inner hair cells is due to failed K⁺ and metabolite homeostasis. However, OHCs can develop normally in some mutants, suggesting that the hair cells death is not the universal mechanism. In search for alternatives, we have developed an in silico large scale three-dimensional model of electrical current flow in the cochlea in the small signal, linearised, regime. The effect of mutations was analysed by varying the magnitude of resistive components representing the GJ network in the organ of Corti. The simulations indeed show that reduced GJ conductivity increases the attenuation of the OHC transmembrane potential at frequencies above 5 kHz from 6.1 dB/decade in the wild-type to 14.2 dB/decade. As a consequence of increased GJ electrical filtering, the OHC transmembrane potential is reduced by up to 35 dB at frequencies >10 kHz. OHC electromotility, driven by this potential, is crucial for sound amplification, cochlear sensitivity and frequency selectivity. Therefore, we conclude that reduced OHC electromotility may represent an additional mechanism underlying deafness in the presence of Cx26 mutations and may explain lowered OHC functionality in particular reported Cx26 mutants.     

Intracochlear pressure and organ of corti impedance from a linear active three-dimensional model

Intracochlear pressure and basilar membrane (BM) velocity are calculated from a physiologically based chinchilla cochlea model . The model includes three-dimensional viscous fluid and the pectinate zone of the elastic BM with dimensional and material property variation along its length. The passive response shows excellent agreement with measurement at high sound pressure levels. The active process is represented by adding the motility of the outer hair cells (OHCs) to the passive model with the feed-forward approximation of the organ of Corti (OC), as was done previously. The current model explains recent observations including: (1) agreement with characteristic frequency (CF)-to-place map, (2) CF shift in the active model, (3) BM displacement gain from OHC motility, (4) lower intracochlear pressure gain than BM displacement gain, and (5) OC impedance (Z(OC)).     

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 (greater than 1 kHZ) are quite realistic: they have properly shaped amplitude curves with large tip-to-tail ratios (30-50 dB), Q10'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 -6 pi 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.