Imaging electrically evoked micromechanical motion within the organ of corti of the excised gerbil cochlea

The outer hair cell (OHC) of the mammalian inner ear exhibits an unusual form of somatic motility that can follow membrane-potential changes at acoustic frequencies. The cellular forces that produce this motility are believed to amplify the motion of the cochlear partition, thereby playing a key role in increasing hearing sensitivity. To better understand the role of OHC somatic motility in cochlear micromechanics, we developed an excised cochlea preparation to visualize simultaneously the electrically-evoked motion of hundreds of cells within the organ of Corti (OC). The motion was captured using stroboscopic video microscopy and quantified using cross-correlation techniques. The OC motion at approximately 2-6 octaves below the characteristic frequency of the region was complex: OHC, Deiter's cell, and Hensen's cell motion were hundreds of times larger than the tectorial membrane, reticular lamina (RL), and pillar cell motion; the inner rows of OHCs moved antiphasic to the outer row; OHCs pivoted about the RL; and Hensen's cells followed the motion of the outer row of OHCs. Our results suggest that the effective stimulus to the inner hair cell hair bundles results not from a simple OC lever action, as assumed by classical models, but by a complex internal motion coupled to the RL.     

Cochlear implants: the view from the brain

The cochlear implant arguably is the most successful neural prosthesis. Studies of the responses of the central auditory system to prosthetic electrical stimulation of the cochlea are revealing the success with which electrical stimulation of a deaf ear can mimic acoustic stimulation of a normal-hearing ear. Understanding of the physiology of central auditory structures can lead to improved restoration of hearing with cochlear implants. In turn, the cochlear implant can be exploited as an experimental tool for examining central hearing mechanisms isolated from the effects of cochlear mechanics and transduction.     

Basilar membrane velocity in a cochlea with a modified organ of Corti

Many cochlear models assign zero longitudinal coupling in the cochlea. Although this is consistent with the transverse basilar membrane (BM) fibers, the cochlear partition contains cellular longitudinal coupling. In cochlear models, longitudinal coupling diminishes passive BM tuning; however, it has recently been employed in theories of active mechanics to enhance tuning. Our goal in this study was to probe passive longitudinal coupling by comparing BM responses in damaged cochleae with passive responses in normal cochleae. The cochleae of gerbils were damaged with intratympanic neomycin followed by a waiting period to ensure that all of the cells of the partition were missing or severely disrupted. We then measured BM motion and examined the cochleae histologically. In comparison with passive responses in normal cochleae, we observed a downward shift in characteristic frequency, an expected consequence of reduced stiffness from cellular damage. However, we did not observe enhanced passive tuning in the damaged cochleae, as would be expected if longitudinal coupling were substantially greater in the normal cochleae. Thus, we conclude that cell-based longitudinal coupling is not large enough to influence passive cochlear mechanics. This finding constrains theories of active mechanics.     

In Vivo Impedance of the Gerbil Cochlear Partition at Auditory Frequencies

The specific acoustic impedance of the cochlear partition was measured from 4 to 20 kHz in the basal turn of the gerbil cochlea, where the best frequency is ∼40 kHz. The acoustic impedance was found as the ratio of driving pressure to velocity response. It is the physical attribute that governs cochlear mechanics and has never before been directly measured, to our knowledge. The basilar membrane velocity was measured through the transparent round window membrane. Simultaneously, the intracochlear pressure was measured close to the stapes and quite close to the cochlear partition. The impedance phase was close to −90° and the magnitude decreased with frequency, consistent with stiffness-dominated impedance. The resistive component of the impedance was relatively small. Usually the resistance was negative at frequencies below 8 kHz; this unexpected finding might be due to other vibration modes within the cochlear partition.     

Response to a pure tone in a nonlinear mechanical-electrical-acoustical model of the cochlea

In this article, a nonlinear mathematical model is developed based on the physiology of the cochlea of the guinea pig. The three-dimensional intracochlear fluid dynamics are coupled to a micromechanical model of the organ of Corti and to electrical potentials in the cochlear ducts and outer hair cells (OHC). OHC somatic electromotility is modeled by linearized piezoelectric relations whereas the OHC hair-bundle mechanoelectrical transduction current is modeled as a nonlinear function of the hair-bundle deflection. The steady-state response of the cochlea to a single tone is simulated in the frequency domain using an alternating frequency time scheme. Compressive nonlinearity, harmonic distortion, and DC shift on the basilar membrane (BM), tectorial membrane (TM), and OHC potentials are predicted using a single set of parameters. The predictions of the model are verified by comparing simulations to available in vivo experimental data for basal cochlear mechanics. In particular, the model predicts more amplification on the reticular lamina (RL) side of the cochlear partition than on the BM, which replicates recent measurements. Moreover, small harmonic distortion and DC shifts are predicted on the BM, whereas more significant harmonic distortion and DC shifts are predicted in the RL and TM displacements and in the OHC potentials.     

Power amplification in the mammalian cochlea

It was first suggested by Gold in 1948 [1] that the exquisite sensitivity and frequency selectivity of the mammalian cochlea is due to an active process referred to as the cochlear amplifier. It is thought that this process works by pumping energy to augment the otherwise damped sound-induced vibrations of the basilar membrane [2-4], a mechanism known as negative damping. The existence of the cochlear amplifier has been inferred from comparing responses of sensitive and compromised cochleae [5] and observations of acoustic emissions [6, 7] and through mathematical modeling [8, 9]. However, power amplification has yet to be demonstrated directly. Here, we prove that energy is indeed produced in the cochlea on a cycle-by-cycle basis. By using laser interferometry [10], we show that the nonlinear component of basilar-membrane responses to sound stimulation leads the forces acting on the membrane. This is possible only in active systems with negative damping [11]. Our finding provides the first direct evidence for power amplification in the mammalian cochlea. The finding also makes redundant current hypotheses of cochlear frequency sharpening and sensitization that are not based on negative damping.     

Detection of Cochlear Amplification and Its Activation

The operation of the mammalian cochlea relies on a mechanical traveling wave that is actively boosted by electromechanical forces in sensory outer hair cells (OHCs). This active cochlear amplifier produces the impressive sensitivity and frequency resolution of mammalian hearing. The cochlear amplifier has inspired scientists since its discovery in the 1970s, and is still not well understood. To explore cochlear electromechanics at the sensory cell/tissue interface, sound-evoked intracochlear pressure and extracellular voltage were measured using a recently developed dual-sensor with a microelectrode attached to a micro-pressure sensor. The resulting coincident in vivo observations of OHC electrical activity, pressure at the basilar membrane and basilar membrane displacement gave direct evidence for power amplification in the cochlea. Moreover, the results showed a phase shift of voltage relative to mechanical responses at frequencies slightly below the peak, near the onset of amplification. Based on the voltage-force relationship of isolated OHCs, the shift would give rise to effective OHC pumping forces within the traveling wave peak. Thus, the shift activates the cochlear amplifier, serving to localize and thus sharpen the frequency region of amplification. These results are the most concrete evidence for cochlear power amplification to date and support OHC somatic forces as its source.     

Detection of Cochlear Amplification and Its Activation

The operation of the mammalian cochlea relies on a mechanical traveling wave that is actively boosted by electromechanical forces in sensory outer hair cells (OHCs). This active cochlear amplifier produces the impressive sensitivity and frequency resolution of mammalian hearing. The cochlear amplifier has inspired scientists since its discovery in the 1970s, and is still not well understood. To explore cochlear electromechanics at the sensory cell/tissue interface, sound-evoked intracochlear pressure and extracellular voltage were measured using a recently developed dual-sensor with a microelectrode attached to a micro-pressure sensor. The resulting coincident in vivo observations of OHC electrical activity, pressure at the basilar membrane and basilar membrane displacement gave direct evidence for power amplification in the cochlea. Moreover, the results showed a phase shift of voltage relative to mechanical responses at frequencies slightly below the peak, near the onset of amplification. Based on the voltage-force relationship of isolated OHCs, the shift would give rise to effective OHC pumping forces within the traveling wave peak. Thus, the shift activates the cochlear amplifier, serving to localize and thus sharpen the frequency region of amplification. These results are the most concrete evidence for cochlear power amplification to date and support OHC somatic forces as its source.     

Microstructures in the Organ of Corti Help Outer Hair Cells Form Traveling Waves along the Cochlear Coil

According to the generally accepted theory of mammalian cochlear mechanics, the fluid in the cochlear scalae interacts with the elastic cochlear partition to generate transversely oscillating displacement waves that propagate along the cochlear coil. Using a computational model of cochlear segments, a different type of propagating wave is reported, an elastic propagating wave that is independent of the fluid-structure interaction. The characteristics of the propagating wave observed in the model, such as the wavelength, speed, and phase lag, are similar to those observed in the living cochlea. Three conditions are required for the existence of the elastic propagating wave in the cochlear partition without fluid-interaction: 1), the stiffness gradient of the cochlear partition; 2), the elastic longitudinal coupling; and 3), the Y-shaped structure in the organ of Corti formed by the outer hair cell, the Deiters cell, and the Deiters cell phalangeal process. The elastic propagating waves in the cochlear partition disappeared without the push-pull action provided by the outer hair cell and Deiters cell phalangeal process. The results suggest that the mechanical feedback of outer hair cells, facilitated by the organ of Corti microstructure, can control the tuning and amplification by modulating the cochlear traveling wave.     

Microstructures in the Organ of Corti Help Outer Hair Cells Form Traveling Waves along the Cochlear Coil

According to the generally accepted theory of mammalian cochlear mechanics, the fluid in the cochlear scalae interacts with the elastic cochlear partition to generate transversely oscillating displacement waves that propagate along the cochlear coil. Using a computational model of cochlear segments, a different type of propagating wave is reported, an elastic propagating wave that is independent of the fluid-structure interaction. The characteristics of the propagating wave observed in the model, such as the wavelength, speed, and phase lag, are similar to those observed in the living cochlea. Three conditions are required for the existence of the elastic propagating wave in the cochlear partition without fluid-interaction: 1), the stiffness gradient of the cochlear partition; 2), the elastic longitudinal coupling; and 3), the Y-shaped structure in the organ of Corti formed by the outer hair cell, the Deiters cell, and the Deiters cell phalangeal process. The elastic propagating waves in the cochlear partition disappeared without the push-pull action provided by the outer hair cell and Deiters cell phalangeal process. The results suggest that the mechanical feedback of outer hair cells, facilitated by the organ of Corti microstructure, can control the tuning and amplification by modulating the cochlear traveling wave.     

Voltage oscillations and ionic conductances in hair cells isolated from the alligator cochlea

Summary Tall hair cells were isolated by enzymatic and mechanical dissociation from selected regions of the apical half of the alligator (A. mississippiensis) cochlea. Single cells were subjected to voltage-clamp and current-clamp using the tight-seal whole-cell recording technique. Most hair cells isolated from the apex of the cochlea produced slowly regenerative depolarizations or Na action potentials during current injection, whereas hair cells isolated from more basal regions usually produced voltage oscillations (ringing) in response to depolarizing current injection, an indication of electrical resonance. Resonant frequencies ranged from 50 to 157 Hz in different cells. The higher-frequency cells tended to have larger and more rapidly activating outward currents than did the lower-frequency cells. An inward Ca current and an outward Ca-activated K current were present in all hair cells. In addition, an inwardly rectifying current and a small, transient outward current were often seen. Thus, we conclude that an electrical tuning mechanism is present in alligator hair cells. The role of the ionic conductances in shaping hair cell responses to current injection, and the possible contributions of these electrical responses to cochlear function are discussed.     

Auditory brain-stem responses evoked by electrical stimulation of the cochlear nucleus in human subjects

When auditory nerve function is lost due to surgical removal of bilateral acoustic tumors, a sense of hearing may be restored by means of an auditory brain-stem implant (ABI), which electrically stimulates the auditory pathway at the level of the cochlear nucleus. Placement of the stimulating electrodes during surgical implantation may be aided by electrically evoked auditory brain-stem responses (EABRs) recorded intra-operatively. To establish preliminary standards for human EABRs evoked by electrical stimulation of the cochlear nucleus, short-latency evoked potentials were recorded from 6 ABI patients who were either already implanted or undergoing implantation surgery. Neural responses were distinguished from stimulus artifact and equipment artifact by their properties during stimulus polarity reversal and amplitude variation. Other properties contributed to further identification of the evoked potentials as auditory responses (EABRs). The response waveforms generally had 2 or 3 waves. The peak latencies of these waves (approximately 0.3, 1.3, and 2.2 msec) and the brain-stem localization of the region from which they could be elicited are consistent with auditory brain-stem origin.     

Coupling active hair bundle mechanics, fast adaptation, and somatic motility in a cochlear model

One of the central questions in the biophysics of the mammalian cochlea is determining the contributions of the two active processes, prestin-based somatic motility and hair bundle (HB) motility, to cochlear amplification. HB force generation is linked to fast adaptation of the transduction current via a calcium-dependent process and somatic force generation is driven by the depolarization caused by the transduction current. In this article, we construct a global mechanical-electrical-acoustical mathematical model of the cochlea based on a three-dimensional fluid representation. The global cochlear model is coupled to linearizations of nonlinear somatic motility and HB activity as well as to the micromechanics of the passive structural and electrical elements of the cochlea. We find that the active HB force alone is not sufficient to power high frequency cochlear amplification. However, somatic motility can overcome resistor-capacitor filtering by the basolateral membrane and deliver sufficient mechanical energy for amplification at basal locations. The results suggest a new theory for high frequency active cochlear mechanics, in which fast adaptation controls the transduction channel sensitivity and thereby the magnitude of the energy delivered by somatic motility.     

Reduction in Noise-Induced Functional Loss of the Cochleae in Mice with Pre-Existing Cochlear Dysfunction Due to Genetic Interference of Prestin

Various cochlear pathologies, such as acoustic trauma, ototoxicity and age-related degeneration, cause hearing loss. These pre-existing hearing losses can alter cochlear responses to subsequent acoustic overstimulation. So far, the knowledge on the impacts of pre-existing hearing loss caused by genetic alteration of cochlear genes is limited. Prestin is the motor protein expressed exclusively in outer hair cells in the mammalian cochlea. This motor protein contributes to outer hair cell motility. At present, it is not clear how the interference of prestin function affects cochlear responses to acoustic overstimulation. To address this question, a genetic model of prestin dysfunction in mice was created by inserting an internal ribosome entry site (IRES)-CreER^T2-FRT-Neo-FRT cassette into the prestin locus after the stop codon. Homozygous mice exhibit a threshold elevation of auditory brainstem responses with large individual variation. These mice also display a threshold elevation and a shift of the input/output function of the distortion product otoacoustic emission, suggesting a reduction in outer hair cell function. The disruption of prestin function reduces the threshold shifts caused by exposure to a loud noise at 120 dB (sound pressure level) for 1 h. This reduction is positively correlated with the level of pre-noise cochlear dysfunction and is accompanied by a reduced change in Cdh1 expression, suggesting a reduction in molecular responses to the acoustic overstimulation. Together, these results suggest that prestin interference reduces cochlear stress responses to acoustic overstimulation.     

Comparative actions of salicylate on the amphibian lateral line and guinea pig cochlea

1. Salicylate actions on afferent nerve activity in the Xenopus lateral line and on cochlear potentials in guinea pig were investigated. 2. In the lateral line, salicylate (0.3-2.5 mM) suppressed spontaneous activity, water motion evoked excitation and responses to L-glutamate (1-2 mM) and kainate (10-20 microM). 3. In the guinea pig, salicylate (0.6-10 mM) suppressed the compound action potential (CAP) and increased N1 latency at low but not high sound intensities. 4. In the lateral line salicylate action may involve an antagonism of the hair-cell transmitter on the afferent nerve. 5. In the cochlea salicylate may suppress the active process or cochlear amplifier.     

Three-dimensional motion of the organ of Corti

The vibration of the organ of Corti, a three-dimensional micromechanical structure that incorporates the sensory cells of the hearing organ, was measured in three mutually orthogonal directions. This was achieved by coupling the light of a laser Doppler vibrometer into the side arm of an epifluorescence microscope to measure velocity along the optical axis of the microscope, called the transversal direction. Displacements were measured in the plane orthogonal to the transverse direction with a differential photodiode mounted on the microscope in the focal plane. Vibration responses were measured in the fourth turn of a temporal-bone preparation of the guinea-pig cochlea. Responses were corrected for a "fast" wave component caused by the presence of the hole in the cochlear wall, made to view the structures. The frequency responses of the basilar membrane and the reticular lamina were similar, with little phase differences between the vibration components. Their motion was rectilinear and vertical to the surface of their membranes. The organ of Corti rotated about a point near the edge of the inner limbus. A second vibration mode was detected in the motion of the tectorial membrane. This vibration mode was directed parallel to the reticular lamina and became apparent for frequencies above approximately 0.5 oct below the characteristic frequency. This radial vibration mode presumably controls the shearing action of the hair bundles of the outer hair cells.     

Uptake and Release of γ-Aminobutyric Acid in the Guinea Pig Cochlear Nucleus After Axotomy of Cochlear and Centrifugal Fibers

Abstract: This study attempts to determine if γ-aminobutyric acid (GABA) may be a transmitter of cochlear nerve fibers projecting from the cochlea to the cochlear nucleus, and of centrifugal fibers projecting to the cochlear nucleus via the trapezoid body and the acoustic striae of the medulla. The uptake and the electrically evoked release of exogenous [¹⁴C]GABA were measured, in vitro, in the three major subdivisions of the guinea pig cochlear nucleus: the anteroventral, posteroventral, and dorsal cochlear nuclei. These activities were compared using unlesioned animals, animals with bilateral cochlear ablations, and animals whose trapezoid body and acoustic striae were interrupted on the right side of the medulla. Subdivisions from unlesioned animals took up [¹⁴C]GABA, achieving concentrations in the tissues that were 11–19 times that in the medium. Electrical stimulation evoked a Ca²⁺-dependent release of [¹⁴C]GABA from each subdivision. Bilateral cochlear ablation, which presumably destroyed the cochlear nerve fibers, had no effect on [¹⁴C]GABA uptake and release. Section of the trapezoid body and the acoustic striae on the right side of the medulla typically severed all known connections of the right posteroventral and dorsal cochlear nuclei with the rest of the brain, but left intact many connections involved with the right anteroventral cochlear nucleus. This lesion partially depressed [¹⁴C]GABA uptake and release in the right posteroventral and dorsal cochlear nuclei, but not in the right anteroventral cochlear nucleus. These findings suggest that one or more of the centrifugal tracts projecting to the cochlear nucleus may be GABAergic, 88% or more of the cochlear nerve fibers probably are not GABAergic, and some neurons of the cochlear nucleus are probably GABAergic.     

Regeneration of Stereocilia of Hair Cells by Forced Atoh1 Expression in the Adult Mammalian Cochlea

The hallmark of mechanosensory hair cells is the stereocilia, where mechanical stimuli are converted into electrical signals. These delicate stereocilia are susceptible to acoustic trauma and ototoxic drugs. While hair cells in lower vertebrates and the mammalian vestibular system can spontaneously regenerate lost stereocilia, mammalian cochlear hair cells no longer retain this capability. We explored the possibility of regenerating stereocilia in the noise-deafened guinea pig cochlea by cochlear inoculation of a viral vector carrying Atoh1, a gene critical for hair cell differentiation. Exposure to simulated gunfire resulted in a 60–70 dB hearing loss and extensive damage and loss of stereocilia bundles of both inner and outer hair cells along the entire cochlear length. However, most injured hair cells remained in the organ of Corti for up to 10 days after the trauma. A viral vector carrying an EGFP-labeled Atoh1 gene was inoculated into the cochlea through the round window on the seventh day after noise exposure. Auditory brainstem response measured one month after inoculation showed that hearing thresholds were substantially improved. Scanning electron microscopy revealed that the damaged/lost stereocilia bundles were repaired or regenerated after Atoh1 treatment, suggesting that Atoh1 was able to induce repair/regeneration of the damaged or lost stereocilia. Therefore, our studies revealed a new role of Atoh1 as a gene critical for promoting repair/regeneration of stereocilia and maintaining injured hair cells in the adult mammal cochlea. Atoh1-based gene therapy, therefore, has the potential to treat noise-induced hearing loss if the treatment is carried out before hair cells die.     

A mechanism for active hearing

The remarkable sensitivity, frequency selectivity, and nonlinearity of the cochlea have been attributed to the putative 'cochlear amplifier', which consumes metabolic energy to amplify the cochlear mechanical response to sounds. Recent studies have demonstrated that outer hair cells actively generate force using somatic electromotility and active hair-bundle motion. However, the expected power gain of the cochlear amplifier has not been demonstrated experimentally, and the measured location of cochlear nonlinearity is inconsistent with the predicted location of the cochlear amplifier. We instead propose a 'cochlear transformer' mechanism to interpret cochlear performance.