Modelling the Generation of the Cochlear Microphonic

<p>The human ear is a remarkable sensory organ. A normal healthy human ear is able to process sounds covering a wide range of frequencies and intensities, while distinguishing between different components of complex sounds such as a musical chord. In the last four decades, knowledge about the cochlea and the mechanisms involved in its operation has greatly increased, but many details about these mechanisms remain unresolved and disputed. The cochlea has a vulnerable structure. Consequently, measuring and monitoring its mechanical and electrical activities even with contemporary devices is very difficult. Modelling can be used to fill gaps between those measurements that are feasible and actual cochlear function. Modelling techniques can also help to simplify complex cochlear operation to a tractable and comprehensible level while still reproducing certain behaviours of interest. Modelling therefore can play an essential role in developing a better understanding of the cochlea. The Cochlear Microphonic (CM) is an electrical signal generated inside the cochlea in response to sound. This electrical signal reflects mechanical activity in the cochlea and the excitation processes involved in its generation. However, the difficulty of obtaining this signal and the simplicity of other methods such as otoacoustic emissions have discouraged the use of the cochlear microphonic as a tool for studying cochlear functions. In this thesis, amodel of the cochlea is presented which integrates bothmechanical and electrical aspects, enabling the interaction between them to be investigated. The resulting model is then used to observe the effect of the cochlear amplifier on the CM. The results indicate that while the cochlear amplifier significantly amplifies the basilar membrane displacement, the effect on the CM is less significant. Both of these indications agree with previous physiological findings. A novel modelling approach is used to investigate the tuning discrepancy between basilar membrane and CMtuning curves. The results suggest that this discrepancy is primarily due to transversal phase cancellation in the outer hair cell rather than longitudinal phase cancellation along the basilar membrane. In addition, the results of the model suggest that spontaneous cochlear microphonic should exist in the cochlea. The existence of this spontaneous electrical signal has not yet been reported.</p>

Modelling the Generation of the Cochlear Microphonic

<p>The human ear is a remarkable sensory organ. A normal healthy human ear is able to process sounds covering a wide range of frequencies and intensities, while distinguishing between different components of complex sounds such as a musical chord. In the last four decades, knowledge about the cochlea and the mechanisms involved in its operation has greatly increased, but many details about these mechanisms remain unresolved and disputed. The cochlea has a vulnerable structure. Consequently, measuring and monitoring its mechanical and electrical activities even with contemporary devices is very difficult. Modelling can be used to fill gaps between those measurements that are feasible and actual cochlear function. Modelling techniques can also help to simplify complex cochlear operation to a tractable and comprehensible level while still reproducing certain behaviours of interest. Modelling therefore can play an essential role in developing a better understanding of the cochlea. The Cochlear Microphonic (CM) is an electrical signal generated inside the cochlea in response to sound. This electrical signal reflects mechanical activity in the cochlea and the excitation processes involved in its generation. However, the difficulty of obtaining this signal and the simplicity of other methods such as otoacoustic emissions have discouraged the use of the cochlear microphonic as a tool for studying cochlear functions. In this thesis, amodel of the cochlea is presented which integrates bothmechanical and electrical aspects, enabling the interaction between them to be investigated. The resulting model is then used to observe the effect of the cochlear amplifier on the CM. The results indicate that while the cochlear amplifier significantly amplifies the basilar membrane displacement, the effect on the CM is less significant. Both of these indications agree with previous physiological findings. A novel modelling approach is used to investigate the tuning discrepancy between basilar membrane and CMtuning curves. The results suggest that this discrepancy is primarily due to transversal phase cancellation in the outer hair cell rather than longitudinal phase cancellation along the basilar membrane. In addition, the results of the model suggest that spontaneous cochlear microphonic should exist in the cochlea. The existence of this spontaneous electrical signal has not yet been reported.</p>

Evaluation of cochlear implant electrode scalar position by 3 Tesla magnet resonance imaging

The estimation of scalar electrode position is a central point of quality control during the cochlear implant procedure. Ionic radiation is a disadvantage of commonly used radiologic estimation of electrode position. Recent developments in the field of cochlear implant magnets, implant receiver magnet position, and MRI sequence usage allow the postoperative evaluation of inner ear changes after cochlear implantation. The aim of the present study was to evaluate the position of lateral wall and modiolar cochlear implant electrodes using 3 T MRI scanning. In a prospective study, we evaluated 20 patients (10× Med-El Flex 28; 5× HFMS AB and 5× SlimJ AB) with a 3 T MRI and a T2 2D Drive MS sequence (voxel size: 0.3 × 0.3 × 0.9 mm) for the estimation of the intracochlear position of the cochlear implant electrode. In all cases, MRI allowed a determination of the electrode position in relation to the basilar membrane. This observation made the estimation of 19 scala tympani electrode positions and a single case of electrode translocation possible. 3 T MRI scanning allows the estimation of lateral wall and modiolar electrode intracochlear scalar positions.

The Disorder of Mitochondrial Dynamics Causes Deafness By Promoting Macrophage-Mediated Hair Cell Death

Mitochondrial dynamics are essential for maintaining the physiological function of the mitochondrial network, and the disorder of mitochondrial dynamics leads to neurodegenerative diseases. However, how mitochondrial dynamics affects auditory function in the inner ear remains unclear. FAM73a and FAM73b are mitochondrial outer membrane proteins that mediate mitochondrial fusion. Here, we found that FAM73a or FAM73b deficiency resulted in elevated oxidative stress and apoptosis of hair cells. Additionally, mitochondrial fission also causes an increase expression of IL-12 in basilar membrane macrophages through accumulating IRF1. As a bridge between innate and adaptive immune responses, hyperproduction of IL-12 further promoted the polarization of Th1 and tissue damage. Our data highlighted an important role of mitochondrial dynamics in maintaining cochlear homeostasis and hair cell survival. Mitochondrial dynamics not only disturbed hair cell function, but also induced the disorder of immune responses.

Functional Analyses of Peripheral Auditory System Adaptations for Echolocation in Air vs. Water

The similarity of acoustic tasks performed by odontocete (toothed whale) and microchiropteran (insectivorous bat) biosonar suggests they may have common ultrasonic signal reception and processing mechanisms. However, there are also significant media and prey dependent differences, notably speed of sound and wavelengths in air vs. water, that may be reflected in adaptations in their auditory systems and peak spectra of out-going signals for similarly sized prey. We examined the anatomy of the peripheral auditory system of two species of FM bat (big brown bat Eptesicus fuscus; Japanese house bat Pipistrellus abramus) and two toothed whales (harbor porpoise Phocoena phocoena; bottlenose dolphin Tursiops truncatus) using ultra high resolution (11–100 micron) isotropic voxel computed tomography (helical and microCT). Significant differences were found for oval and round window location, cochlear length, basilar membrane gradients, neural distributions, cochlear spiral morphometry and curvature, and basilar membrane suspension distributions. Length correlates with body mass, not hearing ranges. High and low frequency hearing range cut-offs correlate with basilar membrane thickness/width ratios and the cochlear radius of curvature. These features are predictive of high and low frequency hearing limits in all ears examined. The ears of the harbor porpoise, the highest frequency echolocator in the study, had significantly greater stiffness, higher basal basilar membrane ratios, and bilateral bony support for 60% of the basilar membrane length. The porpoise’s basilar membrane includes a “foveal” region with “stretched” frequency representation and relatively constant membrane thickness/width ratio values similar to those reported for some bat species. Both species of bats and the harbor porpoise displayed unusual stapedial input locations and low ratios of cochlear radii, specializations that may enhance higher ultrasonic frequency signal resolution and deter low frequency cochlear propagation.

A flexible anatomical set of mechanical models for the organ of Corti

We build a flexible platform to study the mechanical operation of the organ of Corti (OoC) in the transduction of basilar membrane (BM) vibrations to oscillations of an inner hair cell bundle (IHB). The anatomical components that we consider are the outer hair cells (OHCs), the outer hair cell bundles, Deiters cells, Hensen cells, the IHB and various sections of the reticular lamina. In each of the components we apply Newton’s equations of motion. The components are coupled to each other and are further coupled to the endolymph fluid motion in the subtectorial gap. This allows us to obtain the forces acting on the IHB, and thus study its motion as a function of the parameters of the different components. Some of the components include a nonlinear mechanical response. We find that slight bending of the apical ends of the OHCs can have a significant impact on the passage of motion from the BM to the IHB, including critical oscillator behaviour. In particular, our model implies that the components of the OoC could cooperate to enhance frequency selectivity, amplitude compression and signal to noise ratio in the passage from the BM to the IHB. Since the model is modular, it is easy to modify the assumptions and parameters for each component.

The origin of mechanical harmonic distortion within the organ of Corti in living gerbil cochleae

Although auditory harmonic distortion has been demonstrated psychophysically in humans and electrophysiologically in experimental animals, the cellular origin of the mechanical harmonic distortion remains unclear. To demonstrate the outer hair cell-generated harmonics within the organ of Corti, we measured sub-nanometer vibrations of the reticular lamina from the apical ends of the outer hair cells in living gerbil cochleae using a custom-built heterodyne low-coherence interferometer. The harmonics in the reticular lamina vibration are significantly larger and have broader spectra and shorter latencies than those in the basilar membrane vibration. The latency of the second harmonic is significantly greater than that of the fundamental at low stimulus frequencies. These data indicate that the mechanical harmonics are generated by the outer hair cells over a broad cochlear region and propagate from the generation sites to their own best-frequency locations.