Comparison of the tuning of outer hair cells and the basilar membrane in the isolated cochlea

NUMERICAL SIMULATION ON THE MECHANICAL BEHAVIOR OF OUTER STEREOCILIA IN CORTI

In this paper, a two-dimensional model of the organ of Corti (OC) which includes basilar membrane (BM), tectorial membrane (TM), inner and outer hair cells and reticular lamina (RL) is established by Comsol. Based on experimental data that sinusoidal excitation was applied on the pectinate zone of the BM, the incentives are added to the corresponding position of the model. Transient analysis is made and the displacement of six different positions is achieved. The results are in good agreement with experimental data, which confirms the validity of the FE model. Based on time-domain and frequency-domain analysis, the relative motion between stereocilia and TM and RL is studied under sinusoidal excitation. The results show that, under time-domain analysis, the whole trend of relative displacement and velocity difference of Inner is similar to that of Outer, while there is little different when comparing with Middle. The relative velocity difference of Middle and Inner lag behind Outer with roughly 0.01[Formula: see text]s. Under frequency-domain analysis, at characteristic frequency, the deviation of each stereocilium is the largest. In the meantime, the frequency that the maximum value of outer stereocilia achieved is different. This new finding may cause the difference of lateral vibration of BM and indicate the frequency sensitivity of BM.

Minimal basilar membrane motion in low-frequency hearing

Low-frequency hearing is critically important for speech and music perception, but no mechanical measurements have previously been available from inner ears with intact low-frequency parts. These regions of the cochlea may function in ways different from the extensively studied high-frequency regions, where the sensory outer hair cells produce force that greatly increases the sound-evoked vibrations of the basilar membrane. We used laser interferometry in vitro and optical coherence tomography in vivo to study the low-frequency part of the guinea pig cochlea, and found that sound stimulation caused motion of a minimal portion of the basilar membrane. Outside the region of peak movement, an exponential decline in motion amplitude occurred across the basilar membrane. The moving region had different dependence on stimulus frequency than the vibrations measured near the mechanosensitive stereocilia. This behavior differs substantially from the behavior found in the extensively studied high-frequency regions of the cochlea.

The response of the apical turn of cochlea modeled with a tuned amplifier with negative feedback

In an earlier study [Hear. Res. 149 (2000) 55] velocity amplitudes of the outer Hensen's cell (HC) and basilar membrane (BM) were measured before, and at different times, after, sacrificing the animal. The velocity amplitude changed in a way that was characteristic of a negative feedback amplifier. A simple negative feedback amplifier model was proposed to explain the magnitude of the HC and BM velocity changes at CF. In the experiment tuning changed as well, both at the HC and BM. The model has now been extended to include tuning changes. The model response is compared with the experimental observations. The model is able to account quantitatively for the following experimental observations: (i) At the HC the tuning broadens and velocity decreases slowly after sacrifice. (ii) At the BM tuning sharpens and velocity increases at a faster rate. (iii) The velocity increase at BM is much larger than the decrease at HC.

Amplification in the apical turn of the cochlea with negative feedback

The apical turn of the anesthetized guinea pig cochlea was opened to examine the basilar membrane optically through the intact Reissner's membrane. Vibrations of the outer Hensen's cell and the basilar membrane (BM) adjacent to and about 130 microm below the level of the Hensen's cell were measured. Outer Hensen's cell vibration at the characteristic frequency was up to 900 times higher compared to the BM amplitude. After sacrifice BM vibration increased while Hensen's cell vibration decreased. The magnitude and sequence of change after sacrifice can best be explained by the presence of negative feedback between reticular lamina and BM. In other experiments using ototoxic drugs that damage outer hair cells, similar changes in Hensen's cell and BM vibration were observed. These results show that the apical turn behavior is different from that observed by other investigators in the basal turn. The potential benefits of the negative feedback are discussed. The presence of negative feedback would explain the linearity at the fundamental frequency observed in the apical turn of cochlea.

Reticular lamina vibrations in the apical turn of a living guinea pig cochlea

The reticular lamina of the apical turn of a living guinea pig cochlea was viewed through the intact Reissner's membrane using a slit confocal microscope. Vibrations were measured at selected identified locations with a confocal heterodyne interferometer, in response to tones applied with an acoustic transducer coupled to the ear canal. The position coordinates of each location were recorded. Mechanical tuning curves were measured along a radial track at Hensen's cells, outer hair cells, inner hair cells and at the osseous spiral lamina, over a frequency range of 3 kHz, using five sound pressure levels (100, 90, 80, 70 and 60 dB SPL). The carrier to noise ratio obtained throughout the experiments was high. The response shape at any measuring location was not found to change appreciably with signal level. The response shape also did not change significantly with the radial position on the reticular lamina. However, the response magnitude increased progressively from the inner hair cell to the Hensen's cell. The observed linearity of response at the fundamental frequency is explained by the presence of negative feed back in the apical turn of the cochlea.

Psychophysical measures of auditory nonlinearities as a function of frequency in individuals with normal hearing

In order to gain a better understanding of how auditory nonlinear phenomena vary as a function of location along the cochlea, several psychophysical measures of nonlinearity were examined as a function of signal frequency. Six normal-hearing individuals completed three experiments, each designed to measure one aspect of nonlinear behavior: (1) the effects of level on frequency selectivity in simultaneous masking, measured using notched-noise maskers at spectrum levels of 30 and 50 dB, (2) two-tone suppression, measured using forward maskers at the signal frequency (fs) and suppressor tones above fs, and (3) growth of masking, measured using forward maskers below fs at a signal/masker frequency ratio of 1.44. Four signal frequencies (375, 750, 1500, and 3000 Hz) were tested to sample the nonlinear behavior at different locations along the basilar membrane, in order to test the hypothesis that the apical (low-frequency) region of the cochlea behaves more linearly than the basal (high-frequency) region. In general, all three measures revealed a progressive increase in nonlinear behavior as signal frequency increased, with little or no nonlinearity at the lowest frequency, consistent with the hypothesis.

Vibration of reflective beads placed on the basilar membrane

Most investigators place reflective beads on the basilar membrane to measure its vibration with optical methods. It is therefore important to find out if the beads faithfully follow the motion of the structures on which they are placed. Vibration of the beads on the basilar membrane and basilar membrane adjacent to the beads are measured in the third turn of the guinea pig cochlea in a temporal bone preparation. It is shown that the beads do not follow the motion of the organ. The mechanism by which this departure may occur is investigated by modeling the motion of the beads on the Claudius' cells.

Mechanical responses of the mammalian cochlea

Recent findings in auditory research have significantly changed our views of the processes involved in hearing. Novel techniques and new approaches to investigate the mammalian cochlea have expanded our knowledge about the mechanical events occurring at physiologically relevant stimulus intensities. Experiments performed in the apical, low-frequency regions demonstrate that although there is a change in the mechanical responses along the cochlea, the fundamental characteristics are similar across the frequency range. The mechanical responses to sound stimulation exhibit tuning properties comparable to those measured intracellularly or from nerve fibres. Non-linearities in the mechanical responses have now clearly been observed at all cochlear locations. The mechanics of the cochlea are vulnerable, and dramatic changes are seen especially when the sensory hair cells are affected, for example, following acoustic overstimulation or exposure to ototoxic compounds such as furosemide. The results suggest that there is a sharply tuned and vulnerable response related to the hair cells, superimposed on a more robust, broadly tuned response. Studies of the micromechanical behaviour down to the cellular level have demonstrated significant differences radially across the hearing organ and have provided new information on the important mechanical interactions with the tectorial membrane. There is now ample evidence of reverse transduction in the auditory periphery, i.e. the cochlea does not only receive and detect mechanical stimuli but can itself produce mechanical motion. Hence, it has been shown that electrical stimulation elicits motion within the cochlea very similar to that evoked by sound. In addition, the presence of acoustically-evoked displacements of the hearing organ have now been demonstrated by several laboratories.

Modeling the active process of the cochlea: phase relations, amplification, and spontaneous oscillation

The high sensitivity and sharp frequency selectivity of acoustical signal transduction in the cochlea suggest that an active process pumps energy into the basilar membrane's oscillations. This function is generally attributed to outer hair cells, but its exact mechanism remains uncertain. Several classical models of amplification represent the load upon the basilar membrane as a single mass. Such models encounter a fundamental difficulty, however: the phase difference between basilar-membrane movement and the force generated by outer hair cells inhibits, rather than amplifies, the modeled basilar-membrane oscillations. For this reason, modelers must introduce artificially either negative impedance or an appropriate phase shift, neither of which is justified by physical analysis of the system. We consider here a physical model based upon the recent demonstration that the basilar membrane and reticular lamina can move independently, albeit with elastic coupling through outer hair cells. The mechanical model comprises two resonant masses, representing the basilar membrane and the reticular lamina, coupled through an intermediate spring, the outer hair cells. The spring's set point changes in response to displacement of the reticular lamina, which causes deflection of the hair bundles, variation of outer hair cell length and, hence, force production. Depending upon the frequency of the acoustical input, the basilar membrane and reticular lamina can oscillate either in phase or in counterphase. In the latter instance, the force produced by hair cells leads basilar-membrane oscillation, energy is pumped into basilar-membrane movement, and an external input can be strongly amplified. The model is also capable of producing spontaneous oscillation. In agreement with experimental observations, the model describes mechanical relaxation of the basilar membrane after electrical stimulation causes outer hair cells to change their length.

Nonlinear mechanics at the apex of the guinea-pig cochlea

A heterodyne laser interferometer was used to observe the sound-evoked displacement patterns of Reissner's membrane and various other structures in the apical turn of the guinea-pig cochlea. Most structures (including the basilar membrane) were similarly tuned, and had best frequencies in the 200-350Hz range. A distinct notch was usually observed approximately 0.7 octaves above the best frequency, and amplitude- and phase-plateaus were observed at higher frequencies. In most other respects, however, the mechanical tuning resembled the frequency-threshold curves of low frequency cochlear nerve fibers. In five reasonably intact, in vivo preparations, the frequency of the mechanical sensitivity notch was intensity-dependent: Compressive nonlinearities were observed above approximately 80 dB SPL on the low-frequency side of the notch, with antagonistically expansive nonlinearities on the high-frequency side. Two-tone suppression was observed in one of these preparations. Stimulus-related baseline position shifts were observed in another in vivo preparation. No such nonlinearities were observed in structurally damaged and/or > 1 hour post-mortem preparations. However, more robust nonlinearities were observed in all preparations at higher levels of stimulation (e.g. > 100-110 dB SPL). These high-level nonlinearities diminished only slowly after death, and gave rise to various effects, including time-dependent (i.e. adapting) and severely distorted (e.g. peak-split and/or dc-shifted) responses.

Fine structure of the lamina basilaris of guinea pig cochlea

The lamina basilaris of guinea pig cochlea was studied with SEM after trypsin treatment, and with TEM of resin sections and deep-etching replicas. The lamina consists of radial, evenly compacted filaments in the zona arcuata, and radial, discretely bundled filaments in the zona pectinata. In both zones, elementary filaments measured about 12 nm in thickness on the replica. The filaments formed more or less irregular passing bridges with each other and, eventually, a three-dimensional network which was continuous with the basement membrane under the supporting cells.

A realizable cochlear model using feedback from motile outer hair cells

A physically realizable form of a recent cochlear model using feedback forces from motile outer hair cells [Geisler (1991) Hear. Res. 54, 105-117] has been developed. The model was computer-simulated in the frequency domain (necessarily linear). Its responses to pure tones are very realistic in terms of sharpness (Q10s of 3-5) and in terms of tip-to-tail ratios (50-60 dB). These large tips are due to the feedback forces, which act as negative resistances (energy-supplying elements) over restricted spatial ranges. Nyquist-criterion analysis indicates that the model is stable. The spatial patterns of the model's output also bear qualitative resemblances to several other phenomena observed in cochleas, both living and excised.

Changes in the mechanical tuning characteristics of the hearing organ following acoustic overstimulation

An in vitro preparation of the guinea-pig temporal bone was used to study the effects of acoustic overstimulation on the mechanical tuning characteristics of the inner ear. Using laser heterodyne interferometry, the vibratory responses of selected sensory and supporting cells within the hearing organ were measured in response to acoustic signals applied to the ear to obtain mechanical tuning curves before and after applying acoustic overstimulation. Following overstimulation the frequency at which the maximal vibration response occurred moved towards lower frequencies, the vibration amplitude generally increased and the shape of the mechanical tuning curves became considerably flatter. These effects were seen within minutes of overstimulation. The micromechanical changes were accompanied by distinct morphological changes mainly affecting the first row of outer hair cells, which were swollen and shortened. Hensen bodies and swelling of the subsurface cisternae were observed in the affected cells. Apart from this, most of the shortened cells looked structurally intact, had undamaged sensory hair bundles and made synaptic contacts to both afferent and efferent nerve fibres. The results demonstrate that the outer hair cells play a key role in determining the tuning of the hearing organ.

Fibronectin-like immunoreactivity of the basilar membrane of young and aged rats

Dysfunction of cochlear mechanics has been hypothesized to be a source of age-related hearing loss and the basilar membrane mass and stiffness contribute to normal cochlear mechanics. Fibronectin, a large, extracellular matrix protein and a major component of the basilar membrane, may contribute to both the mass and stiffness of the membrane. Mesothelial cells underlying the basilar membrane may produce the fibronectin and also contribute to the mass of the membrane. Changes in either the fibronectin or the mesothelial cells might, therefore, have an effect on cochlear mechanics. In order to assess basilar membrane changes in aged animals, young adult (2-4 months) and aged (24-26 months) Sprague-Dawley rats were evaluated for the presence of fibronectin-like protein and mesothelial cells. The basilar membrane in the young animals had strong fibronectin-like immunoreactivity throughout its length. The old animals, on the other hand, showed normal fibronectin immunoreactivity in the basilar membrane of the basal turn, but little or no reactivity in the apical cochlear turn. The number of mesothelial cells was reduced throughout the length of the membrane in aged animals, with the greatest loss in the basal turn (60% fewer cells). These two degenerative changes, which appear to be independent of each other, may contribute to the observed threshold shifts in aged cochleas.

Responses to sound of the basilar membrane of the mammalian cochlea

Recent evidence shows that the frequency-specific non-linear properties of auditory nerve and inner hair cell responses to sound, including their sharp frequency tuning, are fully established in the vibration of the basilar membrane. In turn, the sensitivity, frequency selectivity and non-linear properties of basilar membrane responses probably result from an influence of the outer hair cells.

Effects of opening and resealing the cochlea on the mechanical response in the isolated temporal bone preparation

The isolated temporal bone preparation has been used previously for studying the micromechanical behaviour of the cochlea. Mechanical tuning curves have been obtained from several cells and structures within the hearing organ. In order to obtain access to the apical turns the bony shell of the cochlea has to be opened. To study how the opening affects the mechanical response of the cochlea, experiments were performed in which the cochlea was opened and then sealed with a glass window. Responses were measured from the same identified cells in the opened and in the sealed cochlea. The opening of the cochlea reduced the vibration amplitude mainly at frequencies below 300 Hz. Below the mechanical resonance frequency the slope of the tuning curve became steeper. The shape was not affected appreciably above the resonance frequency. The relative vibration amplitude of different cells remained unchanged by opening and closing the cochlea.

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.

The organ of Corti in the bat Hipposideros bicolor

The bat Hipposideros bicolor (Hipposideridae, Microchiroptera) is the mammalian species with the highest upper limit of hearing in which the structure of the organ of Corti has been studied. H. bicolor emits pure tone echo-locating signals of 153 kHz, compensates for Doppler shifts in the echo and hears ultrasonic frequencies up to 200 kHz (Neuweiler et al., 1984). The organ of Corti was investigated qualitatively and quantitatively using the technique of semi-thin sectioning. Some complementary ultra-thin sections were also examined. Length, width and cross-sectional area of the basilar membrane, the tectorial membrane, the hair cells with their stereocilia and the organ of Corti were measured at equi-distant positions on the basilar membrane. The organ of Corti of H. bicolor is composed of elements similar to those found in the cochleae of other eutherian mammals studied. However, in H. bicolor some of these elements show species-specific differences when compared to auditorily unspecialized mammals. The most basal region of the cochlea is characterized by miniaturization and re-inforcement of macro- and micro-mechanically important elements. This is interpreted as an adaptation for hearing extremely high frequencies. Specialized structures as well as local maxima of 'normal' elements in the basal and middle cochlear region are associated with evaluation of the echos of emitted pure tones. Besides the basal specializations. Hipposideros also shows specializations in the apical, low frequency, region which can be correlated with passive acoustic orientation.

The effects of quinine on the cochlear mechanics in the isolated temporal bone preparation

Quinine is known to induce a reversible hearing loss and to evoke motile responses of isolated outer hair cells. To study the effect of quinine, mechanical tuning curves of the Hensen's cells were measured in the isolated cochlea preparation in response to acoustical stimuli applied to the ear before and after application of the drug. It was shown that 0.5-4 mM quinine increased the vibration amplitude at the peak of the mechanical resonance curves and increased the sharpness of tuning. The time course of the event depended on whether the scala media was opened or not. The results show that quinine alters the micromechanical tuning of the organ of Corti.

Application of a commercially-manufactured Doppler-shift laser velocimeter to the measurement of basilar-membrane vibration

A commercially-available laser Doppler-shift velocimeter has been coupled to a compound microscope equipped with ultra-long-working-distance objectives for the purpose of measuring basilar membrane vibrations in the chinchilla. The animal preparation is nearly identical to that used in our laboratory for similar measurements using the Mössbauer technique. The vibrometer head is mounted on the third tube of the microscope's trinocular head and its laser beam is focused on high-refractive-index glass microbeads (10-30 microns) previously dropped, through the perilymph of scala tympani, on the basilar membrane. For equal sampling times, overall sensitivity of the laser velocimetry system is at least one order of magnitude greater than usually attained using the Mössbauer technique. However, the most important advantage of laser-velocimetry vis-à-vis the Mössbauer technique is its linearity, which permits undistorted recording of signals over a wide velocity range. Thus, for example, we have measured basilar-membrane responses to clicks whose waveforms have dynamic ranges exceeding 60 dB.

Laser Doppler velocimetry of basilar membrane vibration

A method is described for the measurement of basilar membrane (BM) vibration velocimeter (LDV). The instrumentation was coupled to a compound microscope which served to visualize reflective glass microbeads placed on the BM. The laser beam of the LDV was focused in the microscope object plane and positioned over the reflective bead. We show examples of frequency tuning curves and displacement input/output intensity functions obtained with the technique.