The gene causative for the human nonsyndromic recessive form of deafness DFNB22 encodes otoancorin, a 120-kDa inner ear-specific protein that is expressed on the surface of the spiral limbus in the cochlea. Gene targeting in ES cells was used to create an EGFP knock-in, otoancorin KO (Otoa(EGFP/EGFP)) mouse. In the Otoa(EGFP/EGFP) mouse, the tectorial membrane (TM), a ribbon-like strip of ECM that is normally anchored by one edge to the spiral limbus and lies over the organ of Corti, retains its general form, and remains in close proximity to the organ of Corti, but is detached from the limbal surface. Measurements of cochlear microphonic potentials, distortion product otoacoustic emissions, and basilar membrane motion indicate that the TM remains functionally attached to the electromotile, sensorimotor outer hair cells of the organ of Corti, and that the amplification and frequency tuning of the basilar membrane responses to sounds are almost normal. The compound action potential masker tuning curves, a measure of the tuning of the sensory inner hair cells, are also sharply tuned, but the thresholds of the compound action potentials, a measure of inner hair cell sensitivity, are significantly elevated. These results indicate that the hearing loss in patients with Otoa mutations is caused by a defect in inner hair cell stimulation, and reveal the limbal attachment of the TM plays a critical role in this process

Frequency tuning in the cochlea is determined by the passive mechanical properties of the basilar membrane and active feedback from the outer hair cells, sensory-effector cells that detect and amplify sound-induced basilar membrane motions. The sensory hair bundles of the outer hair cells are imbedded in the tectorial membrane, a sheet of extracellular matrix that overlies the cochlea's sensory epithelium. The tectorial membrane contains radially organized collagen fibrils that are imbedded in an unusual striated-sheet matrix formed by two glycoproteins, -tectorin (Tecta) and -tectorin (Tectb). In Tectb-/- mice the structure of the striated-sheet matrix is disrupted. Although these mice have a low-frequency hearing loss, basilar-membrane and neural tuning are both significantly enhanced in the high-frequency regions of the cochlea, with little loss in sensitivity. These findings can be attributed to a reduction in the acting mass of the tectorial membrane and reveal a new function for this structure in controlling interactions along the cochlea.

Distortion product otoacoustic emissions (DPOAE) were recorded from wild-type mice and mutant Tecta(deltaENT/deltaENT) mice with detached tectorial membranes (TM) under combined ketamine/xylaxine anesthesia. In Tecta(deltaENT/deltaENT) mice, DPOAEs could be detected above the noise floor only when the levels of the primary tones exceeded 65 dB SPL. DPOAE amplitude decreased with increasing frequency of the primaries in Tecta(deltaENT/deltaENT) mice. This was attributed to hair cell excitation via viscous coupling to the surrounding fluid and not by interaction with the TM as in the wild-type mice. Local minima and corresponding phase transitions in the DPOAE growth functions occurred at higher DPOAE levels in wild-type than in Tecta(deltaENT/deltaENT) mice. In less-sensitive Tecta(deltaENT/deltaENT) mice, the position of the local minima varied nonsystematically with frequency or no minima were observed. A bell-like dependence of the DPOAE amplitude on the ratio of the primaries was recorded in both wild-type and Tecta(deltaENT/deltaENT) mice. However, the pattern of this dependence was different in the wild-type and Tecta(deltaENT/deltaENT) mice, an indication that the bell-like shape of the DPOAE was produced by a combination of different mechanisms. A nonlinear low-frequency resonance, revealed by nonmonotonicity of the phase behavior, was seen in the wild-type but not in Tecta(deltaENT/deltaENT) mice.

The mammalian cochlea is a structure comprising a number of components connected by elastic elements. A mechanical system of this kind is expected to have multiple normal modes of oscillation and associated resonances. The guinea pig cochlear mechanics was probed using distortion components generated in the cochlea close to the place of overlap between two tones presented simultaneously. Otoacoustic emissions at frequencies of the distortion components were recorded in the ear canal. The phase behavior of the emissions reveals the presence of a nonlinear resonance at a frequency about a half octave below that of the high-frequency primary tone. The location of the resonance is level dependent and the resonance shifts to lower frequencies with increasing stimulus intensity. This resonance is thought to be associated with the tectorial membrane. The resonance tends to minimize input to the cochlear receptor cells at frequencies below the high-frequency primary and increases the dynamic load to the stereocilia of the receptor cells at the primary frequency when the tectorial membrane and reticular lamina move in counterphase. ©2003 Acoustical Society of America.

This review is concerned with experimental results that reveal multiple roles for the tectorial membrane in active signal processing in the mammalian cochlea. We discuss the dynamic mechanical properties of the tectorial membrane as a mechanical system with several degrees of freedom and how its different modes of movement can lead to hair-cell excitation. The role of the tectorial membrane in distributing energy along the cochlear partition and how it channels this energy to the inner hair cells is described.

Electrically evoked otoacoustic emissions are sounds emitted from the inner ear when alternating current is injected into the cochlea. Their temporal structure consists of short- and long-delay components and they have been attributed to the motile responses of the sensory-motor outer hair cells of the cochlea. The nature of these motile responses is unresolved and may depend on either somatic motility, hair bundle motility, or both. The short-delay component persists after almost complete elimination of outer hair cells. Outer hair cells are thus not the sole generators of electrically evoked otoacoustic emissions. We used prestin knockout mice, in which the motor protein prestin is absent from the lateral walls of outer hair cells, and Tecta(Delta ENT/Delta ENT) mice, in which the tectorial membrane, a structure with which the hair bundles of outer hair cells normally interact, is vestigial and completely detached from the organ of Corti. The amplitudes and delay spectra of electrically evoked otoacoustic emissions from Tecta(Delta ENT/Delta ENT) and Tecta(+/+) mice are very similar. In comparison with prestin(+/+) mice, however, the short-delay component of the emission in prestin(-/-) mice is dramatically reduced and the long-delay component is completely absent. Emissions are completely suppressed in wild-type and Tecta(Delta ENT/Delta ENT) mice at low stimulus levels, when prestin-based motility is blocked by salicylate. We conclude that near threshold, the emissions are generated by prestin-based somatic motility.

The sensitivity, large dynamic range and narrow frequency tuning of the mammalian cochlea is determined by the passive mechanical properties of the basilar membrane (BM) and active feedback from the outer hair cells (OHCs). Two mechanisms have been proposed to provide amplification: Hair bundle motility, and OHC somatic-motility. Acoustically- and electrically-elicited mechanical responses were measured from the BMs of the cochleae of wild type and genetically modified mice where the hair bundles are freestanding and cannot react against the tectorial membrane (TM) to contribute to amplification. We found the electrically elicited responses in mutant mice, where only somatic motility can provide amplification, to be remarkably similar to acoustical and electrical responses in the wild type animals. We, therefore, conclude that somatic, not stereocilia motility is the basis of the cochlear amplifier.

Purpose of review: The review is both timely and relevant, as recent findings have shown the tectorial membrane plays a more dynamic role in hearing than hitherto suspected, and that many forms of deafness can result from mutations in tectorial membrane proteins. Recent findings: Main themes covered are (i) the molecular composition, structural organisation and properties of the tectorial membrane, (ii) the role of the tectorial membrane as a second resonator and a structure within which there is significant longitudinal coupling, and (iii) how mutations in tectorial membrane proteins cause deafness in mice and men. Implications: Findings from experimental models imply that the tectorial membrane plays multiple, critical roles in hearing. These include coupling elements along the length of the cochlea, supporting a travelling wave and ensuring the gain and timing of cochlear feedback are optimal. The clinical findings suggest stable, moderate-to-severe forms hereditary hearing loss may be diagnostic of a mutation in TECTA, the gene encoding one of the major, non-collagenous proteins of the tectorial membrane.

Distortion product otoacoustic emissions (DPOAEs) were recorded from guinea pigs in response to simultaneous increases in the levels of high frequency primary tones in the presence of a low frequency biasing tone of 30 Hz at 120 dB SPL. The DPOAE amplitudes plotted as functions of the biasing tone phase angle show distinctive repeatable minima, which are identical to the amplitude notches observed for the distortion products at the output of a single saturating non-linearity. The number of the amplitude minima grows with increasing order of the DPOAE, a feature that is also reproduced by the model. The model of DPOAE generation due to a single saturating non-linearity does not explain the experimentally observed asymmetry of the response of the DPOAEs to rising and falling half cycles of the biasing tone. This asymmetry is attributed to a hypothetical mechanism, which adjusts the operating point of the outer hair cell¿s mechanoelectrical transducer. Experimental data were consistent with a hypothesis that, for the parameters of stimulation used in this study, both lower and upper sideband DPOAEs are dominated by emission generated from a single and spatially localized place in the cochlea.

Sensitivity, dynamic range and frequency tuning of the cochlea are attributed to amplification involving outer hair cell stereocilia and/or somatic motility. We measured acoustically and electrically elicited basilar membrane displacements from the cochleae of wild-type and TectaDeltaENT/DeltaENT mice, in which stereocilia are unable to contribute to amplification near threshold. Electrically elicited responses from TectaDeltaENT/DeltaENT mice were markedly similar to acoustically and electrically elicited responses from wild-type mice. We conclude that somatic, and not stereocilia, motility is the basis of cochlear amplification.

A tenet of cochlear physiology is that sharp tuning and sensitivity are directly interrelated. Here we show a reciprocal interdependence between tuning and sensitivity in the mammalian cochlea from measurements of basilar membrane (BM) mechanical tuning and neural Suppression tuning curves of wild-type (Tectb(+/+)) and beta-tectorin mutant (Tectb(-/-)) mice. The tectorial membrane (TM) of the mutants lacks striated-sheet matrix, which is likely to decrease longitudinal elastic coupling. Mechanical and neural tuning curves recorded in mutants are slightly less sensitive, although more sharply tuned. The inverse relationship between sensitivity and tuning observed in the mutants Could be attributed to smaller numbers of the outer hair cells responding in synchrony due to reduced longitudinal coupling in the TM. We suggest that frequency tuning and high sensitivity are not necessarily concomitant but reciprocal properties of the cochlea.

A laser-diode forms the basis of a displacement sensitive homodyne interferometer suitable for measurements from poorly reflective surfaces. The compact and cost-effective interferometer utilizes the self-mixing effect when laser light reflected from a moving target re-enters the laser cavity and causes phase dependent changes of the lasing intensity. A piezo positioner was used to displace the interferometer with known frequency and amplitude as a basis for real-time calibration of the interferometer's sensitivity. The signal-processing algorithm is described that allows measurements in presence of high amplitude noise leading to variation of the interferometer's operating point. Measurements of sound-induced basilar membrane displacements were made in the intact cochleae of rodents by focusing the laser beam of the interferometer through the transparent round window membrane. The interferometer provides a viable means for making subnanometre mechanical measurements from structures in the inner ears of small mammals, where opening of the cochlea is not practicable.

The motor protein prestin in the outer hair cells is a prime candidate for the molecular amplifier that ensures the sensitivity, frequency tuning and dynamic range of the mammalian cochlea. Absence of prestin results in a 40-60 dB reduction in cochlear neural sensitivity. Here we show that sound-evoked basilar membrane (BM) vibrations in the basal cochleae of prestin(-/-) mice are as sensitive as those of their prestin(+/+) siblings. BM vibrations in prestin(-/-) mice are, however, broadly tuned to a frequency similar to a half octave below the characteristic frequency (CF) of similar BM locations in prestin(+/+) mice. The peak sensitivity of prestin(+/+) BM tuning curves matches the neural thresholds, while prestin(-/-) BM tuning curves at the best frequency are > 50 dB more sensitive than the neural responses. We conclude that prestin influences properties of the cochlear partition that are crucial for BM frequency tuning and for converting its vibrations into neural excitation.

a-tectorin (encoded by Tecta) is a component of the tectorial membrane, an extracellular matrix of the cochlea. In humans, the Y1870C missense mutation in TECTA causes a 50- to 80-dB hearing loss. In transgenic mice with the Y1870C mutation in Tecta, the tectorial membrane's matrix structure is disrupted, and its adhesion zone is reduced in thickness. These abnormalities do not seriously influence the tectorial membrane's known role in ensuring that cochlear feedback is optimal, because the sensitivity and frequency tuning of the mechanical responses of the cochlea are little changed. However, neural thresholds are elevated, neural tuning is broadened, and a sharp decrease in sensitivity is seen at the tip of the neural tuning curve. Thus, using TectaY1870C/+ mice, we have genetically isolated a second major role for the tectorial membrane in hearing: it enables the motion of the basilar membrane to optimally drive the inner hair cells at their best frequency.

The design principles and specific proteins of the dynein¿tubulin motor, which powers the flagella and cilia of eukaryotes, have been conserved throughout the evolution of life from algae to humans. Cilia and flagella can support both motile and sensory functions independently, or sometimes in parallel to each other. In this paper we show that this dual sensory¿motile role of eukaryotic cilia is preserved in the most sensitive of all invertebrate hearing organs, the Johnston's organ of the mosquito. The Johnston's organ displays spontaneous oscillations, which have been identified as being a characteristic of amplification in the ears of mosquitoes and Drosophila. In the auditory organs of Drosophila and vertebrates, the molecular basis of amplification has been attributed to the gating and adaptation of the mechanoelectrical transducer channels themselves. On the basis of their temperature-dependence and sensitivity to colchicine, we attribute the molecular basis of spontaneous oscillations by the Johnston's organ of the mosquito Culex quinquefasciatus, to the dynein¿tubulin motor of the ciliated sensillae. If, as has been claimed for insect and vertebrate hearing organs, spontaneous oscillations epitomize amplification, then in the mosquito ear, this process is independent of mechanotransduction.

Mechanically coupled cochlear structures are likely to form a resonator with several degrees of freedom. Consequently one can expect complex, frequency-dependent relative movements between these structures, particularly between the tectorial membrane and reticular lamina. Shearing movement between these two structures excites the cochlear receptors. This excitation should be minimal at the frequency of the hypothesized tectorial membrane resonance. In each preparation, simultaneous masking neural tuning curves and distortion product otoacoustic emissions were recorded. The position of the low-frequency minima in the tuning curves, frequency dependence of the emission bandpass structure, and level-dependent phase reversal were compared to determine if they were generated by a common phenomenon, for example the tectorial membrane resonance. The notch in the masking curves and the phase inversion of the emission growth functions at the auditory thresholds are both situated half an octave below the probe frequency and the high-frequency primary, respectively, and show similar frequency dependence. The emission bandpass structure is, however, likely to be generated by a combination of mechanisms with different ones dominating at different stimulus parameters.

The mammalian inner ear contains sense organs responsible for detecting sound, gravity and linear acceleration, and angular acceleration. Of these organs, the cochlea is involved in hearing, while the sacculus and utriculus serve to detect linear acceleration. Recent evidence from birds and mammals, including humans, has shown that the sacculus, a hearing organ in many lower vertebrates, has retained some of its ancestral acoustic sensitivity. Here we provide not only more evidence for the retained acoustic sensitivity of the sacculus, but we also found that acoustic stimulation of the sacculus has behavioral significance in mammals. We show that the amplitude of an elicited auditory startle response is greater when the startle stimuli are presented simultaneously with a low-frequency masker, including masker tones that are outside the sensitivity range of the cochlea. Masker-enhanced auditory startle responses were also observed in otoconia-absent Nox3 mice, which lack otoconia but have no obvious cochlea pathology. However, masker enhancement was not observed in otoconia-absent Nox3 mice if the low-frequency masker tones were outside the sensitivity range of the cochlea. This last observation confirms that otoconial organs, most likely the sacculus, contribute to behavioral responses to low-frequency sounds in mice.

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

The driver responsible for spontaneous oscillations of the mosquito (Culex quinquefasciatus) antennae was investigated. The activation energy derived from the temperature dependence of the spontaneous oscillation frequency is 30 kJ/mol suggesting a dynein ATPase is responsible. Colchicine application abolished spontaneous oscillations but left transduction intact. Hence, the transduction apparatus is thought not to be responsible for the spontaneous oscillations of the antennae.

Distortion product otoacoustic emissions (DPOAE) were recorded from wild-type mice and mutant Tecta(deltaENT/deltaENT) mice with detached tectorial membranes (TM) under combined ketamine/xylaxine anesthesia. In Tecta(deltaENT/deltaENT) mice, DPOAEs could be detected above the noise floor only when the levels of the primary tones exceeded 65 dB SPL. DPOAE amplitude decreased with increasing frequency of the primaries in Tecta(deltaENT/deltaENT) mice. This was attributed to hair cell excitation via viscous coupling to the surrounding fluid and not by interaction with the TM as in the wild-type mice. Local minima and corresponding phase transitions in the DPOAE growth functions occurred at higher DPOAE levels in wild-type than in Tecta(deltaENT/deltaENT) mice. In less-sensitive Tecta(deltaENT/deltaENT) mice, the position of the local minima varied nonsystematically with frequency or no minima were observed. A bell-like dependence of the DPOAE amplitude on the ratio of the primaries was recorded in both wild-type and Tecta(deltaENT/deltaENT) mice. However, the pattern of this dependence was different in the wild-type and Tecta(deltaENT/deltaENT) mice, an indication that the bell-like shape of the DPOAE was produced by a combination of different mechanisms. A nonlinear low-frequency resonance, revealed by nonmonotonicity of the phase behavior, was seen in the wild-type but not in Tecta(deltaENT/deltaENT) mice.

The remarkable power amplifier [1] of the cochlea boosts low-level and compresses high-level vibrations of the basilar membrane (BM) [2]. By contributing maximally at the characteristic frequency (CF) of each point along its length, the amplifier ensures the exquisite sensitivity, narrow frequency tuning, and enormous dynamic range of the mammalian cochlea. The motor protein prestin in the outer hair cell (OHC) lateral membrane is a prime candidate for the cochlear power amplifier [3]. The other contender for this role is the ubiquitous calcium-mediated motility of the hair cell stereocilia, which has been demonstrated in vitro and is based on fast adaptation of the mechanoelectrical transduction channels [4,5]. Absence of prestin [6] from OHCs results in a 40¿60 dB reduction in cochlear neural sensitivity [7]. Here we show that sound-evoked BM vibrations in the high-frequency region of prestin-/- mice cochleae are, surprisingly, as sensitive as those of their prestin+/+ siblings. The BM vibrations of prestin-/- mice are, however, broadly tuned to a frequency approximately a half octave below the CF of prestin+/+ mice at similar BM locations. The peak sensitivity of prestin+/+ BM tuning curves matches the neural thresholds. In contrast, prestin-/- BM tuning curves at their best frequency are >50 dB more sensitive than the neural responses. We propose that the absence of prestin from OHCs, and consequent reduction in stiffness of the cochlea partition, changes the passive impedance of the BM at high frequencies, including the CF. We conclude that prestin influences the cochlear partition's dynamic properties that permit transmission of its vibrations into neural excitation. Prestin is crucial for defining sharp and sensitive cochlear frequency tuning by reducing the sensitivity of the low-frequency tail of the tuning curve, although this necessitates a cochlear amplifier to determine the narrowly tuned tip.