Micromechanical responses to tones in the auditory fovea of the greater mustached bat's cochlea. (1/139)

An extended region of the greater mustached bat's cochlea, the sparsely innervated (SI) zone, is located just basally to the frequency place of the dominant 61-kHz component of the echolocation signal (CF2). Anatomic adaptations in the SI zone are thought to provide the basis for cochlear resonance to the CF2 echoes and for the extremely sharp tuning throughout the auditory system that allows these bats to detect Doppler shifts in the echoes caused by insect wing beat. We measured basilar membrane (BM) displacements in the SI zone with a laser interferometer and recorded acoustic distortion products at the ear drum at frequencies represented in the SI zone. The basilar membrane in the SI region was tuned both to its characteristic frequency (62-72 kHz) and to the resonance frequency (61-62 kHz). With increasing stimulus levels, the displacement growth functions are compressive curves with initial slopes close to unity, and their properties are consistent with the mammalian cochlear amplifier working at high sound frequencies. The sharp basilar membrane resonance is associated with a phase lag of 180 degrees and with a shift of the peak resonance to lower frequencies for high stimulus levels. Within the range of the resonance, the distortion product otoacoustic emissions, which have been attributed to the resonance of the tectorial membrane in the SI region, are associated with an abrupt phase change of 360 degrees. It is proposed that a standing wave resonance of the tectorial membrane drives the BM in the SI region and that the outer hair cells enhance, fine tune, and control the resonance. In the SI region, cochlear micromechanics appear to be able to work in two different modes: a conventional traveling wave leads to shear displacement between basilar and tectorial membrane and to neuronal excitation for 62-70 kHz. In addition, the SI region responds to 61-62 kHz with a resonance based on standing waves and thus preprocesses signals which are represented more apically in the CF2 region of the cochlea.  (+info)

Cochlear function: hearing in the fast lane. (2/139)

The cochlea amplifies sound over a wide range of frequencies. Outer hair cells have been thought to play a mechanical part in this amplification, but it has been unclear whether they act rapidly enough. Recent work suggests that outer hair cells can indeed work at frequencies that cover the auditory range.  (+info)

Direct visualization of organ of corti kinematics in a hemicochlea. (3/139)

The basilar membrane in the mammalian cochlea vibrates when the cochlea receives a sound stimulus. This mechanical vibration is transduced into hair cell receptor potentials and thereafter encoded by action potentials in the auditory nerve. Knowledge of the mechanical transformation that converts basilar membrane vibration into hair cell stimulation has been limited, until recently, to hypothetical geometric models. Experimental observations are largely lacking to prove or disprove the validity of these models. We have developed a hemicochlea preparation to visualize the kinematics of the cochlear micromechanism. Direct mechanical drive of 1-2 Hz sinusoidal command was applied to the basilar membrane. Vibration patterns of the basilar membrane, inner and outer hair cells, supporting cells, and tectorial membrane have been recorded concurrently by means of a video optical flow technique. Basilar membrane vibration was driven in a direction transversal to its plane. However, the direction of the resulting vibration was found to be essentially radial at the level of the reticular lamina and cuticular plates of inner and outer hair cells. The tectorial membrane vibration was mainly transversal. The transmission ratio between cilia displacement of inner and outer hair cells and basilar membrane vibration is in the range of 0.7-1.1. These observations support, in part, the classical geometric models at low frequencies. However, there appears to be less tectorial membrane motion than predicted, and it is largely in the transversal direction.  (+info)

Three-dimensional motion of the organ of Corti. (4/139)

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.  (+info)

The spatial and temporal representation of a tone on the guinea pig basilar membrane. (5/139)

In the mammalian cochlea, the basilar membrane's (BM) mechanical responses are amplified, and frequency tuning is sharpened through active feedback from the electromotile outer hair cells (OHCs). To be effective, OHC feedback must be delivered to the correct region of the BM and introduced at the appropriate time in each cycle of BM displacement. To investigate when OHCs contribute to cochlear amplification, a laser-diode interferometer was used to measure tone-evoked BM displacements in the basal turn of the guinea pig cochlea. Measurements were made at multiple sites across the width of the BM, which are tuned to the same characteristic frequency (CF). In response to CF tones, the largest displacements occur in the OHC region and phase lead those measured beneath the outer pillar cells and adjacent to the spiral ligament by about 90 degrees. Postmortem, responses beneath the OHCs are reduced by up to 65 dB, and all regions across the width of the BM move in unison. We suggest that OHCs amplify BM responses to CF tones when the BM is moving at maximum velocity. In regions of the BM where OHCs contribute to its motion, the responses are compressive and nonlinear. We measured the distribution of nonlinear compressive vibrations along the length of the BM in response to a single frequency tone and estimated that OHC amplification is restricted to a 1.25- to 1.40-mm length of BM centered on the CF place.  (+info)

A targeted deletion in alpha-tectorin reveals that the tectorial membrane is required for the gain and timing of cochlear feedback. (6/139)

alpha-tectorin is an extracellular matrix molecule of the inner ear. Mice homozygous for a targeted deletion in a-tectorin have tectorial membranes that are detached from the cochlear epithelium and lack all noncollagenous matrix, but the architecture of the organ of Corti is otherwise normal. The basilar membranes of wild-type and alpha-tectorin mutant mice are tuned, but the alpha-tectorin mutants are 35 dB less sensitive. Basilar membrane responses of wild-type mice exhibit a second resonance, indicating that the tectorial membrane provides an inertial mass against which outer hair cells can exert forces. Cochlear microphonics recorded in alpha-tectorin mutants differ in both phase and symmetry relative to those of wild-type mice. Thus, the tectorial membrane ensures that outer hair cells can effectively respond to basilar membrane motion and that feedback is delivered with the appropriate gain and timing required for amplification.  (+info)

Hair cell death in a hearing-deficient canary. (7/139)

Cell death has been documented in bird auditory inner ear epithelia after induced damage. This cell death is quickly followed by an increase in supporting cell division and regeneration of the epithelium, thereby suggesting a possible relationship between these two processes. However, aspects of this relationship still need to be better understood. The Belgian Waterslager (BWS) canary is an ideal system in which to study cell death and subsequent cell division. In contrast to mixed breed (MB) canaries, cell division normally occurs in the auditory end organ of the BWS without any external manipulation. In addition, some of the cells in the auditory epithelium may be dying through an apoptotic-like process. In the present study two methods were used to quantify dying cells in the BWS and MB canary auditory epithelia: morphological criteria and TUNEL. Results confirm that some of the abnormal hair cells in the BWS auditory epithelium are apoptotic-like. The presence of both cell death and cell division indicates that these processes act concurrently in the adult end organ. Future studies are needed to determine if cell death is a stimulus for the observed cell division.  (+info)

Development of the gerbil inner ear observed in the hemicochlea. (8/139)

A frequency-dependent change in hearing sensitivity occurs during maturation in the basal gerbil cochlea. This change takes place during the first week after the onset of hearing. It has been argued that the mass of a given cochlear segment decreases during development and thus increases the best frequency. Changes in mass during cochlear maturation have been estimated previously by measuring the changes in cochlear dimensions. Fixed, dehydrated, embedded, or sputter-coated tissues were used in such work. However, dehydration of the tissue, a part of most histological techniques, results in severe distortion of some aspects of cochlear morphology. The present experiments, using a novel preparation, the hemicochlea, show that hydrated structures, such as the tectorial membrane and the basilar membrane hyaline matrix, are up to 100% larger than estimated previous studies. Therefore, the hemicochlea was used to study the development of cochlear morphology in the gerbil between the day of birth and postnatal day 19. We used no protocols that would have resulted in severe distortion of cochlear elements. Consequently, a detailed study of cochlear morphology yields several measures that differ from previously published data. Our experiments confirm growth patterns of the cochlea that include a period of remarkably rapid change between postnatal day 6 and 8. The accelerated growth starts in the middle of the cochlea and progresses toward the base and the apex. In particular, the increase in height of Deiters' cells dominated the change, "pushing" the tectorial membrane toward scala vestibuli. This resulted in a shape change of the tectorial membrane and the organ of Corti. The tectorial membrane was properly extended above the outer hair cells by postnatal day 12. This time coincides with the onset of hearing. The basilar membrane hyaline matrix increased in thickness, whereas the multilayered tympanic cover layer cells decreased to a single band of cells by postnatal day 19. Before and after the period of rapid growth, the observed gross morphological changes are rather small. It is unlikely that dimensional changes of cochlear structures between postnatal days 12 and 19 contribute significantly in the remapping of the frequency-place code in the base of the cochlea. Instead, structural changes affecting the stiffness of the cochlear partition might be responsible for the shift in best frequency.  (+info)

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