High-frequency resolution is one of the salient features of peripheral sound processing in the mammalian cochlea. The sensitivity originates in the active amplification of the travelling wave on the basilar membrane by the outer hair cells (OHCs), where electrically induced mechanical action of the OHC on a cycle-by-cycle basis is believed to be the crucial component. However, it is still unclear if this electromechanical action is sufficiently fast and can produce enough force to enhance mechanical tuning up to the highest frequencies perceived by mammals. Here we show that isolated OHCs in the microchamber configuration are able to overcome f luid forces with almost constant displacement amplitude and phase up to frequencies well above their placefrequency on the basilar membrane. The high-frequency limit of the electromotility, defined as the frequency at which the amplitude drops by 3 dB from its asymptotic low-frequency value, is inversely dependent on cell length. The frequency limit is at least 79 kHz. For frequencies up to 100 kHz, the electromotile response was specified by an overdamped (Q ‫؍‬ 0.42) second-order resonant system. This finding suggests that the limiting factor for frequencies up to 100 kHz is not the speed of the motor but damping and inertia. The isometric force produced by the OHC was constant at least up to 50 kHz, with amplitudes as high as 53 pN͞mV being observed. We conclude that the electromechanical transduction process of OHCs possesses the necessary high-frequency properties to enable amplification of the travelling wave over the entire hearing range.The discovery of electrically induced length changes of isolated outer hair cells (OHCs) (1), the so-called electromotility, provided the experimental basis for a previously proposed mechanism for the action of OHCs as putative cochlear amplifiers (2): Electrical energy from mechanoelectrical transduction in the stereocilia is converted into mechanical energy by electromechanical transduction in the cell wall. This mechanical energy, converted on a cycle-by-cycle basis, is fed back into the cochlear partition, thereby enhancing mechanoelectrical transduction. One of the most important questions concerns the frequency response of the electromotility and the mechanical force generated by the OHC (3). The frequency range of auditory signals perceived by mammals extends from several Hz to more than 100 kHz for small insect-hunting bats (4). Therefore, any mechanism proposed for amplifying mechanical signals in the cochlea, such as electromotility, must act at all frequencies in this range. Because of the capacitance of the lateral cell membrane, the OHC receptor potential, which drives the electromotility (5-8), is low-pass filtered with corner frequencies between 30 Hz for apical OHCs and 1 kHz for basal OHCs (9,10). This electrical filtering, together with the high-frequency increase of the resistive and inertial fluid forces, is expected to cause severe attenuation of the electromotility at high frequencies in vivo. By holdi...

The tectorial membrane has long been postulated as playing a role in the exquisite sensitivity of the cochlea. In particular, it has been proposed that the tectorial membrane provides a second resonant system, in addition to that of the basilar membrane, which contributes to the amplification of the motion of the cochlear partition. Until now, technical difficulties had prevented vibration measurements of the tectorial membrane and, therefore, precluded direct evidence of a mechanical resonance. In the study reported here, the vibration of the tectorial membrane was measured in two orthogonal directions by using a novel method of combining laser interferometry with a photodiode technique. It is shown experimentally that the motion of the tectorial membrane is resonant at a frequency of 0.5 octave (oct) below the resonant frequency of the basilar membrane and polarized parallel to the reticular lamina. It is concluded that the resonant motion of the tectorial membrane is due to a parallel resonance between the mass of the tectorial membrane and the compliance of the stereocilia of the outer hair cells. Moreover, in combination with the contractile force of outer hair cells, it is proposed that inertial motion of the tectorial membrane provides the necessary conditions to allow positive feedback of mechanical energy into the cochlear partition, thereby amplifying and tuning the cochlear response.Understanding the micromechanical mechanisms underlying the extraordinary sensitivity of the cochlea is a cardinal goal of auditory physiology. It is generally agreed that motion of the tectorial membrane (TM) relative to the cuticular plate of a sensory hair cell stimulates transduction channels in its stereocilia-directly through physical contact to the TM of the longest stereocilia of the outer hair cells (OHCs) and indirectly by fluid motion around the stereocilia of the inner hair cells (1-5). Moreover, because OHCs undergo somatic length changes in response to electrical (6-8) and chemical (9) stimuli, OHCs and their stereocilia are supposed to feed mechanical energy back into the cochlear partition, thereby reducing its impedance (10-12). Therefore, the TM is expected to be functionally connected not only to the input of mechanoelectrical transducers in hair-cell stereocilia, but also to the output of electromechanical transducers in the OHC membrane. Technical difficulties have prevented measurements of TM vibration. Therefore, functional information has been inferred from morphological investigations (13-15), stiffness measurements post mortem (16) and in vivo (17), a physical model (18), mathematical models (5,10,11,(19)(20)(21)(22)(23)(24), together with the frequency tuning properties of evoked otoacoustic emissions (25, 26) and cochlear microphonic potentials (17). In general, the latter models (5,10,(18)(19)(20)(21)(22)(23)(24)(25)(26) require that the TM be mechanically resonant.The aim of the present study was to experimentally characterize the vibration response of the TM. This was achieved by deve...

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.

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Dynamic material properties of the tectorial membrane (TM) have been measured at audio frequencies in TMs excised from the apical portions of mouse cochleae. We review, integrate, and interpret recent findings. The mechanical point impedance of the TM in the radial, longitudinal, and transverse directions is viscoelastic and has a frequency dependence of the form 1/(K(j2pif)(alpha)) for 10