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Recent advances in optical techniques for vibration measurement have made it feasible to purchase commercially available laser Doppler velocimeters that, in combination with a light microscope, are capable of detecting velocities of poorly reflecting biological materials down to fractions of a micrometers s-1. Thus, in our particular application, the aim is to understand the micromechanics of the inner ear, particularly in the nanometer region where the ear is first able to detect sound. With currently available devices, it is now possible to make the necessary vibration measurements without need of introducing reflecting materials onto the object of interest. This not only avoids inertial loading, but also allows focusing through cellular layers onto otherwise inaccessible structures. Here we present the results of micromechanical experiments for the in vitro inner ear of the guinea pig, the most commonly used model in auditory physiology. Our experiments concentrate on the third and fourth cochlear turns because in this region the different cellular structures of the organ of Corti are optically accessible with present technology. The mechanical responses of all examined structures, including the basilar membrane, were equally frequency selective, but linear below 90 dB SPL, suggesting that active mechanical tuning is not as pronounced in the apex as in the base of the cochlea. This has important consequences for signal processing in the inner ear.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Der Grundmechanismus des Horens — die mechano-elektrische Transduktion — und damit auch die dazugehorige Pathophysiologie sind noch nicht vollstandig erforscht. Experimente an isolierten auseren Haarsinneszellen (AHZ) zeigen, das sie fahig sind, auf elektrische, akustische und chemische Reize mit einer Langenanderung ihres Zellkorpers zu antworten. Es wurde deshalb postuliert, das AHZ mechanische Energie in das Cortische Organ einspeisen konnen, dem passiven Widerstand in der Cochlea entgegenwirken und dadurch die auserordentlich hohe Frequenzselektivitat und Empfindlichkeit des Innenohres erzeugen und modulieren. Zu klaren ist jedoch noch, ob AHZ in situ im Cortischen Organ bei physiologisch relevanten Schallintensitaten Motilitat zeigen. Um diese Frage zu beantworten, haben wir an einem Felsenbeinpraparat die Bewegung zellularer Strukturen in der dritten und vierten Windung der Meerschweinchencochlea bei lufthaltigem Mittelohr untersucht.

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.

Microscale mechanical probes were designed and bulk-fabricated for applying shearing forces to biological tissues. These probes were used to measure shear impedance of the tectorial membrane (TM) in two dimensions. Forces were applied in the radial and longitudinal directions at frequencies ranging from 0.01-9 kHz and amplitudes from 0.02-4 microN. The force applied was determined by measuring the deflection of the probes' cantilever arms. TM impedance in the radial direction had a magnitude of 63 +/- 28 mN x s/m at 10 Hz and fell with frequency by 16 +/- 0.4 dB/decade, with a constant phase of -72 +/- 6 degrees . In the longitudinal direction, impedance was 36 +/- 9 mN x s/m at 10 Hz and fell by 19 +/- 0.4 dB/decade, with a constant phase of -78 +/- 4 degrees . Impedance was nearly constant as a function of force except at the highest forces, for which it fell slightly. These results show that the viscoelastic properties of the TM extend over a significant range of audio frequencies, consistent with a poroelastic interpretation of TM mechanics. The shear modulus G' determined from these measurements was 17-50 kPa, which is larger than in species with a lower auditory frequency range. This value suggests that hair bundles cannot globally shear the TM, but most likely cause bulk TM motion.

Die auseren Haarsinneszellen spielen fur die Funktion des Innenohrs eine wesentliche Rolle: Sie verstarken die Schwingung der Basilarmembran bis zu 1000-fach und verscharfen dabei ihre Erregungsmuster. Diese Muster werden von den inneren Haarsinneszellen in Aktionspotenziale des auditorischen Nervs codiert. Bei einer Schadigung der filigranen auseren Haarsinneszellen, z. B. durch Larm, kann ein Horgerat die Amplitude der Erregungsmuster anheben, die fehlende Frequenzselektivitat aber nicht wiederherstellen. Zudem zerstort Larm zunachst Nervenfasern mit niedriger Spontanrate, die fur die Codierung von Schall in Storgerauschen wichtig sind. Dieser Verlust kann mit einer Reintonaudiometrie nicht diagnostiziert werden.