Resonant Tectorial Membrane Motion in the Inner
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Proc. Natl. Acad. Sci. USA Vol. 93, pp. 8727-8732, August 1996 Neurobiology Resonant tectorial membrane motion in the inner ear: Its crucial role in frequency tuning (cochlear amplifier/outer hair cell/basilar membrane/two-dimensional motion) ANTHONY W. GUMMER*, WERNER HEMMERT, AND HANS-PETER ZENNER Section of Physiological Acoustics and Communication, Department of Otolaryngology, University of Tubingen, Silcherstrasse 5, 72076 Tubingen, Germany Communicated by JozefJ. Zwislocki, Syracuse University, Syracuse, NY, May 22, 1996 (received for review February 26, 1996) ABSTRACT The tectorial membrane has long been pos- The aim of the present study was to experimentally charac- tulated as playing a role in the exquisite sensitivity of the terize the vibration response of the TM. This was achieved by cochlea. In particular, it has been proposed that the tectorial developing an optical system that detected vibrations in two membrane provides a second resonant system, in addition to orthogonal directions: one perpendicular to the basilar mem- that of the basilar membrane, which contributes to the brane (BM), in the transverse direction, and the other parallel amplification of the motion of the cochlear partition. Until to the BM, in the radial direction (Fig. 1). Recordings were now, technical difficulties had prevented vibration measure- made in the apical turn of the cochlea where the upper surface ments of the tectorial membrane and, therefore, precluded of the TM is optically accessible (27-29). An isolated temporal direct evidence of a mechanical resonance. In the study bone preparation of the guinea pig cochlea was developed for reported here, the vibration of the tectorial membrane was this purpose. Provided appropriate precautions are taken, this measured in two orthogonal directions by using a novel region of the cochlea has the advantage that the frequency method of combining laser interferometry with a photodiode selectivity of the in vitro vibration responses of Hensen's cells, technique. It is shown experimentally that the motion of the Reissner's membrane, and the cuticular plate of hair cells, tectorial membrane is resonant at a frequency of 0.5 octave measured in the transverse direction (30), is similar to that of (oct) below the resonant frequency of the basilar membrane tuning curves for primary auditory nerve fibers (31, 32) and and polarized parallel to the reticular lamina. It is concluded receptor potentials (1, 33) measured at a sound pressure level that the resonant motion of the tectorial membrane is due to (SPL) of 20-50 dB above threshold. a parallel resonance between the mass of the tectorial mem- Two experimental protocols were employed. First, the trans- brane and the compliance of the stereocilia of the outer hair verse component in response to intracochlear current injection cells. Moreover, in combination with the contractile force of (34) was used to uncover resonant motion of the TM and also outer hair cells, it is proposed that inertial motion of the to locate the resonant frequency of the BM. Second, responses tectorial membrane provides the necessary conditions to allow to sound were measured in both the transverse and the radial positive feedback of mechanical energy into the cochlear directions to describe the dynamics of TM motion. partition, thereby amplifying and tuning the cochlear re- In our experiments, the rationale for the current-injection sponse. experiments was based on the premise that sinusoidal current injected locally into the organ of Corti causes the OHCs to Understanding the micromechanical mechanisms underlying exert synchronous, sinusoidal forces of equal magnitude on the the extraordinary sensitivity of the cochlea is a cardinal goal of TM-stereocilia complex and on the BM-Deiters cell complex. auditory physiology. It is generally agreed that motion of the If these structures can be considered to be in series mechan- tectorial membrane relative to the cuticular of a ically, so that each experiences the same force delivered by the (TM) plate OHC, then measurement of the velocity of the TM and BM sensory hair cell stimulates transduction channels in its stereo- yields the TM impedance relative to the BM impedance. cilia-directly through physical contact to the TM of the According to experiments of Mammano and Ashmore (35), for longest stereocilia of the outer hair cells (OHCs) and indirectly positive current injection in scala media the OHCs hyperpo- by fluid motion around the stereocilia of the inner hair cells larize and elongate, causing the TM to move toward scala (1-5). Moreover, because OHCs undergo somatic length vestibuli and the BM toward scala tympani. Moreover, the changes in response to electrical (6-8) and chemical (9) electromotors in the OHC wall function independent of fre- stimuli, OHCs and their stereocilia are supposed to feed quency, at least up to 22 kHz (36). Therefore, instead of the mechanical energy back into the cochlear partition, thereby pressure transducer (the loudspeaker) being located at the reducing its impedance (10-12). Therefore, the TM is ex- external ear canal, as it normally is for acoustical stimulation, pected to be functionally connected not only to the input of it is located within the cochlear partition for electrical stimu- mechanoelectrical transducers in hair-cell stereocilia, but also lation; the OHCs become the frequency-independent, elec- to the output of electromechanical transducers in the OHC tromechanical transducers. membrane. Technical difficulties have prevented measure- ments of TM vibration. Therefore, functional information has been inferred from morphological investigations (13-15), stiff- MATERIALS AND METHODS ness measurements post mortem (16) and in vivo (17), a Temporal bones were removed from guinea pigs (250-400 g), physical model (18), mathematical models (5, 10, 11, 19-24), which were decapitated after cervical dislocation. The tem- together with the frequency tuning properties of evoked poral bone was cemented (Harvard dental cement) to a delrin otoacoustic emissions (25, 26) and cochlear microphonic po- sound delivery cone and the ventral bulla region opened to tentials (17). In general, the latter models (5, 10, 18-26) expose the apical end of the cochlea. A sheet of Parafilm was require that the TM be mechanically resonant. Abbreviations: TM, tectorial membrane; OHC, outer hair cell; BM, The publication costs of this article were defrayed in part by page charge basilar membrane; BF, best frequency; oct, octave; SPL, sound payment. This article must therefore be hereby marked "advertisement" in pressure level. accordance with 18 U.S.C. §1734 solely to indicate this fact. *To whom reprint requests should be addressed. 8727 Downloaded by guest on October 2, 2021 8728 Neurobiology: Gummer et al. Proc. Natl. Acad. Sci. USA 93 (1996) the scalae, but also to avoid light reflections and refractions at an air interface by placing the microscope objective directly on the fluid (16). Vibration measurements began about 20 min post mortem, lasted 1-1.5 h, and were done in different order on different animals to exclude the possibility of time artifacts. Velocity in the transverse direction was measured with a laser Doppler velocimeter (model OFV-302; Polytec, Wald- bronn, Germany) coupled into the side-arm of an epifluores- cence microscope (Leitz Aristomet), in a manner similar to that described in refs. 39 and 40; the transverse direction coincides with the optical axis of the microscope (Fig. 1). Displacement in the radial direction was measured with a double photodiode (model BPX 48; Siemens, Munich, Ger- many) mounted on the microscope parallel to the focal plane. The need for a small hole in the cochlear wall meant that it was possible to adjust the focal plane of the microscope parallel to the BM but rarely possible to adjust it parallel to the reticular lamina. Therefore, the preparation was placed such that the transverse direction was approximately orthogonal to the BM. The focal plane was no more than ± 100 from the plane of the BM; it was determined from the transverse and radial distances between a focal point on a border cell of the internal spiral sulcus and one on a Claudius cell, with distances calculated from turns of the micrometers on the microscope. Data were not corrected to place the radial direction parallel to the reticular lamina because the possibility of more than one degree of vibrational freedom would require data interpreta- FIG. 1. Optical measurement of the vibration of the organ of Corti. tion before presentation. The microscope lens was a Zeiss x40, Velocity in the transverse direction (T) was measured with a laser 0.75 NA water immersion with working distance of 1.92 mm Doppler velocimeter (LDV) coupled into the side-arm of an epiflu- orescence microscope (M) and displacement in the radial direction (R) and focal depth of 1.4 ,um. For the laser Doppler velocimeter, with a double photodiode (PD) mounted on the microscope. The the wavelength was 633 nm, the output power was 1 mW, and drawing of the cross-section of the organ of Corti was made from the diameter of the scattered laser beam was about 5 ,tm, which light-microscopic observations of histological sections of the basal part is less than that of an OHC (7-10 Am). For the photodiode, the of the fourth cochlear turn. The orientation of the "radial" fibers in object was illuminated from above with the microscope light the TM is derived from ref. 15; they represent collagenous, type A and the magnified image (X203) of an edge of the object protofibrils. Note that (i) the stereocilia of the OHCs are orthogonal projected onto the double photodiode. Cochlear structures in to the reticular lamina and the lower margin of the TM; (ii) the long the focal plane were discerned on a video display using a axis of the OHCs are inclined at about 550 to the reticular lamina; (iii) Hamamatsu camera (C3077) with contrast enhancement the reticular lamina is inclined at about 350 to the BM or 145° to the positive radial direction; and (iv) at the upper and lower margins of the (C2400).