Longitudinal pattern of basilar membrane vibration in the sensitive cochlea

Tianying Ren

Proceedings of the National Academy of Sciences, USA, early edition, Dec 2, 2002 (pnas.262663699)

Oregon Hearing Research Center (NRC 04), Department of Otolaryngology and Head and Neck Surgery, Oregon Health and Science University, 3181 Southwest Sam Jackson Park Road, Portland, OR 97239-3098, USA

Abstract:

In the normal mammalian ear, sound vibrates the eardrum, causing the tiny bones of the middle ear to vibrate, transferring the vibration to the inner ear fluids. The vibration propagates from the base of the cochlea to its apex along the cochlear partition. As
essential as this concept is to the theory of hearing, the waveform of cochlear partition vibration has yet to be measured in vivo. Here I report a ‘‘snapshot’’ (the instantaneous waveform of cochlear partition vibration) measured in the basal turn of the sensitive gerbil cochlea using a scanning laser interferometer. For 16-kHz tones, the phase delay is up to 6p radians over the observed cochlear length (<1,000 mm), and instantaneous waveforms show sound propagation along the cochlear partition, supporting the existence of the cochlear traveling wave. The detectable basilar membrane response to a low-level 16-kHz tone occurs over a very restricted (~600 µm) range. The observed vibration shows compressive nonlinear growth, a shorter wavelength, and a slower propagation velocity along the cochlear length than previously reported. Data obtained at different frequencies show the relationship between the longitudinal pattern and frequency tuning, demonstrating that the observed localized traveling wave in this study is indeed the spatial representation of the sharp tuning observed in the frequency domain..
(Bold text emphasis by Martin Braun)

Comment:

The vibration of the mammalian cochlear partition has now for the first time been measured with a scanning laser. Tianying Ren recorded basilar membrane (BM) motion of an uninterrupted longitudinal section of 900 µm in the gerbil cochlea. He collected data for the complete range in a single recording using a scanning laser that automatically operated in steps of 2.5 µm. Major results were (1) that up to about 50 dB SPL the longitudinal range of BM excitation is symmetrical around CF, and (2) that at 10 dB SPL the total longitudinal range of BM excitation is ca. 600 µm, that is 300 µm on either side of the given point of characteristic frequency (CF). These results agree with earlier ones reported by Russell and Nilsen (1997, PNAS 94, 2660-2664) and Nilsen and Russell (2000, PNAS 97, 11751-11758). They indicate that at low sound levels BM motion reflects a local resonance in the organ of Corti, probably originating from intrinsically tuned, motile outer hair cells (OHC). Ren's new technique has large potentials for future explorations. The speed of the BM traveling wave (TW) as a function of longitudinal location can now be determined with high precision at many different sound levels. Thus it will be possible to obtain a detailed analysis of both the passive and the active, OHC-driven, components of the TW, which is necessary for an understanding of cochlear function. (Comment Martin Braun)

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