Longitudinally propagating traveling waves of the mammalian tectorial membrane

doi:10.1073/pnas.0703665104

Longitudinally propagating traveling waves of the mammalian tectorial membrane

*Speech and Hearing Bioscience and Technology Program, Harvard–MIT Division of Health Sciences and Technology, Cambridge, MA 02139;
^†Research Laboratory of Electronics and
^‡Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139; and
^§Eaton–Peabody Laboratory of Auditory Physiology, Massachusetts Eye and Ear Infirmary, Boston, MA 02114

Edited by David P. Corey, Harvard Medical School, Boston, MA, and accepted by the Editorial Board August 27, 2007 (received for review April 23, 2007)

Abstract

Sound-evoked vibrations transmitted into the mammalian cochlea produce traveling waves that provide the mechanical tuning necessary for spectral decomposition of sound. These traveling waves of motion that have been observed to propagate longitudinally along the basilar membrane (BM) ultimately stimulate the mechano-sensory receptors. The tectorial membrane (TM) plays a key role in this process, but its mechanical function remains unclear. Here we show that the TM supports traveling waves that are an intrinsic feature of its visco-elastic structure. Radial forces applied at audio frequencies (2–20 kHz) to isolated TM segments generate longitudinally propagating waves on the TM with velocities similar to those of the BM traveling wave near its best frequency place. We compute the dynamic shear storage modulus and shear viscosity of the TM from the propagation velocity of the waves and show that segments of the TM from the basal turn are stiffer than apical segments are. Analysis of loading effects of hair bundle stiffness, the limbal attachment of the TM, and viscous damping in the subtectorial space suggests that TM traveling waves can occur in vivo. Our results show the presence of a traveling wave mechanism through the TM that can functionally couple a significant longitudinal extent of the cochlea and may interact with the BM wave to greatly enhance cochlear sensitivity and tuning.

Footnotes

^¶To whom correspondence should be addressed at:
Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 36-889, Cambridge, MA 02139.
E-mail: freeman{at}mit.edu
Author contributions: R.G., A.J.A., and D.M.F. designed research; R.G. performed research; R.G. and A.J.A. analyzed data; and R.G., A.J.A., and D.M.F. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. D.P.C. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/cgi/content/full/0703665104/DC1.
Abbreviations:

BM,

basilar membrane;

TM,

tectorial membrane;

OHC,

outer hair cell;

BF,

best frequency;

IHC,

inner hair cell.
Freely available online through the PNAS open access option.