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The basilar membrane is a stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani, .

Basilar membrane.
Organ of corti.svg
Section through organ of corti, showing basilar membrane
Cross section of the cochlea.
Latinmembrana basilaris ductus cochlearis
Anatomical terminology


The basilar membrane is a pseudo-resonant structure[1] that, like the strings on an instrument, varies in width and stiffness. But unlike the parallel strings of a guitar, the basilar membrane is a single structure with different width, stiffness, mass, damping, and the duct dimensions at different points along its length. The motion of the basilar membrane is generally described as a traveling wave.[2] The properties of the membrane at a given point along its length determine its characteristic frequency (CF), the frequency at which it is most sensitive to sound vibrations. The basilar membrane is widest (0.42–0.65 mm) and least stiff at the apex of the cochlea, and narrowest (0.08–0.16 mm) and stiffest at the base (near the round and oval windows).[3] High-frequency sounds localize near the base of the cochlea, while low-frequency sounds localize near the apex.


Sinusoidal drive through the oval window (top) causes a traveling wave of fluid–membrane motion. A modeled snapshot of fluid streamlines is shown. The wavelength is long compared to the duct height near the base, in what is called the long-wave region, and short (0.5 to 1.0 mm in typical observations[4][5]) near the place where the displacement and velocity are maximized, just before cutoff, in the short-wave region.

Endolymph/perilymph separationEdit

Along with the vestibular membrane, several tissues held by the basilar membrane segregate the fluids of the endolymph and perilymph, such as the inner and outer sulcus cells (shown in yellow) and the reticular lamina of the organ of Corti (shown in magenta). For the organ of Corti, the basilar membrane is permeable to perilymph. Here the border between endolymph and perilymph occurs at the reticular lamina, the endolymph side of the organ of Corti.[6]

A base for the sensory cellsEdit

The basilar membrane is also the base for the hair cells. This function is present in all land vertebrates. Due to its location, the basilar membrane places the hair cells adjacent to both the endolymph and the perilymph, which is a precondition of hair cell function.

Frequency dispersionEdit

A third, evolutionarily younger, function of the basilar membrane is strongly developed in the cochlea of most mammalian species and weakly developed in some bird species:[7] the dispersion of incoming sound waves to separate frequencies spatially. In brief, the membrane is tapered and it is stiffer at one end than at the other. Furthermore, sound waves travelling to the far, "floppier" end of the basilar membrane have to travel through a longer fluid column than sound waves travelling to the nearer, stiffer end. Each part of the basilar membrane, together with the surrounding fluid, can therefore be thought of as a "mass-spring" system with different resonant properties: high stiffness and low mass, hence high resonant frequencies at the near end, and low stiffness and high mass, hence low resonant frequencies, at the far end.[8] This causes sound input of a certain frequency to vibrate some locations of the membrane more than other locations. As shown in experiments by Nobel Prize laureate Georg von Békésy, high frequencies lead to maximum vibrations at the basal end of the cochlear coil, where the membrane is narrow and stiff, and low frequencies lead to maximum vibrations at the apical end of the cochlear coil, where the membrane is wider and more compliant. This "place–frequency map" can be described quantitatively by the Greenwood function and its variants.

Sound-driven vibrations travel as waves along this membrane, along which, in humans, lie about 3,500 inner hair cells spaced in a single row. Each cell is attached to a tiny triangular frame. The 'hairs' are minute processes on the end of the cell, which are very sensitive to movement. When the vibration of the membrane rocks the triangular frames, the hairs on the cells are repeatedly displaced, and that produces streams of corresponding pulses in the nerve fibers, which are transmitted to the auditory pathway.[9] The outer hair cells feed back energy to amplify the traveling wave, by up to 65 dB at some locations.[10][11]

Additional imagesEdit

See alsoEdit


  1. ^ M. Holmes and J. D. Cole, "Pseudoresonance in the cochlea, ' in: Mechanics of Hearing, E. de Boer and M. A. Viergever (editors), Proceedings of the IUTAM/ICA Symposium, Delft (1983), pp. 45-52.
  2. ^ Richard R. Fay; Arthur N. Popper & Sid P. Bacon (2004). Compression: From Cochlea to Cochlear Implants. Springer. ISBN 0-387-00496-3.
  3. ^ Oghalai JS. The cochlear amplifier: augmentation of the traveling wave within the inner ear. Current Opinion in Otolaryngology & Head & Neck Surgery. 12(5):431-8, 2004
  4. ^ Shera, Christopher A. (2007). "Laser amplification with a twist: Traveling-wave propagation and gain functions from throughout the cochlea". Journal of the Acoustical Society of America. 122 (5): 2738–2758. doi:10.1121/1.2783205. Archived from the original on 3 July 2013. Retrieved 13 April 2013.
  5. ^ Robles, L.; Ruggero, M. A. (2001). "Mechanics of the mammalian cochlea". Physiological Reviews. 81 (3): 1305–1352. PMC 3590856. Retrieved 13 April 2013.
  6. ^ Salt, A.N., Konishi, T., 1986. The cochlear fluids: Perilymph and endolymph. In: Altschuler, R.A., Hoffman, D.W., Bobbin, R.P. (Eds.), Neurobiology of Hearing: The Cochlea. Raven Press, New York, pp. 109-122
  7. ^ Fritzsch B: The water-to-land transition: Evolution of the tetrapod basilar papilla; middle ear, and auditory nuclei. In: Douglas B. Webster; Richard R. Fay; Arthur N. Popper, eds. (1992). The Evolutionary biology of hearing. Berlin: Springer-Verlag. pp. 351–375. ISBN 0-387-97588-8.
  8. ^ Schnupp J.; Nelken I.; King A. (2011). Auditory Neuroscience. Cambridge MA: MIT Press. ISBN 0-262-11318-X.
  9. ^ Beament, James (2001). "How We Hear Music: the Relationship Between Music and the Hearing Mechanism". Woodbridge: Boydell Press: 97. Cite journal requires |journal= (help)
  10. ^ Nilsen KE, Russell IJ (1999). "Timing of cochlear feedback: spatial and temporal representation of a tone across the basilar membrane". Nat. Neurosci. 2 (7): 642–8. doi:10.1038/10197. PMID 10404197.
  11. ^ Nilsen KE, Russell IJ (2000). "The spatial and temporal representation of a tone on the guinea pig basilar membrane". Proc. Natl. Acad. Sci. U.S.A. 97 (22): 11751–8. doi:10.1073/pnas.97.22.11751. PMC 34345. PMID 11050205.

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