The basilar membrane
within the cochlea
of the inner ear
is a stiff structural element that separates two liquid-filled tubes that run along the coil of the cochlea, the scala media
and the scala tympani
The fluids in these two tubes, the endolymph
and the perilymph
are very different chemically, biochemically, and electrically. Therefore they are kept strictly separated. This separation is the main function of Reissner's membrane
(between scala vestibuli
and scala media), and is one of the functions of the basilar membrane in the hearing organ of all land vertebrates
A base for the sensory cells
The basilar membrane is also the base
for the sensory cells of hearing, the hair cells
(see figure). This function gave the basilar
membrane its name, and it is again present in all land vertebrates. Due to its location, the basilar membrane places the hair cells in a position where they are adjacent to both the endolymph and the perilymph, which is a precondition of hair cell function.
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. It is the function of frequency dispersion of incoming sound waves. In brief, the membrane is tapered and it is stiffer at one end than at the other. The dispersion of fluid waves 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 (narrow, stiff membrane), and low frequencies lead to maximum vibrations at the apical end of the cochlear coil (wide, more compliant membrane). This "place-frequency map" can be described quantitatively by the Greenwood Function and its variants.
The basilar membrane is a pseudo-resonant structure that, like strings on an instrument, varies in width and stiffness. The "string" of the basilar membrane is not a set of parallel strings, as in a guitar, but a long structure that has different properties (width, stiffness, mass, damping, and the dimensions of the ducts that it couples to) at different points along its length. The motion of the basilar membrane is generally described as a traveling wave. The parameters 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 taut at the apex of the cochlea, and narrowest (0.08–0.16 mm) and most taut at the base. High-frequency sounds localize near the base of the cochlea (near the round and oval windows), while low-frequency sounds localize near the apex.
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