Basilar membrane

Separation of fluids
The basilar membrane within the cochlea of the inner ear separates two liquid filled tubes that run along the coil of the cochlea, the scala media and the scala tympani (see figure). The fluids in these two tubes, the endolymph and the perilymph are very different chemically, biochemically, and electrically. Therefore they have to be kept strictly separated. This separation is the main function of the basilar membrane in the hearing organ of all land vertebrates. A leakage between the two tubes, due to an injury, causes a disruption or even a total breakdown of hearing in that ear.

A base for the sensory cells
Closely associated with the function of separating the two fluids is the function of providing a 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.

Frequency dispersion
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. This causes sound input of a certain frequency to vibrate a particular location of the membrane more than other locations due to the physical property of resonance. 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 membrane), and low frequencies lead to maximum vibrations at the apical end of the cochlear coil (wide membrane).

The error of von Békésy
The benefit that animals have from this third function is still a matter of research. The hypothesis of von Békésy that it would provide frequency selectivity for the hair cells turned out to be in error. Research results of recent decades showed that frequency selectivity in hearing remains intact even with an immobilized basilar membrane. Therefore one had to conclude that also mammals, as all other land vertebrates, hear frequency-selectively due to intrinsically tuned hair cells. This conclusion was later confirmed by an extensive study where a large-scale pharmacological knockout of one type of hair cells resulted in a complete loss of frequency-selective hearing, even though the basilar membrane had remained fully functional.