Basilar membrane

Basilar membrane
Section through organ of corti, showing basilar membrane
Cross section of the cochlea.
Details
Identifiers
Latin	membrana basilaris ductus cochlearis
MeSH	A09.246.631.246.125
Anatomical terminology []

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 (see figure).

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 stiff at the apex of the cochlea, and narrowest (0.08–0.16 mm) and most stiff 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.

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 it is also the function of tissue held by the basilar membrane 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.

The basilar membrane is also the base for the sensory cells of hearing, the hair cells that are equipped with "Stereocilia". There are approximately 15,000 hair cells in each human ear (see figure). This function as base of the sensory cells 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.