The inner ear (labyrinth) is made up of two parts; one within the other, the (bony labyrinth) is a series of channels in the petrous portion of the temporal bone. Inside these channels, surrounded by a fluid called (perilymph) is the (membrane labyrinth). This membrane structure more or less duplicates the shape of the bony channels. It is filled with a fluid called (endolymph), and there is no communication between the space filled with endolymph and those filled with perilymph.
The cochlear portion of the labyrinth is a coiled tube which in human is 3.5 mm long and makes 2¼ turns.
Throughout its length, the basilar membrane and Reissner`s membrane divided it into three chamber’s (scalae).
The upper scala vestibuli and the lower scala tympani contain perilymph and communicate with each other at the apex of the cochlea through a small opening called the helicortrema. At the base of the cochlea, the cochlea, the scala vestibuli ends at the oval window, which is closed by the footplate of the stapes. The scala tympani end at the round window, a foramen on the medial wall of the middle ear that is closed by the flexible secondary tympanic membrane.
The scala media, the middle cochlear chamber, is continuous with the membranous labyrinth and does not communicate with the other two scala. It contains endolymph.
The scala vestibuli and scala media are separated from each other by Reissner`s membrane (also called the vestibular membrane); the scala tympani and scala media are separated from each other by the basilar membrane.
On the surface of the basilar membrane lies the organ of Corti, which contains a series of electromechanically sensitive cells, the hair cells. The Reissner`s membrane is so thin and so easily moved that it does not obstruct the passage of sound vibrations through fluid from the scala vestibuli into scala media.
The basilar membrane contains 20,000 to 30,000 basilar fibers that project from the bony center of the cochlea, the modiolus, toward the outer wall. These fibers are stiff, elastic, reed like structure that are fixed at their basal ends in the central bony structure of the (the modiolus) but not fixed at their distal ends, except that the distal ends are embedded in the loose basilar membrane. Because the fibers are stiff and free at one end, they can vibrate like the reeds of a harmonicaالهواء في الهارمونكة الريشة التي تتحرك عند نفخ.
The basilar membrane characterized by:
1. The lengths of the basilar fibers increase progressively beginning at the oval window and going from the base of the cochlea to the apex, increasing from a length of about 0.04 millimeter near the oval and round windows to 0.5 millimeter at the tip of the cochlea (the “helicotrema”), a 12-fold increase in length
2. The diameters of the fibers, however, decrease from the oval window to the helicotrema, so their overall stiffness decreases more than 100-fold. As a result, the stiff, short fibers near the oval window of the cochlea vibrate best at a very high frequency, whereas the long, limber fibers near the tip of the cochlea vibrate best at a low frequency.
3. Another feature of the traveling wave is that it travels fast along the initial portion of the basilar membrane but becomes progressively slower as it goes farther into the cochlea. The cause of this difference is the high coefficient of elasticity of the basilar fibers near the oval window and a progressively decreasing coefficient farther along the membrane.
Low-frequency resonance occurs near the helicotrema, mainly because of the less stiff fibers and also because of increased “loading” with extra masses of fluid that must vibrate along the cochlear tubules.
The high-frequency resonance of the basilar membrane occurs near the base, where the sound waves enter the cochlea through the oval window.