The cochlea is a system of coiled tubes, shown in Figure 1 and in cross section in Figures 2 and 3. It consists of three tubes coiled side by side: (1) the scala vestibuli, (2) the scala media, and (3) the scala tympani. The scala vestibuli and scala media are separated from each other by Reissner’s membrane (also called the vestibular membrane), shown in Figure 2; 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. They are the receptive end organs that generate nerve impulses in response to sound vibrations.
Fig1. The tympanic membrane, ossicular system of the middle ear, and inner ear.
Fig2. The cochlea. (Modified from Drake RL, Vogl AW, Mitchell AWM: Gray’s Anatomy for Students, ed 2, Philadelphia, 2010, Elsevier.)
Fig3. Section through one of the turns of the cochlea.
Figure 4 diagrams the functional parts of the uncoiled cochlea for conduction of sound vibrations. First, note that Reissner’s membrane is missing from this figure. This membrane is so thin and so easily moved that it does not obstruct the passage of sound vibrations from the scala vestibuli into the scala media. Therefore, as far as fluid conduction of sound is concerned, the scala vestibuli and scala media are considered to be a single chamber. (The importance of Reissner’s membrane is to maintain a special kind of fluid in the scala media that is required for normal function of the sound-receptive hair cells.)
Fig4. Movement of fluid in the cochlea after forward thrust of the stapes.
Sound vibrations enter the scala vestibuli from the faceplate of the stapes at the oval window. The faceplate covers this window and is connected with the window’s edges by a loose annular ligament so that it can move inward and outward with the sound vibrations. Inward movement causes the fluid to move forward in the scala vestibuli and scala media, and outward movement causes the fluid to move backward.
Basilar Membrane and Resonance in the Cochlea. The basilar membrane is a fibrous membrane that separates the scala media from the scala tympani. It 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, reedlike structures that are fixed at their basal ends in the central bony structure of the cochlea (the modiolus) but are 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 harmonica.
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.
The diameters of the fibers, however, decrease from the oval window to the helicotrema, so their overall stiff ness 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.
Thus, high-frequency resonance of the basilar mem brane occurs near the base, where the sound waves enter the cochlea through the oval window. However, low frequency resonance occurs near the helicotrema, mainly because of the less stiff fibers but also because of increased “loading” with extra masses of fluid that must vibrate along the cochlear tubules.
