Functional Anatomy of the Cochlea

The cochlea is a system of coiled tubes, shown in Figure 52-1 and in cross section in Figures 52-2 and 52-3. 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 52-3; 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.

Figure 52-4 diagrams the functional parts of the uncoiled cochlea for conduction of sound vibrations. First, note that Reissner's membrane is missing from

Figure 52-3

Section through one of the turns of the cochlea. (Drawn by Sylvia Colard Keene. From Fawcett DW: Bloom & Fawcett: A Textbook of Histology, 11th ed. Philadelphia: WB Saunders, 1986.)

Section through one of the turns of the cochlea. (Drawn by Sylvia Colard Keene. From Fawcett DW: Bloom & Fawcett: A Textbook of Histology, 11th ed. Philadelphia: WB Saunders, 1986.)

Cochlea Physiology
Movement of fluid in the cochlea after forward thrust of the stapes.

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, as discussed later in the chapter.)

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.

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