Think of the xylophone you had as a child. This allows each segment to vibrate at a different frequency. Structurally, it is narrow and stiff near the oval window and as it moves toward the helicotrema it becomes wider and more limber. As we will explain later, this membrane is responsible for detecting sound waves of different frequencies. The spiral organ sits on the membrane that separates the cochlear duct from the scala tympani, the basilar membrane. This structure is the Organ of Corti or Spiral Organ (see the images below for a cross section of the cochlea and a close up of the spiral organ). Within the cochlear duct is the organ that converts mechanical vibrations to electrical action potentials. This chamber is filled with endolymph, which unlike the perilymph, resembles intracellular fluid in composition, and thus has a high K + concentration. Between these two chambers and also running the length of the cochlea is the cochlear duct. The scala vestibuli and scala tympani are filled with perilymph, a fluid that is similar to extracellular fluids. It is therefore acting as a pressure release valve, allowing the fluids in these chambers to vibrate (recall that fluids do not compress). Thus, when the oval window is pushed in by the stapes, the round window bulges out and when the oval window is pulled out, the round window moves in. The round window is a thin membrane between the scala tympani and the middle ear. The scala tympani runs parallel to the scala vestibuli and ends at the round window. Although they have different names, they are actually one long chamber that folds back on itself. At the very tip of the cochlea, the helicotrema, the scala vestibuli makes a U-turn and becomes the scala tympani. When the stapes vibrates it causes the fluids in the scala vestibuli to vibrate. The oval window (recall this is a membrane attached to the stapes) communicates with the first chamber, the scala vestibuli, which runs the entire length of the cochlea. It is composed of three parallel chambers that are filled with fluid. The cochlea is a spiral-shaped structure about 3.5 cm long (1.5 inches) that makes 2 ½ turns from top to bottom. This structure gets its name from its shape, cochlea means spiral, or snail shell. Before we discuss how sound waves are converted to action potentials, we need to understand the structure of the cochlea.
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