What Is the Central Membrane of the Cochlea? The cochlea is a long coiled tube, with three channels divided by two thin membranes. The top tube is the scala vestibuli, which is connected to the oval window. The bottom tube is the scala tympani, which is connected to the round window. The middle tube is the scala media, which contains the Organ of Corti. The Organ of Corti sits on the basilar membrane, which forms the division between the scalae media and tympani.
Figure 12.3 illustrates a cross section through the cochlea. The three scalae (vestibuli, media, tympani) are cut in several places as they spiral around a central core. The cochlea makes 2-1/2 turns in the human (hence the 5 cuts in midline cross section). The tightly coiled shape gives the cochlea its name, which means snail in Greek (as in conch shell). As explained in Tonotopic Organization, low frequency sounds stimulate the base of the cochlea, whereas high frequency sounds stimulate the apex. This feature is depicted in the animation of Figure 12.3 with neural impulses (having colors from red to blue representing low to high frequencies, respectively) emerging from different turns of the cochlea. The activity in Figure 12.3 would be generated by white noise that has all frequencies at equal amplitudes. The moving dots are meant to indicate afferent action potentials. Low frequencies are transduced at the apex of the cochlea and are represented by red dots. High frequencies are transduced at base of the cochlea and are represented by blue dots. A consequence of this arrangement is that low frequencies are found in the central core of the cochlear nerve, with high frequencies on the outside.
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