The sensory epithelium of the inner ear is called the organ of Corti after the Italian scientist who first described it. Its orderly rows of outer hair cells is unique among the organs of the body. Figure 5 shows a short section of the organ of Corti as it spirals in the cochlea. The organ of Corti is larger and the basilar membrane on which it sits is longer as it gets further away from the base of the cochlea. This difference in size is consistent with the fact that different frequencies of sound result in greater vibrations of the organ of Corti depending on where along the length of the cochlea you are measuring. The shorter, smaller structures near the base of the cochlea respond best to high frequencies, while the longer, larger structures near the top of cochlea respond best to low frequencies. This is similar to the organization of a piano where the longer, larger strings produce the lower frequency sounds.
The wide base of the cochlea from which this segment comes is towards the bottom of the page. The central axis of the spiraling cochlea is to the left of the drawing. Eighth nerve fibers pass through a bony shelf on their way to the hair cells (orange). The organ of Corti is made up of hair cells and supporting cells (purple and blue, respectively) that sit on a flexible basilar membrane which is anchored to the bony shelf on the left and a ligament (not shown) on the right. A single flask shaped inner hair cell is shown on the left and three rows of cylindrically shaped outer hair cells are seen on the right. The tips of the outer hair cell stereocilia are imbedded in a gelatinous mass called the tectorial membrane which lies on top of the organ of Corti and is secreted from cells (not shown) on the left. When sound is transmitted to the inner ear the organ of Corti begins to vibrate up and down. Since the basilar membrane is attached to bone and ligament at its two ends, the area of maximal vibration is near the third (furthest right) row of outer hair cells. The overlying tectorial membrane is not as flexible so the stereocilia are bent as the organ of Corti moves up and down against it. The electrical potential inside the hair cells changes as the stereocilia are bent.