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Organ of Corti
In l851, Alfonso Corti, an Italian anatomist, published an article describing for the first time a small, spiraling organ in the cochlea that contained the microscopic outer and inner hair cells. Little did he know then the importance of these fuzzy-looking cells to auditory function. He discovered the structure just in time to have it named the Organ of Corti in his honor. That same year, at the age of 29, he inherited and father's estate and lost all interest in scientific endeavors.
In Italian, the word corti means "short," and Corti's organ is indeed a short one, spiraling around the core of the cochlea for less than one inch from base to apex.
Outer and Inner Hair Cells
We have long known that the outer and inner hair cells are critical to hearing function, but in different ways. There is one row of the 4,000 or so pear-shaped inner hair cells and three rows of the 12,000 or so test-tube shaped outer hair cells. The inner hair cells contact 95 percent of the auditory fibers and are responsible for sending information up the 8th nerve to the brain that produces the sensation of hearing.
Why are the outer hair cells, which contact only 5 percent of the ascending auditory fibers, far more numerous? Read on, to refresh your memory and extend your insight into these miraculous cells and their unique environment.
Function of Outer Hair Cells
The mystery of outer hair cell function remained until the discovery that the cells were not only sensory but also motor. When an outer hair cell is stimulated through movement of the underlying basilar membrane, the cells contract like tiny muscles. Such mechanical contractions of sensory cells are found nowhere else in your body's estimated 100 trillion cells. They serve to amplify the motion of the cochlear fluids and to increase the sensitivity of our hearing by 40-60 dB. Without the outer hair cells, we would all need hearing aids. In addition to serving as a cochlear amplifier, the cells also serve to fine tune our ability to detect small frequency and intensity changes.
Prestin
The electromotility of the outer hair cells is dependent upon a motor protein called prestin,1 which is expressed in the lateral plasma membrane of the cells. The prestin molecule was patented in 2003 and was named for the musical note "presto," which in Italian means "very fast." Since prestin allows the outer hair cells to vibrate up to 20,000 Hz in humans and 60,000 Hz in mice, the molecule in retrospect could well have been named prestis, for the musical note "prestissimo," meaning "very, very fast."
The vibrations of the outer hair cells lead to sounds, which can be picked up by sensitive microphones in the outer ear canal. These tiny sounds, called Otoacoustic Emissions (OAEs), are used routinely by audiologists for diagnostic and screening purposes. The production of sounds by the outer hair cells in the cochlea is a surprising phenomenon, which some say is equivalent to the production of light by the retina. So far, this light phenomenon has not been observed outside the pages of science fiction.
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