The presence of two types of hair cells, the inner and outer hair cells, in the organ of Corti was appreciated nearly a 100 years ago but the function of the two has only become clear in the last 20 years. The first clue that they might play different roles in hearing came about 40 years ago as the result of a painstaking anatomical investigation which revealed that most of the nerve fibers that carry information to the brain contact only the inner hair cells. This meant that most the information about the acoustic world reached the brain via the inner hair cells. What then was the role of outer hair cells which are over three times more numerous? The mystery was compounded by the discovery that neural fibers originating from neurons deep in the brain, which send information back to the hair cells, only touch outer hair cells. It was later determined that outer hair cell stereocilia are firmly embedded in the overlying tectorial membrane while inner hair cell stereocilia make only a tenuous connection. The outer hair cells are located near the center of the basilar membrane where vibrations will be greatest while the basilar membrane is anchored under the inner hair cells (see Figure 5). These observations suggest that the movement of stereocilia and the resulting modulation of their ionic currents is likely to be greater for outer hair cells than for inner hair cells. Several studies that had examined the inner ears of deaf people shortly after they died demonstrated that outer hair cells were required for hearing. It was clear that the inner hair cells served to transmit information to the brain but the role of the outer hair cells remained a mystery.
The mechanical vibrations of the organ of Corti had been analyzed by engineers since the 1940s. Their analysis was able to explain the frequency selectivity originally measured by von Bekesy in cochlea obtained from cadaver ears. It was known at the time that the measured frequency selectivity and the frequency selectivity computed from the engineering analysis did not approach the frequency selectivity of the human hearing or the frequency selectivity that could be measured from individual nerve fibers. Shortly after WW2 an American astrophysicist who had worked on radar during the war suggested that the frequency selectivity of the cochlea could be enhanced if a source of mechanical energy were present in the cochlea. His suggestion was largely ignored until several engineering groups in the late 70s rediscovered the potential benefits of this hypothetical energy source. They were forced to consider the possible existence of this "cochlear amplifier" when improved measures from living (as opposed to dead) ears revealed that the mechanical frequency selectivity in the living ear began to approach that of human hearing. The concept that a source of mechanical energy exists in the cochlea appeared validated when in the late 70s it was discovered that sound is produced by the inner ear. These sounds can be measured by placing a sensitive microphone in the ear canal. They were called otoacoustic emissions and they are now routinely measured in the clinic to assess hearing. Their discovery was amazing for sensory physiology because it was equivalent to finding that light comes out of the eye (which has never been observed). Within five years it was discovered that the outer hair cell could be made to elongate and shorten by electrical stimulation. The function of the outer hair cell in hearing is now perceived as that of a cochlear amplifier that refines the sensitivity and frequency selectivity of the mechanical vibrations of the cochlea.
Next chapter: Outer Hair Cell Electromotility
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