|
|
|
 |
 |
How the Inner Ear Recognizes Sound |
In the seventeenth century G.J. Duvérney, a French anatomist, proposed that the ear used a set of resonators. In the nineteenth century most scientists believed that some form of "resonance" was behind our ability to distinguish pitch. Resonance theory was most fully developed by the German scientist Hermann von Helmholtz. He believed that tuned fibers in the basilar membrane, on which the organ of Corti rests, vibrate in response to particular sound frquencies, just as a specific piano string will begin to vibrate in response to a sound at just the right frequency. He was correct that different frequencies are "heard" by different sections of the organ of Corti, with the parts nearest the ossicles sensitive to high tones and the parts farthest from the ossicles sensitive to low tones, but there were still many unanswered questions about how the cochlea functions. It took an elegant series of experiments by Hungarian physicist Georg von Bèkèsy to shed light on what was going on in the cochlea.
Tiny, opaque, spiral-shaped, and embedded in the temporal bone--the hardest bone in the body--the cochlea is very difficult to study. Beginning his experiments in 1928, von Bèkèsy built enlarged models of the cochlea. He used straight glass tubes-in effect unrolling the cochlea's spiral and making it transparent. Down the middle of a tube he attached a rubber membrane to simulate the basilar membrane, a flexible membrane that separates the cochlea into two segments. He filled the tube with water and introduced sound vibrations through one end, making the fluid within vibrate much as the ossicles of the middle ear make the fluid in the cochlea vibrate.
He noticed that the introduction of each sound sent a wave down the model's basilar membrane. He called this the "traveling wave." Although the traveling wave for any given tone deformed the entire simulated basilar membrane, von Bèkèsy observed that the cochlea is arranged tonotopically--high tones produced the largest deformation at the near end, and low tones produced the largest deformation at the far end. Using techniques two decades ahead of his time, von Bèkèsy confirmed his model by observing the same deformations in the basilar membranes of cochleas he dissected from cadavers. He observed that when the basilar membrane was deformed, the tiny stereocilia on top of the hair cells bent against another membrane called the tectorial membrane. The point at which the basilar membrane  was deformed the most was the point at which the stereocilia were bent the most. This, he concluded, is how different tones are "heard" at different points along the organ of Corti. For his seminal work in the biophysics of hearing, von Bèkèsy was awarded the Nobel Prize in Physiology or Medicine in 1961. |
|
|
|
|
|
|
