When you play a piano, the strings may supply the musical tones, but if it weren't for the wooden hammers striking the strings, you wouldn't hear a note. New research from the National Institute on Deafness and Other Communication Disorders (NIDCD) suggests that the ear may be built in much the same way. While most of the credit for detecting sound is paid to the small sensory cells in our inner ear, called hair cells, a membrane that lies immediately above these cells could be playing a larger role in the hearing process than scientists once thought.
A "column" of three outer hair cells and one inner hair cell beneath the tectorial membrane (blue). Roughly 3,500 such columns wind their way along the cochlear spiral, with each column processing a different sound frequency.
In order for you to hear, sound vibrations travel through the ear canal, past the eardrum and a series of small bones, to the cochlea, a snail-shaped organ in the inner ear. The vibrations cause fluid in the cochlea to move and a wave to form along a membrane running down its length. Hair cells sitting on top of the membrane ride the wave and bump up against a second membrane—the tectorial membrane—which lies overhead. When this happens, bundles of bristly structures protruding from their tops tilt to one side, causing channels to open up, ions to rush in, and an electrical signal to be generated that travels to the brain.
Not all sounds are detected equally by each hair cell, however. Hair cells are arranged like a spreadsheet with three rows and roughly 3,500 columns. Each column processes a different sound frequency. Three outer hair cells with V-shaped hair bundles make up one column. (One inner hair cell is also part of that column, but it's located by itself, farther down the spreadsheet.) The column of hair cells found at the opening to the cochlea detects the highest-pitched sound you can hear;the column at the end of the cochlea—at the centermost part of the spiral—detects the lowest-pitched sound;and a gradual progression of pitches from high to low is detected in between. On average, humans are able to detect sounds ranging from frequencies as high as 20,000 hertz, which is in the same range as a dog whistle, to as low as 20 hertz, which is roughly the frequency of some whale songs.
Richard Chadwick, Ph.D., and Nuria Gavara, Ph.D., both of the Auditory Mechanics Section in NIDCD's Division of Intramural Research, wanted to learn more about how the tectorial membrane assists hair cells in accomplishing this feat. Specifically, they wanted to test how soft or stiff the membrane is along the cochlea and how these properties might affect a hair cell's ability to convert sound vibrations into an electrical signal.
According to a new study published in the March 18, 2009, issue of the journal PLoS ONE, Drs. Gavara and Chadwick used nanotechnology techniques to devise a new method for studying the properties of the tectorial membrane. One hurdle in studying this membrane is that it contains long collagen fibers connected to one another by shorter, more randomly dispersed proteins. The current technology for studying the mechanical properties of a material at an extremely small scale—called an atomic force microscope—is not able to measure materials made up of two or more substances with different structural orientations.
Under normal conditions, the microscope uses a cantilever to apply force to the material and measures how much the material gives in response to that force. With the revised method, however, Drs. Gavara and Chadwick placed fluorescent microbeads on top of the tectorial membranes of guinea pigs at varying distances from the cantilever. As they pressed up and down on the cantilever, the amount of give from the membrane caused the microbeads to move in various directions. Microbeads located on stiffer portions of membrane moved smaller distances than microbeads located on softer portions, where the give was much greater. Based on these measurements, the researchers were able to calculate the stiffness of the collagen fibers and their connecting fibers running throughout the membrane. They also used other imaging techniques to determine how close or far apart the individual fibers were from one another along the cochlea.
Among their findings, the researchers discovered:
- Each collagen fiber is paired to a single column of outer hair cells, much like a hammer is paired to a piano string. When sound vibrations cause one hair cell to move back and forth, its associated fiber moves all three hair cell bundles in the same column, thus amplifying the signal.
- The stiffness of the fibers varies along the gradient of the cochlea. As the hair cell bundles increase in stiffness in the high-frequency range, so do the fibers increase in thickness. This enables the hair bundles along the entire length of the cochlea to tilt at the same angle when they detect a sound, thus initiating the electrical signal.
- The distance between the individual fibers changes along the cochlea, with fibers located closer together in the higher frequencies than in the lower frequencies. The larger the gap, the better the ear can distinguish between sounds of differing frequencies, since the movement of one fiber is less likely to cause a neighboring fiber—and its associated hair cells—to move. This finding helps explain why we are better able to distinguish between adjacent pitches in the low-frequency range than in the high-frequency range.
