In mammalian hearing, outer hair cells within the tunnel of Corti (a spiral structure within the inner ear) are stimulated by sound vibrations. These hair cells convert vibrations into nerve impulses that are transmitted by the inner ear to the brain. Known to exhibit voltage changes in response to acoustic stimulation, these cells also exhibit length changes in response to changes in membrane voltage.
Until now, however, scientists have not understood how such structures can increase the ability to hear faint sounds. The reason, according to David Mountain, professor of biomedical engineering at Boston University (Boston, MA, USA; www.bu.edu), is that when the outer hair cells contract, they push fluid back and forth through a channel in the tunnel of Corti. If this fluid flow is synchronized with sound-induced motions in the cochlea, hearing sensitivity is increased 100-fold.
To show this effect, Mountain and his colleagues used a number of off-the-shelf components to image the small but rapid vibrations present in the cochlea. By using stroboscopic illumination at rates up to 10,000 times a second, images were captured with a computer-controlled video system mounted on a microscope. At the same time, the outer hair cells were electrically stimulated to change their membrane voltages at frequencies that span the hearing range. By changing the time delay between the strobe flashes and the electrical stimulus, a series of images were captured that were later animated on a computer. This animation produces a slow-motion video of vibrations that take place at a rate of hundreds to thousands of times per second.
To image the movement within the ear, a high-intensity LED is coupled to a custom-made current source. Mounted on a holder designed to replace the light source of the microscope, the LED is driven by input pulses generated by a Tucker-Davis Technologies (Alachua, FL, USA; www.tdt.com) data-acquisition system. These pulses occur at fixed phases within the period of the stimulus with duration equal to 10% of the period.
Mounted in a custom-made chamber, the test setup was placed on the stage of a BX50WI upright microscope from Olympus America (Melville, NY, USA; www.olympusamerica.com) sitting on a vibration-isolation table. A 20x (Olympus 20x, 0.5 NA) or a 60x (Olympus 60x, 0.9 NA) water-immersion lens with an additional 2x magnification is used for detailed observation of the regions of interest. To capture images from the microscope, a C2400-77 CCD camera from Hamamatsu (Bridgewater, NJ, USA; www.hps-systems.com) is mounted on the phototube of the microscope.
Analog contrast enhancement and brightness enhancement were accomplished by using an Argus-20 image processor, also from Hamamatsu. The output of the image processor is connected to an AG-5, an externally triggered PC-based frame grabber from Scion (Frederick, MD, USA www.scioncorp.com) real-time frame capture and averaging. According to Mountain, most of the software was written in Matlab from The MathWorks (Natick, MA, USA; www.mathworks.com) and Microsoft C.
Captured images of the outer hair cells showed that the first and third rows of outer hair cells moved apart when the cells contracted. This suggests that hair-cell contractions cause a pressure increase within the tunnel of Corti that causes the cells to spread apart in the radial dimension. Longitudinal displacements of nerve fibers that traverse the tunnel of Corti were synchronized with the outer hair cell contractions.
Such displacements were also related to the longitudinal movement of fluid within the tunnel. This motion of the fibers is observed in regions several millimeters away from the region where the outer hair cells are contracting. When the outer hair cells contract, they squeeze the tunnel of Corti and pump fluid into the tunnel. As a result of outer hair-cell contractions, fluid flows in the tunnel of Corti leading to increased hearing sensitivity . — Andrew Wilson
