Do Microfluid Pumps Give Humans Their Sensitive Hearing?

New images of movements inside the cochlea, the part of the inner ear responsible for auditory function, suggest that the incredible sensitivity of mammalian hearing may be the result of hair cells that act as electromechanical fluid pumps. Arranged in a spiral structure known as the organ of Corti, the cochlea's outer hair cells exhibit voltage changes in response to sound, and change their length in response to an electrical voltage. At the Acoustical Society of America in Austin earlier this month, researchers (David Mountain, Boston University, and Domenica Karavitaki, now at Harvard Medical School) presented visual evidence of contracting hair cells pushing fluid back and forth. The fluid traveled through a tiny channel in the sensory organ known as the tunnel of Corti. According to theoretical calculations by Mountain and colleagues, hearing sensitivity is increased 100-fold if this fluid flow is properly synchronized with sound-induced motions in the cochlea. To image small but very rapid vibrations in the cochlea, Karavitaki used stroboscopic illumination flashing at rates 10,000 times a second to "freeze" the motion of the cells. This visual evidence of outer hair cells acting as electromechanical fluid pumps supports the researchers' theory of cochlear function, which states that an increase in hearing sensitivity cannot take place without fluid flow through the tunnel of Corti. Among all vertebrates, only mammals have a tunnel of Corti, and only mammal ears have hair cells that change their lengths in response to an electrical voltage. (Paper 4pABa1 at meeting; lay-language paper with diagrams and movies)