In the 11 May issue of Nature , neurobiologists Peter Dallos, Jing Zheng, and their team at Northwestern University in Evanston, Illinois, reported that they had isolated the protein, which they dubbed prestin, that gives the outer hair cells their unique ability to contract. Score one for electromotility. Every hair cell, however, also wears a crown of stiff fibers called stereocilia, which tip to one side as vibrations in the basilar membrane push it up against the overriding tectorial membrane. In response, trapdoorlike ion channels open in the stereocilia and let in potassium and calcium ions. This is the mechanism that converts vibrations into the chemical signals that fire nerves.
Some researchers think it also amplifies the vibrations.
The calcium ions bind to the channels, snapping them shut, and some believe this pulls the stereocilia in the opposite direction, causing them to push the tectorial membrane and amplify the motion of the basilar membrane. Recently, neuroscientist Jim Hudspeth, physicist Marcelo Magnasco, and their colleagues at The Rockefeller University in New York City developed a mathematical model of such a trapdoor amplifier. In the 29 May issue of Physical Review Letters , they argue that a single property of the model can explain some puzzling characteristics of hearing, such as why the ear registers soft tones and pitches more effectively than loud ones. The wrangling over how the ear amplifies sound goes back more than 50 years. At that time, scientists agreed that the cochlea could only respond passively to incoming sounds, much as the strings of a piano will ring if you shout into the guts of the instrument.