Currently, the function of hair cells can be replaced by using cochlear implants. However, research into the function of hair cells may someday allow them to be repaired. It has been suspected since 1851 that hair cells are responsible for translating sound into electric signals that nerves can convey to the brain. But only in the past 30 years have researchers determined how hair cells accomplish this remarkable feat. The latest and most far reaching research on the physiology of hair cells was performed by A. James Hudspeth, now at Rockefeller University, and his colleagues, who initially studied the hearing system of frogs, which have hair cells very similar to those found in the mammalian cochlea. Beginning in 1977, in an exquisitely detailed series of experiments, Hudspeth was able to isolate individual hair cells and penetrate them with minuscule glass electrodes. Hudspeth and his colleagues used the electrodes to record the electric activity within the hair cells as he gently pushed against their stereocilia with a small, precisely controlled probe. They discovered that it does not take much of a push on the stereocilia to get the hair cell to respond. All it takes is a movement of just 100 picometers, 100 trillionths of a meter--a distance smaller than the diameter of some atoms.

Hair cells, like all excitable nerve cells, are tiny batteries, with an excess of negatively charged ions inside and an excess of positively charged ions outside. Moving the stereocilia causes tiny pores on the stereocilia to open, allowing positive ions to rush into the cell, which causes "depolarization." Through a series of biochemical steps, this depolarization causes the hair cell to release neurotransmitter molecules--chemicals that transmit the electric signal from one nerve to another--that drift across a small space to receptors on nerve cells. Contact with the receptors depolarizes nerve fibers and starts an electric signal moving down the auditory nerve toward the brain.