Scientists from the Center for Hearing and Balance at Johns Hopkins have discovered how tiny cells in the inner ear change sound into an electrical signal the brain can understand.
Their finding, published in a recent issue of Nature Neuroscience, could improve the design and programming of hearing aids and cochlear implants by filling in a "black hole" in scientists' understanding of how we hear, the researchers say.
"Sound itself is mechanical, a wave that moves, just like the ripples fanning out from a pebble dropped in a lake," says Paul Fuchs, professor of otolaryngology at the School of Medicine. "When the inner ear detects this wave, a burst of chemicals is released and a nerve sends an electrical signal to the brain that carries information about the original sound. But the nature of the chemical burst has been a mystery until now."
With the help of powerful microscopes, the scientists studied individual cells from rat cochleas, tiny coiled structures deep inside the ear where sound is translated into electricity, the language of the brain. Fuchs and research associate Elisabeth Glowatzki discovered that these so-called "hair cells," named for tiny projections that stick up like a spiky haircut, release a barrage of chemical packets to an adjacent nerve in response to sound.
The finding was unexpected, Fuchs says, because hair cells were thought previously only to communicate to nerves by sending a single packet of these chemical transmitters at a time.
"Most cells in the brain normally move one packet to their edges, releasing a single dollop of transmitter that travels the short distance to the nerve," he says. "But hair cells deliver a dramatic burst of packets."
The scientists suggest that this means of communication with nerves may help hair cells carefully control the signals they send. "Hearing requires smooth signaling to accurately detect and distinguish a wide range of sound frequency (pitch) and intensity (volume)," Fuchs says.
"Nerves connecting to other cells have to collect the chemical messengers for a while before they will send an electrical signal to the brain; those nerves have to reach a threshold level of stimulation. And once the signal is sent, the nerve is quiet again," Fuchs adds. "But for hair cells, their continual pumping of messengers toward the nerve may be a kind of fail-safe device that ensures a ready supply of transmitters should the sound continue or change."
Hearing aids and cochlear implants are designed to boost or replace the sound-detecting function of hair cells in the cochlea. Fuchs and Glowatzki believe their discovery might help improve the range or accuracy of hearing aids and cochlear implants, they say.
The studies were funded by the U.S. National Institute on Deafness and Other Communication Disorders, one of the National Institutes of Health.