JHU Med 2002-42 (how we hear)

[Dennis O'Shea]

Johns Hopkins Medical Institutions
Office of Communications and Public Affairs
Media Contact: Joanna Downer
410-614-5105
E-mail: jdowner1@jhmi.edu
May 2, 2002
  			
FOR IMMEDIATE RELEASE

HOPKINS SCIENTISTS REVEAL HOW SOUND BECOMES ELECTRIC

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, say 
the researchers.

"Sound itself is mechanical, a wave that moves, just like the ripples 
fanning out from a pebble dropped in a lake," says Paul Fuchs, Ph.D., 
professor of otolaryngology at the Johns Hopkins 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 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 awhile 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," adds Fuchs. "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.

                                                                  --JHMI--

Nature Neuroscience February 2002: 5 (2); 147-154.

On the Web:
http://www.nature.com/neuro

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