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The newspaper of The Johns Hopkins University February 7, 2005 | Vol. 34 No. 21
Study: Hearing's 'Off-Switch' Borrowed from Muscles

By Joanna Downer
Johns Hopkins Medicine

In experiments with rats, Johns Hopkins researchers have discovered that the shut-off switch for the auditory system is quite similar to an "on" switch previously known principally in muscle. The findings appear in the Dec. 8 issue of the Journal of Neuroscience.

Calcium was already known to be a key factor in both systems. It helps trigger contraction of muscles, and it helps the brain rapidly shut down the ear's sound-detecting "hair cells," named for the spiky projections that help translate sound waves into electricity. However, the amount of calcium coming into hair cells in response to the brain's signals has always seemed too small to do the trick.

Now, the Hopkins researchers report that the small influx of calcium triggered by the "shut-off" nerve causes a flood of calcium to be released from a reservoir sitting just inside the hair cell. That flood, in turn, quiets the hair cell by stimulating its release of potassium.

"We've known the response to the nerve is practically immediate, and people had suggested that perhaps the cells had a reservoir of calcium that made it possible," said Paul Fuchs, professor of otolaryngology — head and neck surgery, of neuroscience and of biomedical engineering and director of the Cochlear Neurotransmission Laboratory at the Center for Hearing and Balance at Johns Hopkins. "Our evidence strongly supports this idea and shows how it happens. The similarities to the situation in muscle is striking."

In both systems, the reservoir of calcium is just inside the cell and extends along the cell's connection with the nerve, maximizing the impact of the incoming calcium. Moreover, the researchers discovered that the flood gates are so-called ryanodine receptors, which are triggered to open by nicotinic receptors in hair cells as in muscle.

Fuchs says that in the future it could conceivably be possible to manipulate the influx of calcium or the release of calcium from the hair cells' reservoir to try to relieve hypersensitivity to sound or tinnitus.

The research was funded by the National Institute for Deafness and Communication Disorders.


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