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
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
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
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.