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A Protein That Helps Turn Sound into Sense
Scientists say they may have uncovered a key player in how the body turns sound
into sense — that is, how the vibrations called sound waves that pulsate
through the air are turned into the words, music and clamor that our brains sense.
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| Microscopic structures called stereocilia sit atop a
hair cell. Tip links (arrows) connect shorter stereocilia to their taller
neighbors. Reproduced with permission from Nature
Reviews Genetics 5, 489–98, copyright 2004 Macmillan Magazines
Ltd. |
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Researchers have long known that hair cells, small sensory cells in the inner ear,
convert sound energy into electrical signals that travel to the brain. The process
begins when a sound wave enters the ear, causing the eardrum and the three tiny
bones behind it — the hammer, anvil and stirrup — to vibrate. This
vibration creates pressure waves through the fluid of the inner ear. Hair cells
sitting on a membrane ride the waves, causing microscopic projections on the hair
cells called stereocilia to bump against an overlying membrane and deflect to the
side. This movement causes tiny channels on the stereocilia to open up, allowing
potassium molecules to rush in, initiating an electrical signal.
Key to this process at the molecular level are threadlike links that connect
the tips of shorter stereocilia to the sides of neighboring higher ones like
rope bridges. Scientists believe that these so-called "tip links" may be responsible
for opening and closing the channel gates.
In the June 28 issue of the Journal of Neuroscience, researchers report
that they have identified a protein called protocadherin-15 that’s an important
component of the tip links. Protocadherin-15 has previously been linked to a
form of Usher syndrome, the most common cause of deaf-blindness in humans.
The discovery, by an international team led by researchers from NIH’s National
Institute on Deafness and Other Communication Disorders and the University of
Sussex, bodes well for identifying and perhaps one day reversing an underlying
cause of some forms of hearing loss. While researchers still don’t know exactly
how protocadherin-15 works, Dr. James Battey, Director of NIDCD, says that this
finding brings us to “the closest point we have ever been to understanding the
mechanism by which the ear converts mechanical energy into a form of energy that
the brain can recognize as sound.”
This finding not only provides insight into how hearing takes place at the molecular
level, but it may help researchers figure out why some people temporarily lose
their hearing after being exposed to loud noise, only to regain it a day or two
later. Delicate tip links, when broken by noise, could cause temporary hearing
loss until the link re-establishes itself. The scientists hope that this discovery
will lead them to understand how broken tip links can be stimulated to re-form.
The researchers now plan to delve more deeply into the role that protocadherin-15
plays in the tip-link and to investigate what other components the protein interacts
with.
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