A New Twist to the Cochlea: Why It's Shaped the Way
It Is
Editor: Good question! WHY is the cochlea shaped like a snail shell?
This NIDCD (National Institute on Deafness and Other Communication
Disorders) press release provides a surprising answer.
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The mammalian cochlea is spiral-shaped, like a snail shell.
For years, scientists have believed that the distinctive snail shape
of the mammalian cochlea - in contrast to the stretched-out versions
found in birds or reptiles - is useful for packing a slew of hearing
parts into a very small space. Now, Dr. Daphne Manoussaki, Vanderbilt
University, together with NIH researchers Dr. Richard Chadwick, NIDCD,
and Dr. Emilios K. Dimitriadis, Office of Research Services, have
demonstrated another benefit to its tightly-wound shape: the better to
hear low-pitched (or low-frequency) sounds. Their paper is published in
the March 2, 2006, online issue of Physical Review Letters.
Though the cochlea looks fairly simple, it's surprisingly complex. A
fluid-filled organ with an elastic "basilar" membrane running
down its length, the cochlea is responsible for converting sound energy
into electrical signals that travel to the brain. It accomplishes this
in two stages. Sound vibrations enter the cochlea by way of the bones of
the middle ear, causing the fluid inside to ripple and a traveling wave
to form along the basilar membrane. Hair cells, sensory cells sitting
atop the basilar membrane, "ride the wave," bending against an
overlying membrane and generating an electrical signal. Because the
basilar membrane is fine-tuned to respond to certain frequencies at
specific positions, hair cells near the base, or large end, of the
cochlea detect higher-pitched sounds, such as a cell phone ringing, and
those nearer the apex, or centermost point, detect lower-pitched sounds,
such as a kettle drum or the call of a blue whale.
Starting with a simple model of a spiral, the research team
calculated how the cochlea's curve affects the movement of sound energy
inside the inner ear. Like the Whispering Gallery in St. Paul's
Cathedral in London, whereby a whispered voice travels along the curved
wall to be heard on the opposite side of the room, the researchers found
that sound energy preferentially travels along the outer wall of the
cochlea. As this occurs, the fluid sloshes higher on one side, the
basilar membrane wave tilts more to one side, and the hair cells, which
are sensitive to motion at right angles to the direction of the wave,
get an added push, causing them to bend. The effect is strongest at the
center of the spiral, where the lowest frequencies are detected, said
the researchers, who calculated an increase in sensitivity by as much as
20 decibels. The ability to hear low-frequency sounds, which travel the
farthest of all sounds, is especially important to mammals for
communication and survival.
