Among biosensors, the ear
rates highly as an exquisitely sensitive and sophisticated device,
arguably on a par with the eye. The human ear is capable of responding
to frequencies from about 20 Hz to 20 kHz with fine pitch discrimination
and is sensitive to the slightest whisper. Much regarding how the
ear itself actually works, however, remains unknown. NIDCD
researchers Richard Chadwick
and Daphne Manoussaki are trying
to better understand the workings of the cochlea (inner ear). Although
their research is not directly related to a NASA project, it's a
good example of the marriage of the physical and biological sciences
NASA director Daniel Goldin advocates (see story, page
1) and represents a step toward understanding how sophisticated
sensing systems function.
Left: The cochlear partition, showing the path taken by
the basilar membrane through the interior of the cochlea.
A wave is shown traveling through a portion of the basilar
membrane, at bottom. Right: The arrangement of the basilar
membrane (BM), outer hair cells (OHC), inner hair cells (IHC)
and tectorial membrane (TM).
Chadwick, acting chief of the auditory mechanics section in the
Laboratory of Cellular Biology, explains that his section's research
aims to "understand the mechanical and electrical properties
of hearing." Manoussaki, whose background is in applied mathematics,
joined the section last year as a fellow and has focused on producing
a computational model of the mechanical properties of the cochleaa
Daphne Manoussaki and Richard Chadwick
The cochlea is a coiled, fluid-filled structure that receives
its input from a tiny bone called the stapes, or "stirrup."
Vibrations of the eardrum are transmitted through the ossicles
in the middle earthrough the "hammer" and "anvil"
to the stirrup, which strikes the membranous "oval window"
of the cochlea, sending waves through its fluid interior. Winding
through the cochlear interior is the "organ of Corti,"
consisting of the basilar membrane, atop which sit the outer and
inner hair cells, bounded at their apical surfaces by the tectorial
membrane. Vibrations of the basilar membrane cause the hair cells
to deform against the tectorial membrane, stimulating the neurons
that innervate the hair cells.
"There are many basic, interesting things about normal hearing
that are puzzling, things we just don't understand," says
Chadwick. For example, "The transduction process should be
too slowit should take time to charge up the cell membranessomehow,
there's something there that bypasses that." Another curious
phenomenon he points to is that of "otoacoustic emissions"sounds
produced by the ear itself in response to an acoustic stimulus.
He notes that such events are not rare, though typically not noticed,
and that detection of these sounds is used clinically to assess
normal auditory function in infants.
A more obvious feature of the cochlea is its geometry, something
that initially caught the eye of Manoussaki. "When I came
here, I didn't know much about the cochlea, and the first thing
that intrigued me was the shape," she says. She explains
that prevailing ideas concerning the cochlea's coiled shape pointed
to more efficient use of space, or to a presumed benefit related
to the innervation pattern it produced. However, no one had adequately
dealt with the effect of coiling on wave propagation within the
cochlea; most attempts made by others to model the inner ear simply
treated it as a long rectangular tube. But Manoussaki "just
couldn't believe" the coiled shape would not contribute critical
properties to the cochlea. After all, she observed, water flowing
through a coiled hose will force it to uncoil, a result that would
not be produced in a straight hose.
To test her idea, Manoussaki produced a coiled model of the cochlea,
the complex geometry of which, Chadwick suggests, may account
for others' preference to treat the system in simpler terms. Their
findings, presented in a poster at the Research Festival, suggest
that coiling amplifies traveling waves in the cochlea. Manoussaki
is working to achieve a more detailed, realistic physical model
of the cochlea to better discern what happens as sound waves move
through its interior. Chadwick observes that even cochlear implant
technology, which bypasses cochlear mechanics to stimulate auditory
nerve branches, would benefit from a finer understanding of normal
For more information on hearing, see the NIDCD
web site: <
http://www.nih.gov/nidcd/health/hb.htm>; info on
ear anatomy and hearing is also at <http://www.earaces.com/>.