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March 2005 Vol 6 No 3 HIGHLIGHTS
Nature Reviews Neuroscience 6, 171 (2005); doi:10.1038/nrn1632
[545K]
AUDITORY SYSTEM
A moving experience in the cochlea
Rachel Jones
The ongoing debate about the mechanism by which the mammalian inner ear
amplifies incoming sounds now sees the publication of new evidence in
favour of a mechanism driven by an influx of calcium ions into the outer hair
cells. The study uses a new type of in vitro cochlear preparation that allows
researchers to get closer to the process than ever before.
Writing in Nature Neuroscience, Chan and
Hudspeth describe how they removed part of the
cochlea of the clawed jird (also known as the
Mongolian gerbil) and mounted it in a recording
chamber that had two compartments. The
compartments were designed to reproduce the
conditions found in the inner ear, in which the
apical side of the sensory epithelium is bathed in
endolymph -- a fluid that is rich in potassium
ions -- while the basal side is bathed in sodium-
rich perilymph. This set-up allowed the authors to record the responses of
the cochlea to sound stimulation for up to 30 minutes.
The amplification process in the inner ear is an active, frequency-specific
process that optimizes the ear's response to incoming sounds. A
characteristic feature of the active process is compressive nonlinearity -- a
large range of stimulus intensities is compressed into a relatively narrow
range of responses. The active process is also responsible for 'spontaneous
otoacoustic emission' -- sounds that are produced by the active movement
of the inner hair cells and that can be recorded from the ear canal.
There are two theories about how the active process occurs. The first is
active hair-bundle motility, and relies on a calcium current entering the cell
during mechanoelectrical transduction. The calcium current is thought to
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stimulate the closure of transduction channels and the activity of myosin
motors, both of which could contribute to the amplification of the mechanical
stimulation that is produced by sounds. The second theory is known as
somatic electromotility. This is also initiated by electrical activity, but relies
on conformational changes in a protein called prestin.
Chan and Hudspeth set out to test the first theory. They blocked the
mechanoelectrical transduction channels with amiloride, so that calcium ions
could not enter the cells, and found that this abolished the nonlinear
amplification process. By contrast, when the ionic contents of the artificial
endolymph were manipulated so that currents other than the calcium current
were abolished, the nonlinear amplification persisted. These results provide
evidence that calcium influx is both necessary and sufficient for the active
process.
The debate will no doubt continue, with proponents of both theories
continuing to test their ideas. It might be that both mechanisms are
involved, perhaps at different frequencies. But the in vitro preparation used
by Chan and Hudspeth should be a useful tool in the investigation.
References and links
ORIGINAL RESEARCH PAPER
Chan, D. K. & Hudspeth, A. J. Ca2+ current-driven nonlinear amplification by
the mammalian cochlea in vitro. Nature Neurosci. 8, 149-155
(2005) | Article | PubMed | ChemPort |
WEB SITE
Hudspeth Laboratory web page:
http://www.rockefeller.edu/labheads/hudspeth/hudspeth-lab.php
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 2005 Nature Publishing Group
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