Tuned hair cells for hearing, but tuned basilar membrane
for overload protection: evidence from dolphins, bats, and desert rodents
Martin Braun
Hear Res 1994, Jul 78:1, 98-114
Abstract
A cochlear model is presented suggesting that the organ of Corti (OC)
and the basilar membrane (BM) are both tuned resonant systems, but have
different functions. The OC provides frequency filtering and amplification
by means of tuned outer hair cells. The BM provides resonant absorption
of excessive vibrational energy as an overload protection for vulnerable
elements in the OC. Evidence supporting this model is demonstrated in
dolphins, bats, and desert rodents. Specialized auditory capabilities
correlate with cochlear deviations, some of them dramatically changing
BM compliance. In characteristic regions along the cochlea there are BM
thickenings and, on both sides of the OC, hypertrophied supporting cells.
Structures of striking similarity have evolved independently across orders
or families, revealing multiple events of convergent evolution. In all
cases, the locations of deviating structures rule out a BM function in
auditory frequency selectivity but support one in resonant absorption.
Cochlear microphonics and BM responses demonstrate strongest high-level
absorption in the frequency bands most vital for the tested species. The
assumed cause is increased internal damping in the enlarged structures
during BM motion. Species with intermediate specializations supply further
evidence that resonant absorption is universally the genuine function
of BM mechanics in mammals, providing complementary high-level protection
of low-level sensitivity.
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