August 9, 2004 | David F. Coppedge

Inner Ear Hairs Provide Optimum Sensitivity

The inner ear cochlea is lined with hair cells that transduce mechanical vibrations into electrical signals for the auditory nerve.  European scientists publishing in PNAS1 measured the sensitivity of inner ear hair cells to mechanical motion, and considering the noise caused by thermal motion, calculated that the ear operates at the optimum level. 

The ear relies on nonlinear amplification to enhance its sensitivity and frequency selectivity to oscillatory mechanical stimuli…. We find that the magnitude of the fluctuations resulting from the active processes that mediate mechanical amplification remains just below that of thermal fluctuations.  Fluctuations destroy the phase coherence of spontaneous oscillations and restrict the bundle’s sensitivity as well as frequency selectivity to small oscillatory stimuli.  We show, however, that a hair bundle studied experimentally operates near an optimum of mechanosensitivity in our state diagram.

1Nadrowski et al., “Active hair-bundle motility harnesses noise to operate near an optimum of mechanosensitivity,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0403020101, publ. online 8/9/04.

Was this optimum found by trial and error?  Did all the individuals below optimum die until perfection was reached?  Imagine a sound engineer designing the perfectly sensitive receiver electronically.  Now imagine him making it work throughout a double-flipping dive into an Olympic pool without an interruption of response or sensitivity.

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Categories: Amazing Facts, Human Body

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