Relating the ear's minuscule amplifiers to their environment

Hair cells in the auditory, vestibular, and lateral-line systems of
vertebrates receive inputs through a remarkable variety of accessory
structures that impose complex mechanical loads on the
mechanoreceptive hair bundles. Although the physiological and
morphological properties of the hair bundles in each organ are
specialized for detecting the relevant inputs, we propose that the
mechanical load on the bundles also adjusts their responsiveness to
external signals. We use a parsimonious description of active
hair-bundle motility to show how the mechanical environment can
regulate a bundle’s innate behavior and response to input. We find
that an unloaded hair bundle can behave very differently from one
subjected to a mechanical load. Depending on how it is loaded, a hair
bundle can function as a switch, active oscillator, quiescent
resonator, or low-pass filter. Moreover, a bundle displays a sharply
tuned, nonlinear, and sensitive response for some loading conditions
and an untuned or weakly tuned, linear, and insensitive response under
other circumstances. Our simple characterization of active hair-bundle
motility explains qualitatively most of the observed features of
bundle motion from different organs and organisms. The predictions
stemming from this description provide insight into the operation of
hair bundles in a variety of contexts.

Reference:

The diverse effects of mechanical loading on active hair bundles, D.
Ó'Maoiléidigh, E.M. Nicola and A.J. Hudspeth; Proc. Natl. Acad. Sci.
USA, 109: 1943-1948 (2012).