The cochlea, that spiral-shaped structure in the inner ear, is filled with fluid.  In this fluid, tiny hair cells called stereocilia are positioned in bundles along the length of the structure.  These bundles sense vibrations transmitted into the fluid from the bony levers of the inner ear.  The vibrations picked up by the hair cell bundles, each tuned to its own frequency, mechanically transduce the sound impulses by opening ion channels that set up electrical impulses in the auditory nerve, that travel to the brain.  But motion in fluid creates friction known as viscous drag.  How do the hair cell bundles overcome it?  Scientists have figured out that the hair cells in the bundles are not only finely tuned to reduce viscous drag, but actually to employ it for even higher sensitivity to sound.

Publishing in Nature,1 scientists from Howard Hughes Medical Institute, with help from European academies, explained the problem of viscous drag, and two ways the ear deals with it:

The detection of sound begins when energy derived from an acoustic stimulus deflects the hair bundles on top of hair cells.  As hair bundles move, the viscous friction between stereocilia and the surrounding liquid poses a fundamental physical challenge to the ear’s high sensitivity and sharp frequency selectivity. Part of the solution to this problem lies in the active process that uses energy  for frequency-selective sound amplification.  Here we demonstrate that a complementary part of the solution involves the fluid-structure interaction between the liquid within the hair bundle and the stereocilia.

What they found is that the positioning of the individual stereocilia causes them to move in concert, so that viscous drag within the bundle is dramatically reduced: “We find that the close apposition of stereocilia effectively immobilizes the liquid between them, which reduces the drag and suppresses the relative squeezing but not the sliding mode of stereociliary motion.”  They can thus slide as the bundle bends without stirring up the liquid.  Further, “The obliquely oriented tip links couple the mechanotransduction channels to this least dissipative coherent mode, whereas the elastic horizontal top connectors that stabilize the structure further reduce the drag.”  The relative motion is reduced to just a fraction of a billionth of a meter (nanometer).

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