Inner Ear Hair Cells Overcome Friction

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).

Their opening paragraph provides a picturesque view of the workings of this remarkable organ:

A hair bundle is a microscopic array of quasi-rigid, cylindrical stereocilia separated by small gaps filled with viscous endolymph. Like an array of organ pipes, the stereocilia vary monotonically in length across the hair bundle…. The tip of each short stereocilium is attached to the side of the longest adjacent stereocilium by a tip link, the tension in which controls the opening and closing of transduction channels. Adjacent stereocilia are also interconnected along all three hexagonal axes by horizontal top connectors. At the tall edge of the bundle in many species stands a single kinocilium, the process to which mechanical stimuli are applied and that is ligated to the adjacent stereocilia by kinociliary links.

Why are the stereocilia arranged in bundles?  “The small difference between the drag coefficients for a single stereocilium and for an entire hair bundle reveals the striking advantage that grouping stereocilia in a tightly packed array offers to the auditory system.”  Using models from a bullfrog inner ear magnified 12,000 times and various mathematical techniques, they were able to measure the viscous drag of the coherently-arranged stereocilia.  Their conclusion explains how the findings contribute to understanding the remarkable sensitivity of the ear:

In conclusion, because all stereocilia and the liquid between them move in unison over the whole auditory spectrum, with the relative motions apparent only on a sub-nanometre scale, most stereocilia inside the hair bundle are shielded from the external liquid and experience little viscous drag. Although viscous forces might be thought to impair sensitivity and frequency selectivity, the hair bundle’s structure actually minimizes energy dissipation, making it easier for the active process to keep the ear tuned. The tight clustering of stereocilia even transforms liquid viscosity into an asset by using it as a simple means of activating numerous mechanosensitive ion channels in concert.

The authors made no attempt to explain how this arrangement might have evolved. 

1. Koslov, Baumgart et al., “Forces between clustered stereocilia minimize friction in the ear on a subnanometre scale,” Nature  474   (16 June 2011), pp. 376–379, doi:10.1038/nature10073.

The human ear has an extraordinarily large sensitivity range of a trillion to one, allowing us to hear a rocket launch or the footfalls of a cat on a carpet.  According to Werner Gitt, the ear is our highest-precision sense organ, capable of responding over twelve orders of magnitude without switching (The Wonder of Man, p.21).  Some of this sensitivity is amplified by the eardrum and middle ear ossicles, but the paper reported above shows even more fine-tuning inside the cochlea.  Gitt's book is highly recommended for generating a profound feeling of awe over the design of our senses.  Proverbs said, “The seeing eye, and the hearing ear, the Lord has made them both” (Prov. 20:12).