Stiff Links Are Required for High Positive Correlations between Stereocilium Deflections in the Ear's Mechanosensory Cells

Inner-hair-cell hair bundles (IHBs) are the sensory organelles required for mammalian hearing. An IHB comprises tens of stereocilia, filaments emanating from the sensory-cell surface, which deflect by only nanometers in response to piconewton sound-induced forces. Stereocilium deflections promote ion-channel opening and closing, causing receptor currents. This process is limited by thermal-scale fluctuations in the stereocilium deflections, which cause noise in the receptor current. This noise limits the threshold of hearing.

We propose that stiff links between the tallest stereocilia promote positive correlations between its fluctuating stereocilium deflections. To test our proposal, we use a mathematical model that relates the IHB’s mechanical properties to its stereocilium fluctuations. In the absence of stiff links, stereocilium deflections are negatively or weakly correlated over the range of physiological stimulus frequencies. Adding stiff links makes the correlations positive and increasing the link stiffness increases the correlations. Our results suggest that stiff links between stereocilia decrease noise in the receptor current, decreasing the threshold of hearing.