Inner-hair-cell hair bundles (IHBs) are the sensory organelles required for mammalian hearing. Within the ear, IHBs convert sound-induced forces into receptor currents, a key step in the transmission of sound signals to the brain. An IHB comprises tens of stereocilia, filaments emanating from the sensory-cell, which deflect only a few 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 mechanical fluctuations, which cause fluctuations in the receptor current. These fluctuations compete with the sound signal driving firing of the auditory neurons, limiting the threshold of hearing. In our recently published paper, we use an experimentally constrained mathematical model to determine how the IHB responds to thermal-scale noise. Our mathematical model relates the IHB’s mechanical properties to its stereocilium fluctuations. The model parameters are based on published experimental observations. We find that, to overcome deflection fluctuations, sound-induced deflections are <1.5 nm at threshold. We also find that increasing the stiffness of links between stereocilia decreases the fluctuation power 40 fold, which we expect to decrease the threshold of hearing by 16 dB SPL. Intriguingly, eliminating the stiff links causes negative correlations between neighboring stereocilia. These and other predictions of the mathematical model are experimentally testable.
Marrocchio et al. (2026) studied this question.