Human hearing is extraordinary in its technical specifications. We can perceive frequencies as great as 20 kHz and discriminate between different tones with a precision of 0.2%. At the acoustical threshold, the inner ear responds to vibrations of only ± 0.3 nm, an atomic dimension. Finally, our auditory system can register sound-pressure levels from 0 dB to 120 dB, representing a millionfold range in amplitude and a trillionfold range in power.
The defining feature of a hair cell is its mechanoreceptive organelle, the hair bundle. Extending less than 1 mm to more than 100 mm from the flattened apical surface, the bundle comprises from a dozen to over 300 cylindrical protrusions called stereocilia. Each stereocilium consists of a core of parallel actin filaments that are cross-linked into a rigid fascicle. The streocilia are not of equal size, but display a monotonic decrease in length from one edge to another so that the bundle's top edge is beveled like a hypodermic needle. When the top of a hair bundle is displaced during stimulation, the adjacent stereocilia shear with respect to one another. This movement is communicated to a tip link, a fine filament comprising four cadherin molecules that extends from the tip of each short stereocilium to the side of the longest adjacent one. Each tip link probably contacts a pair of mechanically sensitive ion channels whose opening initiates the hair cell's electrical response.
