Cochlea

The cochlea is the part of the inner ear that coils in a spiral, like a snail shell, and the organ of Corti lives inside it. That organ runs the full length of the spiral and rests on the basilar membrane, a sheet that runs inside the snail shell.

When a sound arrives, the pressure sends a wave traveling along that membrane. The detail that explains everything is where that wave rises highest: the point of peak displacement depends on the frequency of the sound. Each pitch moves one specific point on the membrane the most. That is how the ear tells a low note from a high one.

The next step belongs to the stereocilia, tiny stiff hairs. As the membrane moves, those hairs bend toward the tallest one; bending tightens the very fine links that join them at their tips; and that tension opens ion channels, gates that let ions in. Right there, exactly there, the mechanical energy of sound becomes electrical activity.