How the cochlea sorts sound
Inside the cochlea, a curled, fluid-filled structure in the inner ear, sound is separated into different pitches before the brain interprets it. A flexible strip called the basilar membrane runs through the cochlea. Its base is narrow and stiff, so it moves most in response to high pitches; farther along, it is wider and more flexible, responding best to lower pitches. This creates a physical map of frequency: different places move most for different notes.
From movement to nerve signals
Sensory cells called hair cells sit along that membrane. Each has a bundle of tiny projections, or stereocilia. When the membrane moves, the surrounding fluid shifts and bends the bundles. That bend opens microscopic channels in the hair cell, allowing charged particles to flow in and changing the cell’s electrical state. The cell then releases chemical messengers that activate fibers of the auditory nerve.
The nerve does not simply send a miniature copy of the sound wave. It conveys patterns: which fibers are active, how strongly they respond, and when their signals occur. The brain uses those patterns to distinguish pitch, loudness, and timing.
A built-in amplifier
Other hair cells help sharpen the process. These outer hair cells change length as their electrical state changes, adding a small amount of motion back into the cochlea. That boost makes faint sounds easier to detect and helps tune the response to nearby pitches. The inner hair cells do most of the signaling to the auditory nerve.
Because this system depends on delicate cells and precisely controlled motion, damage can make particular pitches harder to hear or blur the detail needed to distinguish sounds. The cochlea’s first contribution to hearing is not merely detecting vibration: it is sorting and encoding it before the brain gets the message.
