Inside the uncoiled cochlea runs the basilar membrane, narrow and stiff at the base where sound enters and wider and floppier toward the apex.The cochlea is a bony tube coiled like a snail shell; uncoiled it runs 33-35 mm. Inside, the basilar membrane runs its length — and it is not uniform: narrow and stiff at the base (where sound enters), wide and floppy toward the apex.
2 · Where the traveling wave peaks
Sound sends a traveling wave along the basilar membrane that peaks at the base for high frequencies and at the apex for low ones, turning frequency into place.When sound arrives, a traveling wave runs from base to apex, growing and slowing until it peaks at one position. High frequencies peak at the base, low at the apex. Frequency is mapped to place — this is tonotopy.
3 · Outer hair cells amplify actively
Prestin in outer hair cells makes the cells change length with membrane potential, actively amplifying vibration and raising mammalian sensitivity by more than 40 dB.The passive gradient only sketches the map — resolution is not sharp enough. Prestin in outer hair cells changes cell length with membrane potential, actively amplifying vibration, boosting sensitivity by 40+ dB in mammals (about 100x sound pressure, not 100x loudness).
4 · Damage maps to a location
Noise damage and age-related loss both start at the base of the cochlea, so high-frequency hearing goes first, and once outer hair cells die they are not regenerated.Remember this line: noise damage starts at the base (high frequencies), so high-frequency hearing goes first; age-related loss also starts high. Trouble with consonants or speech in noise maps back to this line. Once outer hair cells die, mammals cannot regenerate them.