What is The cochlea?

Sound is a pressure wave; the ear’s problem is to detect pressure changes of a few tens of micropascals in air and to sort them by frequency. The eardrum and the three middle-ear bones concentrate the force from the drum’s $55\,\mathrm{mm}^{2}$ onto the $3\,\mathrm{mm}^{2}$ of the oval window, raising the pressure some twentyfold — the impedance match between air and the fluid of the inner ear, without which most of the sound would reflect. Inside the cochlea, a coiled tube $35\,\mathrm{mm}$ long, the pressure wave travels along the basilar membrane, which is narrow and stiff at the base and wide and floppy at the apex; each frequency makes the membrane vibrate most at one place — high frequencies near the base, low near the apex — a tonotopic map that the brain reads as pitch. On the membrane sit the hair cells: $3500$ inner hair cells in a single row, each crowned with a bundle of stereocilia joined tip to tip by fine tip links; bending the bundle toward its tallest cilium stretches the links and pulls open cation channels within microseconds, without any second messenger, and potassium flows in from the endolymph, whose unusual composition and $+80\,\mathrm{mV}$ potential give a driving force of $150\,\mathrm{mV}$. The inner hair cells signal the auditory nerve; the $12\,000$ outer hair cells, driven by the same motion, contract and lengthen with the sound through the motor protein prestin and pump energy back into the membrane’s vibration — an amplifier that sharpens tuning a hundredfold and is what fails in most deafness. Intensity is measured in decibels: $L = 10\log_{10} (I/I_{0})$, with $I_{0} = 1 \times 10^{-12}\,\mathrm{W}/\mathrm{m}^{2}$ the threshold of hearing; the ear works over $120\,\mathrm{dB}$, a trillionfold in intensity.