4Cochlear mechanics & tonotopy

FTInside the cochlea

The cochlea is a fluid-filled tube coiled two-and-a-half times into the shape that gives it its name (Latin for snail). Along its length run three parallel compartments, the scalae: scala vestibuli and scala tympani (filled with perilymph, an ordinary low-potassium extracellular fluid) sandwich the scala media (filled with endolymph, an unusual high-potassium fluid actively generated by the stria vascularis).[2012]

Dividing the scalae is the basilar membrane, and on it sits the sensory organ of Corti (the next module). The basilar membrane is the mechanical star of this module: its physical properties change gradually along its length, and that gradient is what sorts sound by frequency. The stapes pushes on the oval window, the pressure travels through the fluid, and the basilar membrane moves in response.

TThe travelling wave

When the stapes drives the cochlear fluid, the basilar membrane does not move all at once. Instead a travelling wave sweeps along it from base to apex, growing in amplitude as it goes, reaching a peak at one place, and then dying away rapidly. Georg von Békésy first observed these waves directly in cadaver cochleae — work that won him the Nobel Prize — and showed that the place of the peak depends on the stimulus frequency.[1960]

The reason is a gradient of stiffness and mass. At the base the membrane is narrow and stiff, so it resonates to high frequencies; toward the apex it becomes wider and floppier, resonating to low frequencies. A useful analogy is a set of strings: short, tight strings sound high notes, long, slack ones sound low. Drag a tone along the cochlea below and watch its travelling-wave envelope find its place.

TCTonotopy — place codes frequency

This orderly arrangement — each frequency mapped to a characteristic place — is called tonotopy, and it is the single most important organising principle in all of auditory physiology. A complex sound, with energy at many frequencies, sets up a complex pattern of vibration with peaks at several places at once: the cochlea performs a rough mechanical spectral analysis, laying the spectrum out along its length. Every stage above the cochlea — the auditory nerve, the brainstem nuclei, the cortex — preserves this map.[2009]

CThe Greenwood map

The place-to-frequency relationship is not vague: Donald Greenwood reduced it to an equation that holds across species. For the human cochlea, the characteristic frequency at a given position follows F = 165.4 × (10^2.1x − 0.88), where x is the fractional distance from the apex (0 at the apex, 1 at the base). The function is the dashed scale under the membrane in the widget above; it is also the equation a cochlear-implant fitting system implicitly uses when it decides which frequencies to send to which electrode.[1990]

CSharp tuning needs an active cochlea

There is a catch in Békésy's story. The travelling waves he saw in dead cochleae were broadly tuned — each place responded to a wide range of frequencies. But the living cochlea is exquisitely sharply tuned, able to separate frequencies a few percent apart. The difference is that the living cochlea is active: the outer hair cells inject mechanical energy back into the travelling wave, sharpening and amplifying its peak. The passive mechanics set up the map; an active process makes it precise.[2012]

This matters clinically because the active process is fragile. When outer hair cells are damaged — noise, ageing, ototoxic drugs — tuning broadens and sensitivity falls together, the signature of most sensorineural hearing loss. The mechanism is the subject of Module 7.

FTTonotopy and the cochlear implant

Here is the pay-off for the whole atlas. Tonotopy is a property of the basilar membrane and the cochlea's layout, not of the hair cells — so it survives even when the hair cells are gone. A cochlear-implant electrode array, threaded into the cochlea from the base, lies along the tonotopic map. Stimulating a basal electrode produces a high-pitched percept; stimulating an apical one produces a lower pitch. The implant does not recreate the travelling wave — it exploits the place map the travelling wave would have used.[2009]

With the cochlea's mechanical frequency analysis in place, the next module looks at the structure that rides on the basilar membrane and turns its motion into a signal: the organ of Corti and its hair cells.