Hearing & the Cochlea

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Tinnitus
Reduced input from one frequency band of the cochlea is one of the most common triggers of tinnitus: the gap arises in the ear, while the sound itself is kept going by the brain. That is why hearing loss and tinnitus so often come together, even though the ringing is not in the ear.

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The cochlea spreads pitch along a line

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The cochlea lays sound out by pitch: high notes land at one end, low notes at the other, and each pitch has its own place along a single membrane.

Sound entering the inner ear goes into a bony tube coiled like a snail shell — the cochlea. Uncoiled, it is about 33 to 35 mm long, and that length varies noticeably between people. A membrane runs the length of the tube: the basilar membrane. It is not uniform. The end where sound comes in (the base) is narrow and stiff; toward the far end (the apex) it grows wider and softer.

When sound arrives, a wave runs along the membrane from base toward apex — a traveling wave — growing taller and slower until it peaks at one spot. High frequencies peak near the base, low frequencies near the apex. Frequency has been turned into place, which is what tonotopy means. The stiffness gradient alone would give only a blunt map; what sharpens it is a group of cells in the cochlea that actively lengthen and shorten.

The use of this line is location: every kind of damage described later in this story lands on some stretch of frequencies along it.

One thing unrelated to the map is worth knowing now: if one ear turns muffled and unclear over a few hours to a few days, see a doctor promptly. The chapter on sudden hearing loss, near the end, explains why.

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Educational content, not medical advice — consult a clinician.

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