Inside the Cochlea: How Tiny Hair Cells Turn Vibration Into Nerve Signals

Deep inside the skull, close to the side of the head, sits an organ about the size of a pea and shaped like a snail shell. The cochlea is where hearing stops being mechanical and becomes biological. Up to this point, sound has been nothing more than a series of pushes travelling through air, skin and bone. Inside the cochlea, those pushes are read, sorted and translated into the electrical language of nerves.

How is the cochlea built?

If you could uncoil the cochlea, you would see a narrow tube that tapers from a wider end to a narrower tip. The wide end, near the middle ear, is called the base. The far tip, at the centre of the spiral, is called the apex. The tube is filled with fluid and divided along its length by a flexible strip of tissue, the basilar membrane.

Resting on that membrane is a thin ribbon of sensory tissue, sometimes called the organ of Corti. It holds rows of hair cells, each topped with a cluster of microscopic projections called stereocilia. Above them lies another, jelly-like membrane. The stage is set for a very small but very precise movement.

How does the cochlea tell high from low?

When the stirrup bone of the middle ear rocks against the cochlea, it sets the fluid moving. That movement becomes a wave that rolls along the basilar membrane from the base towards the apex.

The membrane is not the same along its length. Near the base it is narrow and stiff; towards the apex it is wider and more flexible. Because of this, each pitch makes the wave peak at its own spot. High-pitched sounds peak close to the base, and low-pitched sounds travel further and peak near the apex. Specialists call this layout tonotopic, which simply means arranged by tone, a little like the keys of a piano laid along a spiral.

The result is that the cochlea analyses sound before it sends anything onward. A voice, a bird and a passing bus each produce their own pattern of peaks along the membrane, and the hair cells at each spot report what is happening there.

What do inner and outer hair cells each do?

There are two kinds of hair cells, and they have quite different roles.

• Outer hair cells act like a built-in amplifier. They change length rapidly in time with the sound, adding energy to the wave at exactly the right place. This makes the cochlea more sensitive to quiet sounds and sharpens its tuning, so that nearby pitches can be told apart.
• Inner hair cells are the main messengers. When the membrane moves, their stereocilia bend, tiny channels open, and the cell releases chemical signals onto the fibres of the hearing nerve. From there, the message is on its way to the brain.

A useful way to picture it is a microphone with an automatic sharpening circuit. The inner cells are the microphone; the outer cells keep it sensitive and precise.

Why does damage to hair cells matter so much?

In people, cochlear hair cells do not grow back once they are lost. Loud noise, the gradual changes of ageing, some medicines and some illnesses can all damage them.

When outer hair cells are affected, quiet sounds become harder to catch and the fine tuning that separates similar pitches weakens. Speech may still be audible yet seem blurred, which is one reason people describe others as mumbling. Many common changes, including those described in noise and hearing damage, tend to show up first in the high-pitch region near the base of the cochlea. Because consonants such as "s" and "f" live in that range, clarity can suffer before overall loudness does.

The cochlea also shares its fluid system with the balance organs next door. That is why some inner-ear conditions can affect hearing and balance together, and why a doctor may ask about dizziness when you mention a hearing change.

Key takeaways

• The cochlea is a coiled, fluid-filled tube that turns vibration into nerve signals.
• A wave travels along the basilar membrane and peaks at different places for different pitches.
• Outer hair cells boost and sharpen the response; inner hair cells pass the signal to the hearing nerve.
• Lost hair cells do not regrow in people, so many cochlear changes are long-term.
• High-pitch regions are often affected first, which can blur consonants before sound seems quieter.

Keep learning: age-related hearing loss describes how gradual cochlear change feels in daily life, and how the ear works shows where the cochlea fits in the whole system. Questions about your own hearing are for an ENT doctor or hearing professional.