Cochlea

The cochlea is the part of the inner ear that helps us hear. It looks like a spiral shell and sits inside a bony area called the bony labyrinth. In humans, the cochlea makes about 2.75 turns around a central point called the modiolus.

Inside the cochlea is an important structure called the organ of Corti. This is the main part that lets us hear. It sits along a special wall inside the cochlea, helping to change sound vibrations into signals for our brain.

Etymology

The word "cochlea" comes from a Latin word that means "snail shell." This Latin word came from an Ancient Greek word, kokhlias, which means "snail" or "screw." The cochlea is named this because it has a spiral shape, like a snail shell, in most animals.

Structure

The cochlea is a part of the inner ear that helps us hear. It looks like a spiral, similar to a snail shell. Inside this spiral are three spaces filled with fluid. These spaces help carry sound vibrations through the ear.

The cochlea has special cells called hair cells. When sound vibrations move, these hair cells change the vibrations into signals. These signals go to the brain, where they are understood as sound. The cochlea’s shape helps it work properly to detect different sounds.

Function

The cochlea is filled with a special liquid called endolymph. When sounds make the middle ear vibrate, this liquid moves. Inside the cochlea, tiny hair cells feel these movements through their stereocilia. They change the movement into electrical signals, which travel through nerves to the brain. This is how we hear.

The small bones in the middle ear, called the stapes, send vibrations to the cochlea. These vibrations move fluids inside the cochlea, which then move a special part called the organ of Corti. This organ has hair cells that can sense different sound pitches. Very loud noises can hurt these hair cells, which can cause some loss of hearing. That’s why people wear earmuffs or earplugs around loud machines or fireworks.

When we hear, the hair cells in the cochlea turn the movement of sound into electrical signals. These signals travel through special nerves to the brain, where they are understood as sound. The cochlea can also make tiny movements that help us hear very quiet sounds.

Clinical significance

Damage to the cochlea can happen from a hard hit to the head, a growth called a cholesteatoma, an infection, or very loud noise. This damage can hurt the tiny cells that help us hear.

When these cells are hurt or die, it can cause hearing loss. Damage can make it harder to hear quiet sounds or tell different sounds apart. Scientists are still learning more about this.

In 2009, scientists at the Massachusetts Institute of Technology made a special electronic chip that works like a cochlea. It can study many radio frequencies using very little power.

Main article: Hearing loss

Other animals

The coiled shape of the cochlea is special to mammals. In birds and other non-mammalian vertebrates, the part that holds hearing cells is sometimes called a "cochlea" too, but it is not coiled. Instead, it looks like a tube that ends in a point.

Mammals can hear a wider range of sounds partly because of a special way their cells move to boost sound. While mammals and birds can tell similar kinds of sounds apart, mammals can often hear much higher sounds.

When scientists study the cochlea, they look closely at hair cells. Different animals have hair cells that work in slightly different ways. In birds, there are tall and short hair cells instead of the inner and outer hair cells found in mammals.

All hair cells in birds connect to a special structure called the tectorial membrane.