Cochlea

The cochlea is the part of the inner ear that helps us hear. It is shaped like a spiral and is found inside a bony area called the bony labyrinth. In humans, the cochlea makes about 2.75 turns around a central axis known as the modiolus.

Inside the cochlea is a very important structure called the organ of Corti. This organ is the main sensory part that allows us to hear. It is located along a special partition that separates fluid-filled spaces inside the cochlea's coiled tube. This helps turn sound vibrations into signals our brain can understand.

Etymology

The word "cochlea" comes from a Latin word that means "snail shell." This Latin word itself came from an Ancient Greek word, kokhlias, which means "snail" or "screw." The cochlea is called this because it has a spiral shape, like a snail shell, in most animals, but not in a special group called monotremes.

Structure

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

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

Function

The cochlea is filled with a special liquid called endolymph. When sounds vibrate the middle ear, this liquid moves. Inside the cochlea, thousands of 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 are sensitive to different sound pitches depending on where they are located. 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. Sometimes, these movements can be measured coming back out of the ear, which helps doctors check how well someone can hear.

Clinical significance

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

When these cells are hurt or die, it can cause hearing loss. Cells on the outside are easier to damage, which can make it harder to hear quiet sounds. Damage to the cochlea can also make it tricky to tell different sounds apart. Scientists are still learning more about how this damage affects hearing.

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. This difference happened as the frequency range of hearing changed over time between mammals and other animals. 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. Most birds cannot hear above 4–5 kHz, with the highest known being about 11 kHz in the barn owl. Some sea mammals can hear up to 200 kHz. Having a long, coiled shape gives more room for hearing higher sounds, which has helped mammals develop special ways of using their hearing.

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. Tall hair cells in birds work like inner hair cells in mammals, and short hair cells are similar to outer hair cells but do not connect to the nerves that carry sound signals. One big difference is that all hair cells in birds connect to a special structure called the tectorial membrane, while only outer hair cells in mammals do.