Ears: hearing and balance

Three tiny bones in the middle ear pass sound on to the inner ear, and human hearing covers sounds from about 20 to 20,000 vibrations per second.

From air to fluid

When someone speaks, the air in front of their mouth is squeezed and stretched many times a second. Those pressure waves travel to your head, and your ear has to turn them into electrical signals that a nerve can carry. The National Institute on Deafness and Other Communication Disorders (NIDCD) describes the process as a chain of steps, and each part of the ear handles one of them.

Sound waves enter the outer ear and travel along the ear canal to the eardrum, also called the tympanic membrane. The eardrum vibrates and passes the vibrations to three tiny bones in the middle ear: the malleus, incus and stapes. Their Latin names roughly mean hammer, anvil and stirrup. The bones amplify the vibrations and pass them to the cochlea in the inner ear through a membrane-covered opening called the oval window. The middle ear is also connected to the throat by the Eustachian tube, which helps keep the air pressure equal on both sides of the eardrum.

Inside the cochlea

The cochlea is a snail-shaped tube filled with fluid. An elastic partition, the basilar membrane, runs along its length and splits it into an upper and a lower part. When the stapes pushes on the oval window, the fluid ripples and a travelling wave forms along the basilar membrane. Sensory hair cells sit on top of it. Each has a bundle of tiny projections called stereocilia, arranged from tallest to shortest. As the wave moves the hair cells, the stereocilia bump against an overlying membrane and bend. Bending opens channels at their tips, ions rush in, and an electrical signal starts. The auditory nerve carries the signal to the brain.

How does the ear tell a high note from a low one? The basilar membrane is not the same all along. High-frequency waves move the part near the base of the cochlea, close to the oval window, while low-frequency waves move the part near the tip. So a hair cell's position tells the brain the pitch. OpenStax compares this to a prism that spreads light into its colours. The range runs from about 20 Hz at the tip to about 20,000 Hz at the base. One hertz is one vibration per second.

The balance organs

Next to the cochlea, in the same bony maze, is the vestibular system. It uses the same kind of hair cell to sense the head instead of sound. The utricle and saccule sense head position and straight-line movement. Their hair cells poke into a gel topped with small crystals of calcium carbonate called otoliths. When you tilt your head or speed up in a car, the heavy layer lags behind and bends the stereocilia.

The three semicircular canals sense rotation. They are fluid-filled loops set roughly at right angles to one another, so together they cover every direction the head can turn. Inside each canal a jelly-like sail, the cupula, sits on a cluster of hair cells. When you turn your head the fluid lags behind, the cupula flexes and the hair cells signal which way you turned. The NIDCD points out that balance also relies on the eyes and on muscle and touch sensors in the legs, and the brain blends all of them.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.