Division of Audiology and Speech - Language Pathology

Hearing: How We Hear

Within the relatively small structure of the ears, nose, and throat are several very complex mechanisms that allow us not only to make sound but also to hear, to keep our balance, to smell, to breathe in and filter air, and to swallow food and water. These mechanisms are interrelated and generally carry out their functions without our being aware of the processes at work. Although these structures are complex, they are in many ways not far removed from the more primitive form in which they once existed-first in aquatic animals and later in more primitive land species.


diagram courtesy of ENTAA Care

Anatomy of the Ear

The ear is divided into three parts: the external ear, the middle ear, and the inner ear.

The outer ear consists of the auricle, or pinna (the cartilage and skin that we see on the outside of the skull), and the outer ear canal, which conducts sound to the middle ear. At the entrance to the middle ear is the eardrum, a thin membrane that is stretched across the end of the outer ear canal.

The middle ear is a cavity that contains three connected bones, collectively called the ossicles. Each is named for the object it resembles: the malleus (hammer), the incus (anvil), and the stapes (stirrup). Leading from the bottom of the middle ear cavity to the back of the nose is a narrow channel called the eustachian tube. When an individual swallows or yawns, the muscles attached to this tube pull on its edges and open it so that the air outside may replenish the air supply in the middle ear. The Eustachian tube thus serves to equalize the air pressure on the inside of the eardrum with that on the outside.

The middle ear opens into the inner ear via the oval window, which is covered by the footplate of the stapes bone. On the inner ear of that plate, the stapes is bathed with the fluid of the vestibule, which is contiguous with the fluid of the cochlea. This structure, shaped something like a snail shell and lined with tiny hairs, is the major part of the hearing mechanism. The second part of the inner ear is the labyrinth, which is responsible for balance.

The inner ear also contains other very important structures. Three semi-circular canals (the labyrinth) function as the body뭩 gyroscope, regulating balance. The Eustachian tube, or auditory tube, extends from the middle ear to the passages behind the nose and upper part of the throat: when a person swallows or yawns, outside air replenishes the air supply in the middle ear, equalizing the air pressure in the middle ear. 

How Hearing Works

The primary function of the outer ear (or pinna) is to collect and carry sounds (which are essentially vibrations) to the middle ear. Sound waves travel through the outer ear canal and strike the eardrum (tympanic membrane), which vibrates like a drum and converts the waves to mechanical energy.

This energy resonates to the middle ear, where tiny bones (the ossicles ? these include the malleus, incus, and stapes) vibrate to the rhythm of the eardrum, amplify the sound, and pass the sound waves on to the inner ear.

The inner ear (or cochlea) is fluid-filled and lined with tiny hairs. Vibrating sound waves cause ripples in the fluid, which then bend the tiny hairs. This process converts the sound into nerve impulses, which then travel along a network of nerve cells (the auditory or eighth cranial nerve) to the brain, which perceives these impulses as sound.

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