How we hear: Structure of the ear

The ears comprise three anatomical sections; the outer ear, the middle ear and the inner ear. Only a small part of the outer ear is normally visible.

the human auditory system



The outer ear

the pinnaThe outer ear consists of the pinna, the ear canal and the eardrum. The pinna is the part you can see on the side of your head. Its main purpose is to collect sounds and direct them down your ear canal – rather like a funnel. The pinna also plays a role in enabling us to decide from which direction a sound is coming – a process known as sound localisation.

The ear canal is a passage that leads to the eardrum (also known as the tympanic membrane). The average ear canal is about 26mm in length and 7mm in diameter – although this varies from person to person.

The ear canal is lined with skin. Ear wax (also called cerumen) is produced by glands in the skin of the outer portion of the ear canal.

Sounds enter the ear and travel down the ear canal until they reach the eardrum.

The eardrum (tympanic membrane) is approximately 8-10mm in diameter and is formed of three layers of skin. The eardrum is a bit like a drum-skin, which vibrates when sound strikes it. When this happens, the sound vibrations are passed into an area known as the middle ear.

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The middle ear

the ossicles with size referenceBeyond the eardrum is an air filled space known as the middle ear or tympanic cavity. This space houses the ossicles – a group of three tiny bones that link the outer and inner ear.

These bones are the smallest in the human body and their job is to pass the vibrations of the eardrum through the middle ear to the inner (sensory) part of the ear.

Because of their distinctive shapes, these bones are sometimes called the “hammer”, “anvil” and “stirrup”. They are also commonly referred to by their equivalent Latin names: malleus, incus, and stapes respectively. The malleus (or “hammer”) is partially embedded in the eardrum and is responsible for transferring the eardrum vibrations to the remaining ossicles.

There are also two very small muscles inside the middle ear (stapedius and tensor tympani) which have two roles: they help to suspend and retain the ossicles within the middle ear and they contract in response to loud sound, which in turn tightens the chain of ossicles. This contraction is known as the acoustic reflex. This process makes it difficult for sound vibrations to pass across the chain of ossicles - thereby helping to protect the sensory part of the ear from damage by loud sounds.

The middle ear cavity is also connected to the back of the throat by a passage called the Eustachian tube. This passage helps to keep the pressure of the air in the middle ear equal to that of the outside world. The tube is opened naturally by swallowing or yawning, for example, to enable this to happen. In this way, the ear drum has equal pressure on either side and is able to work at its best. Many people experience an unpleasant sensation in their ears when flying, and need to swallow from time to time to help equalise the pressure across their eardrums.

When a person suffers from a cold, the Eustachian tube can become clogged with mucus. If this happens, air and fluid get trapped inside the ear – something that can temporarily impair hearing as well as allowing painful ear infections to develop. This is especially common in young children, as their smaller Eustachian tubes are more prone to blockage.

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The inner ear

the cochlea with size referenceThe inner ear has two parts - the cochlea and the vestibule. The cochlea is the part involved in hearing, while the vestibule forms part of your balance or vestibular system.

The cochlea is a small spiral shaped structure (rather like the shell of a snail) that is embedded in bone. It is filled with fluid. Sound is transmitted as ‘waves’ in this cochlea fluid by vibration of the stapes bone in the ‘oval window’.Inside the cochlea is an important structure known as the organ of Corti, which supports rows of special cells known as hair cells. These hair cells detect sound waves moving through the cochlea fluid and turn them into electrical signals that travel up the auditory nerve to the brain. When these signals reach the ‘auditory cortex’ of the brain, they are processed and perceived as a sound.

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