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THE AUDITORY SYSTEM

Being able to hear sounds is of major importance!

Not only for being able to recognise sounds and to talk with others, but for your own safety as well. Besides, hearing is closely related to our emotions.

Sound is a vibration of molecules that moves through the air or through water.

The ears pick up sounds. The auditory organ consists of three parts: the outer ear, the middle ear and the inner ear.

OUTER EAR

The outer ear consists of the auricle and the outer part of the auditory duct. The auricle is made of cartilage covered with skin. The ridges and the asymmetric shape of the auricle have a function. The auricle picks up the sound waves. They enable us to perceive sounds from various directions, from behind and from the front as well as from above and from below, and make sure that the sound of the wind does not bother us too much.

Through the auricle the sound waves are transferred to the thin light blue transparent ear drum and they make the ear drum vibrate.

MIDDLE EAR

The middle ear is found behind the ear drum. This is an area filled with air in which the ossicles, the oval window, the round window and the Eustachian tube are found. The ossicles, being the hammer, the anvil and the stirrup, are the smallest bones in our bodies. From the ear drum the ossicles are set in motion, making sure that the sound vibrations are without ‘loss of energy’ transferred through the oval window in the cochlea.

The round window separates the middle ear from the inner ear. The Eustachian tube connects the middle ear and the pharynx with each other. This tube opens when we swallow or yawn and controls the overpressure and underpressure in the middle ear.

INNER EAR

The inner ear is the most complex part of the ear into which a lot of research is being done in order to grasp the exact functioning of this. The inner ear consists of the cochlea and the organ of balance (vestibulum).

The cochlea is a curled tube with a length of about 3.5 cm and a diameter of 1 cm. Three channels can be distinguished in this: the scala media, the scala vestibule and the scala tympani. The latter two are seamlessly interlinked. The scala media contains the basilar membrane with the organ of Corti. The organ of Corti includes hair cells, also called the cilia. The sound waves coming from the middle ear sets the fluid in the cochlea in motion and this makes the basilar membrane vibrate. The vibration of the membrane sets the hair cells in the cochlea in motion and they can pass on electrical currents to the auditory nerve.


Cochlea – Etching by André de Regt A cross section of the entire cochlea.

AUDITORY NERVE

The ear picks up sounds and passes these on to the auditory nerve. It is the task of the auditory nerve to lead the sound signal to the hearing centres in the brain. If the auditory nerve sends an incomplete signal, the brain has to work harder to recognise the sounds or to convert these into words.

BRAIN

Our brain converts the sounds into sound recognition and into words, which enables us to understand each other.

“Hearing is done with the ears. Understanding is done with the brain!”

CILIA

The organ of Corti with the basilar membrane, which is found in the cochlea, contains the hair cells. There are internal and external hair cells. Usually, these hair cells are neatly arranged in a V-shape. The hair cells are surrounded by support cells. Someone with normal hearing has as many as 15,000 hair cells that pass on sound vibrations to the auditory nerve. The basilar membrane is wider at the end than it is at the basis. Also, it is more rigid at the beginning than at the end. Therefore the hair cells at the beginning of the basilar membrane will be more sensitive to higher frequencies than the hair cells positioned further away from the oval window. In other words, the higher tones are detected at the beginning of the cochlea and the low tones at the end.

With children who were born deaf because of Usher Syndrome, the cochlea and the vestibulum have not properly been developed. With children who are hard of hearing this has been built, but the hair bundles in the cochlea are not intact. Many cilia are missing and an increasing number of cilia can die in the course of the years.

The first figure below shows the nice and intact hair bundles of cilia, whereas in the second figure you see the weak hair bundles and cilia that have fallen down.


Hair cells of inner ear. Credit: Dr David Furness.

Scanning electron microscopy showing human hair cells and stereocilia. Credit: Rask-Anderson