Structure and functions of ear

PUBLISHED : Thursday, 26 April, 2001, 12:00am
UPDATED : Thursday, 26 April, 2001, 12:00am
 

Our ear helps us to hear and to keep our balance. Each ear has three parts: the outer ear, middle ear and inner ear.


The outer ear comprises the pinna (external ear flap) and auditory canal. The pinna collects sound waves and directs them through the external auditory canal towards the eardrum which is a thin 'skin' across the entrance of the middle ear.


The eardrum vibrates when it is hit by the sound waves. The sound vibrations are amplified at the same time and are transmitted to the tiny ear bones in the middle ear. These ear bones amplify the sound vibrations greatly and transmit them to the oval window of the inner ear.


The vibrations of the oval window cause the perilymph and endolymph of the cochlea to vibrate. When the endolymph vibrates, sensory nerve endings in the cochlea are stimulated and nerve impulses are generated. These impulses are relayed to the brain through the auditory nerve.


The auditory centre in the cerebral cortex of the brain interprets the nerve impulses as sounds.


When we are gaining height rapidly as we going up in a fast- moving lift or in a Peak tram, we may experience a short period of temporary deafness. This is because the eardrum can only vibrate properly when the pressure on its two sides is equal. As we are going up quickly, the atmospheric pressure decreases, but the pressure in the middle ear cannot adjust in pace. Therefore, the eardrum cannot vibrate properly, leading to a temporary deafness.


Normal hearing can be restored simply by blowing out air so that air leaves the middle ear to lower the pressure inside.


The same condition appears when we are going down quickly. But this time, the atmospheric pressure is increasingly quickly. To restore normal hearing, we need to swallow air so that air is forced into the middle ear to increase the pressure inside.


Detection of body position during movement


The inner ear has three semi- circular canals arranged in different planes at right angles to each other. They are responsible for detecting changes in positions of the body during movement. The brain makes use of this information to co-ordinate contraction of the muscles to maintain balance.


At one end of each semi-circular canal is a swelling called an ampulla. When the head moves, the endolymph moves in an opposite direction due to its own inertia. This displaces the gelatinous liquid and pulls on the sensory hair cells. This stimulates the sensory hair cells which then generate nerve impulses. The nerve impulses are then transmitted to the brain to be interpreted.


To read articles previously published in this section, go to 'Subjects Pages' at http://ypost.scmp.com/archive


Graphic: SPBIOLGLO


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