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The Hearing System

All sounds originate from movement. For example, when we speak the air coming from our mouth pushes the molecules in the air, making them vibrate. These vibrations are called sound waves and can be perceived by the ear. Slow vibrations (low frequency) are heard as deep tones (bass), while fast vibrations (high frequency) are heard as high tone (treble).

The complete hearing system is responsible for our sense of hearing. It picks up mostly acoustic waves and transforms them into neural codes, which can be interpreted by the brain. (PICTURE)

How the ear works?

The ear consists of three main sections: the external ear, the middle ear and the inner ear.

The outer ear

The outer ear is made up of the external cartilaginous part of the ear and the canal. The eardrum is located at the end of the ear canal and forms the boundary to the middle ear. The outer ear picks up sound waves and directs the sound to the eardrum. The sound wave pressure causes the eardrum to vibrate and the middle ear comes into action.

The middle ear

The middle ear is a room filled with air. The Eustachian tube, which connects the middle ear to the throat functions to keep the air pressure in the middle ear equal to the external air pressure. There are three tiny bones in the middle ear cavity commonly called the hammer, the anvil and the stirrup. This chain of bones forms a lever mechanism conducting movements of the eardrum to the fluids of the inner ear.

Two small muscles, the stapedius and the tensor tympani are attached to the bones. These muscles are activated by a reflex when loud sounds reach the ear. This muscle reflex reduces excessive sound pressure before it reaches the inner ear.

The inner ear

The inner ear or the cochlea is shaped like a snail and filled with fluid. The oval window connects the middle ear and the inner ear. The base of the stapes is attached to the oval window and functions as a piston moving the fluid of the inner ear.

This movement of the fluid activates the hair cells in the inner ear (there are about 20000 of these sensory cells). When the hair cells are activated, they send impulses via the auditory nerve to the brain, which perceives these impulses as sound.

Via these fantastic twisting ways the ear is able to pick up sound waves, transform them to mechanical energy (ossicles) then to hydraulic energy (wave movement in fluid) and finally to an electrical energy (nerve impulses) that can be interpreted by the brain. Even the slightest flaw in this complex system can compromise hearing ability.

© 2011 JISH Jeddah Institute for Speech and Hearing, all rights reserved.