How does the sound reach our ears?
The human auditory system is composed of 3 parts: the outer ear, the middle ear, and the inner ear.
The functioning in detail
Sound waves are picked up by the pinna of the outer ear. They are then amplified and transmitted to the middle ear through the external ear canal. This movement of sound causes the tympanic membrane to vibrate. The tympanic membrane is a small, flexible circular membrane that begins to vibrate when sound waves reach it. These vibrations are transmitted to the ossicles located in the middle ear.
The ossicles are the smallest bones in the human body. They are composed of the hammer, which transmits its vibrations to the anvil, and the stirrup. The latter plays the role of a piston that compresses the liquid of the inner ear.
This whole is known as the ossicular chain and connects the tympanic membrane to the entrance of the inner ear.
The cochlea, which resembles a snail shell, is the main organ of auditory perception. It contains between 15 and 20,000 ciliated cells that detect the vibrations of the liquid and generate the nerve impulses that are sent to the brain by the auditory nerve.
However, this system is fragile and can fail. The two most common types of hearing loss are conduction loss and sensorineural loss.
The loss of conduction may be due to a foreign body that obstructs the ear canal. For example, a wax plug can cause a perforation of the eardrum, which results in a malfunction of the ossicles, which can lead to an infection in the middle ear. This type of loss can often be resolved with medical intervention and accounts for only 10% of hearing loss cases.
The second type of loss, sensorineural loss, which accounts for 90% of cases, is the result of the destruction of the ciliated cells of the cochlea. It is often due to aging, but can be genetic. Or it may be the result of repeated exposure to very loud sounds.
This type of loss is irreversible, but can often be compensated by hearing aids.