How do we hear?
Sound consists of vibrations of air in the form of waves which it is picked up by the ear; the ear consists of three parts: the outer ear, the middle ear and the inner ear.
The outer ear has a shape that assists in collecting sound waves from the air which are then funnelled down the ear canal. At the end of the ear canal passage the sound waves impinge on the eardrum or tympanic membrane. The eardrum separates the outer ear from the middle ear. The middle ear is a small cavity which is ventilated from the back of the nose through the Eustachian tube. The Eustachian tube allows for equalization of pressure on both sides of the eardrum enabling it to vibrate efficiently. The muffled sound with a head cold is often associated with Eustachian tube blockage.
The middle ear cavity is normally air filled. It contains three ossicles or bones which are the smallest bones in the human body. They are generally termed the hammer (malleus), anvil (incus) and the stirrup (stapes).
The three middle ear bones are connected to one another and act as a lever system to transmit and amplify movements of the tympanic membrane produced by sound waves. These sound waves are then transmitted to the oval window of the inner ear. The oval window is a membrane which separates the middle ear from the inner ear.
The organ of hearing in the inner ear is the cochlea which is a fluid-filled system. The fluid called endolymph and perilymph are in contact with the inner side of the oval window.
Vibrations of the middle ear bones are passed through the oval window and are transmitted through the cochlear fluid to nerve receptors in the cochlea. The nerve receptors in the cochlea respond to the frequency of the sound vibrations. The human ear has nerve receptors sometimes called hair cells. There are two types of hair cells, outer hair cells and inner hair cells. Each human ear has about 13,000 outer hair cells and 3,500 inner hair cells. Nerves from these receptors carry impulses to the brain, which interprets them as a sound of a certain pitch or loudness. The complex wave patterns of speech sound produce a pattern of nerve impulses. The brain learns by experience to attach some particular significance to the impulse pattern. The brain stores this pattern in memory and when it is received again, it is recognized.
If any part of the hearing system fails to develop adequately or is damaged which will interrupt the normal process of the sound transmission and will cause a hearing loss.