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How Hearing Works
Anatomy of the Human Ear
The ear is comprised of three parts: an outer ear (external), a middle ear, and an inner ear (containing hearing and balance chambers). Each part performs an important function in the process of hearing and balance.
The outer earThe outer (external) ear consists of the auricle and the ear canal. These structures gather the sound waves and direct them towards the eardrum (tympanic membrane).
The middle ear
The middle ear chamber lies between the external ear and inner ear, behind the eardrum. This air filled chamber receives air through the Eustachian tube. It is a tubular structure connecting to the ear from the back of the throat (pharynx) and above the palate. The Eustachian tube is responsible for equalizing the pressure in the middle ear with the atmospheric pressure. The middle ear consists of an eardrum and three small bones (ossicles): the malleus, incus, and stapes. These structures are connected to each other and transmit sound vibrations to the inner ear. In so doing they act as a transformer, converting sound vibrations in the external ear canal into fluid waves in the inner ear. The smallest bone in the human body (stapes) actually moves in and out of the inner ear opening, causing fluid waves to occur in the inner ear. Any disturbance of the Eustachian tube, eardrum, or the ear bones may result in a conductive hearing loss, meaning impairment of sound conduction to the inner ear. This type of impairment is usually correctable medically or surgically.
The inner earThe inner ear contains microscopic hearing nerve endings (hair cells), which are bathed in fluid. Inner ear fluid waves move the delicate nerve endings, which in turn transmit sound energy information by the hearing nerve to the brain, where it is interpreted into sound. A disturbance in the inner ear fluids, hair cells, nerve endings, or hearing nerve may result in a sensorineural hearing loss. This type of impairment cannot be restored to normal with surgery. The inner ear is also responsible for our balance.
The mechanics of hearing
Hearing occurs as sound enters the outer ear canal and causes the tympanic membrane (eardrum) to vibrate and subsequently the moves the middle ear bones. The piston-like movement of the stapes bone initiates a fluid wave within the inner ear. This “traveling wave” in turn activates the hair cells inside the cochlea that reside in the Organ of Corti, causing the hearing nerve to send electrical signals to the brain. The hair cells are responsible for converting the mechanical energy of the fluid waves into electrical signals, which will be processed by the brain. The hearing nerve (auditory nerve, cochlear nerve or cranial nerve VIII) then takes the signal to the brain for interpretation. There are two types of hair cells in the cochlea, the outer and inner hair cells. The outer hair cells are responsible for amplifying the traveling wave signal, and “fine tuning” the signal to a frequency-specific region of the cochlea. Inner hair cells are the special sensory receptors that receive the traveling wave information and relay it to the hearing nerve.