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How Do We Hear?

The auditory system is divided into three distictive parts. The outer ear, middle ear and inner ear. In the process of hearing, each division has a distinct and specific function. The mechanism of hearing involves the transmission of vibrations, the generation of nerve impulses, and sound recognition in the brain. The following graphic shows the basic areas of the ear.

Ear graphics

The sense of hearing involves mechanical action as sound waves enter the external auditory meatus, where they are transformed into impulses. As the impulses pass through the external auditory canal, they impinge on the tympanic membrane. Energy from the sound waves cause the tympanic membrane to vibrate. These vibrations are transferred directly to the middle ear bones, called ossicles, causing them to vibrate in a chain reaction. The first ossicle, known as the malleus, is attached to the tympanic membrane. As the vibrations hit the tympanic membrane the malleus vibrates causing the the second ossicle, the incus, to vibrate. The generated vibrations cause the innermost ossicle, the stapes to vibrate. The foot plate of the stapes fits into the oval window, a small opening in the wall between the middle and inner ear. This lever like system amplifies the sound waves as the vibrations move through the middle ear into the inner ear. The vibrations are now passed into the fluid of the inner ear into the cochlea. The cochlear duct which is located just inside the cochlea, houses the Organ of Corti. The Organ of Corti contains the auditory receptor cells/hair cells, supporting cells and nerve fibers. The receptor cells are embedded in the basilar membrane of the cochlear duct. A gelatinous membrane called the tectorial membrane overhangs and touches the haircells of the basilar membrane. Fluid movement caused by the sound waves entering the cochlea leads to movement of the basilar membrane. This causes the thousands of hair cells covering it to excite the receptor nerve endings and send an electrical signal through the auditory nerve to the temporal lobe of the brain. The brain decodes the impulses and recognizes them as sound.