The structure and function of human Ear


Human ear has three portions, outer, middle and inner ear. Outer ear is called external pinna which curved in shape and catches sound waves and passes it into middle ear.

Middle ear:
It has the auricle and external auditory canal. Middle ear begins at the tympanic membrane and ends inside the skull where two small membranous openings, the oval and round windows are located. Three small ossicles are between tympanic membrane and oval window. They include malleus (hammer), incus (anvil) and stapes (stirrup), so named for their shapes. The malleus adheres to the tympanic membrane and connects to the incus. The incus connects to the stapes which adhere to the oval window. The auditory (Eustachian) tube extends from the middle ear to the nasopharynx and equalizes air pressure between middle ear and throat.

Inner ear:
It has three components. First two the vestibule and semi circular canals are concerned with equilibrium and third the cochlea is involved with hearing. Semicircular canals are arranged so that one is in each dimension of space. The process of hearing is as under:

Sound waves enter the outer ear and create pressure waves that reach the tympanic membrane. Air molecules under pressure vibrate the tympanic membrane. The vibrations move the malleus on the other side of the membrane. The handle of malleus articulates with incus vibrating it. Vibrating incus moves the stapes back and forth against the oval window. The movements of the oval windows setup pressure changes that vibrate the fluid in the inner ear. 

These vibrations are transmitted to the basilar membrane causing it to triple. Receptor hair cells of the organ of corti that are in contact with the overlying tectorial membrane are bent, causing a generator potential which leads to an action potential that travel along the vestibule cochlear nerve to the brain for interpretation. Vibrations in the cochlear fluid dissipate as a result of movements of the round window.

Humans are not able to hear low pitched sounds below 20 cycles per second. Young children can hear high pitched sounds up to 20,000 cycles per second but this ability decreases with age. Sense of equilibrium can be divided into two separate senses. Static equilibrium refers to sensory movement in one plane and dynamic equilibrium refers to angular or rotational movement.

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