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There are three systems that assist with hearing: the actual ear, the
brain's central auditory system, and the brain's other processes that
are not necessarily directly related to hearing. Hearing occurs when sound
waves (e.g. air vibrations) set off movements in the various structures
of the ear. A chain of vibrations moves from one structure to the next
and eventually causes neurons to send electrical impulses to the brain
which we interpret as sound.
Sound waves first come into contact with the external portion of the
outer ear, called the pina, which helps to direct sounds into the auditory
canal. The sound waves travel through the auditory canal until they meet
the ear drum, the first component of the middle ear. Then they are passed
from the vibrating eardrum to three small bones, called the ossicles,
and known as the hammer, the anvil and the stirrup. The stirrup passes
the vibrations to the inner ear, where fluid vibrates in a spiral bony
chamber, called the cochlea.
In a portion of the cochlea, the basilar membrane vibrates. The basilar
membrane contains sensitive cells called hair cells. Different frequency
sounds cause different parts of the basilar membrane to vibrate maximally
which then stimulate subsets of hair cells to spur neurons into action.
A neural impulse is fired down the auditory nerve and reaches the auditory
center in the cerebrum of the brain.
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