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The inner
ear is made up of the fluid filled cochlea and vestibular
organs. Cochlear organs are responsible for hearing and vestibular
organs are responsible for our sense of balance and motion;
they share the same fluid supply and are interrelated.
The cochlea
is named for its snail-shell shape and it is really a
space
that is hollowed-out of the skull's temporal bone. Located
a few inches behind your eye sockets, your cochleae are
about
the size of your smallest fingernail.
The cochlea
has three ducts separated by thin membranes that wind 2 1/2
turns with the apex pointing towards the front of the head.
Fluid does not flow from the cochlea into the middle ear space
because the oval window (opening hole to the cochlea) is covered
by a protective membrane. The oval window is pulsed by the
movement of the stapes, the final bone in the middle ear's
ossicular chain. The movement of the stapes transfers signals
in the form of kinetic energy into the cochlear fluid, causing
a series of traveling waves. In this way the signal is transduced
once again, from kinetic energy into hydraulic energy (fluid).
Traveling
waves move the two membranes as they progress through the
spiraling ducts of the cochlea. Between the membranes are
four rows of microscopic cells called haircells.
The haircells
move with the membrane and this triggers bioelectrical nerve
impulses in nearby neurons. Once again, there is a transduction,
as the signal is changed from the waves' hydraulic energy
into electrical energy.
The neural
impulses pass up the auditory nerve to the brain, where they
are processed and perceived as sound.
A healthy
cochlea houses approximately 15,000 haircells which are coupled
to 30,000 individual nerve fibres. Loud sounds trigger more
haircells than soft sounds and tones of different pitches
trigger different haircells along the length of the cochlear
duct. The human hearing system is an amazing example of anatomical
engineering. It changes acoustic energy into kinetic energy,
then into hydraulic energy and finally into neural energy
(electrical). This happens almost instantaneously over an
incredibly wide range of signals and intensities.
Each
and every sound we hear passes through this series of changes
while we, taking this marvel for granted, are completely oblivious
to them!
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The human hearing mechanism is a complicated, exquisitely
sensitive system. Capable of processing an extremely wide
range of intensities and tones, our auditory system also handles
several different signals simultaneously and delivers them
to the brain with fidelity and intelligibility.
