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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 it's 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.