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The vestibular system is located in the inner ear and
contains the following structures: vestibular labyrinth,
semicircular canals, vestibule, utricle, and saccule. These
structures work in tandem with the vestibular areas of the
brain to help us maintain balance.
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The semicircular canals and vestibule of the inner ear
contain a fluid called endolymph that moves in response to
head movement.
The vestibular labyrinth is located behind the eardrum.
The labyrinth's most striking feature is a group of three
semicircular canals or tubes that arise from a common base. At
the base of the canals is a rounded chamber called the
vestibule. The three canals and the vestibule are hollow and
contain a fluid called endolymph which moves in response to
head movement.
Within the vestibule and the semicircular canals are
patches of special nerve cells called hair cells. Hair cells
are also found in two fluid-filled sacs, the utricle and
saccule, located within the vestibule These cells are aptly
named: rows of thin, flexible, hairlike fibers project from
them into the endolymph.
Also located in the inner ear are tiny calcium stones
called otoconia. When you move your head or stand up, the hair
cells are bent by the weight of the otoconia or movement of the
endolymph. The bending of the hair cells transmits an electrical
signal about head movement to the brain. This signal travels
from the inner ear to the brain along the eighth cranial
nerve--the nerve involved in balance and hearing. The brain
recognizes the signal as a particular movement of the head and
is able to use this information to help maintain balance.
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In the inner ear, rows of hairlike fibers project from tiny
patches of nerve cells called hair cells.
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These tiny calcium stones called otoconia are part of the
inner ear's balance system.
A thorough exam reveals the cause of most dizziness.
The senses are also important in determining balance.
Sensory input from the eyes as well as from the muscles and
joints is sent to the brain, alerting us that the path we are
following bends to the fight or that our head is tilted as we
bend to pick up a dime. The brain interprets this
information--along with cues from the vestibular system--and
adjusts the muscles so that balance is maintained.
Dizziness can occur when sensory information is distorted.
Some people feel dizzy at great heights, for instance, partly
because they cannot focus on nearby objects to stabilize
themselves. When one is on the ground, it is normal to sway
slightly while standing. A person maintains balance by
adjusting the body's position to something close by. But when
someone is standing high up, objects are too far away to use to
adjust balance. The result can be confusion, insecurity, and
dizziness, which is sometimes resolved by sitting down.
Some scientists believe that motion sickness, a malady
that affects sea, car; and even space travelers, occurs when
the brain receives conflicting sensory information about the
body's motion and position. For example, when someone reads
while riding in a car, the inner ear senses the movement of the
vehicle, but the eyes gaze steadily on the book that is not
moving. The resulting sensory conflict may lead to the typical
symptoms of motion sickness: dizziness, nausea, vomiting, and
sweating.
Another form of dizziness occurs when we turn around in a
circle quickly several times and then stop suddenly. Turning
moves the endolymph. The moving endolymph tells us we are still
rotating but our other senses say we've stopped. We feel dizzy.
Now go play in traffic.