dizzy. Although you have stopped, the fluid in the semi-
circular canal continues moving for a while. It stimu-
lates hair cells and gives you the impression that you
are still in motion. When the fluid stops moving, you feel
stationary again.
The movements of hair cells within the vestibular ap-
paratus generate action potentials and send nerve impuls-
es through the vestibular branch of the vestibulocochlear
nerve to various parts of the brain including the medulla,
pons, cerebellum, and vestibular area of the cortex in the
parietal lobe. The vestibular nuclei in the pons also re-
ceive and integrate sensory information from the eyes and
proprioceptors in the head and neck muscles. This infor-
mation goes to the cortex and cerebellum, which coordi-
nate muscle movements.
CONCEPT CHECK
1.
Which
structure in the ear first vibrates with
incoming sound waves?
2.
How
does the ear change sound waves into
nerve impulses?
3.
What
does the otolithic membrane do?
4.
What
type of equilibrium would be used to
determine the head position when an individual
is doing a somersault?
b. Dynamic equilibrium maintains the head position
relative
to
rotational acceleration
or
deceleration.
The ampullae within the semicircular canals
contain sites of
hair
cells in cristae involved in
dynamic equilibrium. The semicircular canals
cover acceleration in three dimensions.
How Ampullae
Work
Turning the head accelerates fluid within the
semicircular canals. The fluid drags the
cupula, which bends the hair cells. The
bending hair cells send electrical signals to
the sensory neurons in the vestibular branch
of the vestibulocochlear (VIII) nerve.
Cupula
Cupula -
jellylike mass into which
the hair bundles protrude
Crista -
elevation within the
ampulla containing
the following:
Hair
bundle - sensory
portion of hair cells that
protrude into cupula
Hair
cells -
sensory receptors
• Supporting cells -
support hair cells
Ampullary nerve
Ampulla
Ampullary
Head in still position
As head
rotates in one direction, cupula
is dragged through endolymph
and bent in opposite direction.
Head rotating
The Ear is involved in Hearing and Equilibrium 245
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