Equilibrium Comes in Two Varieties:
Static and Dynamic
Sometimes ballet dancers stumble, but on these rare oc-
casions, they usually regain their balance (equilibrium)
quickly. There are two types of equilibrium: static and dy-
namic. Static equilibrium maintains body position, spe-
cifically the position of the head, relative to the direction
of gravity, as when a ballerina is holding a pose. Dynamic
equilibrium maintains head and body positions when the
body is in motion (accelerating or decelerating), as might
occur when a ballerina is doing a pirouette. Different parts
of the vestibular apparatus are involved in the different
types of equilibrium.
The saccule and utricle of the vestibule are involved in
static equilibrium. They contain structures called
m aculae,
which contain hair cells that are covered by a crystal and
gelatinous layer (Figure 8.14a). The force of gravity pulls
on the crystal layer and bends the hair cells, which elicit
action potentials relaying position information.
The
a m p u lla
at the base of each semicircular canal is
involved in dynamic equilibrium. Within each ampulla is
a structure called a crista (KRIS-ta), which has embed-
ded sensory hair cells (Figure 8.14b). When you move
your head, the change of position causes the fluid within
the semicircular canal to move and bend the hair cells,
which elicit action potentials conveying movement in-
formation. The semicircular canals are oriented at right
angles to one another in three dimensions, so that you
can sense motion in all three dimensions of space. For
example, if you spin around and then stop, you may feel
Hair cells w ithin stru c tu re s o f th e v estib u lar a p p a ra tu s se n se equilibrium • Figure 8.14
membrane
a.
Static equilibrium maintains the head position
relative
to
the force of gravity. Both the utricle and
saccule contain
hair
cells in maculae involved in
static equilibrium. These
two
parts cover the
position of the head in three dimensions.
Otoliths -
dense layer of calcium
carbonate crystals
Otolithic membrane ■
jellylike substance
containing the otoliths
Hair
bundle -
sensory portion of hair
cells that protrude into
otolithic membrane
Hair
cells -
sensory
receptors
ncle
V
'
1
Saccule
j t /
Supporting cells -
support hair cells
Vestibular branches of vestibulocochlear (VIII) nerve
Key:
Sensory fiber
Motor fiber
Head upright
Force of
gravity
How Otoliths
Work
The force of gravity pulls on the
otolithic membrane and bends the
hair cells. The bending hair cells send
electrical signals to the sensory
neurons in the vestibular branch of
the vestibulocochlear (VIII) nerve.
Head tilted forward
244 CHAPTER 8
Somatic Senses and Special Senses
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