580
UNIT 3
Regulation and Integration of the Body
15
called our sense of
static equilibrium
. Te semicircular canals
monitor changes in head rotation, sometimes called our sense
of
dynamic equilibrium
.
The Maculae
Te
maculae
(mak
9
u-le; “spots”), one in each saccule wall and
one in each utricle wall
(Figure 15.33)
, are sensory receptor
organs that monitor the position of the head in space. In so do-
ing, they play a key role in controlling posture. Tey respond to
linear
acceleration, that is, changes in straight-line speed and
direction, but not rotation.
Anatomy of a Macula
Each macula is a flat epithelial patch
containing
hair cells
. Tese hair cells, like all those of the inter-
nal ear, have stereocilia plus one kinocilium that project into a
gel-like mass.
Supporting cells
surround the macula’s scattered
hair cells (Figure 15.33). Te “hairs” of the hair cells are em-
bedded in the overlying
otolith membrane
(o
9
to-lith), a jelly-
like mass studded with tiny stones (calcium carbonate crystals)
called
otoliths
(“ear stones”). Te otoliths, though small, are
dense and they increase the membrane’s weight and its inertia
(resistance to changes in motion).
In the utricle, the macula is horizontal, and the hairs are ver-
tically oriented when the head is upright (Figure 15.33). For
this reason, the utricular maculae respond best to acceleration
in the horizontal plane and tilting the head to the side, because
vertical (up-down) movements do not displace their horizontal
otolith membrane.
In the saccule, on the other hand, the macula is nearly vertical,
and the hairs protrude horizontally into the otolith membrane.
Te saccular maculae respond best to vertical movements, such
as the sudden acceleration of an elevator.
Te hair cells synapse with fibers of the
vestibular nerve
,
whose endings coil around their bases. Like the cochlear nerve,
the vestibular nerve is a subdivision of the vestibulocochlear
nerve (VIII). Te cell bodies of the sensory neurons are located
in the nearby
superior
and
inferior vestibular ganglia
(see Fig-
ure 15.26).
Activating Macula Receptors
What happens in the maculae
that leads to sensory transduction? When your head starts or
stops moving in a linear direction, inertia causes the otolith
membrane to slide backward or forward like a greased plate
over the hair cells, bending the hairs. For example, when you
start to run, the otolith membranes of the utricle maculae lag
behind, bending the hairs backward. When you suddenly stop,
the otolith membrane slides abruptly forward (just as you slide
forward in your car when you brake), bending the hair cells
forward. Likewise, when you nod your head or fall, the otoliths
roll inferiorly, bending the hairs of the maculae in the saccules.
Te hair cells release neurotransmitter continuously but
movement of their hairs modifies the amount they release
(Figure 15.34)
.
When the hairs bend toward the kinocilium, the hair cells
depolarize, stepping up their pace of neurotransmitter re-
lease, and more impulses travels up the vestibular nerve to
the brain.
Macula of
utricle
Macula of
saccule
Otoliths
Hair bundle
Kinocilium
Stereocilia
Otolith
membrane
Vestibular
nerve fibers
Hair cells
Supporting
cells
Figure 15.33
Structure of a macula.
The “hairs” of the receptor
cells of a macula project into the gelatinous otolith membrane.
Vestibular nerve fibers surround the base of the hair cells.
Equilibrium and Orientation
Explain how the balance organs of the semicircular canals
and the vestibule help maintain equilibrium.
Te equilibrium sense is not easy to describe because it does not
“see,” “hear,” or “feel,” but responds (frequently without our aware-
ness) to various head movements. Furthermore, this sense de-
pends not only on inputs from the internal ear but also on vision
and information from stretch receptors of muscles and tendons.
Te equilibrium receptors in the semicircular canals and ves-
tibule are collectively called the
vestibular apparatus
. Under
normal conditions, they send signals to the brain that initiate
reflexes needed to make the simplest changes in position as well
as more complex moves such as serving a tennis ball precisely to
the right spot (your opponent’s backhand).
Te equilibrium receptors of the vestibular apparatus can be
divided into two functional arms. Te receptors in the vestibule
monitor linear acceleration and the position of the head with
respect to gravity. Because gravity is constant, this is sometimes
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