Chapter 15
The Special Senses
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15
direction causes hyperpolarization and generates fewer im-
pulses. Because the axes of the hair cells in the complementary
semicircular ducts are opposite, rotation in a given direction
depolarizes the receptors in one ampulla of the pair, and hyper-
polarizes the receptors in the other.
If the body continues to rotate at a constant rate, the endo-
lymph eventually comes to rest—it moves along at the same
speed as the body and the hair cells are no longer stimulated.
Consequently, if we are blindfolded, we cannot tell whether we
are moving at a constant speed or not moving at all aFer the first
few seconds of rotation. However, when we suddenly stop mov-
ing, the endolymph keeps on going, in effect reversing its direc-
tion within the canal. Tis sudden reversal in the direction of
hair bending alters membrane voltage in the receptor cells and
modifies the rate of impulse transmission, which tells the brain
that we have slowed or stopped (±igure 15.35c, right panel).
Te key point to remember when considering both types of
equilibrium receptors is that the rigid bony labyrinth moves
with the body, while the fluids (and gels) within the membra-
nous labyrinth are free to move at various rates, depending
on the forces (such as gravity, acceleration, and so on) acting
on them.
Vestibular Nystagmus
Impulses transmitted from the semi-
circular canals are particularly important to reflex movements
of the eyes.
Vestibular nystagmus
is a complex of rather strange
eye movements that occurs during and immediately aFer
rotation.
As you rotate, your eyes slowly driF in the opposite direction,
as though fixed on some object in the environment. Tis reac-
tion relates to the backflow of endolymph in the semicircular
canals. Ten, because of CNS compensating mechanisms, the
eyes jump rapidly toward the direction of rotation to establish a
new fixation point. Tese alternating eye movements continue
until the endolymph comes to rest.
When you stop rotating, at first your eyes continue to move
in the direction of the previous spin, and then they jerk rapidly
in the opposite direction. Tis sudden change is caused by the
change in the direction in which the cristae bend aFer you stop.
Nystagmus is oFen accompanied by vertigo.
When the hairs bend away from the kinocilium, the recep-
tors hyperpolarize and release less neurotransmitter, gener-
ating fewer impulses.
In either case, the brain is informed of the changing position of
the head in space.
It is important to understand that the maculae respond to
changes
in the velocity of head movement (linear acceleration or
deceleration). Because most hair cells adapt quickly (resuming
their basal level of neurotransmitter release), they do not report
on unchanging head positions. In this way, the maculae help us
to maintain normal head position with respect to gravity.
The Cristae Ampullares
Te receptor for rotational acceleration, called the
crista am-
pullaris
or simply
crista
, is a minute elevation in the ampulla
of each semicircular canal (see ±igure 15.26). Like the macu-
lae, the cristae are excited by head movement (acceleration and
deceleration), but in this case the major stimuli are rotational
(angular) movements. When you twirl on the dance floor or
suffer through a rough boat ride, these gyroscope-like receptors
are working overtime. Since the semicircular canals are located
in all three planes of space, all rotational movements of the head
disturb one or another
pair
of cristae (one in each ear).
Anatomy of a Crista Ampullaris
Each crista is composed of
supporting cells and hair cells whose structure and function are
basically the same as the hair cells of the cochlea and maculae. In
this case, the gelled mass is an
ampullary cupula
(ku
9
pu-lah),
which resembles a pointed cap
(Figure 15.35)
. Te cupula is
a delicate, loosely organized network of gelatinous strands that
radiate outward to contact the “hairs” of each hair cell. Den-
drites of vestibular nerve fibers encircle the base of the hair cells.
Activating Crista Ampullaris Receptors
Te cristae respond
to
changes
in the velocity of rotational movements of the head.
Because of its inertia, the endolymph in the semicircular ducts
moves briefly in the direction
opposite
the body’s rotation, de-
forming the crista in the duct. As the hairs bend, the hair cells
depolarize and impulses reach the brain at a faster rate (±ig-
ure 15.35c, middle panel). Bending the cilia in the opposite
Otolith
membrane
Kinocilium
Stereocilia
Receptor potential
Nerve impulses generated
in vestibular fiber
When hairs bend toward
the kinocilium, the hair cell
depolarizes, exciting the
nerve fiber, which generates
more frequent action potentials.
When hairs bend away
from the kinocilium, the hair cell
hyperpolarizes, inhibiting the nerve
fiber, and decreasing the action
potential frequency.
Depolarization
Hyperpolarization
Figure 15.34
The effect of gravitational pull on a macula receptor cell in the utricle.
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