Sensory Physiology
223
skull, and because of inertia, tends to retain its original posi-
tion (i.e., to be “left behind”). Thus, the moving ampulla is
pushed against the stationary fl uid, which causes bending of
the stereocilia and alteration in the rate of release of a chemi-
cal transmitter from the hair cells. This transmitter activates
the nerve terminals synapsing with the hair cells.
The speed and magnitude of rotational head movements
determine the direction in which the stereocilia are bent and
the hair cells stimulated. Movement of these mechanorecep-
tors causes depolarization of the hair cell and neurotrans-
mitter release as in cochlear hair cells. Neurotransmitter is
released from the hair cells at rest, and the release increases or
decreases from this resting rate according to the direction in
which the hairs are bent. Each hair cell receptor has one direc-
tion of maximum neurotransmitter release; when its stereo-
cilia are bent in this direction, the receptor cell depolarizes.
When the stereocilia are bent in the opposite direction, the
cell hyperpolarizes (
Figure 7–42
). The frequency of action
potentials in the afferent nerve fi
bers that synapse with the
hair cells is related to both the amount of force bending the
stereocilia on the receptor cells and to the direction in which
this force is applied.
When the head continuously turns at a steady velocity,
the duct fl uid begins to move at the same rate as the rest of the
head, and the stereocilia slowly return to their resting posi-
tion. For this reason, the hair cells are stimulated only dur-
ing
changes
in the rate of rotation (i.e., during acceleration or
deceleration) of the head.
The Utricle and Saccule
The utricle and saccule (see Figure 7–39) provide information
about
linear
—up and down, back and forth—acceleration and
changes in head position relative to the forces of gravity. Here,
too, the receptor cells are mechanoreceptors sensitive to the dis-
placement of projecting hairs. The hair cells in the utricle point
nearly straight up when you stand, and they respond when you
tip your head away from the horizontal, or to linear accelerations
in the horizontal plane. In the saccule, hair cells project at right
angles to those in the utricle, and they respond when you move
from a lying to a standing position, or to vertical accelerations
like those produced when you jump on a trampoline. The utricle
and saccule are slightly more complex than the ampullae.
The stereocilia projecting from the hair cells are covered
by a gelatinous substance in which tiny stones, or
otoliths,
are
embedded. The otoliths, which are calcium carbonate crystals,
make the gelatinous substance heavier than the surrounding
uid. In response to a change in position, the gelatinous oto-
lithic material moves according to the forces of gravity and
pulls against the hair cells so that the stereocilia on the hair
cells bend and the receptor cells are stimulated.
Figure 7–43
demonstrates how otolith organs are stimulated by a change
in head position.
Vestibular Information and Pathways
Vestibular information is used in three ways. One is to control
the eye muscles so that, in spite of changes in head position,
the eyes can remain fi xed on the same point.
Nystagmus
is
a large, jerky, back-and-forth movement of the eyes that can
occur in response to unusual vestibular input in normal people;
it can also be a pathological sign. Nystagmus is noticeable
when a person spins in a swiveling chair for about 20 sec-
onds, then abruptly stops the chair. For a short time after the
motion ceases, the fl uid in the semicircular canals continues to
spin and the person’s eyes will involuntarily move as though
attempting to track objects spinning past the fi eld of view.
The second use of vestibular information is in refl ex
mechanisms for maintaining upright posture and balance.
The vestibular apparatus plays a role in the support of the
head during movement, orientation of the head in space, and
refl exes accompanying locomotion. Very few postural refl exes,
Ampulla wall
Ampulla
(a)
(b)
At rest
Cupula
Hair cell
Rotation
of head
Semicircular
duct
Stereocilia
Hair cell
Support
cell
Cupula
Pressure
exerted by
stationary
fluid
Figure 7–41
(a) Organization of a cupula and ampulla. (b) Relation of the cupula
to the ampulla when the head is at rest and when it is accelerating.
(a)
(b)
(c)
Resting activity
Stimulation
(depolarization)
Inhibition
(hyperpolarization)
Discharge rate of vestibular nerve
Figure 7–42
The relationship between the position of hairs in the ampulla and
action potential fi ring in afferent neurons. (a) Resting activity.
(b) Movement of hairs in one direction increases the action potential
frequency in the afferent nerve activated by the hair cell.
(c) Movement in the opposite direction decreases the rate relative to
the resting state.
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