Ganong's Review of Medical Physiology, 23rd Edition

Frequency of impulses/s
60
40
20
0
60
40
20
0
Angular
velocity
Rotation
0 10 20 30 40 50 60
Time (s)
FIGURE 13–14 Ampullary responses to rotation. Average
time course of impulse discharge from the ampulla of two semicircular
canals during rotational acceleration, steady rotation, and deceleration.
Movement of the cupula in one direction increases the firing rate
of single nerve fibers from the crista, and movement in the opposite
direction inhibits neural activity. (Reproduced with permission from Adrian
ED: Discharge from vestibular receptors in the cat. J Physiol [Lond] 1943;101:389.)
increase in the firing rate of single nerve fibers from the crista,
whereas movement in the opposite direction commonly inhibits
neural activity (Figure 13–14).
Rotation causes maximal stimulation of the semicircular
canals most nearly in the plane of rotation. Because the canals
on one side of the head are a mirror image of those on the
other side, the endolymph is displaced toward the ampulla on
one side and away from it on the other. The pattern of stimulation
reaching the brain therefore varies with the direction as
well as the plane of rotation. Linear acceleration probably fails
to displace the cupula and therefore does not stimulate the
cristae. However, there is considerable evidence that when
one part of the labyrinth is destroyed, other parts take over its
functions. Clinical Box 13–2 describes the characteristic eye
movements that occur during a period of rotation.
RESPONSES TO LINEAR ACCELERATION
In mammals, the utricular and saccular maculae respond to
linear acceleration. In general, the utricle responds to horizontal
acceleration and the saccule to vertical acceleration. The
otoliths are more dense than the endolymph, and acceleration
in any direction causes them to be displaced in the opposite direction,
distorting the hair cell processes and generating activity
in the nerve fibers. The maculae also discharge tonically in
the absence of head movement, because of the pull of gravity
on the otoliths.
The impulses generated from these receptors are partly
responsible for labyrinth righting reflexes. These reflexes are
a series of responses integrated for the most part in the nuclei
of the midbrain. The stimulus for the reflex is tilting of the
head, which stimulates the otolithic organs; the response is
compensatory contraction of the neck muscles to keep the
head level. In cats, dogs, and primates, visual cues can initiate
optical righting reflexes that right the animal in the absence
of labyrinthine or body stimulation. In humans, the operation
CHAPTER 13 Hearing & Equilibrium 215
CLINICAL BOX 13–2
Nystagmus
The characteristic jerky movement of the eye observed at the
start and end of a period of rotation is called nystagmus. It is
actually a reflex that maintains visual fixation on stationary
points while the body rotates, although it is not initiated by
visual impulses and is present in blind individuals. When rotation
starts, the eyes move slowly in a direction opposite to the
direction of rotation, maintaining visual fixation (vestibuloocular
reflex, VOR). When the limit of this movement is
reached, the eyes quickly snap back to a new fixation point
and then again move slowly in the other direction. The slow
component is initiated by impulses from the vestibular labyrinths;
the quick component is triggered by a center in the
brain stem. Nystagmus is frequently horizontal (ie, the eyes
move in the horizontal plane), but it can also be vertical
(when the head is tipped sidewise during rotation) or rotatory
(when the head is tipped forward). By convention, the direction
of eye movement in nystagmus is identified by the direction
of the quick component. The direction of the quick component
during rotation is the same as that of the rotation, but
the postrotatory nystagmus that occurs owing to displacement
of the cupula when rotation is stopped is in the opposite
direction. Clinically, nystagmus is seen at rest in patients
with lesions of the brain stem. Nystagmus can persist for
hours at rest in patients with acute temporal bone fracture affecting
semicircular canals or after damage to the flocculonodular
lobe or midline structures such as the fastigial nucleus.
Nystagmus can be used as a diagnostic indicator of the integrity
of the vestibular system. Caloric stimulation can be used
to test the function of the vestibular labyrinth. The semicircular
canals are stimulated by instilling warm (40 °C) or cold
(30 °C) water into the external auditory meatus. The temperature
difference sets up convection currents in the endolymph,
with consequent motion of the cupula. In normal subjects,
warm water causes nystagmus that bears toward the stimulus,
whereas cold water induces nystagmus that bears toward the
opposite ear. This test is given the mnemonic COWS (Cold
water nystagmus is Opposite sides, Warm water nystagmus is
Same side). In the case of a unilateral lesion in the vestibular
pathway, nystagmus is reduced or absent on the side of the lesion.
To avoid nystagmus, vertigo, and nausea when irrigating
the ear canals in the treatment of ear infections, it is important
to be sure that the fluid used is at body temperature.
of these reflexes maintains the head in a stable position and
the eyes fixed on visual targets despite movements of the body
and the jerks and jolts of everyday life. The responses are initiated
by vestibular stimulation, stretching of neck muscles, and
movement of visual images on the retina, and the responses
are the vestibulo-ocular reflex and other remarkably precise
reflex contractions of the neck and extraocular muscles.