Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 13Hearing & Equilibrium 215

increase in the firing rate of single nerve fibers from the crista,
whereas movement in the opposite direction commonly in-
hibits 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 stimu-
lation 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 horizon-
tal 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 di-
rection, distorting the hair cell processes and generating activ-
ity 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

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 initi-

ated 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.

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 decelera-
tion. 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.)

0 10 20 30 40 50 60

Time (s)

Rotation

Angular

velocity

Frequency of impulses/s

60

40

20

0

60

40

20

0

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 rota-

tion starts, the eyes move slowly in a direction opposite to the

direction of rotation, maintaining visual fixation (vestibulo-

ocular 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 laby-

rinths; 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 direc-

tion of eye movement in nystagmus is identified by the direc-

tion of the quick component. The direction of the quick com-

ponent during rotation is the same as that of the rotation, but

the postrotatory nystagmus that occurs owing to displace-

ment of the cupula when rotation is stopped is in the oppo-

site 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 af-

fecting semicircular canals or after damage to the flocculon-

odular lobe or midline structures such as the fastigial nucleus.

Nystagmus can be used as a diagnostic indicator of the integ-

rity of the vestibular system. Caloric stimulation can be used

to test the function of the vestibular labyrinth. The semicircu-

lar canals are stimulated by instilling warm (40 °C) or cold

(30 °C) water into the external auditory meatus. The tempera-

ture 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 le-

sion. 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.