C h a p t e r 3 8
Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function
985
motion. When the head is tilted, the gelatinous mass
shifts its position because of the pull of the gravitational
field, bending the cilia of the macular hair cells (see
Fig. 38-21D). In a condition called
benign paroxysmal
positional vertigo
(sensation of whirling or spinning
motion), the otoliths become dislodged from their
gelatinous base, causing a vertigo that is precipitated by
changes in the recumbent head position (to be discussed).
The response to body imbalance, such as stumbling,
must be fast and reflexive. Hence, information from the
vestibular system goes directly to vestibular nuclei in the
brain stem or to the cerebellum. The vestibular nuclei,
which form the main integrative center for balance, also
receive input from visual and somatic receptors, par-
ticularly from stretch receptors in the neck muscles that
report the angle or inclination of the head. The vestibular
nuclei integrate this information and then send impulses
to the brain stem centers that control the extraocular
eye movements and reflex movements of the neck, limb,
and trunk muscles. These reflex movements include the
vestibulo-ocular reflexes
that keep the eyes still as the
head moves and the
vestibulospinal reflexes
that allow
the body to maintain or regain balance. Neurons of the
vestibular nuclei also project to the thalamus and cor-
tex, providing the basis for the subjective experiences
of position in space and of rotation. Moreover, connec-
tions with the chemoreceptor trigger zone of the brain
stem stimulate the vomiting center, accounting for the
nausea and vomiting that often are associated with
motion sickness and vestibular disorders.
Nystagmus
The term
nystagmus
refers to the involuntary eye
movements that preserve eye fixation on stable objects
in the visual field during angular and rotational
movements of the head.
16
As the body rotates, the
vestibulo-ocular reflexes cause a slow compensatory
FIGURE 38-21.
Vestibular labyrinth.
(A)
Osseous and membranous vestibular labyrinth of the
middle ear showing the utricle and saccule with their maculae and three semicircular ducts and their
ampullae.
(B)
Diagram of the crista ampullaris within the semicircular duct and location of the cupula
and movement of the hair cells with head movement.
(C)
Relationship of the otoliths to the sensory
hair cells, which synapse with the sensory endings of the vestibular nerve (CNVIII) in the maculae of
the utricle and saccule.
(D)
Movement of the otoliths and bending of the macular hair cells when the
head is tilted due to the forces of gravity.
Osseous labyrinth
(otic capsule)
Utricle
Maculae
Saccule
Endolymphatic sac
Ductus
reuniens
Ampullae
Lateral
(horizontal) canal
Posterior
(inferior) duct
Anterior
(superior) duct
Semicircular ducts:
A
Head rotation
Utricle
Cupula
Ampulla
Hair cells
CN VIII
B
Macula
Otoliths
Otolithic
membrane
Hair cells
C
Supporting
cells
CN VIII nerve fiber
D
Force of
gravity
