Porth's Essentials of Pathophysiology, 4e

985

Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function

C h a p t e r 3 8

Head rotation

Osseous labyrinth (otic capsule)

Utricle

Utricle

Cupula

Ampulla

Anterior (superior) duct Semicircular ducts:

Maculae

Hair cells

CN VIII

Saccule

Posterior (inferior) duct

Ductus reuniens

Lateral (horizontal) canal

Ampullae

Endolymphatic sac

B

A

Otoliths

Force of gravity

Macula

Otolithic membrane Hair cells Supporting cells

D

CN VIII nerve fiber

C

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.

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