Human Physiology, 14th edition (2016)

282 Chapter 10

cord stimulate movements of the head, neck, and limbs. Move-
ments of the eyes and body produced by these pathways serve
to maintain balance and “track” the visual field during rotation.

Nystagmus and Vertigo

When a person first begins to spin, the inertia of endolymph
within the semicircular ducts causes the cupula to bend in the
opposite direction. As the spin continues, however, the iner-
tia of the endolymph is overcome and the cupula straightens.
At this time, the endolymph and the cupula are moving in the
same direction and at the same speed. If movement is suddenly
stopped, the greater inertia of the endolymph causes it to con-
tinue moving in the previous direction of spin and to bend the
cupula in that direction.
Bending of the cupula affects muscular control of the eyes
and body through the neural pathways previously discussed.
During a spin, this produces smooth movements of the eyes
in a direction opposite to that of the head movement so that
a stable visual fixation point can be maintained. When the
spin is abruptly stopped, the eyes continue to move smoothly
in the former direction of the spin (because of the continued
bending of the cupula) and then are jerked rapidly back to the
midline position. This produces involuntary oscillations of
the eyes called vestibular nystagmus. People experiencing
this effect may feel that they, or the room, are spinning. The
loss of equilibrium that results is called vertigo.
Vertigo as a result of spinning is a natural response of the
vestibular apparatus. Pathologically, vertigo may be caused by
anything that alters the firing rate of one of the vestibuloco-
chlear nerves (right or left) compared to the other. This is usu-
ally due to a viral infection causing vestibular neuritis. Severe
vertigo is often accompanied by dizziness, pallor, sweating,
nausea, and vomiting due to involvement of the autonomic
nervous system, which is activated by vestibular input to the
brain stem.

Figure 10.17 Neural pathways involved in the
maintenance of equilibrium and balance. Sensory input
enters the vestibular nuclei and the cerebellum, which coordinate
motor responses.

Cerebellum

Eyes

Vestibular

apparatus

Joint, tendon,

muscle, and

cutaneous receptors

Vestibular nuclei

(brain stem)

Oculomotor

center

(control of eye

movements)

Spinal cord

(control of body

movements)

CLINICAL APPLICATION

A person with Meniere’s disease has rotational vertigo (a

sense of spinning), possibly vestibular nystagmus, and tinni-

tus (ringing in the ears). Tinnitus and hearing loss, which can

become permanent as the disease progresses, accompany

the vertigo because endolymph of the vestibular apparatus

and cochlea are continuous through a tiny canal. Exces-

sive endolymph fluid, producing dilation and damage to the

membranous labyrinth, has been suggested as a cause of

Meniere’s disease.

Clinical Investigation CLUES

Susan had a ringing in her ears and a feeling that the

room was spinning.

• What are the medical terms for these conditions?


• What portion of the ear is affected?

| CHECKPOINT

7a. Describe the structure of the utricle and saccule and

explain how linear acceleration results in stimulation

of the hair cells within these organs.

7b. Describe the structure of the semicircular canals

and explain how they provide a sense of angular

acceleration.

10.5 The Ears and Hearing

Sound causes movements of the tympanic membrane and

middle-ear ossicles that are transmitted into the fluid-filled

cochlea. This produces vibrations of the basilar membrane,

which is coated with hair cells. Bending of hair cell stereo-

cilia causes the production of action potentials, which are

interpreted by the brain as sound.

LEARNING OUTCOMES

After studying this section, you should be able to:

8. Explain how sound waves result in movements of the
   oval window and then the basilar membrane.


9. Explain how movements of the basilar membrane at
   different sound frequencies (pitches) affect hair cells.


10. Describe how action potentials are produced, and
    their neural pathways.

Sound waves are alternating zones of high and low pressure

traveling in a medium, usually air or water. (Thus, sound waves

cannot travel in space.) Sound waves travel in all directions

from their source, like ripples in a pond where a stone has been