Essentials of Anatomy and Physiology

Semicircular Canals
The three semicircular canalsare fluid-filled mem-
branous ovals oriented in three different planes. At the
base of each is an enlarged portion called the ampulla
(see Fig. 9–8), which contains hair cells (the crista) that
are affected by movement. As the body moves for-
ward, for example, the hair cells are bent backward
at first and then straighten (see Fig. 9–10). The bend-
ing of the hair cells generates impulses carried by the
vestibular branch of the 8th cranial nerve to the
cerebellum, midbrain, and temporal lobes of the cere-

brum. These impulses are interpreted as starting or

stopping, and accelerating or decelerating, or chang-

ing direction, and this information is used to maintain

equilibrium while we are moving (see Box 9–6: Motion

Sickness).

In summary then, the utricle and saccule provide

information about the position of the body at rest,

while the semicircular canals provide information

about the body in motion. Of course, there is some

overlap, and the brain puts all the information together

to create a single sense of body position.

The Senses 215

Utricle

Saccule

Otoliths

Hair cells

Gravity

Head tilted

8th cranial

nerve

Head upright

Ampulla

Crista

Ampulla

Hair

cells

8th cranial

nerve

1.

At rest

2.

Starting

3.

Moving

4.

Stopping

5.

At rest

A

B

Figure 9–10. Physiology of
equilibrium. (A) Utricle and
saccule. (B) Semicircular canals.
See text for description.
QUESTION: In part A, what
causes the hair cells to bend? In
part B, what causes the hair
cells to sway?