HUMAN BIOLOGY

274 Chapter 14

A common disorder of the vestibular apparatus

is called vertigo. It produces a sensation that

the surroundings are spinning or whirling. A viral

infection, head injury, or other conditions can cause

vertigo. Research this topic on the Web or in the

library. Is vertigo differerent from dizziness? How is

the disorder treated?

ThInk ouTSIde The book

semicircular canals
Organs having sensory
receptors attuned to the
head’s position in space.

vestibular apparatus
Inner ear region containing
the semicircular canals plus
sacs where receptors moni-
tor straight-line movements.

Balance: sensing the Body’s natural position

n    A balance sense helps the brain assess changes from the

body’s natural or “equilibrium” position.

Our sense of balance relies partly on messages from

receptors in our eyes, skin, and joints. In addition, there

are organs of equilibrium located in a part of the inner

ear called the vestibular apparatus. This “appara-

tus” is a closed system of sacs and three fluid-filled

semicircular canals (Figure 14.10). The canals are

positioned at right angles to one another, corresponding

to the three planes of space. Inside them, some sensory

receptors monitor dynamic equilibrium—that is, rotat-

ing head movements. Elsewhere in the vestibular appa-

ratus are the receptors that monitor the straight-line

movements of acceleration and deceleration.

The receptors attuned to rota-

tion are on a ridge of the swollen

base of each semicircular canal

(Figure 14.11). As in the cochlea,

these receptors are sensory hair

cells; their delicate hairs project

up into a jellylike cupula (“lit tle

cap”). When your head rotates

horizontally or vertically or

tilts diagonally, fluid in a canal

corresponding to that direction moves in the opposite

direction. As the fluid presses against the cupula, the

hairs bend. This bending is the first step leading to nerve

impulses that travel to the brain—in this case, along the

vestibular nerve.

The receptors attuned to the head’s position in space are

located in two fluid-filled sacs in the vestibular apparatus,

the utricle and saccule shown in Figure 14.10. Each sac con-

tains an otolith organ, which has hair cells embedded in a

jellylike “membrane.” The material also contains hard bits

of calcium carbonate called otoliths (“ear stones”). Move-

ments of the membrane and otoliths signal changes in the

head’s orientation relative to gravity, as well as straight-line

acceleration and deceleration. For example, if you tilt your

head, the otoliths slide in that direction, the membrane

gelatinous membrane

in a semicircular canal

hair cells with their cilia

embedded in membrane

sensory neurons

semicircular

canals

saccule

vestibular

nerve

utricle

A

B

F i g u r e 14.10 Animated! The vestibular apparatus is

an organ of equilibrium. (© Cengage Learning)

sensory

neuron

hair

cell

stereocilium otolith

cupula

F i g u r e 14.11 Otoliths move when the head tilts. (© Cengage Learning)

14.6

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