SenSory SyStemS 275What is the sense of balance?- Balance is the sense of the natural position of the body or
its parts. - Our balance sense relies mainly on signals from the vestibular
apparatus, a system of fluid-filled canals and sacs in the inner ear. - The semicircular canals lie at angles that correspond to the three
planes of space. Sensory receptors inside them detect rotation,
acceleration, and deceleration of the head. - Otolith organs contain sensory hair cells embedded in a
jellylike membrane. Movements of the membrane and otoliths
signal changes in the head’s orientation relative to gravity, as
well as straight-line acceleration and deceleration.
taKe-Home message
Although the hearing apparatus of our ears is remarkably
sturdy, a variety of illnesses and injuries can damage it.
Children have short eustachian tubes, so they especially
are susceptible to otitis media—a painful inflammation
of the middle ear that usually is caused by the spread of
a respiratory infection such as a cold. An antibiotic is the
usual treatment, although resistant infections are now
common. In some cases pus and fluid can build up and
cause the eardrum to tear. The rupture usually will heal
on its own.
Ear infections, taking lots of aspirin, and genetic factors
can cause the ringing, whirring, or buzzing in the ears
known as tinnitus. While the condition is not a serious
health threat, it can be extremely annoying.
Deafness is the partial or complete inability to hear.
Some people suffer from congenital (inborn) deafness, and
in other cases aging, disease, or environmentally caused
damage is the culprit. About one-third of adults in the
United States will suffer significant hearing loss by the
time they are 65. Researchers believe that most cases of this
progressive deafness are due to the long-term effects of liv-
ing in a noisy world.
The loudness of a sound is measured in decibels. A
quiet conversation occurs at about 50 decibels. Rustling
papers make noise at a mere 20 decibels. The delicate
sensory hair cells in the inner ear (Figure 14.13) begin to
be damaged when a person is exposed to sounds louder
than about 75–85 decibels over long periods. Some MP3
players can crank out sound at well over 100 decibels. At
130 decibels—typical of a rock concert or shotgun blast—
permanent damage can occur much more quickly. Protec-
tive earwear is a must for anyone who regularly operates
noisy equipment or who works around noisy machinery
such as aircraft.Disorders of
the ear
F i g u r e 14.13 Noise is a danger to the ear’s hair cells.
A Healthy sensory hair cells of the inner ear. b Hair cells
damaged by exposure to loud noise. (© Robert E. Preston, courtesy
Joseph E. Hawkins, Kresge Hearing Research Institute, University of Michigan
Medical School)A BOuter Hair
CellsScarsmass shifts, and tips of the hair cells bend (Figure 14.11).
The otoliths also press on hair cells if your head accelerates,
as when you start running or are riding in an accelerating
vehicle.
Nerve impulses from the vestibular apparatus travel
to reflex centers in the brain stem. As the signals are pro-
cessed along with information from your muscles and eyes,
the brain orders compensating movements that help you
keep your balance when you stand, walk, dance, or move
your body in other ways. It also helps you maintain “a
sense of where you are” when you turn a cartwheel or ride
in a roller coaster (Figure 14.12).
Motion sickness can result when extreme or continuous
motion overstimulates hair cells in the balance organs. It
can also be caused by conflicting signals from the ears and
eyes about motion or the head’s position. If you are prone to
motion sickness, you know all too well that nerve impulses
triggered by the sensory input can reach a brain center that
governs the vomiting reflex.F i g u r e 14.12 Riding in a roller coaster gives the vestibular
apparatus a workout. It must process rapidly changing signals
from the eyes, otoliths, and muscles. In some people a roller
coaster ride produces motion sickness leading to nausea or
even vomiting.© Jabiru/Shutterstock.com14.7
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