459
CHAPTER TWELVE
Nervous System III
S
everal factors can impair hearing, includ-
ing interference with transmission of
vibrations to the inner ear (
conductive
deafness
) or damage to the cochlea or the audi-
tory nerve and its pathways (
sensorineural deaf-
ness
). Disease, injury, and heredity all can impair
hearing. There are more than 100 forms of inher-
ited deafness, and many are part of syndromes.
About 8% of people have some degree of hear-
ing loss.
About 95% of cases of hearing loss are con-
ductive. One cause is accumulated dry wax or a
foreign object in the ear, which plugs the acous-
tic meatus. Changes in the tympanic membrane
or auditory ossicles can also block hearing. The
tympanic membrane may harden as a result of
disease, becoming less responsive to sound
waves, or an injury may tear or perforate it.
A common disorder of the auditory ossicles
is
otosclerosis,
in which new bone is deposited
abnormally around the base of the stapes. This
interferes with the movement of the ossicles that
is necessary to transmit vibrations to the inner ear.
Surgery often can restore some hearing to a per-
son with otosclerosis by chipping away the bone
that holds the stapes in position or replacing the
stapes with a wire or plastic substitute.
Two tests used to diagnose conductive deaf-
ness are the Weber test and the Rinne test. In the
Weber test, the handle of a vibrating tuning fork
is pressed against the forehead. A person with
normal hearing perceives the sound coming from
directly in front, whereas a person with sound
conduction blockage in one middle ear hears the
sound coming from the impaired side.
In the Rinne test, a vibrating tuning fork is
held against the bone behind the ear. After the
sound is no longer heard by conduction through
the bones of the skull, the fork is moved to just in
front of the external acoustic meatus. In middle
ear conductive deafness, the vibrating fork can no
longer be heard, but a normal ear will continue to
hear its tone.
Very loud sounds can cause sensorineural
deafness. If exposure is brief, hearing loss may
be temporary, but when exposure is repeated
and prolonged, such as occurs in foundries, near
jackhammers, or on a firing range, impairment
may be permanent. Such hearing loss begins
as the hair cells develop blisterlike bulges that
eventually pop. The tissue beneath the hair cells
swells and softens until the hair cells, and some-
times the neurons, leaving the cochlea become
blanketed with scar tissue and degenerate. Other
causes of sensorineural deafness include tumors
in the CNS, brain damage as a result of vascular
accidents, and the use of certain drugs.
Hearing loss and other ear problems can
begin gradually, so be aware of their signs, which
may include the following:
diF
culty hearing people talking softly
inability to understand speech when there is
background noise
ringing in the ears
dizziness
loss of balance
New parents should notice whether their
infant responds to sounds in a way that indi-
cates normal hearing. Hearing exams are part
of a well-baby visit to a doctor. If the baby’s
responses indicate a possible problem, the next
step is to see an audiologist, who identi±
es and
measures hearing loss.
Often a hearing aid can help people with
conductive hearing loss. A hearing aid has a tiny
microphone that picks up sound waves and con-
verts them to electrical signals, which are then
ampli±
ed. An ear mold holds the device in place,
either behind the outer ear, in the outer ear, or in
the ear canal.
12.5
CLINICAL APPLICATION
Hearing Loss
The utricle and saccule each has a small patch of hair
cells and supporting cells called a
macula
(mak
u-lah) on its
wall. When the head is upright, the hairs of the macula in
the utricle project vertically, while those in the saccule proj-
ect horizontally. In both the utricle and saccule, the hairs
contact a sheet of gelatinous material (otolithic membrane)
that has crystals of calcium carbonate (otoliths) embedded
on its surface. These particles add weight to the gelatinous
sheet, making it more responsive to changes in position. The
hair cells, which are sensory receptors, have nerve F bers
wrapped around their bases. These F
bers are associated with
the vestibular portion of the vestibulocochlear nerve.
Gravity stimulates hair cells to respond. This usually
happens when the head bends forward, backward, or to
one side. Such movements tilt the gelatinous mass of one
or more maculae, and as the material sags in response to
gravity, the hairs projecting into it bend. This action stim-
ulates the hair cells, and they signal their associated nerve
F
bers
(f
gs. 12.18
and
12.19)
. The resulting nerve impulses
travel into the CNS by means of the vestibular branch of the
vestibulocochlear nerve, informing the brain of the head’s
Sense of Equilibrium
The feeling of equilibrium derives from two senses—
static
equilibrium
(stat
ik e
kwı˘-lib
re-um) and
dynamic equilib-
rium
(di-nam
ik e
kwı˘-lib
re-um). Different sensory organs
provide these two components of equilibrium. The organs
associated with static equilibrium sense the position of the
head, maintaining stability and posture when the head and
body are still. When the head and body suddenly move or
rotate, the organs of dynamic equilibrium detect the motion
and aid in maintaining balance.
Static Equilibrium
The organs of static equilibrium are in the vestibule, a bony
chamber between the semicircular canals and the cochlea.
More speciF cally, the membranous labyrinth inside the vesti-
bule consists of two expanded chambers—a
utricle
(u
trı˘-kl)
and a
saccule
(sak
u
¯l). The larger utricle communicates with
the saccule and the membranous portions of the semicircular
canals; the saccule, in turn, communicates with the cochlear
duct
(f g. 12.17)
.
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