584
UNIT 3
Regulation and Integration of the Body
15
Recent evidence suggests that tinnitus is analogous to phan-
tom limb pain. In other words, it is “phantom cochlear noise”
caused by destruction of some neurons in the auditory pathway
and the subsequent ingrowth of nearby neurons whose signals
are interpreted as noise by the CNS.
Ménière’s Syndrome
Classic
Ménière’s syndrome
(men
0
ē-ārz
9
) is a labyrinth disor-
der that seems to affect all three parts of the internal ear. Te
afflicted person has repeated attacks of vertigo, nausea, and
vomiting. Balance is so disturbed that standing erect is nearly
impossible. A “howling” tinnitus is common, so hearing is im-
paired (and ultimately lost) as well. Te cause is uncertain, but
it may result from excessive endolymph that distorts the mem-
branous labyrinth, or a membrane rupture that allows the peri-
lymph and endolymph to mix.
Mild cases can usually be managed by antimotion drugs or a
low-salt diet and diuretics to decrease endolymph fluid volume.
In severe cases, draining the excess endolymph from the inter-
nal ear may help. A last resort is removal of the entire malfunc-
tioning labyrinth, which is usually deferred until hearing loss is
complete.
Check Your Understanding
16.
Six-year-old Mohammed has a cold and says his ears feel
“full” and he “can’t hear well.” Explain what has happened
in Mohammed’s ears. Which type of deafness does
Mohammed have—conduction or sensorineural?
For answers, see Appendix H.
Developmental Aspects
of the Special Senses
List changes that occur in the special sense organs with
aging.
Taste and Smell
All the special senses are functional, to a greater or lesser degree,
at birth. Smell and taste are sharp, and infants relish food that
adults consider bland or tasteless. Some researchers claim that
smell is just as important as touch in guiding newborn infants
to their mother’s breast. However, very young children seem
indifferent to odors and can play happily with their own feces.
Tere are few problems with the chemical senses during
childhood and young adulthood. Women generally have a more
acute sense of smell than men, and nonsmokers have a sharper
sense of smell than smokers.
Beginning in the fourth decade of life, our ability to taste and
smell declines due to the gradual loss of receptors, which are
replaced more slowly than in younger people. More than half
of people over age 65 have serious problems detecting odors,
which may explain why some tend to douse themselves with
large amounts of cologne, or pay little attention to formerly
Check Your Understanding
15.
For each of the following phrases, indicate whether it applies
to a macula or a crista ampullaris: inside a semicircular
canal; contains otoliths; responds to linear acceleration
and deceleration; has a cupula; responds to rotational
acceleration and deceleration; inside a saccule.
For answers, see Appendix H.
Homeostatic Imbalances
of Hearing and Equilibrium
Deafness
Any hearing loss, no matter how slight, is
deafness
of some sort.
Deafness is classified as conduction or sensorineural deafness,
according to its cause.
Conduction deafness
occurs when something hampers
sound conduction to the fluids of the internal ear. For example,
compacted earwax can block the external acoustic meatus, or a
perforated
(
ruptured
) eardrum can prevent sound conduction
from the eardrum to the ossicles. But the most common causes
of conduction deafness are middle ear inflammations (otitis
media) and
otosclerosis
(o
0
to-sklĕ-ro
9
sis) of the ossicles.
Otosclerosis (“hardening of the ear”) occurs when over-
growth of bony tissue fuses the base of the stapes to the oval
window or welds the ossicles to one another. In such cases,
vibrations of the skull bones conduct sound to the receptors
of that ear, which is far less satisfactory. Otosclerosis is treated
surgically.
Sensorineural deafness
results from damage to neural struc-
tures at any point from the cochlear hair cells to and including
the auditory cortical cells. Tis type of deafness typically results
from the gradual loss of hair cells throughout life. Hair cells can
also be destroyed at an earlier age by a single explosively loud
noise or prolonged exposure to high-intensity sounds, such as
rock bands or airport noise, which literally tear off their cilia.
Other causes of sensorineural deafness are degeneration of the
cochlear nerve, strokes, and tumors in the auditory cortex.
Hair cells don’t normally regenerate in mammals, but re-
searchers are seeking ways to prod supporting cells into dif-
ferentiating into hair cells. For congenital defects or age- or
noise-related cochlear damage, cochlear implants (devices that
convert sound energy into electrical signals) can be inserted
into a drilled recess in the temporal bone. Te original cochlear
implants made human voices sound tinny and robotlike, but
modern versions are so effective that even children born deaf
can hear well enough to learn to speak well.
Tinnitus
Tinnitus
(tĭ-ni
9
tus) is a ringing or clicking sound in the ears in
the absence of auditory stimuli. It is usually a symptom rather
than a disease. For example, tinnitus is one of the first symptoms
of cochlear nerve degeneration. It may also signal inflammation
of the middle or internal ears and is a side effect of some medi-
cations, such as aspirin.
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