570
UNIT 3
Regulation and Integration of the Body
15
infections, head injuries, chemicals or medications, or head and
neck radiation for cancer treatment. Zinc supplements may
help some cases of radiation-induced taste disorders.
Check Your Understanding
10.
Name the five taste modalities. Name the three types of
papillae that have taste buds.
11.
Olfactory sensory neurons have cilia and gustatory epithelial
cells have hairs. How do these structures help the cells
perform their functions?
For answers, see Appendix H.
The Ear: Hearing and Balance
Describe the structure and general function of the outer,
middle, and internal ears.
Describe the sound conduction pathway to the fluids of
the internal ear, and follow the auditory pathway from the
spiral organ to the temporal cortex.
Explain how we are able to differentiate pitch and
loudness, and localize the source of sounds.
List possible causes and symptoms of otitis media, deafness,
and Ménière’s syndrome.
At first glance, the machinery for hearing and balance appears
very crude—fluids must be stirred to stimulate the mechanore-
ceptors of the internal ear. Nevertheless, our hearing appara-
tus allows us to hear an extraordinary range of sound, and our
equilibrium (balance) receptors continually inform the nervous
system of head movements and position. Although the organs
serving these two senses are structurally interconnected within
the ear, their receptors respond to different stimuli and are acti-
vated independently of one another.
Structure of the Ear
Te ear is divided into three major areas: external ear, middle
ear, and internal ear
(Figure 15.24a)
. Te external and middle
ear structures are involved with hearing only and are rather sim-
ply engineered. Te internal ear functions in both equilibrium
and hearing and is extremely complex.
External Ear
Te
external (outer) ear
consists of the auricle and the external
acoustic meatus. Te
auricle
, or
pinna
, is what most people call
the ear—the shell-shaped projection surrounding the opening of
the external acoustic meatus. Te auricle is composed of elastic
cartilage covered with thin skin and an occasional hair. Its rim,
the
helix
, is somewhat thicker, and its fleshy, dangling
lobule
(“earlobe”) lacks supporting cartilage. Te function of the auricle
is to funnel sound waves into the external acoustic meatus.
Te
external acoustic meatus
(auditory canal) is a short,
curved tube (about 2.5 cm long by 0.6 cm wide) that extends
from the auricle to the eardrum. Near the auricle, its framework
is elastic cartilage; the remainder of the canal is carved into the
Homeostatic Imbalances
of the Chemical Senses
Most olfactory disorders, or
anosmias
(an-oz
9
me-ahz; “with-
out smells”), result from head injuries that tear the olfactory
nerves, the aFereffects of nasal cavity inflammation, and neuro-
logical disorders such as Parkinson’s disease. Some people have
uncinate fits
(un
9
sih-nāt), olfactory hallucinations during which
they experience a particular (usually unpleasant) odor, such as
rotting meat.
Olfactory auras
experienced by some epileptics
just before they have a seizure are transient uncinate fits. Tese
hallucinations can result from irritation of the olfactory path-
way by brain surgery or head trauma.
±aste disorders are less common than disorders of smell, in
part because the taste receptors are served by three different
nerves and thus are less likely to be “put out of business” com-
pletely. Causes of taste disorders include upper respiratory tract
Gustatory
cortex
(in insula)
Thalamic
nucleus
(ventral
posteromedial
nucleus)
Pons
Solitary nucleus
in medulla
oblongata
Facial
nerve (VII)
Glossopharyngeal
nerve (IX)
Vagus nerve (X)
Figure 15.23
The gustatory pathway.
Taste signals are relayed
from the taste buds to the gustatory area of the cerebral cortex.
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