446
UNIT THREE
Olfactory Receptors
Olfactory
receptors,
used to sense odors, are similar to
those for taste in that they are chemoreceptors sensitive
to chemicals dissolved in liquids. The two chemical senses
function closely together and aid in food selection, because
we smell food at the same time we taste it. It is often dif-
F
cult to tell what part of a food sensation is due to smell
and what part is due to taste. ±or this reason, an onion
tastes different when sampled with the nostrils closed,
because much of the usual onion sensation is due to odor.
Similarly, if copious mucous secretions from an upper
respiratory infection cover the olfactory receptors, food
may seem tasteless. About 75% to 80% of fl
avor derives
from the sense of smell.
Olfactory Organs
The olfactory organs, which contain the olfactory recep-
tors, also include epithelial supporting cells. These organs
appear as yellowish brown masses within pinkish mucous
membrane. They cover the upper parts of the nasal cavity,
the superior nasal conchae, and a portion of the nasal septum
(f
g. 12.5)
.
The
olfactory receptor cells
are bipolar neurons sur-
rounded by columnar epithelial cells. These neurons have
knobs at the distal ends of their dendrites covered with
hairlike cilia. The cilia project into the nasal cavity and are
the sensitive portions of the receptor cells
(f
g. 12.6)
. A
person’s 12 million olfactory receptor cells each have ten to
twenty cilia.
Humans smell the world using about 12 million olfactory receptor
cells. Bloodhounds have 4 billion such cells—and hence a much bet-
ter sense of smell. The excellent sense of smell in canines is the basis
of using service dogs to detect impending health problems in their
owners. The dogs sense subtle odors that people emit when becom-
ing ill with certain conditions. Service dogs are used to sense immi-
nent seizures, drops in blood glucose, and accelerated heart rate.
There is some evidence that dogs can sense cancer.
Chemicals that stimulate olfactory receptor cells, called
odorant molecules, enter the nasal cavity as gases, where
they must dissolve at least partially in the watery fl
uids
that surround the cilia before they can bond to receptor
proteins on the cilia and be detected. Odorant molecules
bind to about 400 types of olfactory receptors that are part
of the cell membranes of the olfactory receptor cells, depo-
larizing them and thereby generating nerve impulses. In
addition, signaling proteins inside the receptor cell trans-
late the chemical signal (binding of the odorant molecule to
the receptor protein) into the electrochemical language of
the nervous system.
Sensory receptors are not the same as membrane recep-
tors. Sensory receptors may be as small as individual cells or as
large as complex organs such as the eye or ear. They respond
Visceral Senses
Receptors in internal organs include lamellated corpuscles
and free nerve endings. The information these receptors con-
vey includes the sense of fullness after eating a meal as well
as the discomfort of intestinal gas and the pain that signals a
heart attack.
PRACTICE
11
Describe a muscle spindle.
12
Explain how muscle spindles help maintain posture.
13
Where are Golgi tendon organs located?
14
What is the function of Golgi tendon organs?
12.4
SPECIAL SENSES
Special senses are those whose sensory receptors are part of
large, complex sensory organs in the head. These senses and
their respective organs include the following:
smell
olfactory organs
taste
taste buds
hearing
equilibrium
ears
sight
eyes
Clinical Application 12.2 discusses an unusual type of sen-
sory abnormality.
Sense of Smell
The ability to detect the strong scent of a F sh market, the anti-
septic odor of a hospital, the aroma of a ripe melon—and thou-
sands of other smells—is possible thanks to a yellowish patch
of tissue the size of a quarter high up in the nasal cavity. This
fabric of sensation is a layer of 12 million specialized cells.
TABLE
12.2
|
Receptors Associated
with General Senses
Type
Function
Sensation
Free nerve endings
(mechanoreceptors)
Detect changes in
pressure
Touch, pressure
Tactile corpuscles
(mechanoreceptors)
Detect objects
moving over the skin
Touch, texture
Lamellated corpuscles
(mechanoreceptors)
Detect changes in
pressure
Deep pressure, vibrations,
fullness in viscera
Free nerve endings
(thermoreceptors)
Detect changes in
temperature
Heat, cold
Free nerve endings
(pain receptors)
Detect tissue damage
Pain
Free nerve endings
(mechanoreceptors)
Detect stretching of
tissues, tissue spasms
Visceral pain
Muscle spindles
(mechanoreceptors)
Detect changes in
muscle length
None
Golgi tendon organs
(mechanoreceptors)
Detect changes in
muscle tension
None
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