Regulation and Integration of the Body
at risk for damage, and their typical life span is 30–60 days.
Olfactory stem cells in the olfactory epithelium differentiate to
replace them.
Specificity of Olfactory Receptors
Smell is difficult to research because any given odor (say, tobacco
smoke) may be made up of hundreds of different chemicals.
Taste has been neatly packaged into five taste qualities as you
will see, but science has yet to discover any similar means for
classifying smell. Humans can distinguish 10,000 or so odors,
but research suggests that our olfactory sensory neurons are
stimulated by different combinations of a more limited number
of olfactory qualities.
±ere are about 400 “smell genes” in humans that are active
only in the nose. Each gene encodes a unique receptor protein.
It appears that each protein responds to one or more odors and
each odor binds to several different receptor types. However,
each receptor cell has only one type of receptor protein.
Olfactory neurons are exquisitely sensitive—in some cases,
just a few molecules activate them. Some of what we call smell
is really pain. ±e nasal cavities contain pain and temperature
receptors that respond to irritants such as the sharpness of
membrane (Figure 15.20b). ±e supporting cells contain a
yellow-brown pigment similar to lipofuscin, which gives the
olfactory epithelium its yellow hue. At the base of the epithe-
lium lie the short
olfactory stem cells
±e olfactory sensory neurons are unusual bipolar neurons.
Each has a thin apical dendrite that terminates in a knob from
which several long cilia radiate. ±ese
olfactory cilia
, which
substantially increase the receptive surface area, typically lie flat
on the nasal epithelium and are covered by a coat of thin mu-
cus produced by the supporting cells and by olfactory glands in
the underlying connective tissue. ±is mucus is a solvent that
“captures” and dissolves airborne odorants. Unlike other cilia in
the body, which beat rapidly in a coordinated manner, olfactory
cilia are largely nonmotile.
±e slender, nonmyelinated axons of the olfactory sensory
neurons are gathered into small fascicles that collectively form the
filaments of the olfactory nerve
(cranial nerve I). ±ey project
superiorly through the openings in the cribriform plate of the eth-
moid bone, where they synapse in the overlying olfactory bulbs.
Olfactory sensory neurons are also unusual because they are
one of the few types of
that undergo noticeable turn-
over throughout adult life. ±eir superficial location puts them
Mitral cell
(output cell)
Filaments of
olfactory nerve
Cribriform plate
of ethmoid bone
Lamina propria
connective tissue
Olfactory stem cell
Supporting cell
Olfactory cilia
Olfactory bulb
Olfactory axon
Olfactory sensory
Route of inhaled air
containing odor molecules
Route of
inhaled air
Olfactory tract
Olfactory bulb
Figure 15.20
Olfactory receptors.
Site of olfactory epithelium in the superior nasal
An enlarged view of the olfactory epithelium showing the course of the fibers
[filaments of the olfactory nerve (I)] through the ethmoid bone. These synapse in the glomeruli
of the overlying olfactory bulb. The mitral cells are the output cells of the olfactory bulb.
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