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
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
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
of ethmoid bone
Olfactory stem cell
Route of inhaled air
containing odor molecules
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.