Chapter 15
The Special Senses
567
15
The Olfactory Pathway
As we have already noted, axons of the olfactory sensory neurons
form the olfactory nerves that synapse in the overlying
olfac-
tory bulbs
, the distal ends of the olfactory tracts (see Table 13.2
on p. 494). ±ere, the filaments of the olfactory nerves synapse
with
mitral cells
(mi
9
tral), which are second-order sensory neu-
rons, in complex structures called
glomeruli
(glo-mer
9
u-li; “little
balls”) (see Figure 15.20).
Axons from neurons bearing the same kind of receptor con-
verge on a given type of glomerulus. ±at is, each glomerulus
represents a single aspect of an odor (like one note in a chord)
but each odor activates a unique set of glomeruli (the chord
itself). Different odors activate different subsets of glomeruli
(making different chords which may have some of the same
notes). ±e mitral cells refine the signal, amplify it, and then
relay it. ±e olfactory bulbs also house
amacrine granule cells
,
GABA-releasing cells that inhibit mitral cells, so that only highly
excitatory olfactory impulses are transmitted.
When the mitral cells are activated, impulses flow from the
olfactory bulbs via the
olfactory tracts
(composed mainly of
mitral cell axons) to the piriform lobe of the olfactory cortex.
From there, two major pathways take information to various
parts of the brain. One pathway brings information to part
of the frontal lobe just above the orbit, where smells are con-
sciously interpreted and identified. Only some of this informa-
tion passes through the thalamus.
±e other pathway flows to the hypothalamus, amygdala,
and other regions of the limbic system. ±ere, emotional re-
sponses to odors are elicited. Smells associated with danger—
smoke, cooking gas, or skunk scent—trigger the sympathetic
fight-or-flight response. Appetizing odors stimulate salivation
and the digestive tract, and unpleasant odors can trigger protec-
tive reflexes such as sneezing and choking.
ammonia, the hotness of chili peppers, and the “chill” of men-
thol. Impulses from these receptors reach the central nervous
system via afferent fibers of the trigeminal nerves.
Physiology of Smell
For us to smell a particular odorant, it must be
volatile
—that
is, it must be in the gaseous state as it enters the nasal cavity.
Additionally, it must dissolve in the fluid coating the olfactory
epithelium.
Activation of Olfactory Sensory Neurons
Dissolved odor-
ants stimulate olfactory sensory neurons by binding to re-
ceptor proteins in the olfactory cilium membranes, opening
cation channels and generating a receptor potential. Ulti-
mately (assuming threshold stimulation) an action potential
is conducted to the first relay station in the olfactory bulb.
Smell Transduction
Transduction of odorants uses a re-
ceptor linked to a G protein. ±e events that follow odorant
binding will be easy for you to remember if you compare
them to what you already know: general mechanisms of re-
ceptors and G proteins (see Figure 11.21) and phototrans-
duction (see Figure 15.17).
As
Figure 15.21
shows, olfactory transduction begins when
an odorant binds to a receptor. ±is event activates G proteins
(G
olf
), which activate enzymes (adenylate cyclases) that synthe-
size cyclic AMP as a second messenger. Cyclic AMP then acts
directly on a plasma membrane cation channel, causing it to
open, allowing Na
1
and Ca
2
1
to enter.
Na
1
influx leads to depolarization and impulse transmission.
Ca
2
1
influx causes the transduction process to adapt, decreasing
its response to a sustained stimulus. ±is
olfactory adaptation
helps explain how a person working in a paper mill or sewage
treatment plant can still enjoy lunch!
Odorant binds
to its receptor.
Receptor
activates G
protein (G
olf
).
G protein
activates adenylate
cyclase.
Adenylate cyclase
converts ATP to cAMP.
cAMP opens a cation
channel, allowing Na
+
and Ca
2+
influx and
causing depolarization.
Odorant
G protein (G
olf
)
Receptor
Adenylate cyclase
ATP
cAMP
cAMP
GTP
GTP
Open cAMP-gated
cation channel
Na
+
Ca
2
+
GTP
GDP
5
4
3
2
1
Figure 15.21
Olfactory transduction process.
A portion of olfactory cilium membrane
is shown.
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