The Special Senses
The Olfactory Pathway
As we have already noted, axons of the olfactory sensory neurons
form the olfactory nerves that synapse in the overlying
, the distal ends of the olfactory tracts (see Table 13.2
on p. 494). ±ere, the ﬁlaments of the olfactory nerves synapse
tral), which are second-order sensory neu-
rons, in complex structures called
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
(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
is, it must be in the gaseous state as it enters the nasal cavity.
Additionally, it must dissolve in the ﬂuid coating the olfactory
Activation of Olfactory Sensory Neurons
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.
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).
shows, olfactory transduction begins when
an odorant binds to a receptor. ±is event activates G proteins
), 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
inﬂux leads to depolarization and impulse transmission.
inﬂux causes the transduction process to adapt, decreasing
its response to a sustained stimulus. ±is
helps explain how a person working in a paper mill or sewage
treatment plant can still enjoy lunch!
to its receptor.
converts ATP to cAMP.
cAMP opens a cation
channel, allowing Na
G protein (G
Olfactory transduction process.
A portion of olfactory cilium membrane