Biomedical Engineering Reference
In-Depth Information
Fig. 2.4
OSNs expressing
dorsal receptors project to the
dorsal portion of the
OB
, and
the same logic applies to the
ventral receptors. The projec-
tions from the
OB
to the
accessory olfactory nucleus
(
AON
) also have the same
pattern with the dorsal glom-
eruli projecting to the dorsal
regions in the
AON
. On the
other hand, the projections
to the piriform seem to be
organized randomly, and the
projections to the
amygdala
are heavily biased towards
the dorsal glomeruli
olfactory nucleus, piriform cortex, lateral enterorhinal cortex, olfactory tubercle and
the amygdala [
99
]. Projections from the primary cortex are diffused over the brain
and project to a large number of regions like the limbic system and the neocor-
tex [
27
]. The dorso-ventral projection pattern is conserved in the anterior olfactory
nucleus, but for the amygdala, projections are mostly traced back to the dorsal por-
tion of the bulb, and no discernable pattern can be traced from the projections to the
piriform cortex [
100
]. (Fig.
2.4
)
2.5
Odor Coding
Precise odor detection and discrimination has its basis in deciphering a combi-
natorial code of activated ORs. A given OR may be activated by a number of
molecules and one odorant is capable of activating a number of ORs, enabling the
olfactory system to detect and discriminate among tens of thousands of odorants
[
101
]. (Fig.
2.5
) The range of unique odorants able to elicit a response from the
OR defines how broadly or narrowly it is tuned. There are ORs that are excited
by a wide range of molecules and ORs that respond to only very specific cues.
One example of a narrowly tuned OR is the human OR7D4, which is activated
very selectively by androstenone and androstadienone. The receptor has common
variants that differ the function and alter the perception of these volatile steroids,
showing an essential role of a single OR in odor perception [
102
-
104
]. The first
step towards the identification of an odorant is the specific set of ORs it can acti-
vate to cause the OSN to depolarize, and hence the pattern of glomeruli it excites.
It has been found that the glomeruli excited by a single odorant are consistent
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