olfactory system
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The Chemical Senses
The olfactory system is one member of the chemical senses. The other two are taste and the
general chemical sense. Although we won’t cover these in this course, you should at least
know a bit about them.
Taste is transduced by receptor cells within taste buds on the tongue (primarily. These cells
e!press a family of receptor proteins that bind families of molecules representing the standard
taste categories" salt, bitter, sweet, sour and unami (glutamate. The receptor cells activate
nerves that pro#ect to the medulla.
The general chemical sense is transduced by unmyelinated somatosensory afferents present
in the mouth$ these are what is activated by capsaicin (hot pepper ingredient$ activating these
receptors on the skin would lead to a sensation of pain and heat. Activating them on the
tongue leads to the sensation of %hot peppers& and is interpreted as a taste.
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The 'lfactory System )omeronasal 'rgan
As * mentioned earlier, all senses process
communication and environmental stimuli in
separate channels. This separation is found atvery high cortical levels in the auditory and visual
system. There is an e!ceptionally clear division of
labour at the very beginning of the olfactory
system. 'lfactory receptors are found at two sites
in the nose" the olfactory epithelium (dorsal nasal
cavity and the vomeronasal organ (small pits of
receptor cells on either side of the nasal septum.The vomeronasal organ has receptors that bind
pheremones chemicals released from the body
and used to convey messages related to
reproduction and territory. The pheremonal
receptors are members of a gene family distinct
from those for general olfactory stimuli.
The vomeronasal organ is innervated by its own
neurons that pro#ect to the accessory olfactory
bulb$ this in turn has its own targets in the brain
devoted to olfactory communication, reproduction
etc. This sense system has not been thoroughly
studied and we will not deal with it any further in
this course.
+ear et al.
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The 'lfactory System" eceptors -
'lfactory receptors are located in a layer of support cells$ they
pro#ect their %dendrites& into the mucosa (where odorants are
trapped and their a!ons through a thin bone to terminate in
the olfactory bulb (part of the CS.
/ifferent receptors respond to different odors and these
receptors are spatially segregated to some degree on the
olfactory epithelium.
+ear et al.
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The 'lfactory System" eceptors 0
There are, in the rat, about -111 odorant receptor genes. Each olfactory receptorexpresses only one of these genes. This is the first critical feature of olfactory coding.
2hen an odorant binds to the olfactory receptor protein it stimulates a 3protein that
activates adenylate cyclase$ cA45 binds to and opens channels permeable to a67Ca06 and
Cl channels. The resulting current flow depolari8es the receptor cell (receptor potential
causing it to spike. *ts a!on terminal in the '+ then releases transmitter (glutamate to
e!cite the target mitral cells.
+ear et al.
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The 'lfactory System" 'lfactory +ulb -
'lfactory receptor a!ons terminate on mitral cell
dendrites in a restricted encapsulated structurecalled a glomerulus$ a glomerulus contains the
dendritic bush of one mitral cell but many
olfactory receptor a!ons. All the ' a!ons
ending in one glomerulus contain are from
receptors e!pressing same olfactory binding
protein.
So each mitral cell codes for one kind of
odorant molecule. This is the primary basis
of olfactory coding.
+ear et al.
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The 'lfactory System" 'lfactory +ulb 0
9eft" 'ptical imaging demonstrates
different parts of the '+ are activated by
different odorants.
ight" :lectrical recording demonstrates
that the same odorant causes different
patterns of spiking in different olfactory
neurons (locust.
It appears likely that the code for
odorant identity is spatio-temporal" an
odorant will activate different butoverlapping populations of '+ neurons
and the activated cells will have different
patterns of spiking discharge.
+ear et al.
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The 'lfactory System" 'lfactory Corte!
The '+ has an e!tensive and
comple! set of pro#ections. 'ne
ma#or target is the olfactory corte!.
'lfactory corte! contains pyramidal cells that receive e!citatory
(glutamate synaptic input from '+ mitral cells. :ach mitral
cell a!on ends on many 5Cs. The 5Cs pro#ect out of
olfactory corte!. +ut they also have collaterals that pro#ect
locally to many other 5Cs (e!citatory glutamate. The
synapses onto the 5Cs use 4/A receptors and are plastic
(9T5. 2hy; -. 4any ob#ects emit numerous volatile odorants (banana,
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opulation !esponse to Odorants
The olfactory corte! (lateral pallium is situated on the ventral aspect of the telencephalon and
not readily accessible for recording in vivo.
*n fish the e>uivalent telencephalic region is called /p and is at the surface.
?aksi et al using two photon confocal Ca06 imaging to investigate the response of /p neurons
to different odorants (8ebrafish. /ifferent subpopulations of /p neurons respond to different
amino acids presented to the nose of the fish.
The population response to natural odorants (from whatever 8ebrafish eat or from whatever
eats 8ebrafish is not known.
@ow to analy8e such population responses is not known and is a ma#or problem inSystems7Theoretical euroscience.
?aksi, 011
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Temporal !esponse to Odorants
Stopfer, 011=
eurons in the locust change their response as the odorant changes or due to changes in the
concentration of a single odorant. *t would seem that the locust would not be able to
discriminate changes in concentration versus changes in odorant.
@owever when the change in response over time (tra#ectory is e!amined the overall shape of
the response is maintained with changes in concentration. +ut different odorants produce
different tra#ectories.
The methods to do this kind of analysis are very sophisticated and still under development
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The 'lfactory System" 5eripheral Stem Cells
'lfactory receptor cells are constantly turned over. The source is
stem cells within the olfactory epithelium. This is a highly
regulated process and is being used as a model of neuronal stem
cell biology. The a!ons of new 's penetrate into the '+.
Special glial cells (ensheathing facilitate this$ ordinary adult glia
block a!onal regeneration$ so the ensheathing cells are of interestto molecular neuroscientists interested in a!onal regeneration.
Burther, the new ' a!ons make correct connections in the '+"
that is, to the glomerulus specified by the receptor type they
e!press. The mechanism for such specific regeneration is
unknown and also of intense interest.
+eites et al.
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The 'lfactory System" Central Stem Cells
Stem cells within the subventricular 8one of thelateral ventricles generate new neurons that
migrate into the '+ where they mature into a
type of inhibitory interneuron (granule cell.
These 3Cs integrate themselves into the '+
circuitry. The control of migration and synapse
formation of new neurons in the adult brain is
an important topic for those interested in
treatment of stroke etc.
2hat is the role of newly generated '+ granule
cells;
"n enriched odor environment leads to
increased survival of ne# granule cells $but
no increase in proliferation%. This iscorrelated #ith an improvement in olfactory
memory.
Saghatelyan et al.