Olfactory System

8/20/2019 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.

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Olfactory System