ns_olfaction ppt.ppt

8/10/2019 NS_olfaction ppt.ppt

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The Olfactory System


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Olfactory System

Chemical sensing systemwith receptor organs in the

nasal passages

Receptors synapsedirectly into the brain;

heavy connections with

the limbic system

Different from othersensory systems in many

ways


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Olfactory System: Peripheral

StructureOlfactory receptors are located on the

olfactory (or nasal) epithelium. The

epithelium hangs down from the roof ofthe nasal sinus. The epithelium

contains olfactory receptor cells and

supporting cells.

Dendrites of olfactory receptor

cells extend into the mucus

coating of the epithelium;

odorant molecules bind toreceptors on the dendrites.

Axons of the olfactory

receptor cells enter the brain

and synapse on cells in the

olfactory bulb.

BRAIN

SINUS


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Olfactory

sensory

neurons

There areabout 10 million

receptors per

side in humans


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Olfactory

sensory

neurons

No circuitry or

synapses in the

epithelium;

receptors have

axons (thin,unmyelinated,

slow) which

project directly

to the brain.

Receptors die

and arereplaced about

every 60 days.

Stem cell

To olfactory bulb


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http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/O/Olfaction.html

Olfactory receptors use a G-protein coupled

transduction mechanism similar to visual receptors


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http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/O/Olfaction.html

Kinase

Olfactory receptors show strong adaptation

Mechanisms: 1. Kinase phosphorolation of receptor protein (desensitization to

odorant molecules); 2. Adjustment of channel sensitivity to cAMP (up or down

depending on odorant concentration)


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What exactly do receptors code?

How odors are encoded by the olfactory receptors was along-standing mystery

Early olfactory researchers suggested that a small number

of receptor types could encode a large number of naturalodors, similar to 3 cones coding all perceived colors: ThePrime Odor theory (7 primes was a popular number)

Difficult to determine what those prime odors might beand how they would be combined to give the smell of anatural substance


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Richard Axel and Linda Buck used molecular techniques to determine

the number of different olfactory receptor types. The concept and

strategy:

1. SPECIFICITY WOULD BE

BASED ON STRUCTURE OF

RECEPTOR-G PROTEIN

COMPLEX; THEREFORE, IF YOUDETERMINE THE NUMBER OF

DIFFERENT RECEPTOR

STRUCTURES, YOU KNOW THE

NUMBER OF DIFFERENT

FUNCTIONAL TYPES, AND

THEREFORE THE NUMBER OF

DIFFERENT PRIME ODORS

2. STRUCTURALLY DIFFERENT

RECEPTOR PROTEINS WOULD

BE CODED BY DIFFERENT

GENES; CLONE, SEQUENCE,CHARACTERIZE GENES

EXPRESSED IN THE

OLFACTORY EPITHELIUM,

LOOK FOR SYSTEMATIC

VARIATION ON G-PROTEIN

TYPES

3. LOCALIZE THE

EXPRESSED

GENES BACK TO

THE OLFACTORY

RECEPTOR

CELLS


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Result: There are 1000 different genes in 4 families; each

codes 7-transmembrane domain G-protein coupled receptor

protein that is expressed in olfactory receptors in mice

About 350 of these are functional genes in humans; the rest

are present as pseudogenes

Each receptor cell in the epithelium expresses only one

receptor gene

Therefore, each receptor is best tuned to one of 1000

different chemical types

What these types are is still not clear, nor is how the code

gets turned into a smell


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http://nobelprize.org/medicine/laureates/2004/buck-slides.pdf

The olfactory

epithelium is

mapped, but not in

a familiar way

The 4 gene families are

expressed in different zones of

the epithelium

Within a zone, different

receptor types appear to

be randomly scattered


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Examples of odorant coding; note that relative levels of

activation in the different receptors might also be

important in coding the odor


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A combinatorial code

means that receptors

can contribute to theperception of very

different smells


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Output of the olfactory epithelium goes to the

Olfactory Bulb: Olfactory bulb is a three layeredstructure. Mitral cells are the principal neurons of the

olfactory bulb.


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Olfactory Bulb Circuitry:

The glomerulus is thebasic processing

component of the

olfactory bulb

Olfactory Bulb Circuitry:

Periglomerular cells in the

glomerulus and granularcells in the deeper layers

mediate local and lateral

inhibition


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Cells expressing

a single type of

receptor are

widely scatteredacross the

olfactory

epithelium.

Axons of all these

cells converge on

a single place(glomerulus) in

the olfactory bulb.


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All the axons terminating in a

glomerulus are from the same type

olfactory receptors. Therefore each

glomerulus codes one odorant type.

Axons from each olfactory receptor

type terminate in very few (maybe

only 1 or 2) glomeruli at one point

in the olfactory bulb.


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Axons from 25,000 olfactory receptors

STRUCTURE OF THE OLFACTORY GLOMERULUS

Dendrites from 25 mitral cells

Periglomerular cells form

inhibitory connections between

glomeruli

A glomerulus is a self-

contained zone of synapticinteractions.

There are about 2000

glomeruli in the olfactory

bulb of each side.

10,000,000

RECEPTORS

2,000

GLOMERULI


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Olfactory system codes odors based on chemical structure of molecules;

specificity is for a molecular structural characteristic, not a particular molecule.

SCIENCE VOL 286 22 OCTOBER 1999


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Dendrodendritic reciprocal synapses form between PG cells

and MT dendritic tufts, and between granular cells and MT

basal dendrites. These both result in local dendritic

inhibition following excitation of the mitral cells by olfactory

nerve inputs. (NOTE: This is in addition to lateral inhibition

of neighboring mitral cells.)

http://flavor.monell.org/%7Eloweg/OlfactoryBulb.htm


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Lateral inhibition through periglomerular cells:

+ ---

-

Looking down on glomerular level;connections form +/- center surround

receptive field

+

_

Oscillations induced through dendrodendritic connections:

Mitral EPSP

Mitral AP

Odorant present

Olfactory pathways out of the bulb are all


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Numerous connections to limbicsystem areas.

Connections to cortical areas do

not depend on relay through a

thalamic nucleus

Olfactory pathways out of the bulb are all

uncrossed.

The piriform cortex is considered the

olfactory sensory cortex.


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SCIENCE VOL 294 9 NOVEMBER 2001

Single glomeruli project to multiple locations in olfactory cortex.

Cortical

representation

of olfactoryinformation


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Glomeruli projections overlap in olfactory cortex, and

individual cortical neurons receive input from multiple

glomeruli (and hence receive input from multiple odorants).



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QUESTION: Why remix

inputs after you have

gone through all the

trouble of separating

them out so effectively?

ANSWER: Olfaction may be based on pattern detection: Cortical neurons are

concerned with specific combinations of inputs, with each combination

corresponding to a percept.


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The Vomeronasal System

A second olfactory system is present in most

vertebrates. It is separate from the main

olfactory system anatomically and

functionally.


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The vomeronasal organ is separate from the

main olfactory epithelium in the nasal cavity

http://bioweb.usc.edu/courses/2002-fall/documents/neur524-olfactory_transduction.pdf


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Vomeronasal

receptors use a

different signal

transduction pathway

than main olfactory

receptors

About 100 different receptor types in two gene families;these families are different from the four in which main

olfactory receptors are coded

Vomeronasal receptors are different from main

olfactory receptors

http://bioweb.usc.edu/courses/2002-fall/documents/neur524-olfactory_transduction.pdf


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The vomeronasal system is specialized for

detecting high molecular weight, relatively

nonvolatile chemicals. Its presence is oftenaccompanied by morphological or behavioral

specializations for moving such odorants to

the vomeronasal epithelium.

LOCATION NEAR

NARES, OR

OPENING INTO

MOUTH CAVITY

VASCULAR

PUMPS

STEREOTYPED

BEHAVIORS: TONGUE

FLICKING IN SNAKES

FLEHMEN RESPONSE

IN HORSES


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The vomeronasal receptors project to a

separate accessory olfactory bulb via aseparate accessory olfactory nerve

Organization of the AOB is similarto that of the MOB. Outputs are

different: the AOB output target only

subcortical limbic areas that

connect in turn to the hypothalamus


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Accessory Olfactory

Bulb

Main Olfactory

Bulb

Vomeronasal Organ Main Olfactory Organ

Septalnuclei

Amygdala Olfactorytubercle

Olfactory

Cortex

PHEROMONES

PREY ODORSGENERAL ODORS

EntorhinalCortex

Hippocampus

PARALLEL OLFACTORY PATHWAYS

Medial,

BNSTCortical

Hypothalamus


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The vomeronasal system is specialized for detecting and

processing biologically important odors, especially

chemical communication signals (pheromones)

Chemical communication is a preeminent social

communication channel in most mammals

Courtship, sexual behavior, aggression, maternalbehavior, kin recognition, pair bonding, territoriality, fear

and predator avoidance all involve chemical signaling and

are controlled by the reception of chemical signals in most

mammals

Lesions of the vomeronasal system at various levels

interfere with normal social behavior mediated by

pheromonal communication


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Vomeronasal and Main Olfactory System May Both

Participate in Chemical Signaling Depending on Experience

In virgin male rodents,

lesioning VNO blocks

sex with a female;

lesioning OE has no

effect

X

XX

X

NO COPULATION WITH A FEMALE

NO COPULATION WITH A FEMALE

NORMAL COPULATION WITH A FEMALE

NORMAL COPULATION WITH A FEMALE

XX

In male rodents with 1

previous sexualexperience, lesioning

VNO or OE alone has

no effect; both must

be lesioned to block

copulation


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Is Chemical Communication Important in Humans?

Do we have a vomeronasal organ? Probably not (nor doOld World primates generally)but does that meananything?

What can we recognize by odor alone? The t-shirtexperiments

Can odors affect reproductive function? The menstrualsynchrony experiments

If human pheromones were controlling our behavior,would we even know it? Look where accessory olfactoryinformation is sent in the brainits all subcortical

ns_olfaction ppt.ppt

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