OlfactionOlfaction

The Sense of SmellThe Sense of Smell

Smell helps us:Smell helps us:

Avoid dangerous situations (fumes, smoke) Avoid dangerous situations (fumes, smoke)

Avoid consuming poisonous substances and Avoid consuming poisonous substances and

spoiled food (sour milk) spoiled food (sour milk)

Choose mates?Choose mates?

What are olfactory stimuli?What are olfactory stimuli?

Odorants (airborne molecules)Odorants (airborne molecules)

Odorants must be volatile (they give off Odorants must be volatile (they give off vapors)vapors)

More vapors are given off when an odorant More vapors are given off when an odorant is heated!! (warm soup smells better than is heated!! (warm soup smells better than cold soup)cold soup)

OlfactionOlfaction

Eric Eric NiederhoffNiederhoff

erer

SIU-SOMSIU-SOM

•Olfaction signaling

•Cyclic nucleotide gated channels

•Inositol trisphosphate/Ca2+/Protein kinase C

Odorants reach olfactory Odorants reach olfactory receptors by:receptors by:

1. Being inhaled through the nose.1. Being inhaled through the nose.

2. Through the mouth (vapors circulate up 2. Through the mouth (vapors circulate up through throat)through throat)

OlfactionOlfaction CNGCOR

Golf

GDP

PDE

AC

Cl-

Ca2+

ORi

RK

O BP

GTPGDP

cAMPATP

GTP

5’-AMP

PLCOR

Gq

GDP

PIP2

PKCRK

Ca2+

GTPGDP

GTP

O BP

CaM

(IP3)

ER

Olfactory ReceptorsOlfactory Receptors

Figure 16.3a, b

Olfactory EpitheliumOlfactory Epithelium

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Olfaction- originates in the nasal Olfaction- originates in the nasal cavitycavity

Olfactory system structuresOlfactory system structures

Olfactory Receptor NeuronsOlfactory Receptor Neurons

Carry impulses directly to the brainCarry impulses directly to the brain

Live for 5- 8 weeks & then die.Live for 5- 8 weeks & then die.

Summary: olfactory pathwaySummary: olfactory pathway Olfactory receptor neurons detect odorants in Olfactory receptor neurons detect odorants in

mucosa.mucosa.

Signals are sent via olfactory receptor neurons to Signals are sent via olfactory receptor neurons to bulb structures (glomeruli).bulb structures (glomeruli).

Mitral and tufted cells carry signals to Mitral and tufted cells carry signals to orbitofrontal cortex, temporal lobe, and the limbic orbitofrontal cortex, temporal lobe, and the limbic system.system.

Which animal is most sensitive to Which animal is most sensitive to smell?smell?

Dogs 300-10,000 times more sensitive to Dogs 300-10,000 times more sensitive to smell than humanssmell than humans

Why? Humans have 10-40 million Why? Humans have 10-40 million olfactory receptor neurons, dogs have over olfactory receptor neurons, dogs have over a billion!!a billion!!

Odor HedonicsOdor Hedonics

What are “pleasant” and “unpleasant” What are “pleasant” and “unpleasant” odors? Variability is high.odors? Variability is high.

Most universally-rated “pleasant” odorant is Most universally-rated “pleasant” odorant is vanilla.vanilla.

How is perception affected by How is perception affected by odor?odor?

Odor enhances our experience of the world Odor enhances our experience of the world (food, surroundings, people).(food, surroundings, people).

We associate memories with certain We associate memories with certain odorants.odorants.

We lose the ability to experience “flavors” We lose the ability to experience “flavors” if our sense of smell is impaired.if our sense of smell is impaired.

AnosmiaAnosmia: loss of : loss of sensesense of smell of smell

May be to May be to specificspecific odors or odors or allall odors. odors.

Caused by infection to nasal cavity or brain Caused by infection to nasal cavity or brain injury (frontal lobe).injury (frontal lobe).

About 2 million people in US are anosmic.About 2 million people in US are anosmic.

Olfaction and behaviorOlfaction and behavior

A. Infant perception & attachmentA. Infant perception & attachment

Babies enthusiastically “orient” to pleasant Babies enthusiastically “orient” to pleasant odors (banana, vanilla) and cry/grimace to odors (banana, vanilla) and cry/grimace to unpleasant odors (shrimp; rotten eggs).unpleasant odors (shrimp; rotten eggs).

Babies can discriminate their mother’s scent Babies can discriminate their mother’s scent from other women (Macfarlane, 1975).from other women (Macfarlane, 1975).

Chemical Senses – Taste and Chemical Senses – Taste and SmellSmell

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Both senses use chemoreceptors

Stimulated by chemicals (odorants) in solution

Taste has four types of receptors

Smell can differentiate a large range of chemicals

Both senses complement each other and respond to many of the same stimuli

Olfaction – The Sense of SmellOlfaction – The Sense of Smell

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Olfactory receptors are in the roof of the nasal cavity

Neurons with long cilia

Chemicals must be dissolved in mucus for detection يكتشف

Impulses are transmitted via the olfactory nerve

Interpretation تفسيرof smells is made in the cortex

Smell (Olfaction)Smell (Olfaction)

Receptors are part of the olfactory Receptors are part of the olfactory epitheliumepithelium

Olfactory epithelium composed of:Olfactory epithelium composed of: Cell bodies of olfactory receptor cellsCell bodies of olfactory receptor cells Supporting cells – columnar cellsSupporting cells – columnar cells Basal cells – form new olfactory Basal cells – form new olfactory

receptor cellsreceptor cells

Smell (Olfaction)Smell (Olfaction)

Olfactory receptors react to their Olfactory receptors react to their stimuli by developing a generator stimuli by developing a generator potential (depolarization) that potential (depolarization) that triggerstriggers يثيريثير one or more nerve one or more nerve impulses.impulses.

Olfaction has a low threshold.Olfaction has a low threshold. Adaptation occurs rapidly.Adaptation occurs rapidly.

The olfactory pathwayThe olfactory pathway Olfactory (I) nerves (bundles of olfactory Olfactory (I) nerves (bundles of olfactory

axons) run on both sides of the nose.axons) run on both sides of the nose. Terminate Terminate ينتهيينتهيin the brain in paired in the brain in paired

masses of grey matter (olfactory bulbs).masses of grey matter (olfactory bulbs). Within the olfactory bulbs the axon Within the olfactory bulbs the axon

terminals of the olfactory receptors (1terminals of the olfactory receptors (1stst order neurons) form synapses with order neurons) form synapses with dendrites & cell bodies of second order dendrites & cell bodies of second order olfactory neurons.olfactory neurons.

Axons of the olfactory bulb neurons extend Axons of the olfactory bulb neurons extend posteriorly and form the olfactory tract.posteriorly and form the olfactory tract.

Axons of the olfactory tract project Axons of the olfactory tract project .to the primary olfactory area.to the primary olfactory areaتروحتروح

Other axons of the olfactory tracts Other axons of the olfactory tracts extend to the limbic system and extend to the limbic system and hypothalamus (emotional responses to hypothalamus (emotional responses to odors).odors).

Axons extend from the primary Axons extend from the primary olfactory area to frontal lobe olfactory area to frontal lobe (odor(odor

رآآئحه.. او رآآئحه.. عطر او .(identification & discrimination).identification & discrimination عطر

The olfactory pathwayThe olfactory pathway