Olfaction 111029153241-phpapp02

Olfaction The Sense of Smell

Smell helps us:
Avoid dangerous situations (fumes, smoke)
Avoid consuming poisonous substances and spoiled food (sour milk)
Choose mates?

What are olfactory stimuli?
Odorants (airborne molecules)
Odorants must be volatile (they give off vapors)
More vapors are given off when an odorant is heated!! (warm soup smells better than cold soup)

Olfaction Eric Niederhoffer SIU-SOM
Olfaction signaling
Cyclic nucleotide gated channels
Inositol trisphosphate/Ca 2+ /Protein kinase C

Odorants reach olfactory receptors by:
1. Being inhaled through the nose.
2. Through the mouth (vapors circulate up through throat)

Olfaction CNGC Cl - Ca 2+ PIP 2 Ca 2+ ER OR G olf   GDP PDE AC OR i RK O BP GTP GDP cAMP ATP  GTP 5’-AMP PLC OR G q GDP PKC RK GTP GDP GTP O BP CaM (IP 3 )

Olfactory Receptors Figure 16.3a, b

Olfaction- originates in the nasal cavity

Olfactory system structures

Olfactory Receptor Neurons
Carry impulses directly to the brain
Live for 5- 8 weeks & then die.

Summary: olfactory pathway
Olfactory receptor neurons detect odorants in mucosa.
Signals are sent via olfactory receptor neurons to bulb structures (glomeruli).
Mitral and tufted cells carry signals to orbitofrontal cortex, temporal lobe, and the limbic system.

Which animal is most sensitive to smell?
Dogs 300-10,000 times more sensitive to smell than humans
Why? Humans have 10-40 million olfactory receptor neurons, dogs have over a billion!!

Odor Hedonics
What are “pleasant” and “unpleasant” odors? Variability is high.
Most universally-rated “pleasant” odorant is vanilla.

How is perception affected by odor?
Odor enhances our experience of the world (food, surroundings, people).
We associate memories with certain odorants.
We lose the ability to experience “flavors” if our sense of smell is impaired.

Anosmia : loss of sense of smell
May be to specific odors or all odors.
Caused by infection to nasal cavity or brain injury (frontal lobe).
About 2 million people in US are anosmic.

Olfaction and behavior
A. Infant perception & attachment
Babies enthusiastically “orient” to pleasant odors (banana, vanilla) and cry/grimace to unpleasant odors (shrimp; rotten eggs).
Babies can discriminate their mother’s scent from other women (Macfarlane, 1975).

Chemical Senses – Taste and Smell 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 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 تفسير of smells is made in the cortex

Smell (Olfaction)
Receptors are part of the olfactory epithelium
Olfactory epithelium composed of:
Cell bodies of olfactory receptor cells
Supporting cells – columnar cells
Basal cells – form new olfactory receptor cells

Smell (Olfaction)
Olfactory receptors react to their stimuli by developing a generator potential (depolarization) that triggers يثير one or more nerve impulses.
Olfaction has a low threshold.
Adaptation occurs rapidly.

The olfactory pathway
Olfactory (I) nerves (bundles of olfactory axons) run on both sides of the nose.
Terminate ينتهي in the brain in paired masses of grey matter (olfactory bulbs).
Within the olfactory bulbs the axon terminals of the olfactory receptors (1 st order neurons) form synapses with dendrites & cell bodies of second order olfactory neurons.
Axons of the olfactory bulb neurons extend posteriorly and form the olfactory tract.

Axons of the olfactory tract project تروح to the primary olfactory area.
Other axons of the olfactory tracts extend to the limbic system and hypothalamus (emotional responses to odors).
Axons extend from the primary olfactory area to frontal lobe (odor عطر .. او رآآئحه identification & discrimination).
The olfactory pathway

Olfaction 111029153241-phpapp02

by Arvinder Singh

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