Sensation of Smell

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Sensation of Smell

• Professor A.M.A Abdel Gader
• MD, PhD, FRCP (Lond., Edin), FRSH (London)
• Professor of Physiology, College of Medicine
• King Khalid University Hospital
• Riyadh, Saudi Arabia

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The Physiology of Smell (Olfaction)

• Smell Taste
• Similar
• Chemical Senses
• Determine food flavor (intake)
• Dissimiliar
• Smell receptors telereceptors- sense projected
  the environment
• Taste is confined to mouth
• Smell pathway does not relay in the thalamus does
  not reach sensory cortex
• Taste pathway finishes in the sensory cortex (PCG)

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Taste Smell cont.

• Chemical Senses
• vital for survival
• Determine appetite
  (Retreat from noxious stimuli)

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Chemical Senses

• Gatekeepers molecule detectors
• identify what the body needs for survival
• identify what is dangerous and should be
  rejected
• Neurogenesis constant renewal of receptors
• Olfactory receptors 30-60 days
• Taste cells 10 days
• Affective component emotions aid in
  discrimination of molecules (good vs. bad)

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Chemical Senses - cont

• Taste plays a vital role in food selection,
• sweet (and umami) are associated with nutritious
  food
• Bitter tastes are associated with the possible
  presence of toxins and are usually avoided.
• Taste and smell are closely linked even though
  they involve different receptors and receptive
  processes. (??overlap in central processing).
• In many species olfactory stimuli play an
  important role in reproduction although not well
  developed in humans (?).

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The Physiology of Smell (Olfaction)

• Sense of Smell species difference
• Macrosmatic animals Dogs highly
  developed sense of smell reliance on olfactory
  system
• Microsmatic Man weakly
  developed sense of smell less reliance on
  olfactory system

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Species differences

• Dog
• Has 40 sq inches of epithelium compared to
• 1 sq inch in humans

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Dogs have about 40 times the area for olfaction
that humans do
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Olfactory Receptor Neurons (ORN)

• Olf. Signal Transducers
• 1000 different types
• Each type found in only 1 zone of mucosa
• Vision
• 3 cone types, 1 type of rod
• 6 million cones, 120 million rods

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Olfactory Receptors

• 1000 different types of receptors each has
  odorant-binding protein
• Only one protein type per ORN
• These membrane bound proteins
• Located in cilia on tips of ORNs
• ORN olfactory receptor neuron

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Olfactory receptors (a) Location of receptors in
the roof of the nasal cavity. (b) Closeup of
olfactory cells.
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• Olfactory epith. at roof
• of nasal cavity
• Warm nasal epithelium
• Conviction currents
• ( by sniffing)

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The Physiology of Smell(Olfaction)

• Olfactory Receptors
• Bipolar neurons with cilia
• Receptors are genuine neurons
• (unlike photoreceptors and hair cells)
• Unlike other neurons, receptors are
• continually regenerated

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Olfactory Epithelium

• 5 cm2 of membrane
• 10-100 million receptors

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Cells of the Olfactory Membrane

• Olfactory epithelium made up of three types of
  cells
• Olfactory receptors
• bipolar neurons with olfactory hairs
• Supporting cells
• columnar epithelium
• Basal stem cells
• replace receptors monthly
• Olfactory (Bowmans) glands
• produce mucus
• Dissolves odorants

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Olfactory Epithelium

• Receptors have four parts cilia, olfactory
  knob, olfactory rod and the axon
• Olfactory nerve - the axons of the olfactory
  receptors form bands which travel to the
  olfactory bulb

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BIPOLAR OLFACTORY NEURONS IN THE NOSE.
RECEPTORS FOR ODOR MOLECULES ARE IN THE CILIA.
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Olfactory System
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Olfactory System
Each sensory cell type connects to a particular
pair of glomeruli
Each sensory cell expresses one receptor type
(indicated by color) Different colors represent
different sensory cells
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The Physiology of Smell - (Olfaction) cont.

• Olfactory Receptors
• Bipolar neurons with cilia
• Mitose throughout life (only part of CNS that is
  known to regenerate)
• Their axons project to the olfactory bulb

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Mechanism of Olfaction
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Physiology of Olfaction Mechanism

• Molecules must dissolve in mucus
• ?
• Combine with receptors on the cilia
• ?
• Stimulation of a G-Protein
• ?
• Activation of Adenyl Cyclase
• ?
• Increase I.C. cAMP
• ?
• Opening of Na Channels
• ?
• Na influx
• ?
• Depolarization (Receptor Potential)

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Threshold to Different Substances
Physiology of Olfaction cont.
Substance Threshold conc mg/L air
Ethyl ether 5.83
Chloroform 3.30
Oil of peppermint 0.02
Propyl mercaptan 0.006
Methyl mercaptan 0.0000004
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Discrimination of Intensity

• Poor
• Requires 30 increase of intensity
• Strong smell highly water and lipid soluble
• Man can distinguish 2000-4000 different odors

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Physiology of Olfaction cont.

• Adaptation to Smell
• Peripheral
• Central

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Olfactory Pathway

• Axons from olfactory receptors synapse in the
  olfactory bulb
• Second-order neurons within the olfactory bulb
  form the olfactory tract
• synapses on primary olfactory area of temporal
  lobe
• conscious awareness of smell begins
• Other pathways lead to the frontal lobe where
  identification of the odor occurs

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Early Olfactory Pathway
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Olfactory Mucosa Olfactory Pathway
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Early Olfactory Pathway
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Central Olfactory Pathways
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Central Olfactory Pathways
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Central Olefactory Pathways
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Central Olefactory Pathways
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Central Olefactory Pathways
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Central Olefactory Pathways
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Physiology of Olfaction cont.

• Clinical Considerations
• Abnormalities of the sense of olfaction
• Anosmia Absence of the sense of smell
• Dysosmia Disturbed
• Hyposomia Reduced
• Vit. A deficiency and hypogonadism
• Hyperosmia Increased sense of smell
• (Adrenal insufficiency)

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Advances in Olfactory Physiology Noble Prize
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How does the sense of smell work?

• Discovered fairly recently (1991)
• Nobel prize in 2004, to Richard Axel and Linda
  Buck
• Discovery in part due to ( thanks to) molecular
  biology and genetic engineering

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How did they do it?

• There are three visual receptors (to distinguish
  different colors)
• Buck got the idea that maybe smell receptors
  might be similar
• She used a genetic technique called the
  Polymerase Chain Reaction (PCR) to find them
• There are about 1000 different smell receptors in
  humans!

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http//nobelprize.org/medicine/laureates/2004/illp
res/4_gene.htmlhttp//nobelprize.org/medicine/laur
eates/2004/illpres/4_gene.html
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