Title: Sensation%20of%20Smell
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2Sensation 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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4The 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)
5Taste Smell cont.
- Chemical Senses
- vital for survival
- Determine appetite
(Retreat from noxious stimuli)
6Chemical 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)
7Chemical 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 (?).
8The 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 -
9Species differences
- Dog
- Has 40 sq inches of epithelium compared to
- 1 sq inch in humans
10Dogs have about 40 times the area for olfaction
that humans do
11Olfactory 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
12Olfactory 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
13Olfactory receptors (a) Location of receptors in
the roof of the nasal cavity. (b) Closeup of
olfactory cells.
14- Olfactory epith. at roof
- of nasal cavity
- Warm nasal epithelium
- Conviction currents
- ( by sniffing)
15The 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
16Olfactory Epithelium
- 5 cm2 of membrane
- 10-100 million receptors
17Cells 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
18Olfactory 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
19BIPOLAR OLFACTORY NEURONS IN THE NOSE.
RECEPTORS FOR ODOR MOLECULES ARE IN THE CILIA.
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21 Olfactory System
22 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
23The 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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25 Mechanism of Olfaction
26Physiology 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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28Threshold 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
29Discrimination of Intensity
- Poor
- Requires 30 increase of intensity
- Strong smell highly water and lipid soluble
- Man can distinguish 2000-4000 different odors
30Physiology of Olfaction cont.
- Adaptation to Smell
- Peripheral
- Central
31Olfactory 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
32Early Olfactory Pathway
33Olfactory Mucosa Olfactory Pathway
34Early Olfactory Pathway
35Central Olfactory Pathways
36Central Olfactory Pathways
37Central Olefactory Pathways
38Central Olefactory Pathways
39Central Olefactory Pathways
40Central Olefactory Pathways
41 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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43Advances in Olfactory Physiology Noble Prize
44How 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
45How 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!
46http//nobelprize.org/medicine/laureates/2004/illp
res/4_gene.htmlhttp//nobelprize.org/medicine/laur
eates/2004/illpres/4_gene.html
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