Organ of smell - different types of cell Nerve supply Olfactory pathway Mechanism at cellular level Abnormalities and testing

1. SPECIAL SENSES
SMELL
DR.CHARUSHILA
08-10-2021 DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR, BMRCI,PALANPUR 1

2. Smell
1. Site
2. Receptor
3. Tract
4. Mechanism
5. Disorders
SPECIFIC LEARNING OBJECTIVES
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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3. • What you know about smell
• What is use of it
• How it is developed
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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4. Smell is made in the air
When you breath tiny particles reach the receptor
React with the chemical dissolved in the mucus in nose
And send message something is rough, shiny, wet, cold,
and many other things.
Basic concept
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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5. Macrosmatics
well developed in animals like
dog, rabbit
(warning of environmental
dangers)
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE
PROFESSOR, BMRCI,PALANPUR
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Microsmatics
less developed in humans, apes
and monkeys (primates),( but
still it is important for pleasure
and for enjoying the taste of
food)
Sense of smell / olfaction

6. Olfactory mucosa of
nose in human
beings
Site of olfaction -
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE
PROFESSOR, BMRCI,PALANPUR
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Vomeronasal organ in
reptiles and certain mammals
Specialized receptors located on free nerve endings of
olfactory nerves, which are located in the:

7. • Upper one third of nasal cavity.
• 5.0 cm2 in area
• Neuron
• 10- 20 million receptor
• Mucous layer covers entire epithelium.
Olfactory mucosa
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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8. Sense of Smell
Figure 15.3

10. Histological structure
1. Receptor cells.
• 10–20 million,Bipolar neurons,which lie between
supporting (sustentacular) cells.
• Dendrites- form mucous layer of the olfactory mucosa.
• Axons - form the olfactory nerves.
• Characteristic features
1. Cell bodies are closest to the external environment.
2. Short life span of about 60 days,Replaced by the
proliferation of basal cells.
3. Bone morphogenic protein (BMP) inhibits the renewal
turnover.
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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11. Histological structure
2. Supporting cells/ sustentacular cells
• Columnar in shape.
• Secrete mucus.
3. Basal cells
• Stem cells from which new receptor cells are formed.
• There is a continuous replacement of receptor cells by
mitosis of the basal cells.
Nerve supply of olfactory mucosa
• Special nerve- 1st nerve
• General nerve- 5th nerve
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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12. Mucus producing Glands
Olfactory mucous
membrane is constantly
covered by mucus
-Mucus is produced by
Bowman’s glands, placed
just under the basal
lamina of the membrane

13. • It is a pouch-like structure in nose of some animals.(rodents and various other
animals).
• Perception of odour that emanates from the pheromones and foodstuffs
• Nerve fibres project to the accessory olfactory bulb to amygdala and
hypothalamus that are concerned with reproduction and eating behaviour.
• Vomeronasal organ is not well developed in humans.
Vomeronasal organ
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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14. • Hormone-like substances, which emit specific odour and produce hormonal,
behavioural or other physiological changes in another animal of same species.
• Secreted by an animal during mating season only.
• Pheromone also exists in humans and that there is close relationship between
smell and sexual function.
Pheromones
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BMRCI,PALANPUR
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15. Nostril
nasal cavity
olfactory receptors (sensory neurons) in membrane
>
through ethmoid (bone) olfactory neurons in olfactory bulb
Brain
OLFACTORY PATHWAYS
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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16. 1. Olfactory nerves.
• Axons of bipolar olfactory neurons,
• 15–20 olfactory nerve filaments
• Pierce the cribriform plate on either side
to reach olfactory bulb.
OLFACTORY PATHWAYS
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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17. 2. Olfactory bulb
• Lying on cribriform plate, receives olfactory nerve.
• Point-to-point representation of olfactory mucosa in the olfactory bulb.
• Upper part of mucosa -represented in anterior part of bulb
• Lower part is represented posteriorly.
OLFACTORY PATHWAYS
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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18. OLFACTORY PATHWAYS
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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19. OLFACTORY PATHWAYS
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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20. 2. Olfactory bulb
• 3 types of cells: mitral cells, tufted cells ,interneurons (granule
cells and periglomerular cells).
• Olfactory neurons - 1st order neurons.
• Mitral and tufted cells - 2nd order neurons.
• It synapses with olfactory neurons & form olfactory glomeruli.
• Granule and periglomerular cells - inhibitory neurons.
• Form synapses with dendrites of mitral cells.
• Axons of the mitral and tufted cells leave the olfactory bulb
and run in the olfactory tract.
OLFACTORY PATHWAYS
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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21. 3. Olfactory tract.
• It lies in olfactory sulcus on frontal lobe and proceeds
backwards
• Here it divides into lateral, intermediate and medial
olfactory striae
• Olfactory trigone - flattened part of olfactory tract,
near the anterior perforated substance before it
divides into the striae.
OLFACTORY PATHWAYS
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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22. 3. Olfactory tract.
Anterior olfactory nucleus
• Made up of scattered neurons within
olfactory tract.
• Neurons receive neurons of olfactory bulb
• and send axons through anterior commissure
to excite inhibitory neurons on contralateral
olfactory bulb
OLFACTORY PATHWAYS
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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23. 3. Olfactory tract.
Olfactory striae
3 Striae-
1. Lateral – Pripiriform Cortex
2. Intermediate- Olfactory Tubercle & Cortex Area
3. Medial- Amygdala , Forbrain Base
OLFACTORY PATHWAYS
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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24. OLFACTORY PATHWAYS
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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25. 4. Olfactory cortex
• It includes anterior olfactory nucleus,
prepiriform cortex, olfactory tubercle and
amygdala.
• All these are the parts of limbic system.
OLFACTORY PATHWAYS
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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26. Transmission of smell signals to CNS
Ethiorhinal
cortex
Piriform
Cortex-
Sniffing
Neocortex-
conscious
discrimination of
odour

27. OLFACTORY PATHWAYS
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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28. OLFACTORY PATHWAYS
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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29. Physiology of Olfaction – Mechanism at cellular level
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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30. Olfactory
Transduction
Process
Odorant
binding
protein
Odorant
chemical
Na+
Cytoplasm
Inactive Active
Na+ influx
causes
depolarization
Adenylate
cyclase
ATP
cAMP
Depolarization of
olfactory receptor
cell membrane
triggers action
potentials in axon
of receptor

31. Binding of odorant molecule to receptors
Activation of receptor.
Depolarization receptor potential.
Action potentials.
Steps in transduction in olfactory receptor neurons
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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32. Properties
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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 Olfactory receptors respond only to
substances in contact with olfactory
epithelium and need to be dissolved in
mucus
 Methyl marcaptan one of the substances
in garlic can be smelled at very low
concentration showing the remarkable
sensitivity of olfactory receptors
 Humans can recognize more than 10,000
different odors, Primary 50 smell
sensetions
 Determination of intensity of odor is poor
Substance mg/L of
Air
Ethyl ether 5.83
Chloroform 3.30
Pyridine 0.03
Oil of peppermint 0.02
Iodoform 0.02
Butyric acid 0.009
Propyl mercaptan 0.006
Artificial musk 0.00004
Methyl mercaptan 0.000000
4

33. Odoriferous stimuli Properties
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BMRCI,PALANPUR
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 Volatile
 Water soluble
 Lipid soluble
 Olfactory pathway is only sensory system that does not have an
obligatory synaptic relay in thalamus before reaching the cortex.
 However, olfactory information does reach the mediodorsal
nucleus of the thalamus, and it is then transmitted to the
prefrontal and orbitofrontal cortex
 Easily adaptable

34. Types of odorant stimuli
• Aromatic or resinous odours, e.g. camphor,
• Fragrant odours, e.g. perfumes and flowers.
• Ethereal odours, e.g. ether, chloroform.
• Garlic odours, e.g. garlic, onion and sulphur compounds.
• Burning odours, e.g. tobacco, burning of feathers, meat
• Nauseating odours, e.g. excreta, decomposed meat
• Goat odours, e.g. sweat, ripe cheese.
• Repulsive odours, e.g. odour of the bed bug.
• Musky odours, e.g. musk.
Odoriferous stimuli Characteristic
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BMRCI,PALANPUR
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35. Role of Pain Fibers in the Nose
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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 Many trigeminal pain fibers
are found in olfactory
membrane
 They are stimulated by
irritating substances
 Are responsible for
initiating sneezing,
lacrimation and other reflex
responses.

36. Testing
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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 Qualitative testing –sense of smell can be tested by asking the
patient to smell common odours, such as onion, peppermint,
rose, garlic or cloves from each side of the nose separately, with
eyes closed
 Olfactrometry – quantitative testing
 Strong odorants, such as ammonia, should be avoided because
they also activate trigeminal nerve fibers.

37. Disorders
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DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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 Anosmia – absence of sense of smell
Basal skull fracture /damage to one or both olfactory bulbs or tracts by a
tumor (olfactory groove meningioma).
 Hyposmia – diminished olfactory sensitivity
 Dysosmia / paraosmia – distorted sense of smell
An aura of a disagreeable odor, often the smell of burning rubber, occurs
during uncinate fits, which are epileptic seizures that originate in the
medial temporal lobe.
More than 75% of humans over the age of 80 have an impaired ability to
identify smells

38.  Organ of smell - different types of cell
 Nerve supply
 Olfactory pathway
 Mechanism at cellular level
 Abnormalities and testing
SUMMARY
08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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39. References
These PPT’s are prepared from the content of following books
1. Guyton and Hall, Textbook of Medical Physiology,John E. Hall
2. Ganong , review of medical physiology
3. Khurana – medical Physiology
4. Berne & lewy physiology
5. Best & Talyor : Physiological basis of medical practice
6. G K Pal : Comprehensive textbook of medica Physiology
7. Harminder singh : Fundamentals of Medical Physiology.
08-10-2021
PREPARED BY DR.CHARUSHILA, ASSISTANT PROFESSOR. ZMCH
DAHOD, GUJARAT
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40. 08-10-2021
DR.CHARUSHILA .PHYSIOLOGY, ASSOCIATE PROFESSOR,
BMRCI,PALANPUR
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Charushila