physiology of olfaction

1. OLFACTION(SMELL
)
Dr LEKSHMI PRIYA J S

2. SPECIFIC LEARNING
OBJECTIVES
1. Site of Olfaction
2. Olfactory Signal Transduction
3. Olfactory Pathway
4. Factors influencing Olfaction
5. Applied Physiology

3. • Smell and taste are generally classified as visceral
senses because of their close association with
gastrointestinal function.
• Both smell and taste receptors are chemoreceptors that
are stimulated by molecules in solution in mucus in the
nose and saliva in the mouth.
• Because stimuli arrive from an external source ,they are
classified as exteroceptors.

4. SENSE OF SMELL - OLFACTION
• Olfactory sensory neurons are located in a specialised portion of the nasal
mucosa,the yellowish pigmented olfactory epithelium.
• MACROSOMATIC  Sense of smell is highly developed,the area covered by this
membrane is large. Eg: DOGS & other animals
• MICROSOMATIC  sense of smell less developed,the area covered by the
membrane is of 10cm2 in the roof of the nasal cavity near the septum.

6. HISTOLOGICAL STRUCTURE

7. HISTOLOGICAL STRUCTURE

8. Innervation
Olfactory nerve (CN I) carries sense of smell from olfactory mucosa
General sensation from mucosa by trigeminal nerve (CN V)
Distinguishing features of olfactory mucosa
Presence of receptor cells
Presence of Bowman’s glands
Presence of a distinctive yellow brown pigment

9. SITE OF OLFACTION
• Yellowish pigmented olfactory epithelium
• 3 types of cells
(i) Olfactory sensory neurons
(ii) Supporting cells
(iii) Basal stem cells

10. (i) Olfactory sensory neurons
• Also called olfactory receptor cells
• Are bipolar neurons that lie in between supporting cells
• Their dendrites terminate in a knob from which 6 -12 fine cilia project
• Axons enter olfactory bulb
• 10 -20 million olfactory receptor cells in humans

11. Characteristic features :-
Only sensory neurons exposed to external environment
Can be regenerated ( short life span of only 1 -2 months)
Functions both as receptor & ganglion cell

12. ii) Supporting cells
• Sustentacular cells
• Secrete mucous which provide appropriate molecular & ionic environment
for odour detection
(iii) Basal cells
• Stem cells / progenitor cells
• Form new receptor cells by mitosis

13. • Each olfactory sensory neuron has a short,thick,dendrite that projects into the nasal
cavity where it terminates in a knob containing 6-12 cilia.
• Humans –
1. cilia are unmyelinated process
2. 5-10µm long
3. 0.1 – 2 µm in dr
4. protrude into the mucus overlying the epithelium.
• ODORANT MOLECULES ( chemicals ) dissolve in the mucus & bind to odorant
receptors on the cilia of olfactory sensory nns .
• Mucus provide molecular & ionic envt for odor detection.

14. •Axons of olfactory sensory nns ( FIRST CRANIAL NERVE )
pass thru cribriform plate of ethmoid bone and enter olfactory
bulbs.
•In the olfactory bulbs,axons of olfactory sensory nns connect
the primary dendrites of MITRAL CELLS & TUFTED CELLS
to form synaptic units called OLFACTORY GLOMERULI(
OG)
•Obs also contain
1. PERIGLOMERULAR CELLS (PG) ,inhibitory nns
connecting one OG to other.
2. GRANULE CELLS – no axons & make reciprocal
synapses with lateral dendrites of M & T cells .here M or
T cells excite granule cell by releasing GLUTAMATE &
granule cells inturn inhibit the M or T cell by releasing
GABA .

16. • Free endings of TRIGEMINAL nerve fibres are found in olfactory epithelium.
• Stimulated by irritating substances which leads to characteresic odor of such
substances as peppermint,menthol & chlorine.
• Activation of these endings by nasal irritants also initiates sneezing,lacrimation,
respiratory inhibition etc.

18. VOMERONASAL ORGAN
• Found in rodents & other mammals .
• Required for successful mating behavior .
• Secrete pheromones which has a peculiar odor that attracts the
mating partner .
• From vomeronasal organ, nerve fibers project to accessory olfactory
bulb .
• And from there to areas in amygdala & hypothalamus that are
concerned with reproduction & eating behavior .

21. OLFACTORY SIGNAL TRANSDUCTION
 Odoriferous stimuli
Olfactory receptors
 Steps in signal transduction

22. ODORIFEROUS STIMULI
• Include smell producing molecules that enter nasal cavity while breathing
• Effective odorant molecule must be volatile, water soluble & lipid soluble

23. COMMON ODOURS
Odours Examples
Aromatic / Resinous Camphor, lavender, cloves
Fragrant Perfumes, flowers
Ethereal Ether , Chloroform
Garlic odour Garlic, onion, Sulphur compounds
Burning odour Tobacco , burning of feathers
Nauseating Excreta, decomposed meat, vegetables
Goat odour Sweat, ripe cheese
Repulsive odour Odour of bed bug
Musky odour Musk

24. OLFACTORY RECEPTORS
• About 1000 olfactory genes in humans.
• Approximately 400 of these genes function as odorant receptors.
• Receptors are activated only when odoriferous substances are dissolved in the
thin layer of mucous covering olfactory epithelium.
• Cilia odour detection

25. OLFACTORY THRESHOLDS AND
DISCRIMINATION•Olfactory receptors respond only to substances in contact with olfactory epithelium and need
to be dissolved in mucus
•Humans can recognize more than 10,000 different odors
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
• Propylmercaptan  0.006
• Artificial musk  0.00004
• Methyl mercaptan 0.0000004

26. STEPS IN SIGNAL TRANSDUCTION
 Binding of odorant molecule to receptors
 Activation of receptor
 Depolarization receptor potential
 Action potential

28. Odoriferous substances
Combine with receptors on surface of cilia
Olf. Receptors are GPCR (α,β & γ subunits)
Bound receptor causes dissociation of α subunit
Activates adenylyl cyclase – increases cAMP
Opens cation channels ( Na+,K+,Ca2+)
Inward diffusion of Na+ & Ca2+ =Depolarisation
Continued in next slide

29. Graded receptor potential
Further opening of Ca2+ activated Cl- channels
Further depolarization
Action potential triggered in olfactory nerve
( when stimulus is sufficient to exceed its
threshold)

30. OLFACTORY PATHWAY
 Olfactory nerves
Olfactory bulb
Olfactory tract
Olfactory cortex

31. OLFACTORY NERVES
• 1st order neurons are bipolar olfactory sensory neurons
• Axons from these neurons gather into 15 – 20 bundles forming olfactory nerve

32. OLFACTORY BULB
• Flattened ovoid strip of grey matter seen on cribriform plate of ethmoid
• Point to point representation
• 3 types of cells
Mitral cells
Tufted cells
Interneurons – Granule cells & Periglomerular short axon cells

34. • Olfactory nerves enter olf. Bulb & synapse with mitral & tufted cells forming 2nd
order neurons
• This synapse forms a complex glomerular mass called olfactory glomeruli
• Periglomerular cells are inhibitory neurons connecting one glomeruli to another
• Granule cells ( no axons) make reciprocal synapses with lateral dendrites of
mitral & tufted cells
• Dendro – dendritic reciprocal synapse

35. • Here mitral / tufted cells excite granule cells by releasing glutamate
• Granule cells in turn inhibit mitral / tufted cells by releasing GABA
• This lateral inhibition helps to sharpen & focus olfactory signals

36. OLFACTORY TRACT
• Axons of mitral & tufted cells leave olf. bulb & run in Olfactory tract
• Olfactory tract lies in the olfactory sulcus on the orbital surface of frontal lobe
• From here, it proceeds backwards to the region of anterior perforated substance
• Lateral olfactory tract / lateral olfactory stria

37. OLFACTORY CORTEX
• Fibers of olfactory tract terminate in the apical dendrites of pyramidal cells of
olfactory cortex
• Mainly in 5 areas
Anterior olfactory nucleus
Olfactory tubercle
Piriform Cortex
Amygdala
Entorhinal cortex

38. • Mitral cells project to all 5 areas
• Tufted cells project to anterior olfactory nucleus & olfactory tubercle
• Mitral cells in accessory olfactory bulb project only to amygdala
• Information from all these areas except anterior olfactory nucleus travel to
frontal cortex, hippocampus, hypothalamus & to orbitofrontal cortex via
thalamus
• Anterior olfactory nucleus projects to contralateral olfactory bulb

40. Orbitofrontal cortex – Conscious perception & analysis of odour
Piriform cortex – Sniffing
Amygdala – emotional responses to olfactory stimuli
Entorhinal cortex – olfactory memories

41.  Olfactory pathway has no direct relay in thalamus. Fibers from
olfactory tract do pass through the dorsomedial nucleus of
thalamus to reach orbitofrontal cortex

42. FACTORS INFLUENCING OLFACTORY FUNCTION
1. Olfactory Threshold
• Odor detection threshold – lowest concentration of a chemical that can be detected .
• Measured by olfactometer (Zwaardimaker’s olfactometer).
• Women more sensitive than men ( ovulation).
2. Intensity / Concentration of odour – concentration diff. must be
changed by 30% to be detected.
3. Structural configuration of odorant

43. 4. Olfactory Adaptation
•Sometimes beneficient phenomenon
• Inactivation of receptors
• Activation of CNG( cyclic nucleotide gated ) ion channels (CNG A4)
• Strong feedback inhibition to olf. bulb from olf.cortex

44. APPLIED PHYSIOLOGY
Anosmia ( absence of smell )
• Causes
• Injuries
• Intracranial lesions
• Nasal obstructions
• Atrophic rhinitis
• Old age
• Kallmann’s syndrome – hyposmia with hypogonadism
Hyposmia ( diminished olfactory sensitivity )
Dysosmia (disorted sense of smell
Hyperosmia( increased olfactory acuity )

45. • Parosmia – change in quality of smell
• Phantosmia – odour sensstion in the absence of an olfactory stimulus
• Olfactory agnosia – inability to recognize odor sensns despite olfactory processing,language &
intellectual function.
• Presbyosmia –smell lose due to ageing
More than 75% of humans over the age of 80 have an impaired ability to identify smells.

46. MEASUREMENT OF SENSE OF
SMELL

47. Qualitative testing – by asking patient to smell common odours with eyes closed.
Olfactometry –quantitative estimation of smell with olfactometer