physiology of smell

Reference - guyton

1. PHYSIOLOGY OF PHYSIOLOGY OF
SMELLSMELL
KASHMEERA.N.AKASHMEERA.N.A
II SEM MSc ZOOLOGYII SEM MSc ZOOLOGY
ROLL NO:37ROLL NO:37

2. Important for enjoyment & Important for enjoyment &
selection of food .selection of food .
Flavours are combinations of Flavours are combinations of
taste and smell (smell contribution taste and smell (smell contribution
is about 80 %)is about 80 %)
Gives warning of harmful Gives warning of harmful
substances or placessubstances or places

3. Primary Sensations of SmellPrimary Sensations of Smell
Based on psychological studies, one attempt to classify
these sensations is the following:
•1. Camphoraceous
•2. Musky
•3. Floral
•4. Pepperminty
•5. Ethereal
•6. Pungent
•7. Putrid
In recent years, specific studies of the genes that encode
for the receptor proteins, suggest the existence of at least
100 primary sensations of smell

4. Anatomy of Olfactory Anatomy of Olfactory
ReceptorsReceptors

5. •The nose contains 10–100 million olfactory receptors The nose contains 10–100 million olfactory receptors
contained within an area called the olfactory epitheliumcontained within an area called the olfactory epithelium..
•olfactory epithelium lies in the superior part of each olfactory epithelium lies in the superior part of each
nostril.nostril.
•In each nostril, the olfactory membrane has a surface In each nostril, the olfactory membrane has a surface
area of about 2.4 square centimeters.area of about 2.4 square centimeters.
•The olfactory epithelium consists of three kinds of cells:The olfactory epithelium consists of three kinds of cells:
 olfactory receptors, olfactory receptors,
supporting cells / sustentacular cellssupporting cells / sustentacular cells
 basal cellsbasal cells

6. Olfactory receptors
•The receptor cells for the smell sensation are the olfactory cells .The receptor cells for the smell sensation are the olfactory cells .
•They are actually bipolar nerve cells derived from the CNS . They are actually bipolar nerve cells derived from the CNS .
•There are about 100 million of these cells in the olfactory epithelium.There are about 100 million of these cells in the olfactory epithelium.

•The mucosal end of the olfactory cell forms a knob .The mucosal end of the olfactory cell forms a knob .
•From knob 4 to 25 olfactory hairs (olfactory cilia), project into the From knob 4 to 25 olfactory hairs (olfactory cilia), project into the
mucus that coats the inner surface of the nasal cavity.mucus that coats the inner surface of the nasal cavity.
•These projecting olfactory cilia form a dense mat in the mucus.These projecting olfactory cilia form a dense mat in the mucus.
•These cilia react to odours in the air and stimulate the olfactory These cilia react to odours in the air and stimulate the olfactory
cellscells

8. Supporting cells/sustentacular cellsSupporting cells/sustentacular cells
The receptor cells in the olfactory epithelium are interspersed among The receptor cells in the olfactory epithelium are interspersed among
sustentacular cells or supporting cells.sustentacular cells or supporting cells.
Supporting cells Supporting cells are columnar epithelial cells.are columnar epithelial cells.
 They provide physical support, nourishment and electrical insulation for They provide physical support, nourishment and electrical insulation for
the olfactory receptors, the olfactory receptors,
They help detoxify chemicals that come in contact with the olfactory They help detoxify chemicals that come in contact with the olfactory
epithelium.epithelium.

9. Basal cellsBasal cells
•Basal cells are stem cells located between
the bases of the supporting cells.
•They continually undergo cell division to
produce new olfactory receptors, which live for
only a month or so before being replaced.
•This process is remarkable - olfactory
receptors are neurons, and mature neurons
are generally not replaced.
•The olfactory renewal process is carefully
regulated - a bone morphogenic protein
(BMP) exerts an inhibitory effect.
•[ BMPs are a large family of growth factors
originally described as promoters of bone
growth]

10. Spaced among the olfactory cells in the olfactory membrane are many small
Bowman’s glandsBowman’s glands that secrete mucus onto the surface of the olfactory membrane
mucus is carried to the surface of the epithelium by ducts.
The secretion moistens the surface of the olfactory epithelium and dissolves
odourants so that transduction can occur.

11. Physiology of OlfactionPhysiology of Olfaction

12. MECHANISM OF EXCITATION OF OLFACTORY CELLS.MECHANISM OF EXCITATION OF OLFACTORY CELLS.
Cilium is the portion which respond to the olfactory chemical stimuli.Cilium is the portion which respond to the olfactory chemical stimuli.
The odourant substance on coming in contact with olfactory surfaceThe odourant substance on coming in contact with olfactory surface
first diffuse in to the mucus which covers the cilia.first diffuse in to the mucus which covers the cilia.
 Then binds with a receptor protein that protrudes through the ciliary Then binds with a receptor protein that protrudes through the ciliary
membrane.membrane.
This receptor is a long molecule, it threads its way through the This receptor is a long molecule, it threads its way through the
membrane 7 times, folding inward and outward. membrane 7 times, folding inward and outward.
Odourant binds with portion of receptor and coupled to Odourant binds with portion of receptor and coupled to G-PROTEIN.G-PROTEIN.
G-PROTEIN –a combination of 3 subunits.G-PROTEIN –a combination of 3 subunits.

13. Odourant receptor
heptahelical receptor/
serpentine receptor/
G protein-linked receptor (GPLR)

15. On excitation of receptor,an alpha subunit breaks away On excitation of receptor,an alpha subunit breaks away
from G-PROTEIN and activates adenylcyclase.from G-PROTEIN and activates adenylcyclase.
Activated cyclase converts many molecules of intracellular Activated cyclase converts many molecules of intracellular
adenosine-tri-phosphate into cyclic-adenosine adenosine-tri-phosphate into cyclic-adenosine
monophosphate(cAMP).monophosphate(cAMP).
This cAMP activates another near by membrane protein,a This cAMP activates another near by membrane protein,a
gated sodium ion channel.gated sodium ion channel.
Allows large number of sodium ions to pour into receptor Allows large number of sodium ions to pour into receptor
cell cytoplasm.cell cytoplasm.
Sodium ions helps in exciting the olfactory neuron and Sodium ions helps in exciting the olfactory neuron and
transmitting action potential in to the CNS through an transmitting action potential in to the CNS through an
olfactory nerve.olfactory nerve.

17. Odourant + receptor proteinOdourant + receptor protein
¯¯
Activation of G proteinActivation of G protein
¯¯
Activation of adenylate Activation of adenylate
cyclasecyclase
¯¯
ATP ATP ®® cAMP cAMP
¯¯
Opening of NaOpening of Na
++
channels channels
¯¯
NaNa
++
influx influx
¯¯
depolarizationdepolarization
Mechanism of olfactory cell stimulationMechanism of olfactory cell stimulation

18. Physical factors affect the degree of Physical factors affect the degree of
stimulation.stimulation.
 Only volatile substancesvolatile substances that can be sniffed into
the nostrils can be smelled.
 The stimulating substance must be at least
slightly water solublewater soluble so that it can pass through
the mucus to reach the olfactory cilia.
 The substance should be at least slightly lipid lipid
solublesoluble, because lipid constituents of the cilium
itself are a weak barrier to non-lipid-soluble
odourants.

20. Cribiform plate separates nasal cavity and cranial cavity.Cribiform plate separates nasal cavity and cranial cavity.
Olfactory bulb lies above cribiform plate.Olfactory bulb lies above cribiform plate.
Small nerves from olfactory membrane in nasalcavity pass through the small Small nerves from olfactory membrane in nasalcavity pass through the small
perforations in the cribiform plate to enter olfactory bulb in the cranial cavity.perforations in the cribiform plate to enter olfactory bulb in the cranial cavity.
Olfactory nerve fibers leading from olf.bulb are called Cranial nerve I or olf. Olfactory nerve fibers leading from olf.bulb are called Cranial nerve I or olf.
Tract.Tract.

21. Short axons from the olfactory cells terminate in multiple globular structures Short axons from the olfactory cells terminate in multiple globular structures
within the olfactory bulb called within the olfactory bulb called glomeruli.glomeruli.
Each bulb has several thousand such glomeruli,each of which is the terminus for Each bulb has several thousand such glomeruli,each of which is the terminus for
about 25,000 axons from olfactory cells. about 25,000 axons from olfactory cells.
Each glomerulus also is the terminus for dendrites from about 25 large Each glomerulus also is the terminus for dendrites from about 25 large mitral mitral
cellscells and about 60 smaller and about 60 smaller tufted cellstufted cells, , the cell bodies of which lie in the olfactory the cell bodies of which lie in the olfactory
bulb superior to the glomeruli. bulb superior to the glomeruli.
These dendrites receive synapses from the olfactory cell neurons,These dendrites receive synapses from the olfactory cell neurons,

 the mitral and tufted cells send axons through the olfactory tract to transmitthe mitral and tufted cells send axons through the olfactory tract to transmit
olfactory signals to higher levels in the central nervous system.olfactory signals to higher levels in the central nervous system.
Some research has suggested that different glomeruli respond to different Some research has suggested that different glomeruli respond to different
odours.odours.

23. Olfactory pathways into theOlfactory pathways into the
Central Nervous SystemCentral Nervous System

24. The olfactory tract enters the brain at the anterior junction between the The olfactory tract enters the brain at the anterior junction between the
mesencephalon and cerebrum;mesencephalon and cerebrum;
there, the tract divides into two pathways, one passing medially into the there, the tract divides into two pathways, one passing medially into the medialmedial
olfactory areaolfactory area of the brain stem, and the other passing laterally into the of the brain stem, and the other passing laterally into the lateral lateral
olfactory area. olfactory area.
The medial olfactory area represents a very old olfactory system, whereas the The medial olfactory area represents a very old olfactory system, whereas the
lateral olfactory area is the input to lateral olfactory area is the input to
(1) A less old olfactory system and (2) a newer system(1) A less old olfactory system and (2) a newer system

25. 25
Olfactory pathway
Olfactory TractOlfactory Tract
Medial Olfactory areaMedial Olfactory area Lateral Olfactory areaLateral Olfactory area
Septal NucleiSeptal Nuclei
HypothalamusHypothalamus
Limbic systemLimbic system
(primitive parts)(primitive parts)
Prepyriform cortexPrepyriform cortex
Pyriform CortexPyriform Cortex
AmygdalaAmygdala
LimbicLimbic systemsystem
(hippocampus)(hippocampus)
ThalamusThalamus
Orbitofrontal Orbitofrontal
CortexCortex
Olfactory receptor cellOlfactory receptor cellOlfactory nerveOlfactory nerve
Olfactory bulbOlfactory bulb
(Very Old (Very Old
Olfactory System)Olfactory System)
(Less Old Olfactory (Less Old Olfactory
System)System)
(Newer System)(Newer System)

26. LIMBIC SYSTEMLIMBIC SYSTEM

27. Very Old Olfactory SystemVery Old Olfactory System
More primitive responses to olfaction
salivation, liking lips and primitive emotional drives
to smell
Less Old Olfactory SystemLess Old Olfactory System
Learned control of food intake
Aversion to food that have caused nausea and
vomiting.
Newer SystemNewer System
Conscious perception & analysis of odour
Odour discrimination

28. Thank You
Take time toTake time to smell the flowers…. smell the flowers….