Smell sensation MBBS Lecture

1. CHEMICALCHEMICAL
SENSESSENSES
Sense of SmellSense of Smell

2. THE SENSES OF TASTE AND SMELL ALLOW US
TO SEPARATE UNDESIRABLE OR EVEN LETHAL
FOODS FROM THOSE THAT ARE PLEASANT TO
AND NUTRITIOUS
The Chemical Senses
Taste and Smell

3. Introduction: Smell & Taste
Smell and taste are generally classified as chemical
senses . Physiologically, they are related to each
other. The flavors of various foods are in large part
a combination of their taste and smell.
Consequently, food may taste "different" if one
has a cold that depresses the sense of smell. Both
taste and smell receptors are chemoreceptors that
are stimulated by molecules in solution in mucus
in the nose and saliva in the mouth.
The smell receptors are distance receptors
(teleceptors), and the smell pathways have no
relay in the thalamus.

4. Smell is the least understood of our senses.
A. Stimulus – Odorant or Odoriferous substances
Physical Factors that affect the degree of stimulation
1. only volatile substances that can be sniffed into the nostril
can be smelled.
2. the stimulating substances must be slightly water soluble.
3. substances must be at least slightly lipid soluble.

6. Olfactory Mucous Membrane
The olfactory receptor cells are located in a
specialized portion of the nasal mucosa, the
yellowish pigmented olfactory mucous
membrane. In dogs and other animals in which
the sense of smell is highly developed
(macrosmatic animals), the area covered by this
membrane is large; in microsmatic animals such
as humans, it is small. In humans, it covers an
area of 5 cm2
in the roof of the nasal cavity near
the septum

7. Smell (olfaction)
 Olfactory apparatus consists of receptor cells, supporting
cells and basal (stem) cells.
 Basal cells generate new receptor cells every 1-2 months.
 Supporting cells contain enzymes that oxidize hydrophobic
volatile odorants.
 Bipolar sensory neurons located within olfactory
epithelium are pseudostratified.
 Axon projects directly up into olfactory bulb of cerebrum.
 Olfactory bulb projects to olfactory cortex, hippocampus, and
amygdaloid nuclei.
 Synapses with 2nd
order neuron.
 Dendrite projects into nasal cavity where it terminates in cilia.
 Neuronal glomerulus receives input from 1 type of olfactory
receptor.

11. Stimulation of the Olfactory Cells
Mechanism of Excitation of the Olfactory Cells
1. Activation of the receptor protein by the odorant
substance activates the G-protein complex
2. This, in turn activates multiple molecules of adenylyl cyclase
inside the olfactory cell membrane
3. This causes the formation of many times more of molecules
of cAMP
4. cAMP opens still many times more sodium channels

12. Olfaction

14. Olfactory receptors
Mitral cell
Olfactory
gland
Olfactory
tract
Olfactory
epithelium
Filaments of
olfactory nerve
Cribriform plate
of ethmoid bone
Lamina propria
connective tissue
Basal cell
Supporting cell
Dendrite
Olfactory cilia
Glomeruli
Axon
Olfactory receptor
cell
Mucus
Route of inhaled
air containing odor
molecules
Frontal lobe
of cerebrum
Nasal
conchae
Route of
inhaled air
Olfactory
epithelium
Olfactory tract
Olfactory
bulb

16. Olfactory Receptors
 Bipolar sensory neurons located within olfactory epithelium
 Dendrite projects into nasal cavity, terminates in cilia
 Axon projects directly up into olfactory bulb of cerebrum
 Olfactory bulb projects to olfactory cortex, hippocampus, and
amygdaloid nuclei

17. Olfactory Epithelium and Olfactory BulbOlfactory Epithelium and Olfactory BulbOlfactory Epithelium and Olfactory BulbOlfactory Epithelium and Olfactory Bulb
OlfactoryOlfactory
epitheliumepithelium
Olfactory BulbOlfactory Bulb
GlomerulusGlomerulus
Mitral cellMitral cell
Tufted cellTufted cell
periglomerularperiglomerular
cellcell
Olfactory tractOlfactory tract

21. Modality:Modality: OlfactionOlfaction
Receptor:Receptor: Olfactory Cell of Olfactory EpitheliumOlfactory Cell of Olfactory Epithelium
Cranial Nerve: I (Olfactory Nerve)Cranial Nerve: I (Olfactory Nerve)
1st Neuron:1st Neuron: Olfactory Bulb --- Mitral & Tufted CellOlfactory Bulb --- Mitral & Tufted Cell
olfactory tractolfactory tract
olfactory striaeolfactory striae
lateral & intermediate olfactory striaelateral & intermediate olfactory striae
Termination:Termination: Primary Olfactory Area (Rhinencephalon)Primary Olfactory Area (Rhinencephalon)
piriform lobe (prepiriform cortex, entorhinal cortex)piriform lobe (prepiriform cortex, entorhinal cortex)
corticomedial amygdalacorticomedial amygdala
anterior perforated substance (olfactory tubercle)anterior perforated substance (olfactory tubercle)
Modality:Modality: OlfactionOlfaction
Receptor:Receptor: Olfactory Cell of Olfactory EpitheliumOlfactory Cell of Olfactory Epithelium
Cranial Nerve: I (Olfactory Nerve)Cranial Nerve: I (Olfactory Nerve)
1st Neuron:1st Neuron: Olfactory Bulb --- Mitral & Tufted CellOlfactory Bulb --- Mitral & Tufted Cell
olfactory tractolfactory tract
olfactory striaeolfactory striae
lateral & intermediate olfactory striaelateral & intermediate olfactory striae
Termination:Termination: Primary Olfactory Area (Rhinencephalon)Primary Olfactory Area (Rhinencephalon)
piriform lobe (prepiriform cortex, entorhinal cortex)piriform lobe (prepiriform cortex, entorhinal cortex)
corticomedial amygdalacorticomedial amygdala
anterior perforated substance (olfactory tubercle)anterior perforated substance (olfactory tubercle)
Olfactory PathwayOlfactory PathwayOlfactory PathwayOlfactory Pathway

23. A. olfactory epitheliumA. olfactory epithelium
B. olfactory bulbB. olfactory bulb
C. nucleus of diagonal bandC. nucleus of diagonal band
of Brocaof Broca
D. periamygdaloid cortexD. periamygdaloid cortex
E. cirticomedial amygdalaE. cirticomedial amygdala
F. entorhinal areaF. entorhinal area
G. septal nucleiG. septal nuclei
I. olfactory nerveI. olfactory nerve
1. olfactory tract1. olfactory tract
2. lateral olfactory stria2. lateral olfactory stria
3. intermediate olfactory stria3. intermediate olfactory stria
4. medial olfactory stria4. medial olfactory stria
Olfactory System - Olfactory PathwaysOlfactory System - Olfactory PathwaysOlfactory System - Olfactory PathwaysOlfactory System - Olfactory Pathways

24. Olfactory PathwaysOlfactory Pathways
I
A. olfactoryA. olfactory
epitheliumepithelium
B. olfdactory bulbB. olfdactory bulb
C. anteriorC. anterior
olfactoryolfactory
nucleus &nucleus &
olfactoryolfactory
tubercletubercle
D. periamygdaloidD. periamygdaloid
areaarea
E. corticomedialE. corticomedial
amygdalaamygdala
F. entorhinal areaF. entorhinal area
II. olfactory nerve. olfactory nerve 2. lateral olfactory striae2. lateral olfactory striae
1. olfactory tract1. olfactory tract 3. intermediate olfactory striae3. intermediate olfactory striae

25. A. olfactory epitheliumA. olfactory epithelium
B. olfactory bulbB. olfactory bulb
C. nucleus of diagonal bandC. nucleus of diagonal band
of Brocaof Broca
D. periamygdaloid cortexD. periamygdaloid cortex
E. cirticomedial amygdalaE. cirticomedial amygdala
F. entorhinal areaF. entorhinal area
G. mediodorsal thalamicG. mediodorsal thalamic
nucleus (MD)nucleus (MD)
I. olfactory nerveI. olfactory nerve
1. olfactory tract1. olfactory tract
2. lateral olfactory stria2. lateral olfactory stria
3. intermediate olfactory stria3. intermediate olfactory stria
4. thalamocortical radiation4. thalamocortical radiation
Olfactory SystemOlfactory System - Connections of Primary Olfactory Cortex- Connections of Primary Olfactory CortexOlfactory SystemOlfactory System - Connections of Primary Olfactory Cortex- Connections of Primary Olfactory Cortex

26. Membrane Potentials and Action Potentials in Olfactory
Cells
Adaptation – Large numbers of centrifugal nerve fibers pass
from the olfactory regions of the brain backward along the
olfactory tract and terminate on special inhibitory cell s in the
olfactory bulb, the granule cells
Primary Sensations of Smell
1. Camphoraceous
2. Musky
3. Floral
4. Pepperminty
5. Ethereal
6. Pungent
7. Putrid

27. Olfactory Cells > Glomerulus > Mitral Cells > Olfactory
Bulb > Olfactory Tract
D. Center
Olfactory Area
1. Medial Olfactory Area – Very Old Olfactory System
*subserves the basic olfactory reflexes such as licking of the
lips, salivation, and other feeding responses caused by smell
of food
2. Lateral Olfactory Area –
A.. Less Old Olfactory system
provides automatic but partially learned control of food
intake and aversion to toxic and unhealthy foods

28. B. Newer Pathway
Used for conscious perception and analysis of olfaction
Adaptation (Centrifugal Control of Activity in the
Olfactory Bulb by the Central Nervous System)
Abnormalities of Olfaction
1. Anosmia – absence of the sense of smell
2. Hyposmia – diminished olfactory sensitivity
3. Dysosmia – distorted sense of smell

29. Adaptation
It is common knowledge that when one is
continuously exposed to even the most
disagreeable odor, perception of the odor decreases
and eventually ceases. This sometimes beneficent
phenomenon is due to the fairly rapid adaptation, or
desensitization, that occurs in the olfactory system.
It is mediated by Ca2+
acting via calmodulin on
cyclic nucleotide-gated (CNG) ion channels.
When CNG A4 is knocked out, adaptation is slowed.

31. Smell (Olfaction)
 Olfactory epithelium with olfactory receptors, supporting cells, basal
cells
 Olfactory receptors are modified neurons
 Surfaces are coated with secretions from olfactory glands
 Olfactory reception involves detecting dissolved chemicals as they
interact with odorant binding proteins

32. Thank You