The document discusses the mechanism of odor perception. It begins by defining key terms like olfaction, odorants, and odor thresholds. It then describes how odors are detected by volatile molecules binding to odor receptors in the nose. The pathway involves odorant receptors on olfactory neurons transmitting signals to the olfactory bulb and brain. Factors like age, gender, chemicals and emotions can impact odor perception. The ability to detect thousands of odors comes from each neuron expressing a single odor receptor protein out of hundreds of possible types.

1. MECHANISM OF
ODOR
PERCEPTION
submitted by :
Bhavna vijayan
Leena
Neha
Saumya Jain
Surbhi Rai

2. INTRODUCTION:-
SMELL is the sense by which organism detect odors and is
known as olfaction.
OLFACTION is the sensory component resulting from the
interaction of volatile food components with olfactory receptors
in the nasal cavity.
 stimulus can be ortho nasal (when the odor stimulus enters
the olfactory region directly from the nose as one sniffs a food)
or it could be retro nasal (which means that the odor stimulus
enters from the oral cavity as one eats a food).
Perception of smell is a physical mechanism. And smelling is
a chemical analysis procedure.
Odor perception is said to be the best when mucous
membrane of the nose is red, swollen and moist but not to
such a degree as to block the air passage.

3. Onset of cold, the olfactory sensitivity increases
strinkingly before it is destroyed by mucus accumulation
which impedes the odoriferous substance to diffuse
through the mucous and thus adversely affects odor
perception.
For a substance to be detected as an odor by the
receptor cells, these criteria must be met :-
The substance must be volatile enough to permeate the
air near the sensory area
The substance must be at least slightly water-soluble to
pass through the mucous layer and to the olfactory cells
The substance must be lipid-soluble because olfactory
cilia are composed primarily of lipid material
A minimum number of odorous particles must be in
contact with the receptors for a minimum length of time.

4. ODOR TERMINOLOGY
Odorant - is a substance capable of eliciting an olfactory
response.
Odor – the sensation resulting from stimulation of the
olfactory organs.
odor threshold – is used to describe the detection threshold
,which indentifies the concentration at which 50 percent of a
human panel can identify the presence of an odor or odorant
without characterizing the stimulus.
Recognition threshold – is the concentration at which 50
percent of the panel can identify the odorant or odor such as
the smell of ammonia or peppermint as example.
Odor adaptation - is the process by which one becomes
accustomed to an odor. When adaptation occurs, the detection
threshold increases. And adaptation occurs differently for each
odor.

5. Odor fatigue- it occurs when total adaptation to a
particular odor has occurred through prolonged exposure.
For example to milkers or daily managers who are exposed
to the smell of dairy manure on a daily basis and appear
virtually unaware of the odor.
Osmophere – is a functional group that imparts odor to
an odorless compound. Strong osmophores include
inorganic compounds such as lead,
arsenic,sulphur,bromide or organic compounds such as
esters, carbonyls, amine, imines, lactones.
Aromachology – is the study of the influence of odors
on behaviour has emerged, which deals with the effect of
smell on feelings.
Compounds with high molecular weight (above 300) and
low volatility are generally odorless.

6. ODOR CLASSIFICATION :-
A. linnaeus proposed seven categories of odor : aromatic, fragrant,
garlicky, musky, goaty, repulsive, and nauseating.
B. Olfactory prism was proposed by Henning as a six-odor grouping for
fundamentals odors . These odors include : spicy, flowery, fowl, fruity,
resinous and burnt.
C. In 1970, Amoore developed a odor classification based on specific
anosmias as they may represent a lack of specific receptor type for a
group of compounds. He proposed a group of eight classes : ethereal,
camphoraceous, musky, floral, minty, pungent, putrid and sweaty.

7. MECHANISM :-

9. MECHANISM OF OLFACTORY CELL STIMULATION
odorous chemical binds to the odorant receptors on cilia of olfactory neurons
Alter the shape of odorant receptor
Activation of G-protein
G-protein activates adenyl cyclase
Formation of cyclic AMP
Activate the cells by opening Na+ ion channel
Na + flux from outside to inside
Depolarisation of cell
Generate action potential in cell
Move through the cribriform plate
Synapse to mitral/tufted cells
To the brain

11. PATHWAY OF ODORANT MOVEMENT
Odorant is taken up by receptor on cilia of olfactory neuron
Neuron in nose has long fibre/axon
Poke through a bone above it cribriform plate
Make connects with other neurons
Synapses form distinct region called Glomerulli
Glomerulli also connect to mitral/tufted cells
Neurons from mitral/tuffed cells forms olfactory tract
Pass the message to brain
In olfactory bulb

14. CELLS INVOLVED IN THE PATHWAY OF ODOR PERCEPTION:-
A. RECEPTOR CELLS
• located in the olfactory epithelium
• They are true neurons that conduct action potentials into the CNS.
• Each olfactory neuron has about 10 hair like cilia, which have on their
surface special protein called odorant receptors.
• The cilia protrude into the thin bath of mucus that covers the olfactory
nerve.
• There are approx. 6 million ciliated olfactory receptor in 4-6 sq. cm area
of the human olfactory tissue .
• Basal cells of olfactory epithelium are undifferentiated stem cells that
continuously turn over and replace the olfactory receptors cells
(neurons). These are the only neurons in the adult human that replace
themselves.
B. CN I
• Carries information from the olfactory receptor cells to the olfactory bulb.
• The axons of the olfactory nerves are unmyelinated C fibres and are
among the smallest and slowest in the nervous system.

15. • Olfactory epithelium is also innervated by CN V (TRIGEMINAL
NERVE), which provide a second set of nerve endings which are
responsible for tactile, pressure, pain and temperature sensation in the
area of the mouth, eyes and nasal cavity.
• The olfactory nerve pass through the cribriform plate on their way to
the olfactory bulb.
C. Mitral cells
• They are second order neurons
• Output of the mitral cells forms the olfactory tract, which projects to the
perpiriform cortex.

16. Factor affecting odor perception :-
A) Age :-
• The perception of odor is more severely compromised with age. Age has a
much more significant effect on olfactory ability than either gender or
smoking.
• The olfactory sensory nerves degenerates from the time of birth to the
extent that only 82 % of the acuity remains at the age of 20 ; 38% at the age
of 60 and 28% at the age of 80.
• our smelling ability increases to reach a plateau at about the age of eight,
and declines in old age.
B) Personality :-
• it can both facilitate and impede detections of both pleasant and unpleasant
odors.
• For e.g., emotionally labile individuals have a greater absolute sensitivity to
some smells and have better odor-naming performance, but have lower
sensitivity to other smells.
C) Gender :-
• men on an average, show age-related decline in odor perception at an earlier
age than did women.
• females tend to have a keener sense of smell than males.

17. D) Chemicals and pollutants
• Ammonia and hydrogen sulfide both can cause olfactory losses as a result of
chronic or prolonged exposure.
•Insecticides can result in loss in olfactory by damaging olfactory receptors.
•Pollutants can alter the normal turnover of olfactory cells (.i.e. 30 days) or disrupt
the integrity of the lipid membranes of olfactory receptors.
E) Odor quality recognition
•Like or dislike of a particular odor can change with odor concentration or intensity.
•The recognition of odor can also gets hampered if individual suffer from any odor
abnormalities like anosmia (smell blindness).
F) Emotion
• perception of smell consists not only of the sensation of the odor themselves but of
the experience and emotions associated with these sensations.
•Smells can evoke strong emotional reactions.

18. HOW CAN ONE KIND OF CELL ENABLE US
TO DISCRIMINATE AMONG SO DIFFERENT
ODORS
 Humans can discriminate between hundreds perhaps
thousands of different odorant molecules each with
its own structure.
 The mammalian genome contains a family of about
thousand related but separate genes encoding
different odour receptors.
 Olfactory epithelium of rats expresses several
hundred genes not expressed in other tissues
 Each gene encodes a transmembrane protein that
resembles but is not identical to the others.
 Each region contains 7 regions of hydrophobic alpha
helix that allow the molecule to pass back and forth
seven times through the plasma membrane.

19.  However many odorant molecules are hydrophobic and could easily
enter the lipid bilayer and binds to the receptors there.possibility is
supporting by the finding that much of the sequence variability from one
receptors to another is found in akhan helices
 Each olfactory neuron expresses only a single type of receptors
 Some evidence gene probes for a single type of receptors bund to only
1 in a thousand sensory neurons in a normal olfactory epithelium.
 Rats made to express a single type of receptors in large numbers of the
olfactory neurons responded much more vigorously to a single type of
odorants than to any of the other 73 tested.
 Cells taken from these rats and placed in tissue culture also responded
to only one kind of odorant molecule.

20. OLFACTORY ABNORMALITIES :-
Hyposmia – is a state when smelling capacity of the individual
reduces.
Anosmia – is complete loss of smell. This is usually temporary
condition that a person may experience after a case of acute influenza.
Somerime referred to as partial anosmia. . Example, people with allicin
anosmia can not smell garlic. Those with eugenol can not smell cloves.
Dysosmia – is a distorted sense of smell. A person sense non-
existent unpleasant odors. It can be caused by medical and mental
conditions.
Hyperosmia – is an increased sensitivity to smell.it can be a
characteristic of someone with a neurotic or histrionic personality.
Presbyosmia – refers to the loss of the olfactory sense acuity that
occurs when a person ages.
Parosmia – is a change in the normal perception of odors such as
when the smell of something familiar is distorted or when something that
normally smells pleasant now smells foul.
Phantosmia – is the sensation of an odor that is not there.

21. INTERESTING FACTS :-
 Olfactory neurons gets replaced every 4 to 8 weeks.
Not everybody smells the same things. For e.g., the ability to smell
asparagus pee. For about a quarter population urinating after eating
asparagus means smelling a distinct odor. The other 75% of population
do not notice.
Another example , for some people the chemical androstenone smells
like vanilla, to others it smells like sweaty urine because androstenone is
commonly found in tasty things like pork.
Astronauts tend to lose their senses of smell and taste. Because of the
congestion in the nose resulting from the increases capillary pressure, as
the heart no longer has to work against gravity.
Mothers recognize their babies by smell, and newborns recognize thrie
mothers in the same way.
Identical twins have the same smell and even dogs cannot distinguish
between them.
The sense of smell does not sleep when we sleep, it acts as a security
alarm.