2. INTRODUCTION
Olfaction, also known as olfactics, is the sense of smell.
This sense is mediated by specialized sensory cells of the
nasal cavity of vertebrates, which can be considered
analogous to sensory cells of the antennae of
invertebrates.
In humans, olfaction occurs when odorant molecules bind
to specific sites on the olfactory receptors.
Many vertebrates, including most mammals and reptiles,
have two distinct olfactory systems—the main olfactory
system, and the accessory olfactory system (used mainly to
detect pheromones).
Olfaction, along with taste, is a form of chemoreception.
3. EVOLUTION OF OLFACTION
• Olfaction is essential for survival in most vertebrates; the degree
to which an animal depends on smell is highly varied.
• Mutations affecting OR genes on the chromosome are primarily
responsible for the evolution of smell.
• Vertebrate olfaction was first derived in an aquatic ecosystem,
where water was the primary medium for odorants.
• Lancelets, a class of fish-like marine chordates, are the most
distantly related ancestors that share the same olfactory receptors
(OR) with humans.
• Tetrapods exhibit both a main and accessory olfactory system.
• Research has suggested that the relative size of the main olfactory
bulb is highly correlated with ecological adaptations.
• while the relative size of the accessory olfactory bulb is related to
sociosexual factors.
4. • The reduction of vomeronasal receptor genes is often attributed to
the transition of tetrapods from water to land.
• Mammals (unlike other tetrapods) utilize a nose to sense volatile
odors.
• The appearance of nasal turbinates and fossae, which resemble
scroll-shaped spongy bones in the nasal passage, is one of the
distinctive features of their evolution.
• Among amniotes, mammals were the first to evolve a complex
system of nasal turbinates, which augment the surface area for
olfactory epithelium.
• Humans have about 10 cm2 (1.6 sq in) of olfactory epithelium,
whereas some dogs have 170 cm2 (26 sq in) and are more
innervated.
5. PHEROMONE
• A pheromone is a secreted or excreted chemical factor
that triggers a social response in members of the
same species.
• There are alarm pheromones, food trail pheromones,
sex pheromones, and many others that affect behavior
or physiology.
• when in close proximity smells also play a role in
sociosexual behavior.
• Experiments have focused on three classes of putative
human pheromones: axillary steroids, vaginal aliphatic
acids, and stimulators of the vomeronasal organ.
6. VOMERONASAL ORGAN
• The vomeronasal organ (VNO), or
the Jacobson's organ, is an
auxiliary olfactory sense organ. It lies close to
the vomer and nasal bones.
• It was discovered by Frederik Ruysch prior to
1732 and later by Ludwig Jacobson in 1813.
• The vomeronasal organ is mainly used to
detect pheromones.
• Its presence in human beings is controversial.
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8. HISTORICAL ASPECT OF OLFACTION
• The extensive doctoral dissertation of Eleanor
Gamble, published in 1898, which compared
olfactory to other stimulus modalitie.
• Roman philosopher Lucretius (1st Century BCE)
speculated, different odors are attributed to
different shapes and sizes of "atoms.
• A modern demonstration of that theory was the
cloning of olfactory receptor proteins by Linda B.
Buck and Richard Axel (who were awarded
the Nobel Prize ).
9. OLFACTORY SYSTEM
• Olfactory epithelium- receptor cells, supporting cells and basal cells
• Receptor cells are bipolar neurons, distal process carries cilia that
project into nasal cavity.
• Cilia have receptors for odoriferous molecules.
• Axons from the olfactory sensory neurons converge in the olfactory
bulb to form clusters called glomeruli.
• These nerve fibers,are lacking in myelin sheaths.
• The glomerulus is a spherical structure located in the olfactory
bulb of the brain where synapses form between the terminals of
the olfactory nerve and the dendrites of mitral, periglomerular
and tufted cell.
10. • The number of glomeruli in a human decreases
with age.
• Mitral cells also project to periglomerular cells
and granular cells that inhibit the mitral cells
surrounding it (lateral inhibition).
• The mitral cells leave the olfactory bulb in
the lateral olfactory tract, which synapses on five
major regions of the cerebrum: the anterior
olfactory nucleus, the olfactory tubercle,
the amygdala, the piriform cortex, and
the entorhinal cortex.
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12. • The piriform cortex is probably the area most
closely associated with identifying the odor.
• The medial amygdala is involved in social
functions such as mating and the recognition
of animals of the same species.
• The entorhinal cortex is associated with
memory, e.g. to pair odors with proper
memories
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22. THEORIES OF OLFACTION
• According to the shape theory, each receptor detects a
feature of the odor molecule.
• Weak-shape theory, known as odotope theory,
suggests that different receptors detect only small
pieces of molecules, and these minimal inputs are
combined to form a larger olfactory perception.
• An alternative theory, the vibration theory proposed
by Luca Turin,states that odor receptors detect the
frequencies of vibrations of odor molecules in the
infrared range by quantum tunnelling.
• Stereochemical theory states that molecules particular
odor is due to lock and key mechanism.
23. MOLECULAR MECHANISM OF
OLFACTION
• Molecules of odorants passing through the superior nasal conchaof
the nasal passages dissolve in the mucus lining the superior portion
of the cavity and are detected by olfactory receptors on
the dendrites of the olfactory sensory neurons.
• This may occur by diffusion or by the binding of the odorant
to odorant binding proteins.
• The mucus overlying the epithelium contains mucopolysaccharides,
salts, enzymes, and antibodies (these are highly important, as the
olfactory neurons provide a direct passage for infection to pass to
the brain).
• This mucus acts as a solvent for odor molecules, flows constantly
and is replaced approximately every 10 minutes.
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28. ROLE AND IMPORTANCE OF
OLFACTION
• Many organisms live in an olfactory rather than a visual
world.
• The sense of smell is closely related to food, for its finding,
correct identification and assessment of its edibility.
• Sensing other animals whether prey or predators is largely
olfactory.
• Bonding between parents and offspring is largely olfactory.
• Olfaction is a very important aspect in sexual
reproduction throughout evolution because it triggers
mating behaviour in many species.
• Olfaction is largely related to emotional behaviour,
olfactory symptoms and disturbed psychological pattern is
well known.
29. • In female humans, the sense of olfaction is strongest
around the time of ovulation, significantly stronger
than during other phases of the menstrual cycle and
stronger than the sense in males.
• The MHC genes (known as HLA in humans) are a group
of genes present in many animals and important for
the immune system; in general, offspring from parents
with differing MHC genes have a stronger immune
system. Fish, mice and female humans are able to
smell some aspect of the MHC genes of potential sex
partners and prefer partners with MHC genes different
from their own.
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31. Genetics Of Olfaction
• Different people smell different odors and most
of these differences are caused by genetic
differences.
• lthough odorant receptor genes make up one of
the largest genes families in the human genome,
only a handful of genes have been linked
conclusively to particular smells.
• The odorant receptor OR5A1- for beta ionone, he
odorant receptor OR2J3 is associated with the
ability to detect the "grassy" smelling odor
32. Interactions Of Olfaction With Other
Senses
• Olfaction and taste-
Olfaction, taste and trigeminal receptors together contribute
to flavor.
The human tongue can distinguish only among five distinct
qualities of taste, while the nose can distinguish among
hundreds of substances, even in minute quantities.
Whereas a flavor results from interactions between smell
and taste.
• Olfaction and audition-
Olfaction and sound information has been shown to converge
in the olfactory tubercles of rodents. his neural
convergence is proposed to give rise to a percept termed
smound
33. DISORDERS OF OLFACTION
• Anosmia - Inability to detect odors
• Hyposmia - Decreased ability to detect odors
• Dysosmia - Distorted identification of smell
• Parosmia - Altered perception of smell in the presence
of an odor, usually unpleasant
• Phantosmia – Perception of smell without an odor
present
• Cacosmia- sensation of unpleasant odor
• Agnosia - Inability to classify or contrast odors,
although able to detect odors
• Presbyosmia- decreased sense of smell due to aging
34. ETIOLOGIES OF OLFACTORY
DISORDERS
Conductive defects
• Inflammatory processes - These may include rhinitis of
various types, including allergic, acute, or toxic (eg, cocaine
use).
• Chronic rhinosinusitis causes progressive mucosal disease
and often leads to decreased olfactory function.
• Masses may block the nasal cavity, preventing the flow of
odorants to the olfactory epithelium.
• These include nasal polyps (most common), inverting
papilloma, or any nasal tumor.
• Developmental abnormalities (eg, encephaloceles, dermoid
cysts) also may cause obstruction.
35. • Patients with laryngectomies or tracheotomies
experience hyposmia because of a reduced or
absent nasal airflow.
• Medications like some antibiotics, antithyroid
drugs, anti cancer drugs etc.
• Tumors – like olfactory neuroblastoma,
meningioma, pitutary tumors etc.
36. Central/sensorineural defects
• Infectious and inflammatory processes
• viral infections (which may damage the
neuroepithelium), sarcoidosis (affecting neural
structures), Wegener granulomatosis, and
multiple sclerosis.
• Head trauma, brain surgery, or subarachnoid
hemorrhage may stretch, damage, or transect the
delicate fila olfactoria or damage brain
parenchyma and result in anosmia.
37. • Sense of smell decreases with age, and it has been shown
that the number of fibers in the olfactory bulb decreases
throughout one's lifetime.
• In one study the average loss in human mitral cells was 520
cells per year with a reduction in bulb volume of 0.19 mm3.
• Congenital syndromes like Kallmann syndrome is one type
of congenital smell loss and is due to failed olfactory
structure ontogenesis and hypogonadotropic
hypogonadism.
• Endocrine disturbances (eg, hypothyroidism,
hypoadrenalism, diabetes mellitus) may affect olfactory
function.
38. • Various neuropsychiatric disorders (eg
depression, schizophrenia, seasonal affective
disorder) have been linked to hyposmia.
• Degenerative processes of the central nervous
system (eg, Parkinson disease, Alzheimer
disease) and other neurologic diseases
(Huntington disease, multiple sclerosis, motor
neuron disease
39. Diagnosis Of Smell Disorders
• The first step in diagnosing any deficit of taste
and smell is obtaining a thorough history and
physical examination.
• Attention should be given to any antecedent
URI, nasal or sinus pathology, history of
trauma, other medical problems, and
medications take.
40. Tests Of Smell
• Threshold tests- a portable kit containing 10
rows of 5 bottles each. Two contains odorant
and 3 only solvents.
• Olfactory olfactogram- based on Amoore’s
description of 7 most primary odours-
ethereal, camphoraceous, musky, minty,
pungent, florid, putrid. Blast olfactometer is
used.
41. • University of Pennsylvania Smell Identification Test(
UPSIT)-The UPSIT involves 40 microencapsulated odors
in a scratch-and-sniff format, with 4 response
alternatives accompanying each odor. The scores are
compared against sex- and age-related norms, and the
results are analyzed. This test has excellent test-retest
reliability.
• Olfactory-evoked response (usually reserved for
research studies)- eye movements,
electroencephalogram (EEG) electrodes and an
electrooculogram measure olfactory-evoked potentials
• BEAM- Brain electric activity monitoring, mapping of
cortical activity in response to sniffing.
• Electro olfactogram- electric potential across olfactory
epithelium,
42. • DNE – Diagnostic nasal endoscopy should be done
• CT scan to rule out nose and paranasal sinuses
pathology
• Intracranial lesions can be ruled out by CT scan and
MRI.
• Tests to evaluate for allergy, diabetes mellitus, thyroid
functions, renal and liver function, endocrine function,
and nutritional deficiencies may be obtained based on
history and the physical examination.
• Olfactory epithelium biopsy
43. TREATMENT OF OLFACTORY
DISORDERS
• Treatment of causative abnormality that has
been identified .
• Local nasal and/or sinus conditions should be
optimally managed with saline lavage,
decongestants, antihistamines, antibiotics,
and/or nasal and systemic steroids, if
applicable.
44. • Polyps and sinus disease that are resistant to
medical management should be surgically
addressed to remove the conductive defect.
• Care must be exercised during surgery to
avoid damage to the olfactory regions.
• In general, conductive olfactory losses are the
most amenable to treatment.
45. • Endocrine disturbances may be addressed by
administration of the deficient hormone, as
with hypothyroidism.
• Control of diabetes mellitus may slow neural
degeneration of the olfactory system.
• Generally, viral processes that damage the
olfactory neuroepithelium, sarcoidosis, and
multiple sclerosis do not have specific
remedies; however, steroids may be
administered in an attempt to limit the
inflammation.
46. • Idiopathic cases of olfactory loss are not
amenable to specific treatment, although
some unproven remedies have been
attempted. The best known of these is zinc
sulfate.
• Other unproven remedies include
pharmacologic doses of vitamins, topical
steroids, and tricyclic antidepressants (for
their effect on CSF catecholamines).
47. • It is now known that olfactory training is safe and
effective therapy for postinfectious olfactory
loss. This method involves the patient choosing 4
known odors and intentionally sniffing these
same 4 odors twice daily. A recent randomized,
controlled study showed improved olfactory
function after 18 weeks,
• Eliminating toxins (eg, cigarette smoke, airborne
pollutants) may help.
48. • Reassurance to the patient
• Psychiatric evaluation and treatment
• Warning and councelling the patient with
olfactory disorders of the hazards associated
with the inability to smell odors such as
smoke, natural gas leaks, and spoiled food.
49. FUTURE PROSPECTIVES
• Stem cell transplant-
Treating olfactory dysfunction is a challenge
for physicians. One of the therapeutic options
could be transplantation of stem cells. neural
stem cells were transplanted into anosmic
mice and recovery of olfactory function was
noted after 4 weeks. These results indicate
that stem cells might be one of the future
modalities for treating olfactory impairment.
50. CONCLUSION
• Smell disorders traditionally have been overlooked in most
aspects of medical practice because these specialized
senses often are not considered critical to life. However,
they affect everyday enjoyment of food, and they impair
detection of the potentially dangerous smells of smoke or
spoiled food.
• Anxiety and depression, as well as anorexia and nutritional
deficiencies, may result from taste and smell disorders.
• Reassurance is one of the most important aspects of
treatment in these disorders because cures are often
difficult to obtain and spontaneous resolution may take
weeks, months, or years.
