The olfactory nerves receive smell signals from olfactory receptor neurons in the nasal cavity and transmit them to the olfactory bulb and other brain regions. The first order neurons are in the olfactory epithelium and project to glomeruli in the olfactory bulb. Second order neurons in the bulb project as the olfactory tract to primary olfactory cortex like the piriform cortex. Higher order processing occurs in other limbic regions. The olfactory system is unique in directly connecting to the brain without relay in the thalamus. Diseases and injuries can cause loss or distortions of smell.

1. OLFACTORY NERVES
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2. Olfactory epithelium

3. Olfactory mucosa

4. Olfactory Nerves (Cranial Nerve I)
Olfactory bipolar receptor nerve cells
(In olfactory mucous membrane in upper part of the nasal cavity above the
level of the superior concha)
peripheral process that passes to the surface
( about 20 fiber bundles)
Short cilia /olfactory hairs

5. Tufted cells
• Mitral cell granular cells
synaptic glomeruli
• Enter cribriform plate of the ethmoid to join the olfactory bulb.
• fine central processes(unmyelinated) form the olfactory nerve
fibers
• Olfactory bipolar receptor nerve cells

7. • Mitral cell Tufted cells medial striae of opp olfac bulb
olfactory tract
medial olfactory striae
paraolfactory area, subcallosal gyrus,
inferior part of the cingulate gyrus
lateral olfactory striae
Primary olfactory cortex secondary olf area
(uncus, anterior hippocampal gyrus) entorhinal area (area 28)
piriform cortex, and amygdaloid nucleus)
(Medial temporal lobe)

10. • FIRST ORDER NEURON:
– From olfactory epithelium to glomerulus
• SECOND ORDER NEURON:
– The olfactory bulb. where the second neurons of the
olfactory pathway (mitral and tufted cells) are located.
– Is considered an extension of the telancephalon.
– The axons of these Second order neurons pass centrally
as the olfactory tract.
• THIRD ORDER NEURON:
– The prepiriform area (area 28) is considered the primary
olfactory cortex which contains the third order neurons.

11. OLFACTORY BULB
– is an elongated oval structure that lies just above the
cribriform plate.
– It is continuous posteriorly with the olfactory tract
through which it is connected to the base of the cerebral
hemisphere.
– The olfactory bulb and tract are not parts of PNS but
Instead constitute the extension of the telencephalon
this contains CNS-specific cell types (oligodendrocytes
and microglia).
– The olfactory bulb and tract share with the
telencephalon share a common meningeal covering.

12. OLFACTORY TRACT
When traced posteriorly the olfactory tract
divides into
– Medial
– Lateral olfactory striae
– Intermediate striais sometimes present.
• The point of bifurcation is expanded and
forms the olfactory trigone.

13. • Some of the axons of the olfactory tract run in the
lateral olfactory stria to the olfactory centers
amygdala, semilunar gyrus, and ambient gyrus.
• Other axons of the olfactory tract run in the medial
olfactory stria to the nuclei in the septal
(subcallosal) area which is part of the limbic system
and to the olfactory tubercle a small elevation in the
anterior perforated substance.
• other axons of the olfactory tract teminate in the
anterior olfactory nucleus where the fibres cross to
the opposite side branch off and relayed.
• This nucleus is located in the olfactory trigone,
which lies between the two olfactory striae in front
of the anterior perforated substance.

14. ANTERIOR PERFORATED SUBSTANCE:
• The olfactory striae are intimately related to a mass
of grey matter called the anterior perforated
substance.
• The medial and lateral striae form the anteromedial
and anterolateral boundaries of this substance.
• The intermediate stria extends into the anterior
perforated substance and ends in a slight elevation
(in the anterior part of the substance) called the
olfactory tubercle.
• Posterolaterally, the anterior perforated substance
is related to the uncus .
• while posteromedially it is bounded by a bundle of
fibres called the diagonal band (of Broca)

16. The uncus
• Is a part of the cerebral hemisphere that lies on the
tentorial surface a little behind and medial to the
temporal pole.
• It represents the anterior end of the
parahippocampal gyrus and is separated from the
temporal pole by the rhinal sulcus.
• The uncus is subdivided into three parts. From
anterior to posterior side these are
– The uncinate gyrus,
– the tail of the dentate gyrus(band of Giacomini)
– The intralimbic gyrus
Entorhinal area (area 28):
– The anterior part of the parahippocampal gyrus, including
the uncus,

19. • When traced backwards the lateral olfactory stria reaches the
limen insulae (in the depth of the stem of the lateral sulcus).
• Here it bends sharply to the medial side and becomes
continuous with a small area of grey matter lying anterior to
the uncus and called the gyrus semilunaris (or
periamygdaloid area).
• The gyrus semilunaris is closely related to the amygdaloid
complex which lies deep (i.e., superior) to it.
• The lateral olfactory stria is covered by a thin layer of grey
matter called the lateral olfactory gyrus.
• When traced backwards this gyrus becomes continuous with
a part of the cortex called the gyrus ambiens.
• The gyrus ambiens lies lateral to the gyrus semilunaris.
• Posteriorly, it becomes continuous with the entorhinal area.
The lateral olfactory gyrus and the gyrus ambiens
collectively form the prepiriform region(or area).

21. Entorhinal area

23. • PRIMARY OLFACTORY CORTEX:
– The piriform cortex is considered the primary olfactory
cortex. 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.
• The exact functions of these higher areas are a matter
of scientific research and debate.
• Olfactory information is processed in primitive areas of
the brain.
• Olfaction is the only sensation not directly processed
in the thalamus

24. • However indirectly the olfactory fibres from the
olfactory cortex pass to the neo cortex passes via
the thalamus terminating in the basal fore brain

26. • The axons that run in the medullary striae of the
thalamus terminate in the habenular nuclei.
• This tract also continues to the brainstem. where it
stimulate salivation in response to smell.

27. Limbic system

28. Amygdala

29. SPECIAL FEATURES OF OLFACTORY
EPITHELIUM
• These neurons have a limited lifespan of up to several
months, but are continiously replenished from the pool
of precursor cells
• In the olfactory mucosa this undergo periodic mitosis.
• New olfactory receptors are thus generated throughout
adult life,
• and their axons enter the olfactory bulb to form new
synapses with existing CNS neurons.
• The regenerative capacity of the olfactory mucosa
gradually diminishes with advancing age.
• Resulting in net loss of receptors and a slow decline in
overall sensory function

30. Olfaction and coding

32. Examination of olfactory nerve
• Examine each nostril separately while occluding
the other. With the patient's eyes closed and one
nostril occluded,
• bring the test substance near the open one. Ask
the patient to sniff and indicate whether she
smells something and,
• if so, to identify it. Repeat for the other nostril
and compare the two sides

33. Inference
• The perception of odor is more important than
accurate identification.
• Perceiving the presence of an odor indicates
continuity of the olfactory pathways.
• Identification of the odor indicates intact cortical
function as
• Since there is bilateral innervation, a lesion central to
the decussation of the olfactory pathways never causes
loss of smell,
• A lesion of the olfactory cortex does not produce
anosmia.
• The appreciation of the presence of a smell, even
without recognition, excludes anosmia.

34. Causes of Persistent Loss of Smell
• Smoking
• Chronic rhinitis
• Deviated nasal septum
• Nasal polyps
• Intranasal tumors (e.g., epidermoid carcinoma)
• Postviral
• General anesthesia
• Dental trauma

35. Causes of Persistent Loss of Smell
• Chemical burns of the olfactory epithelium
• Normal aging
• Pregnancy
• Congenital anosmia
• Chemotherapeutic agents
• Cadmium toxicity
• Antibiotics
• Antihistamines

36. Causes of Persistent Loss of Smell
• Olfactory groove meningioma
• Frontal lobe tumor, especially glioma
• Sellar/parasellar tumor
• Neuro-olfactory tumor
(esthesioneuroblastoma)
• Korsakoff's syndrome
• Vitamin deficiency (B6, B12, A) Zinc or copper
deficiency

37. Causes of Persistent Loss of Smell
• Craniocerebral trauma,
• Surgery- Temporal lobectomy
• Alzheimer's disease
• Parkinson's disease
• Multiple sclerosis
• Meningitis
• Arhinencephaly
• Olfactory dysgenesis Kallmann's syndrome (hereditary
hypogonadism with anosmia)
• Propylthiouracil
• Familial dysautonornia

38. Causes of Persistent Loss of Smell
• Refsum's syndrome
• Psychiatric conditions (depression, conversion
disorder, schizophrenia)
• Cocaine
• Levodopa
• Amphetamines
• Chronic sinus disease
• Radiation therapy

39. Terms and Definitions Related to
Olfactory Abnormalities
• Anosmia -No sense of smell
• Hyposmia -A decrease in the sense of smell
• Hyperosmia- An overly acute sense of smell
• Dysosmia -Impairment or defect in the sense of smell
• Parosmia -Perversion or distortion of smell
• Phantosmia -Perception of an odor that is not real
Presbyosmia Decrease in the sense of smell due to
aging
• Cacosmia- Inappropriately disagreeable odors
• Coprosmia -Cacosmia with a fecal scent
• Olfactory agnosia -Inability to identify or interpret
detected odors

40. • sphenoidal ridge meningioma
• consists of unilateral optic atrophy or papilledema
• exophthalmos,
• ipsilateral anosmia
• The Foster Kennedy syndrome
– classically due to a large tumour involving the
orbitofrontal region, such as an olfactory groove
meningioma.
• anosmia (direct compression)
• unilateral ipsilateral optic atrophy (direct compression)
• contralateral papilledema(d/t inc ICP)
• Pseudo-Foster Kennedy syndrome.
– This ophthalmologic picture, without the anosmia, is
more often due to anterior optic nerve ischemia,
sometimes termed the

41. • Kallmann's syndrome
– a hereditary disorder, usually X-linked,
• hypogonadism
• anosmia,( due to hypoplasia or aplasia of the olfactory bulbs and tracts.)
• Uncinate fits
• Complex partial or temporal lobe seizures preceded by an
olfactory or gustatory aura, usually disagreeable, and often
accompanied, as the patient loses awareness, by smacking of
the lips or chewing movements. Such attacks are typically due
to a seizure focus involving medial temporal lobe structures.
There is never objective loss of smell interictally.