Smell
Olfaction
Dr. Berjin
Jr Resident
Department of Physiology

LEARNING OBJECTIVES
• Olfactory Epithelium
• Olfactory Sensory Pathway
• Odor Detection Threshold
• Abnormalities

• The flavor of foods is a combination of its taste
and smell
• Smell and taste receptors are chemoreceptors as
they are stimulated by molecules in solution
(mucus in the nose and saliva in the mouth)

Receptors for Olfaction
• are the olfactory sensory neurons
also called olfactory receptors
• Olfactory sensory neurons are located in the yellowish
pigmented olfactory epithelium
(a specialized portion of the nasal mucosa)
• It is situated in the roof of the nasal cavity near the septum

• How many olfactory sensory
neurons?
• about 50 million

Olfactory Epithelium
• Bipolar olfactory sensory neuron
• Supporting (sustentacular) cells
• Basal stem cells

Olfactory Epithelium
• Bipolar olfactory sensory neuron
• Responsible for Olfactory transduction
• Dendrites – forms 6 -12 cilia in nasal cavity
• Axons – forms Olfactory bulb (CN I)

Olfactory Epithelium
• Supporting (sustentacular) cells
• Secrete Mucus
• Odorants dissolves in mucus
• Basal stem cells
• Undergoes mitosis to generate new Olfactory
epithelium

Olfactory Pathway
• Olfactory sensory neurons are located in the olfactory
epithelium
• The axon of theses olfactory sensory neurons pierces the
cribriform plate and enter the olfactory bulb

Olfactory Pathway - olfactory bulb
• In the olfactory bulb, the axons
synapse with the dendrites of the
mitral cells and tufted cells and
form globular synaptic units called
called olfactory glomeruli
• The olfactory bulb also contains
periglomerular cells and granule
cells

Olfactory Bulb
Periglomerular
cells
Grannule cells
Axon of
Olfactory
sensory neuron
Mitral cells
Tufted cells
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Olfactory Pathway - olfactory bulb
• The periglomerular cells are
inhibitory neurons connecting one
glomerulus to another
• Granule cells make reciprocal
synapses with the dendrites of the
mitral and tufted cells
• The mitral or tufted cell excites the
granule cell by releasing glutamate,
and the granule cell in turn inhibits
the mitral or tufted cell by releasing
GABA

Olfactory Pathway
• The axons of the mitral and tufted cells pass posteriorly
through the lateral olfactory stria to terminate in five
regions of the olfactory cortex:
• anterior olfactory nucleus, olfactory tubercle, piriform
cortex, amygdala, entorhinal cortex

Olfactory Pathway
• From the olfactory cortex (5 regions), information travels
directly to the frontal cortex or via the thalamus to the
orbitofrontal cortex
• Conscious discrimination of odors is dependent on the
pathway to the orbitofrontal cortex and frontal cortex

Olfactory Pathway
• The pathway to the amygdala and hypothalamus (limbic
system) is probably involved with the emotional responses
to olfactory stimuli
• The pathway to the entorhinal cortex is concerned with
olfactory memories

Odorant receptors
• Odorant receptors are located on the cilia of the
olfactory sensory neurons, that project into the mucus
• There are about 1000 different odorant receptors
• They are G Protein Coupled Receptors
• We can discriminate more than 10,000 different odors

ODOR DETECTION THRESHOLD
• The olfactory receptors are very sensitive
• Low concentrations of chemical that can be
detected is called Odor detection threshold
• Eg - Hydrogen Sulphide, Kerosene
• Women > Men

Abnormalities of smell
• Anosmia – Inability to smell
• Hyposmia – Diminished olfactory sensitivity
• Causes:
- Simple nasal congestion, nasal polyps
– Damage to the olfactory nerves eg fractures,
tumors, infections
– Ageing

Abnormalities of smell
• Hyperosmia : Enhanced Olfactory Sensitivity
Pregnant women are sometimes oversensitive to
smell
• Dysosmia : Distorted sense of smell
– Sinus infections
– Partial damage to the olfactory nerves