This document discusses the sense of smell and olfaction. It describes the key parts of the olfactory system including the olfactory epithelium containing olfactory sensory neurons, and the olfactory pathway from the nose to the olfactory bulb and olfactory cortex. The document outlines how odor molecules stimulate receptors on olfactory neurons, and how the olfactory system allows for detection of thousands of odors. It also briefly discusses abnormalities in smell perception.

1. Smell
Olfaction
Dr. Berjin
Jr Resident
Department of Physiology

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

3. • 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)

4. 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

5. • How many olfactory sensory
neurons?
• about 50 million

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

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

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

9. 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

10. 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

11. Olfactory Bulb
Periglomerular
cells
Grannule cells
Axon of
Olfactory
sensory neuron
Mitral cells
Tufted cells
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12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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