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Sensation of Smell. Professor A.M.A Abdel Gader MD, PhD, FRCP (Lond., Edin), FRSH (London) Professor of Physiology, College of Medicine & King Khalid University Hospital Riyadh, Saudi Arabia. The Physiology of Smell ( Olfaction ). Smell & Taste: Similar : Chemical Senses

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Sensation of smell

Sensation of Smell

Professor A.M.A Abdel Gader

MD, PhD, FRCP (Lond., Edin), FRSH (London)

Professor of Physiology, College of Medicine &

King Khalid University Hospital

Riyadh, Saudi Arabia

The physiology of smell olfaction

The Physiology of Smell(Olfaction)

Smell & Taste:


Chemical Senses

Determine food flavor (intake)


Smell receptors… telereceptors- sense projected the environment

Taste is confined to mouth

Smell pathway does not relay in the thalamus does not reach sensory cortex

Taste pathway finishes in the sensory cortex (PCG)

Taste smell cont
Taste & Smell – cont.

  • Chemical Senses:

    • vital for survival

    • Determine appetite (Retreat from noxious stimuli)

Sensation of smell

Chemical Senses

  • “Gatekeepers”:molecule detectors

    • identify what the body needs for survival

    • identify what is dangerous and should be rejected

  • Neurogenesis: constant renewal of receptors

  • Olfactory receptors = 30-60 days

  • Taste cells = 10 days

  • Affective component: emotions aid in discrimination of molecules (good vs. bad)

Sensation of smell

Chemical Senses - cont

  • Taste plays a vital role in food selection,

    • sweet (and umami) are associated with nutritious food

    • Bitter tastes are associated with the possible presence of toxins and are usually avoided.

  • Taste and smell are closely linked even though they involve different receptors and receptive processes. (??overlap in central processing).

  • In many species olfactory stimuli play an important role in reproduction although not well developed in humans (?).

Sensation of smell

Sense of Smell –species difference:

Macrosmatic animals… Dogs: highly developed sense of smell & reliance on olfactory system

Microsmatic… Man: weakly developed sense of smell & less reliance on olfactory system

The Physiology of Smell(Olfaction)

Species differences
Species differences


Has40 sq inches of epithelium compared to

1 sq inch in humans

Sensation of smell

Dogs have about 40 times the area for olfaction that humans do

Olfactory receptor neurons orn
Olfactory Receptor Neurons (ORN)

  • Olf. Signal Transducers

    • 1000 different types

    • Each type found in only 1 zone of mucosa

    • Vision:

    • 3 cone types, 1 type of rod

    • 6 million cones, 120 million rods

Olfactory receptors
Olfactory Receptors

  • 1000 different types of receptors each has odorant-binding protein

    • Only one protein type per ORN

  • These membrane bound proteins

    • Located in cilia on tips of ORN’s

    • ORN: olfactory receptor neuron

Sensation of smell

Olfactory receptors:

(a) Location of receptors in the roof of the nasal cavity.

(b) Closeup of olfactory cells.

Sensation of smell

Sensation of smell

Olfactory Receptors:

Bipolar neurons with cilia

Receptors are genuine neurons

(unlike photoreceptors and hair cells)

Unlike other neurons, receptors are

continually regenerated

The Physiology of Smell(Olfaction)

Olfactory epithelium
Olfactory Epithelium

  • 5 cm2 of membrane

  • 10-100 million receptors

Cells of the olfactory membrane
Cells of the Olfactory Membrane

  • Olfactory epithelium made up of three types of cells:

    • Olfactory receptors

      • bipolar neurons with olfactory hairs

    • Supporting cells

      • columnar epithelium

    • Basal stem cells

      • replace receptors monthly

  • Olfactory (Bowman’s) glands

    • produce mucus

      • Dissolves odorants

Olfactory epithelium1
Olfactory Epithelium

  • Receptors have four parts cilia,olfactory knob,olfactory rodand the axon

  • Olfactory nerve - the axons of the olfactory receptors form bands which travel to the olfactory bulb

Sensation of smell



Olfactory system
Olfactory System

Each sensory cell type connects to

a particular pair of glomeruli

Each sensory cell expresses

one receptor type (indicated by color)

Different colors represent different sensory cells

Sensation of smell

Olfactory Receptors:

Bipolar neurons with cilia

Mitose throughout life (only part of CNS that is known to regenerate)

Their axons project to the olfactory bulb

The Physiology of Smell - (Olfaction) – cont.

Physiology of olfaction mechanism
Physiology of Olfaction – Mechanism

Molecules must dissolve in mucus

Combine with receptors on the cilia

Stimulation of a G-Protein

Activation of Adenyl Cyclase

Increase I.C. cAMP

Opening of Na Channels

Na influx

Depolarization (Receptor Potential)

Threshold to different substances

Physiology of Olfaction – cont.

Threshold to Different Substances

Discrimination of intensity
Discrimination of Intensity

  • Poor

  • Requires 30% increase of intensity

  • Strong smell highly water and lipid soluble

  • Man can distinguish 2000-4000 different odors

Sensation of smell

Physiology of Olfaction – cont.

Adaptation to Smell

  • Peripheral

  • Central

Olfactory pathway
Olfactory Pathway

  • Axons from olfactory receptors synapse in the olfactory bulb

    • Second-order neurons within the olfactory bulb form the olfactory tract

      • synapses on primary olfactory area of temporal lobe

      • conscious awareness of smell begins

  • Other pathways lead to the frontal lobe where identification of the odor occurs

Olfactory mucosa olfactory pathway
Olfactory Mucosa & Olfactory Pathway

Sensation of smell

Physiology of Olfaction – cont.

Clinical Considerations:

Abnormalities of the sense of olfaction:

  • Anosmia: Absence of the sense of smell

  • Dysosmia; Disturbed “ “ “ “ “

  • Hyposomia: Reduced “ “ “ “ “

    Vit. A deficiency and hypogonadism

  • Hyperosmia: Increased sense of smell

    (Adrenal insufficiency)

Advances in olfactory physiology noble prize
Advances in Olfactory Physiology& Noble Prize

How does the sense of smell work
How does the sense of smell work?

  • Discovered fairly recently (1991)

  • Nobel prize in 2004, to Richard Axel and Linda Buck

  • Discovery in part due to (& thanks to) molecular biology and genetic engineering

How did they do it
How did they do it?

  • There are three visual receptors (to distinguish different colors)

  • Buck got the idea that maybe smell receptors might be similar

  • She used a genetic technique called the Polymerase Chain Reaction (PCR) to find them

  • There are about 1000 different smell receptors in humans!

Sensation of smell