1. Olfaction

2. Smells Fishy
Olfaction is the ability to detect chemicals in the air around you
This ability is one of the most interesting abilities that humans have developed
It also is one of the most important senses due to its ability to affect the subconscious sections of the brain

3. Smells Fishy
The olfactory organs are paired structures that detect chemicals in the air
They are covered in olfactory receptor cells and basal cells
These organs make up a small amount of the space within your nasal cavity

4. Smell That?
Air that comes in to your nose and mouth is designed to swirl inside of an open cavity
This allows particles maximum chance to land on the olfactory organs
Any chemicals that are lipid or water soluble can diffuse into the overlying mucus and stimulate receptor cells

5. Smell That?
However, a normal and relaxed inhalation will cause around 2% of the incoming air to come in contact with the olfactory organs
This does not provide the body with enough of a chance to pick up chemicals in the air
Sniffing causes a more rapid inhalation of air
This creates more swirling of air, and more contact with the surface of the olfactory organs

6. Olfactory Receptors
Think of the olfactory receptors inside of the olfactory organs as modified neurons
These receptors are shaped like neurons and transmit their information down pathways that lead to the brain
As few as 4 molecules of an odorant can send a signal down the line to make an organism aware of a smell

7. Olfactory Receptors
Olfactory receptors are neurons that extend into the surrounding mucus and have tiny cilia on one end
The end with the cilia is designed to pickup odorants
Each cell will contain up to 20 cilia to be able to increase their surface area and effectiveness

8. Olfactory Receptors
Between 5 and 20 million olfactory receptors are packed into your olfactory organs
This is pretty incredible because the olfactory organs contain around 5cm2 of surface area
The cilia do a large amount to increase the surface area of the receptors
It is estimated the cilia create a similar amount of surface area as the surface area of the skin

9. Olfactory Receptors
You might be pretty impressed with your nose right about now…
I would give you a moment of caution because our nose pales in comparison to a dog
A German Shepard has roughly 72 times more surface area inside of their nose

10. Video
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11. Pathways The olfactory system is a very sensitive and efficient system
The olfactory receptors can detect just a few odorant molecules and will send a signal
However, that does not always mean that you will “smell” when you detect just a few particles

12. Pathways
Before a stimulus reaches the olfactory cortex, it might be inhibited
This is because olfactory receptors meet in groups of around 20
These olfactory centers will combine information before they send information to the brain

13. Pathways
This allows low level odorants to be filtered out and higher level odorants to be detected
When there are multiple odorants that are in the air, the nose will get multiple smells coming from different areas and can combine them when they reach the olfactory cortex

14. Pathways
The nerves then deliver messages directly to the olfactory cortex, the hypothalamus and the limbic system
This means that smells are directly tied to emotional states rising from the hypothalamus
It is why the perfume industry spends millions of dollars every year researching what smells can cause different emotions in humans

15. Distinctions
While there are is no structural or chemical difference between olfactory receptors, it is believe they can distinguish between different chemicals
Among those are the primary smells
There are up to 50 primary smells that include citrus, mint, wood and musk

16. Distinctions
Our nose can also determine the differences between smells at very low concentrations
Natural gas, propane and butane (the stuff you cook with) has a distinct odor because of beta- mercaptan
This is a chemical that our nose can distinguish at a few parts per billion
It gives these otherwise odorless gasses a distinct and distinguishable odor

17. Turnover Olfactory receptors will die after a short period of time
Basal cells will replace the dead olfactory cells
This creates a fresh crop of receptors at all times
This is one of the only examples of neural replacement in the human body

18. Turnover
However the basal cells become less effective at replacement as we get older
As people age, there is a greater rate of receptor death to basal cell replacement
This can result in a decreased sense of smell
It is commonly the cause of “old lady smell”