Ahhhhhhh.
“A rose, by any other name, would smell as sweet.”
At least according to good ‘ol Bill Shakespeare.
And he’s right.
The lovely smell of a rose has nothing to do with its name, but rather
the chemicals it releases, and the way your brain perceives them.
Everything we smell, from a delicate rose to delicious, sizzling bacon to fresh coffee to...
flatulence, is thanks to our olfactory system.
Nose goes!
Just as the visual system converts light and
the auditory system converts sound waves to neural signals for our brain,
the olfactory system uses chemical stimuli to tell our brains
something about the world around us.
Mammals tend to have two methods for detecting scents.
The main olfactory system and the accessory olfactory system.
The accessory system is primarily used for detecting pheromones, and we won’t be covering it today.
Instead, we’re going to focus on the main olfactory system and how exactly it translates
the scent of a rose, by any other name, into something so sweet.
When it comes to olfaction, the nose knows.
When I press my nose up against this flower and inhale,
tiny microscopic particles called odorants coming from the rose
are drawn up to a patch of tissue called the olfactory epithelium
located up and back inside your face.
This epithelium contains millions of olfactory receptor cells, and these are
the cells directly responsible for detecting a scent.
Each receptor cell has only one kind of olfactory receptor on it,
and there are nearly one thousand different kinds of olfactory receptors in the human nose.
In fact, there are so many different kinds of receptors that they account for 3% of our genome.
Those different receptors can detect more than one type of odorant,
which means that with all of those different receptors spread out
over all of the olfactory receptor cells,
scientists estimate that humans can detect nearly 1 trillion distinct odors.
Because the olfactory system uses chemical signals,
it’s considered a “chemosensory” system, like taste.
The olfactory receptor neurons are specially designed to maximize their ability to capture
odors and transmit information.
The dendrites of these cells are positioned along the epithelium, and the dendrites project
tiny hair-like cilia into the mucus that coats the inside of your nose.
The olfactory receptors can be found on the cilia.
These receptors are G-protein coupled receptors,
meaning that when an odorant binds to the receptor, it causes the receptor to
activate other changes in the cell that eventually lead it to send a signal.
Once an olfactory neuron has been activated by enough odorants binding to its receptors,
the signal is transmitted away from the epithelium and along the axon,
which together with all the axons from other receptor neurons
forms the olfactory nerve.
The olfactory nerve projects to a structure known as the olfactory bulb, where olfactory
receptor neurons converge into clumps called glomeruli.
Inside the glomeruli, the olfactory nerve cells connect with neurons called mitral cells.
It’s not clear how much processing these cells do on their own, but scientists think
that they may be interpreting the strength of the smell.
In turn, these mitral cells project to several different areas of the brain.
We’re still learning about how exactly your brain interprets smells, but we think that
the piriform cortex is where the majority of the processing happens.
The front part of the piriform cortex, or anterior piriform cortex, is associated with
identifying odorant molecules, while the back part, the posterior piriform cortex,
plays a role in categorizing odors and helping us detect similarities between them.
Projections from the piriform cortex to the orbitofrontal cortex
allow us to consciously perceive smells.
Have you ever noticed how certain smells can trigger strong memories?
You get a whiff of baby powder and suddenly you’re remembering a hug from your grandma,
or the smell of angel food cake reminds you of your eighth birthday.
This is because olfactory information is closely connected with long term and emotional memory,
via signals to your hippocampus and amygdala.
As I said before, humans can smell many different smells,
but we’re still learning how our brains can do all of that processing.
It must involve some pretty complex computing.
This is about as far as we’ve gotten in our understanding of the human perception of smell.
All right, friends, three senses down, and just two left of the five we’ll be covering.
Next time, we’ll talk about taste.
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I’m Alie Astrocyte, you’ve been watching Neuro Transmissions, and until our next transmission.
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