How do we smell? - Rose Eveleth

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An adult human can distinguish up to 10,000 odors. You use your nose to figure out what to eat, what to buy and even when it's time to take a shower. But how do the molecules in the air get translated into smells in your brain? Rose Eveleth charts the smelly journey through your olfactory epithelium and explains why scent can be so subjective.

Lesson by Rose Eveleth, animation by Igor Coric.

It's the first sense you use when you're born.

One out of every fifty of your genes

is dedicated to it.

It must be important, right?

Okay, take a deep breath

through your nose.

It's your sense of smell,

and it's breathtakingly powerful.

As an adult, you can distinguish

about 10,000 different smells.

Here's how your nose does it.

Smell starts when you sniff molecules

from the air into your nostrils.

95% of your nasal cavitity

is used just to filter that air

before it hits your lungs.

But at the very back of your nose

is a region called the olfactory epithelium,

a little patch of skin

that's key to everything you smell.

The olfactory epithelium has a layer

of olfactory receptor cells,

special neurons that sense smells,

like the taste buds of your nose.

When odor molecules hit the back of your nose,

they get stuck in a layer of mucus

covering the olfactory epithelium.

As they dissolve,

they bind to the olfactory receptor cells,

which fire and send signals

through the olfactory tract

up to your brain.

As a side note,

you can tell a lot

about how good an animal's sense of smell is

by the size of its olfactory epithelium.

A dog's olfactory epithelium

is 20 times bigger

than your puny human one.

But there's still a lot we don't know

about this little patch of cells, too.

For example, our olfactory epithelium is pigmented,

and scientists don't really know why.

But how do you actually tell the difference

between smells?

It turns out that your brain has

40 million different olfactory receptor neurons,

so odor A might trigger neurons 3, 427, and 988,

and odor B might trigger neurons 8, 76, and 2,496,678.

All of these different combinations

let you detect a staggeringly broad array of smells.

Plus, your olfactory neurons are always fresh

and ready for action.

They're the only neuron in the body

that gets replaced regularly,

every four to eight weeks.

Once those neurons are triggered,

the signal travels through a bundle

called the olfactory tract

to destinations all over your brain,

making stops in the amygdala,

the thalamus,

and the neocortex.

This is different

from how sight and sound are processed.

Each of those signals goes first

to a relay center

in the middle of the cerebral hemisphere

and then out to other regions of the brain.

But smell, because it evolved

before most of your other senses,

takes a direct route

to these different regions of the brain,

where it can trigger your fight-or-flight response,

help you recall memories,

or make your mouth water.

But even though we've all got

the same physiological set-up,

two nostrils and millions of olfactory neurons,

not everybody smells the same things.

One of the most famous examples of this

is the ability to smell so-called "asparagus pee."

For about a quarter of the population,

urinating after eating asparagus

means smelling a distinct odor.

The other 75% of us don't notice.

And this isn't the only case

of smells differing from nose to nose.

For some people,

the chemical androstenone smells like vanilla;

to others, it smells like sweaty urine,

which is unfortunate

because androstenone is commonly found

in tasty things like pork.

So with the sweaty urine smellers in mind,

pork producers will castrate male pigs

to stop them from making androstenone.

The inability to smell a scent

is called anosmia,

and there are about 100 known examples.

People with allicin anosmia can't smell garlic.

Those with eugenol anosmia can't smell cloves.

And some people can't smell anything

at all.

This kind of full anosmia

could have several causes.

Some people are born without a sense of smell.

Others lose it after an accident

or during an illness.

If the olfactory epithelium gets swollen or infected,

it can hamper your sense of smell,

something you might have experienced

when you were sick.

And not being able to smell anything

can mess with your other senses, too.

Many people who can't smell at all

also can't really taste the same way

the rest of us do.

It turns out that how something tastes

is closely related to how it smells.

As you chew your food,

air is pushed up your nasal passage,

carrying with it the smell of your food.

Those scents hit your olfactory epithelium

and tell your brain a lot

about what you're eating.

Without the ability to smell,

you lose the ability to taste

anything more complicated

than the five tastes

your taste buds can detect:

sweet,

salty,

sour,

bitter,

and savory.

So, the next time you smell exhaust fumes,

salty sea air,

or roast chicken,

you'll know exactly how you've done it

and, perhaps, be a little more thankful that you can.