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  • 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 cavity

  • 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,

  • bitter,

  • sour,

  • 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.

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

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TED-ED】私たちはどうやって匂いを嗅いでいるのか - ローズ・イブレス (【TED-Ed】How do we smell? - Rose Eveleth)

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    Zenn に公開 2021 年 01 月 14 日
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