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It's the first sense you use when you're born.
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One out of every fifty of your genes
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is dedicated to it.
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It must be important, right?
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Okay, take a deep breath
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through your nose.
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It's your sense of smell,
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and it's breathtakingly powerful.
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As an adult, you can distinguish
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about 10,000 different smells.
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Here's how your nose does it.
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Smell starts when you sniff molecules
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from the air into your nostrils.
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95% of your nasal cavity
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is used just to filter that air
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before it hits your lungs.
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But at the very back of your nose
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is a region called the olfactory epithelium,
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a little patch of skin
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that's key to everything you smell.
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The olfactory epithelium has a layer
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of olfactory receptor cells,
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special neurons that sense smells,
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like the taste buds of your nose.
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When odor molecules hit the back of your nose,
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they get stuck in a layer of mucus
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covering the olfactory epithelium.
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As they dissolve,
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they bind to the olfactory receptor cells,
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which fire and send signals
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through the olfactory tract
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up to your brain.
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As a side note,
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you can tell a lot
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about how good an animal's sense of smell is
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by the size of its olfactory epithelium.
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A dog's olfactory epithelium
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is 20 times bigger
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than your puny human one.
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But there's still a lot we don't know
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about this little patch of cells, too.
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For example, our olfactory epithelium is pigmented,
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and scientists don't really know why.
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But how do you actually tell the difference
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between smells?
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It turns out that your brain has
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40 million different olfactory receptor neurons,
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so odor A might trigger neurons 3, 427, and 988,
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and odor B might trigger neurons 8, 76, and 2,496,678.
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All of these different combinations
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let you detect a staggeringly broad array of smells.
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Plus, your olfactory neurons are always fresh
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and ready for action.
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They're the only neuron in the body
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that gets replaced regularly,
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every four to eight weeks.
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Once those neurons are triggered,
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the signal travels through a bundle
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called the olfactory tract
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to destinations all over your brain,
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making stops in the amygdala,
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the thalamus,
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and the neocortex.
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This is different
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from how sight and sound are processed.
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Each of those signals goes first
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to a relay center
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in the middle of the cerebral hemisphere
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and then out to other regions of the brain.
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But smell, because it evolved
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before most of your other senses,
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takes a direct route
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to these different regions of the brain,
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where it can trigger your fight-or-flight response,
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help you recall memories,
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or make your mouth water.
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But even though we've all got
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the same physiological set-up,
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two nostrils and millions of olfactory neurons,
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not everybody smells the same things.
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One of the most famous examples of this
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is the ability to smell so-called "asparagus pee."
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For about a quarter of the population,
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urinating after eating asparagus
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means smelling a distinct odor.
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The other 75% of us don't notice.
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And this isn't the only case
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of smells differing from nose to nose.
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For some people,
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the chemical androstenone smells like vanilla;
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to others, it smells like sweaty urine,
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which is unfortunate
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because androstenone is commonly found
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in tasty things like pork.
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So with the sweaty urine smellers in mind,
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pork producers will castrate male pigs
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to stop them from making androstenone.
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The inability to smell a scent
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is called anosmia,
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and there are about 100 known examples.
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People with allicin anosmia can't smell garlic.
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Those with eugenol anosmia can't smell cloves.
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And some people can't smell anything
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at all.
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This kind of full anosmia
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could have several causes.
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Some people are born without a sense of smell.
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Others lose it after an accident
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or during an illness.
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If the olfactory epithelium gets swollen or infected,
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it can hamper your sense of smell,
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something you might have experienced
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when you were sick.
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And not being able to smell anything
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can mess with your other senses, too.
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Many people who can't smell at all
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also can't really taste the same way
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the rest of us do.
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It turns out that how something tastes
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is closely related to how it smells.
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As you chew your food,
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air is pushed up your nasal passage,
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carrying with it the smell of your food.
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Those scents hit your olfactory epithelium
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and tell your brain a lot
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about what you're eating.
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Without the ability to smell,
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you lose the ability to taste
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anything more complicated
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than the five tastes
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your taste buds can detect:
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sweet,
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salty,
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bitter,
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sour,
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and savory.
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So, the next time you smell exhaust fumes,
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salty sea air,
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or roast chicken,
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you'll know exactly how you've done it
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and, perhaps, be a little more thankful that you can.