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[ ♪ Intro ]
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Whether or not eyes are the windows to the soul... scientifically, pupils are the
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windows of the eyes.
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And sometimes, those windows are more open than usual, like when you've taken a medication
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that causes pupil dilation.
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Or when people use certain recreational drugs.
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It all comes down to how different chemicals can cause changes throughout your whole body,
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which can affect little muscles in your eyes.
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The pupils are openings that let light into the rest of your eye.
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And the colored part of your eye, the iris, controls how much light makes it in.
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The balance of two muscle groups in the iris changes how big your pupils are, like a camera aperture.
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The iris dilator is arranged like bicycle spokes around the center of your eye.
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And when it contracts, it pulls your pupils open wider.
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The iris sphincter, on the other hand, is in a ring around your pupil and acts like
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most of the sphincter muscles in your body.
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It constricts your pupils down to little dots.
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So now you know that you've got a sphincter in your eye.
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That's just a fact for you.
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Anything that blocks or stimulates either muscle can knock that balance out of whack
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and cause some really noticeable changes.
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For example, drugs that block receptors for the neurotransmitter acetylcholine can cause
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impressive dilation.
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These medications are designed to inhibit the parasympathetic nervous system, which
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controls the involuntary muscle contractions that keep things moving in your body, like
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your lungs, or the plumbing that gets rid of waste.
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This system also controls the iris sphincter muscle.
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So when it's inhibited, the iris sphincter can't counteract the pull from the iris
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dilator — and you get wide open, 'blown out' pupils.
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Some drugs, like ones for overactive bladders or excessive sweating, target the entire parasympathetic
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nervous system.
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They often have dilated pupils or blurry vision as side effects, but those aren't the goal.
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Others, like the drops used to dilate pupils for eye exams, are more targeted and don't
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throw your whole body off.
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Now, other drugs can stimulate the sympathetic nervous system, responsible for your body's
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'fight-or-flight' response.
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And these too can cause dilation.
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Take adrenaline, which floods your bloodstream when you're scared but is also a medication
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for extreme allergic reactions — it's what's in the EpiPen.
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Adrenaline can bind to nerve receptors connected to the iris dilator, making it contract and
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pull your pupils open wide.
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If people use drugs like cocaine, amphetamines, or even weed, their brains have more dopamine
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and norepinephrine floating around.
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Usually, either extra neurotransmitters get released, or receptors that naturally suck
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them away get blocked by those drugs.
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These chemicals are involved in the “high” feeling but also cause dilated pupils.
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Scientists think that dopamine makes certain nerve cells release more norepinephrine, which
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works like adrenaline to make the iris dilator contract.
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So wide pupils can be caused by a lot of things.
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But they're a hint that certain chemicals — illicit or not — are bubbling through
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your body.
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Thanks to our Patreon patrons for asking us great questions like this!
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If you want to support us and send us questions, and get some other cool stuff you can go to
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patreon.com/scishow.
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And if you want to learn more about human eyes, check out our video where I explain
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what those pesky little eye floaters actually are.
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[ ♪ Outro ]