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  • Whether or not eyes are the windows to the soul... scientifically, pupils are the

  • windows of the eyes.

  • And sometimes, those windows are more open than usual, like when you've taken a medication

  • that causes pupil dilation.

  • Or when people use certain recreational drugs.

  • It all comes down to how different chemicals can cause changes throughout your whole body,

  • which can affect little muscles in your eyes.

  • The pupils are openings that let light into the rest of your eye.

  • And the colored part of your eye, the iris, controls how much light makes it in.

  • The balance of two muscle groups in the iris changes how big your pupils are, like a camera aperture.

  • The iris dilator is arranged like bicycle spokes around the center of your eye.

  • And when it contracts, it pulls your pupils open wider.

  • The iris sphincter, on the other hand, is in a ring around your pupil and acts like

  • most of the sphincter muscles in your body.

  • It constricts your pupils down to little dots.

  • So now you know that you've got a sphincter in your eye.

  • That's just a fact for you.

  • Anything that blocks or stimulates either muscle can knock that balance out of whack

  • and cause some really noticeable changes.

  • For example, drugs that block receptors for the neurotransmitter acetylcholine can cause

  • impressive dilation.

  • These medications are designed to inhibit the parasympathetic nervous system, which

  • controls the involuntary muscle contractions that keep things moving in your body, like

  • your lungs, or the plumbing that gets rid of waste.

  • This system also controls the iris sphincter muscle.

  • So when it's inhibited, the iris sphincter can't counteract the pull from the iris

  • dilatorand you get wide open, 'blown out' pupils.

  • Some drugs, like ones for overactive bladders or excessive sweating, target the entire parasympathetic

  • nervous system.

  • They often have dilated pupils or blurry vision as side effects, but those aren't the goal.

  • Others, like the drops used to dilate pupils for eye exams, are more targeted and don't

  • throw your whole body off.

  • Now, other drugs can stimulate the sympathetic nervous system, responsible for your body's

  • 'fight-or-flight' response.

  • And these too can cause dilation.

  • Take adrenaline, which floods your bloodstream when you're scared but is also a medication

  • for extreme allergic reactionsit's what's in the EpiPen.

  • Adrenaline can bind to nerve receptors connected to the iris dilator, making it contract and

  • pull your pupils open wide.

  • If people use drugs like cocaine, amphetamines, or even weed, their brains have more dopamine

  • and norepinephrine floating around.

  • Usually, either extra neurotransmitters get released, or receptors that naturally suck

  • them away get blocked by those drugs.

  • These chemicals are involved in thehighfeeling but also cause dilated pupils.

  • Scientists think that dopamine makes certain nerve cells release more norepinephrine, which

  • works like adrenaline to make the iris dilator contract.

  • So wide pupils can be caused by a lot of things.

  • But they're a hint that certain chemicalsillicit or notare bubbling through

  • your body.

  • Thanks to our Patreon patrons for asking us great questions like this!

  • If you want to support us and send us questions, and get some other cool stuff you can go to


  • And if you want to learn more about human eyes, check out our video where I explain

  • what those pesky little eye floaters actually are.

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B2 US iris dilation eye muscle adrenaline nervous system

Why Do Some Drugs Make Your Pupils Wider?

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    Yao Chung Chang posted on 2018/03/28
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