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When it comes to drugs, marijuana is easily one of the most controversial. And today—4/20, a date when weed enthusiasts across America celebrate their favorite recreational (and in many places, still illicit) substance—pot is getting even more ink.

No matter your opinion of this hot topic drug (and its legalization in some states), it's hard to take part in the conversation until you understand it—and the ways that lighting up can impact your brain. The problem is, a lot of those ways still remain a mystery—even to scientists who've spent years researching the drug. What we do know is that today, marijuana can be consumed in foods, drinks, through pill form, a vaporizer, and through smoking.

The hundreds of chemical compounds—many of which are called cannabinoids—impact everyone (and every part of your body and brain) differently. Some cannabinoids have been touted for their medicinal benefits: shown to ease the nausea associated with chemotherapy or lessen the pressures of disease like glaucoma. Other have been linked to anxiety, and the "high" you feel when lighting up with friends.

Below, a recap of just one small part of the pot puzzle: the chemical changes that tend to occur in the brain if you light up recreationally.

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When you inhale pot smoke, you’re sucking a bunch of plant compounds into your lungs, including weed’s famous ingredient: the psychoactive delta-9-tetrahydrocannabinol (THC)—the one that makes you high. According to resources from the National Institutes of Health, THC passes from your lungs into your bloodstream, where it’s carried on to your brain. There, it binds to cells called cannabinoid receptors, which would normally interact with chemicals that exist naturally in your body.

Almost immediately, these receptors trigger a number of responses from your autonomic nervous system, including vasodilation, or the expansion of your blood vessels, explains Ruben Baler, Ph.D., a health scientist with the National Institute on Drug Abuse (NIDA). This irritates your eyes, which is why they may turn red or itchy, and speeds your heart rate by 20 percent or more, shows research from the Kaiser Permanente Medical Care Program. Like most other drugs, THC spurs the release of feel-good chemical dopamine. [Tweet this stat!]

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Those cannabinoid receptors the THC latched onto are part of your noodle’s endocannabinoid system, which helps control memory, mood, concentration, time perception, and motor skills. And THC over-activates and discombobulates it. Put more simply, THC disrupts the way your brain communicates within itself, explains Matthew J. Smith, Ph.D., who researches pot at Northwestern University. That communication disruption leads to the different physical, emotional, and psychological experiences—good or bad—associated with being stoned. Because everyone’s brain is different, the changes caused by THC affect everyone a little differently, research suggests.

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Your brain is like a computer that processes information in a precise, consistent flow. THC and the other chemicals in weed scramble this flow, Baler explains. The more you smoke, the more that flow is scrambled.

Your endocannabinoid system also plays a role in energy regulation and food intake, which may explain why some people get “the munchies” after smoking, Baler says. Because THC also messes with your brain’s pleasure receptors, food (or anything your senses experience) may seem more pleasurable. THC may also disrupt your brain’s mood and emotional pathways in unpleasant ways, triggering paranoia, panic, and other hallmarks of a bad trip, NIDA resources show.

RELATED: The Science Behind Why You Get the Munchies

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As the drug wears off, so does the uptick in feel-good chemicals like dopamine. And that dopamine drop-off can lead to sadness or even depression, research shows. The pot-induced changes in receptor and hormone activity can also leave your brain a little burned out and slow, similar to a night of heavy drinking, NIDA reports explain.

00:24:00 and Beyond
The science gets more controversial when it comes to weed’s long-term effects. It appears, though, that chronic THC activation of those cannabinoid receptors in your brain may leave them unable to function properly, Smith explains. Among teenagers who smoked pot daily for several months or years, Smith and his colleagues at Northwestern discovered memory impairments even two years after the teens had given up weed.

There’s also evidence that, if marijuana use starts at a young age (when your brain is still developing), pot can permanently rewire your noodle, creating “faulty connections” among the areas that rely on seamless coordination to process and store information, Baler adds. (Studies have found the same types of issues among kids who drink alcohol.)

The long-term effects of pot smoking on adult brains are less clear. People who start smoking marijuana in adulthood don’t show significant IQ declines, a NIDA report states. But some recent research published by the Society of Nuclear Medicine shows that grownups who smoke every day suffer a 20 percent drop in the number of receptors that control brain functions ranging from emotion to appetite. But those receptors pop back to normal levels once people quit the drug, the study authors say.

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