Why Is Your First Instinct After Hurting Your Finger to Put It in Your Mouth?

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If you close your fingers in a car door or slam your funny bone into a wall, you might find your first reaction is to suck on your fingers or rub your elbow. Not only is this an instinctive self-soothing behavior, it's a pretty effective technique for temporarily calming pain signals to the brain.

But how and why does it work? To understand, you need to know about the dominant theory of how pain is communicated in the body.

In the 17th century, French scientist and philosopher René Descartes proposed that there were specific pain receptors in the body that "rang a bell in the brain" when a stimulus interacted with the body, Lorne Mendell, a professor of neurobiology and behavior at Stony Brook University in New York, tells Mental Floss. However, no study has effectively been able to identify receptors anywhere in the body that only respond to painful stimuli.

"You can activate certain nerve fibers that can lead to pain, but under other circumstances, they don't," Mendell says. In other words, the same nerve fibers that carry pain signals also carry other sensations.

In 1965, two researchers at MIT, Patrick Wall and Ronald Melzack, proposed what they called the gate control theory of pain, which, for the most part, holds up to this day. Mendell, whose research focuses on the neurobiology of pain and who worked with both men on their pain studies, explains that their research showed that feeling pain is more about a balance of stimuli on the different types of nerve fibers.

"The idea was that certain fibers that increased the input were ones that opened the gate, and the ones that reduced the input closed the gate," Mendell says. "So you have this idea of a gate control sitting across the entrance of the spinal cord, and that could either be open and produce pain, or the gate could be shut and reduce pain."

The gate control theory was fleshed out in 1996 when neurophysiologist Edward Perl discovered that cells contain nociceptors, which are neurons that signal the presence of tissue-damaging stimuli or the existence of tissue damage.

Of the two main types of nerve fibers—large and small—the large fibers carry non-nociceptive information (no pain), while small fibers transmit nociceptive information (pain).

Mendell explains that in studies where electric stimulation is applied to nerves, as the current is raised, the first fibers to be stimulated are the largest ones. As the intensity of the stimulus increases, smaller and smaller fibers get recruited in. "When you do this in a patient at low intensity, the patient will recognize the stimulus, but it will not be painful," he says. "But when you increase the intensity of the stimulus, eventually you reach threshold where suddenly the patient will say, 'This is painful.'"

Thus, "the idea was that shutting the gate was something that the large fibers produced, and opening the gate was something that the small fibers produced."

Now back to your pain. When you suck on a jammed finger or rub a banged shin, you're stimulating the large fibers with "counter irritation," Mendell says. The effect is "a decrease in the message, or the magnitude of the barrage of signals being driven across the incoming fiber activation. You basically shut the gate. That is what reduces pain."

This concept has created "a big industry" around treating pain with mild electrical stimulation, Mendell says, with the goal of stimulating those large fibers in the hopes they will shut the gate on the pain signals from the small fibers.

While counter irritation may not help dull the pain of serious injury, it may come in handy the next time you experience a bad bruise or a stubbed toe.

Lítla Dímun: The Smallest of the Faroe Islands Has Its Very Own Cloud

While some islands are known for their unusual geography or unique history, Lítla Dímun is notable for its weather. The island, which is the smallest of Denmark's Faroe Islands chain, is often capped by a lens-shaped cloud, making it resemble a scene from a fairytale.

According to Mental Floss's own Kerry Wolfe writing for Atlas Obscura, the cloud floating above Lítla Dímun is a lenticular cloud. This type of cloud forms when moist air flows over a protruding geological feature, like a mountain top. When the wind moving up the landmass hits the air current directly above it, a sort of wave is created on the downwind side of the mountain. The moist air falling down this wave evaporates and then condenses into a large, flying-saucer-shaped cloud atop the mountain peak as a result.

Another factor that makes Lítla Dímun distinct is that it's the only one of the 18 main Faroe Islands without human inhabitants. Visitors to the mystical location will instead find a thriving population of sheep. Originally, Lítla Dímun was home to a group of feral sheep likely dating back to the Neolithic era. But they were hunted to extinction in the 19th century. Domesticated sheep were introduced there around the same time, and today, farmers visit the island once a year to round up their flocks.

One of the few signs of human life are the ropes farmers use to scale the cliff faces bordering the island. Even if you have rock-climbing skills, Lítla Dímun may be dangerous to visit. A boat ride to the rocky shore is only possible when the surrounding sea is calm.

[h/t Atlas Obscura]

Why Are Sloths So Slow?

Sloths have little problem holding still for nature photographers.
Sloths have little problem holding still for nature photographers.
Geoview/iStock via Getty Images

When it comes to physical activity, few animals have as maligned a reputation as the sloth. The six sloth species, which call Brazil and Panama home, move with no urgency, having seemingly adapted to an existence that allows for a life lived in slow motion. But what makes sloths so sedate? And what horrible, poop-related price must they pay in order to maintain life in the slow lane?

According to HowStuffWorks, the sloth’s limited movements are primarily the result of their diet. Residing mainly in the canopy vines of Central and South American forests, sloths dine out on leaves, fruits, and buds. With virtually no fat or protein, sloths conserve energy by taking a leisurely approach to life. On average, a sloth will climb or travel roughly 125 feet per day. On land, it takes them roughly one minute to move just one foot.

A sloth’s digestive system matches their locomotion. After munching leaves using their lips—they have no incisors—it can take up to a month for their meals to be fully digested. And a sloth's metabolic rate is 40 to 45 percent slower than most mammals' to help compensate for their low caloric intake. With so little fuel to burn, a sloth makes the most of it.

Deliberate movement shouldn’t be confused for weakness, however. Sloths can hang from branches for hours, showing off some impressive stamina. And because they spend most of their time high up in trees, they have no need for rapid movement to evade predators.

There is, however, one major downside to the sloth's leisurely lifestyle. Owing to their meager diet, they typically only have to poop once per week. Like going in a public bathroom, this can be a stressful event, as it means going to the ground and risking detection by predators—which puts their lives on the line. Worse, that slow bowel motility means they’re trying to push out nearly one-third of their body weight in feces at a time. It's something to consider the next time you feel envious of their chill lifestyle.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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