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How Does Scratching Relieve an Itch?

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If you have an itch, you scratch it. Scratch and itch; they go together like peas and carrots and everyone—humans, apes, dogs and cats—knows it. What we didn't understand for a very long time was the physiological connection between the two—why a good scratch relieves a bad itch.

A study by a group of neuroscientists at the University of Minnesota recently explained the itch-scratch link.* The group hypothesized that the relief mechanism doesn't take place along the nerves of itchy skin, as had been thought, but deep in the central nervous system, in the same area that the itches themselves are communicated. Previous studies showed that neurons in the spinothalamic tract (STT)—a sensory pathway originating in the spinal cord that transmits information about pain, temperature and touch to the thalamus—were activated with the application of itch-inducing chemicals, and these are the neurons that send itch sensations to the brain.

In the new study, the UM researchers implanted recording electrodes in the spinothalmic tracts (STT) of macaques monkeys (the STT is at the base of the spinal cord; most STT neurons respond to pain and some to both pain and itch). Then researchers injected itch-inducing histamines into the monkeys' legs and watched as the STT neurons fired. They then scratched the monkey's itchy legs with a device that mimicked the feel of monkey fingers, and the firing rate of the STT neurons dropped rapidly.

The sudden drop, the researchers said, is the neurological equivalent of the relief you feel after a good scratch, indicating that itching and relief sensations are both rooted in the spinal cord and relief from an itch comes from inhibiting—via scratching—the STT neurons. Scratching basically tells all those tattle-tale neurons who are whining to the brain about an itch to just shut up already.

scratchingOf course, the itch and the scratch still hold plenty of mystery. When the team scratched the monkey's legs without first inducing an itch, the STT neurons fired in a normal response to stimuli, but the scratching didn't slow the firing.

Scratching also had no effect on neurons' response to an application of capsaicin, the spicy component in hot peppers. The STT neurons, it appears, react differently to the sensation of a scratch depending on whether an itch exists, and the nerve-dampening effect of scratching only works when the neurons are firing because of an itch, not pain. Somehow, the neurons know the difference. Itching isn't all physiological, either; it can be caused by emotional and psychological factors and can even be picked up as a "contagious itch" (a study showed that itching can be induced purely by visual stimuli: watching other people scratch).

Once all that is sussed out, though, the UM team's discovery could lead to ways of duplicating the end results and benefits of scratching (quiet, polite STT neurons) without its drawbacks. That's great news for people with the sorts of chronic itching associated with AIDS, Hodgkin's disease and the side effects of some pain medications. Chronic itching, of course, leads to plenty of scratching, which can lead to skin damage, infections and worse (remember the New Yorker article with the woman who scratched right through to her brain?)

* Davidson et al. Relief of itch by scratching: state-dependent inhibition of primate spinothalamic tract neurons. Nature Neuroscience, 2009; 12 (5): 544

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Animals
Owning a Dog May Add Years to Your Life, Study Shows
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We've said that having a furry friend can reduce depression, promote better sleep, and encourage more exercise. Now, research has indicated that caring for a canine might actually extend your lifespan.

Previous studies have shown that dog owners have an innate sense of comfort and increased well-being. A new paper published in Scientific Reports and conducted by Uppsala University in Sweden looked at the health records of 3.4 million of the country's residents. These records typically include personal data like marital status and whether the individual owns a pet. Researchers got additional insight from a national dog registry providing ownership information. According to the study, those with a dog for a housemate were less likely to die from cardiovascular disease or any other cause during the study's 12-year duration.

The study included adults 40 to 80 years old, with a mean age of 57. Researchers found that dogs were a positive predictor in health, particularly among singles. Those who had one were 33 percent less likely to die early than those who did not. Authors didn't conclude the exact reason behind the correlation: It could be active people are more likely to own dogs, that dogs promoted more activity, or that psychological factors like lowered incidences of depression might bolster overall well-being. Either way, having a pooch in your life could mean living a longer one.

[h/t Bloomberg]

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Not Sure About Your Tap Water? Here's How to Test for Contaminants
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In the wake of Flint, Michigan's water crisis, you may have begun to wonder: Is my tap water safe? How would I know? To put your mind at ease—or just to satisfy your scientific curiosity—you can find out exactly what's in your municipal water pretty easily, as Popular Science reports. Depending on where you live, it might even be free.

A new water quality test called Tap Score, launched on Kickstarter in June 2017, helps you test for the most common household water contaminants for $120 per kit. You just need to take a few samples, mail them to the lab, and you'll get the results back in 10 days, telling you about lead levels, copper and cadmium content, arsenic, and other common hazardous materials that can make their way into water via pipes or wells. If you're mostly worried about lead, you can get a $40 test that only tells you about the lead and copper content of your water.

In New York State, a free lead-testing program will send you a test kit on request that allows you to send off samples of your water to a state-certified lab for processing, no purchase required. A few weeks later, you'll get a letter with the results, telling you what kind of lead levels were found in your water. This option is great if you live in New York, but if your state doesn't offer free testing (or only offers it to specific locations, like schools), there are other budget-friendly ways to test, too.

While mailing samples of your water off to a certified lab is the most accurate way to test your water, you can do it entirely at home with inexpensive strip tests that will only set you back $10 to $15. These tests aren't as sensitive as lab versions, and they don't test for as many contaminants, but they can tell you roughly whether you should be concerned about high levels of toxic metals like lead. The strip tests will only give you positive or negative readings, though, whereas the EPA and other official agencies test for the concentration of contaminants (the parts-per-billion) to determine the safety of a water source. If you're truly concerned with what's in your water, you should probably stick to sending your samples off to a professional, since you'll get a more detailed report of the results from a lab than from a colored strip.

In the future, there will likely be an even quicker way to test for lead and other metals—one that hooks up to your smartphone. Gitanjali Rao, an 11-year-old from Colorado, won the 2017 Young Scientist Challenge by inventing Tethys, a faster lead-testing device than what's currently on the market. With Tethys, instead of waiting for a lab, you can get results instantly. It's not commercially available yet, though, so for now, we'll have to stick with mail-away options.

[h/t Popular Science]

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