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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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Health
New Test Can Differentiate Between Tick-borne Illnesses
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Time is of the essence in diagnosing and treating Lyme disease and other tick-borne illnesses. Fortunately, one new test may be able to help. A report on the test was published in the journal Science Translational Medicine.

Ticks and the diseases they carry are on the rise. One 2016 study found deer ticks—the species that carries Lyme disease—in more than half of the counties in the United States.

The two most common tick-borne illnesses in the U.S. are Lyme disease and southern tick-associated rash illness (STARI). Although their initial symptoms can be the same, they’re caused by different pathogens; Lyme disease comes from infection with the bacterium Borrelia burgdorferi. We don’t know what causes STARI.

"It is extremely important to be able to tell a patient they have Lyme disease as early as possible so they can be treated as quickly as possible," microbiologist and first author Claudia Molins of the CDC said in a statement. "Most Lyme disease infections are successfully treated with a two- to three-week course of oral antibiotics." Infections that aren't treated can lead to fevers, facial paralysis, heart palpitations, nerve pain, arthritis, short-term memory loss, and inflammation of the brain and spinal cord.

But to date, scientists have yet to create an accurate, consistent early test for Lyme disease, which means people must often wait until they’re very ill. And it’s hard to test for the STARI pathogen when we don’t know what it is.

One team of researchers led by experts at Colorado State University was determined to find a better way. They realized that, rather than looking for pathogens, they could look at the way a person’s body responded to the pathogens.

They analyzed blood samples from patients with both early-stage Lyme disease and STARI. Their results showed that while all patients’ immune systems had mounted a response, the nature of that response was different.

"We have found that all of these infections and diseases are associated with an inflammatory response, but the alteration of the immune response, and the metabolic profiles aren't all the same," senior author John Belisle of CSU said.

Two distinct profiles emerged. The team had found physical evidence, or biomarkers, for each illness: a way to tell one disease from another.

Belisle notes that there’s still plenty of work to do.

"The focus of our efforts is to develop a test that has a much greater sensitivity, and maintains that same level of specificity," Belisle said. "We don't want people to receive unnecessary treatment if they don't have Lyme disease, but we want to identify those who have the disease as quickly as possible."

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Google Can Warn You When Your Allergies Are About to Go Haywire
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How much allergy medication are you going to need today? Google can tell you. Well, it can give you a forecast, at least, as The Verge reports.

Google announced on August 16 that the search engine will now auto-populate search results for pollen and allergy information with allergy forecasts from The Weather Channel. The integration will include the most recent pollen index and allergy forecast data, showing a 5-day forecast detailing whether you’re likely to feel seasonal allergy symptoms throughout the week.

An animation shows a scroll of Google’s search results for pollen with allergy forecasts.
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If you have the Google app, you can set it to send push notifications when the pollen count is notably high that day, so you know to sequester yourself safely indoors. Hopefully you don't live in a city like Jackson, Mississippi, which in 2016 was named the worst city in the U.S. for allergy sufferers. There, your phone may be pinging every day.

While you can already find this information on sites like Pollen.com, having it show up immediately in search results saves you a few extra clicks, and frankly, it’s far more readable than most allergy and weather forecast sites.

Too bad a search engine can't cure our sneezes and watery eyes, though. Time to stock up on Kleenex, get a jumbo bottle of allergy meds, and maybe buy yourself a robot vacuum.

[h/t The Verge]

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