What is an itch, anyway? Another clue found

By Tia Ghose, LiveScience 

A mysterious source of itchiness has been found. Certain nerve cells are specialized to detect itchy sensations, and those receptors don't detect painful sensations, according to a new study.

The finding, published Dec. 23 in the journal Nature Neuroscience, helps resolve a long-standing debate over whether itchiness is just a weird form of pain. Additionally, now that they have pinpointed the responsible nerve fibers, researchers could silence those nerves to develop better anti-itch treatments, said Ethan Lerner, a neuroscientist at Harvard University who was not involved in the study.

"This is a very convincing piece of work," Lerner told LiveScience. Scientists "can perhaps target this particular type of nerve as a means of treating itch, but still allow you to experience the protective aspect of pain."

For decades, why we itch has been a mystery. While some pain nerves have been found to fire in response to itchy stimulants, nerves that responded solely to itch proved elusive. Some researchers even wondered whether itch and pain were always processed by the same nerve fibers, but interpreted by the brain differently, said study co-author Xinzhong Dong, a neuroscientist at Johns Hopkins University.

But the the urge to scratch seemed different in key ways from the experience of pain. For instance, when a mosquito bites, most people feel a powerful desire to scratch the bite, while the pain of touching a hot stove causes people to recoil, Dong told LiveScience.

To identify cells that sense itch, Dong and his colleagues genetically engineered mice whose nerve cells glowed fluorescent green when firing. The researchers then exposed the mice to irritating compounds, such as histamine and the active ingredient in itching powder, and looked for nerves that fired (and glowed green) as a result.

When the researchers burned out the nerves that lit up, the mice scratched a lot less, suggesting they were less itchy.

But that wasn't enough to prove that the nerves only sense itch, because in theory those neurons could also sense pain. Therefore, the researchers specifically activated just those itch-detecting nerves in the faces of the mice. The animals then scratched their faces with their back paws, which they only do when itchy. (When they are in pain, they wipe their faces with their front legs.)

The newly discovered itch nerves sit inside the spine, near the spinal cord, and only innervate locations within the skin. That explains why people feel the urge to scratch their skin, but don't feel itchy in internal organs, Dong said.

"You can't have an itchy pancreas," he said.

The new findings are important because they provide a target for anti-itch medications. Current options, like anti-histamines or steroids, usually work by reducing inflammation, while many only eliminate the cause of itch for a narrow subset of problems, such as hives, Lerner said.

"Steroids are sort of a shotgun, and antihistamines, almost all the time, are hitting the wrong target," he said.

While the newly discovered nerves can't explain all itchiness (there are probably other nerves which sense both itch and pain ), targeting these nerves could be a huge improvement over current treatments, Lerner said.

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Another woman had simply compartmentalized the anti-sex parts of Christianity and decided to trust her instincts: “I have my body image issues — I don't like sitting in my swimsuit next to someone skinny, stuff like that — but with a guy, naked, I feel really comfortable. I’ve always just known what to do.”

For most of these women, their physical convictions were just as important as their spiritual convictions; if the two came into alignment, all the better. One woman, mentally flirting with the idea of sex, experienced clarity one night in Vegas.

"I met this hot cop,” she told me, “like an actual cop who was hot, not a Chippendale. We started making out in the casino — really going at it, it was amazing — and he persuaded me to come up to his room, where we fell onto his bed. He pulled up my dress and got naked all of a sudden and asked, ‘Can I put it in?’ I was totally horrified. I said absolutely not. Then he just sort of put it on top of me. I pretended I heard my phone ringing and basically ran away.”

“It’s a rule to protect you.”

I asked these women the same question over and over. Why is sex before marriage considered wrong? Essentially, everyone answered the same way: “I believe in the Bible, and the Bible says so.” Most added, “But I’m not going to judge anyone who does it.” Most, of course, were also doing it.

“It has more to do with your identity as a Christian,” one woman said. “How you see yourself, how you want to feel, how you want to be treated. This is hard for me to articulate, but I think that any sin that we commit comes from an internal issue that we have with ourselves — something we’re born with, like pride or greed. With sex, it could maybe be a problem with self-control, or wanting to receive a certain type of attention or feel a certain way.”

“I think it’s a rule to protect you,” another woman said. “To keep you from opening yourself up emotionally to the wrong people, to heartbreak and hurt.”

I remember when I came home from school in fourth grade wearing my very first purity ring. I waved my hand in the air proudly. “Oh Lord,” said my mom, who is an evangelical Christian. “Take it off, take it off now.”

“I was never acting out of an urge that was pure."

Guilt, bargaining, and confusion all played at least minor roles in each woman's story. One talked about a high school boyfriend, saying, "I believed that God wanted the two of us to be together, but that we'd cursed our relationship forever because we'd had sex. There was an inner voice just screaming at me about what I’d done, much louder than the voices that told me not to lie and cheat and steal. I would read books and identify with characters who were prostitutes, that’s how low I felt."

Another brought up middle-school masturbation: "I knew what I was doing, even though I didn’t know the word for it, and I knew it was sinful. I knew even then that I wasn’t taking care of my body in a holy way. I wasn’t acting out of an urge that was pure."

My friend Maya, after her assault: “I was furious at God. I couldn’t understand how I was the only one of our friends who made the decision to stay a virgin, and I loved the decision and defended it, and then He let this happen.”

Purity, this tightly conditioned idea, with so much more to give! In my own life, the times I've felt the purest have involved another trinity — sex, drugs, etc. — and the God that I came to know as a kid, that vague metaphysical presence, was always there in my bones to bless me.

“I have a huge sex drive – it’s how God made me.”

All the women I talked to readily admitted that the evangelical church doesn’t handle sexuality well. From the woman who’d waited until marriage: “It's a big institutional and doctrinal flaw, this idea that sex is bad, sex is wrong. When you're told that your whole life, how are you supposed to just flip that switch when you finally get around to doing it?"

I asked her how long it took to hit her stride with her husband, to feel comfortable having sex. “A while!” she said. “Two or three months, because he was studying for the bar nonstop and we could only really try on weekends. We laughed about it, like, thank goodness we didn’t have anyone else to compare this to.” She added, “But now it’s wonderful. And you know, sex is all over the Bible. God commands us to have communion with each other.”

They all told me that they hoped there would be a generational change in the church, a shifting of priorities. “It’s not our job to grade,” one woman said forcefully. “The emphasis we put on sin is out of proportion. That’s the biggest problem I have with the church.”

Another said, “We should change the conversation. It should be understood that sex is beautiful. It should be more about what you might want to protect yourself against, and how. It should be more about not doing things that could harm you.”

“If I’m truly a Christian, I should be able to understand what grace is. And feeling terrible is not grace,” said another woman, who’d described herself as having “a huge sex drive — it’s how God made me.”

She added, “I went to a bachelorette party where they were asking all the married girls for sex advice for the bride-to-be. I just sat there, listening to them talk about fussy lingerie and complicated games and weird sex menus, and I didn’t say anything, even though I wanted to be like, ‘Girl, just buy a vibrator.’ You know, I have a lot of friends that are waiting, or have waited, and it was great for them. But that’s just not how it’s going to be for me.”

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