Skin might seem like a simple messenger, telling the brain if it's hot, if it's cold, or whether that rock underfoot is sharp.
But scientists are now finding that the nerve-based conversation between the skin and the brain is far more complex than most people realize.
Take, for example, the seemingly simple act of scratching an itch. A child will scratch and scratch a rash to the point of drawing blood and feel relieved. But he'll cry if he scrapes his knee on the ground. What's the difference?
New research published in the Journal of Investigative Dermatology may answer this question, as it reveals part of what's going on in our heads when we can't stop scratching — and may even add to our understanding of how the skin helps define pain for the brain.
"They can't be divorced, itch and scratch," says Dr. Gil Yosipovitch, lead author of the study and a dermatologist at Wake Forest University in Winston Salem, N.C. "Almost all two- and four-legged animals scratch and itch — supposedly, even fish scratch themselves."
Scratching seems like such a natural reaction that it took a while before scientists figured out something more was going on between the skin and the brain.
The first hint for Yosipovitch was long-distance scratching. In a previous study, he showed scratching brings relief even if you miss the itch and scratch far away from the bite or rash.
"It's not just a local reaction," says Yosipovitch. "It's so important to understand there is an interaction between the skin and the brain."
To see this interaction in the brain, researchers hooked up 13 healthy people to MRI scan machines. Then they began to scratch the volunteers on their legs in 30-second intervals.
Simply scratching the skin — even without an induced itch — had a compulsive effect on the brain.
Yosipovitch found scratching activated areas of the brain associated with memory and pleasure, while at the same time it suppressed areas associated with the sensation of pain and emotions.
"It puts in that message to continue it. That's why it's so repetitive," says Yosipovitch. "It's associated with a reward, so the more you do it, the more it feels better."
But that's not license to scratch, says Yosipovitch. He sees many patients with chronic itch from conditions like eczema, who scratch themselves until they bleed.
"Like any vicious cycle, it's not recommended," says Yosipovitch. But our instincts aren't all bad news.
Though scratching can put our brains into a painful cycle, science has found that the instinct to rub an injury can stop pain messages from reaching the brain.
"If you stub your toe, you rub it for a reason," says Dr. Carol Warfield, a professor of anesthesia at Harvard Medical School in Boston, Mass. She says that "Gate Theory," an idea that dates back to the 1960s, explains why.
Imagine there is a little gate within the spinal cord that allows pain messages to travel up to the brain, says Warfield. Usually, smaller nerves send pain signals, the gate opens and the messages of "Ow, I hit my elbow!" make it to the brain.
Larger nerves — which pick up on pressure, touch and position of the limb — will send messages to block the gate, crowding out some of the competing pain messages. Moreover, these larger nerves will trigger the body to release endorphins, the same hormones responsible for the natural high after exercise or sex.