Why It Feels Good to Scratch

"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.

Rubbing

"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.

So while rubbing your skin won't undo any damage of a bonked head, to a certain degree it will close off pain information to the brain and make you feel better.

"Anything you did to increase input into these pressure nerve fibers would allow input of endorphins in the spinal cord," says Warfield.

And "anything" may even include electric catfish.

Electric Shock

It turns out that millennia before a Canadian and British physician explained Gate Theory, the ancient Egyptians stumbled upon an application of it.

"Ancient Egyptians used to pull these electric catfish out of the Nile and put them on the painful area," says Warfield.