Why does scratching an itch bring relief?

Minnesota, April 6: Though for majority of us an itch is only a slight distraction, coping with a bad batch of poison ivy can often lead to pain, discomfort, and misery.

Scratching is known to provide relief, but the logic behind this is still elusive. With just a few vague explanations about the relation of scratching and itching, scientists are trying to find answers.

In a new study, neuroscientists at the University of Minnesota suggest that scratching provides relief to the tingling sensation because it blocks some activity in the spinal cord nerve cells that send signals to the brain. However, the weird part is that the respite happens only when an itch occurs, and at other times rubbing is ineffective.

“The desire to scratch is often overwhelming, but incessant scratching is harmful and leads to itch-scratch cycles that damage the skin and exacerbate the problem. The mechanism by which scratching suppresses an itch is unknown,” the research team reported in the journal Nature Neuroscience.

Glenn J. Giesler, leader of the research, stated the spinothalamic tract is the main part of the spinal cord involved in the itch sensation. This tract becomes active when itchy substances are put on the skin. "We wanted to know how scratch works. How does it relieve itch?"

The researchers conducted an experiment on monkeys by injecting a chemical that leads to an itching sensation. They then studied the monkeys to decipher the outcome of the scratching on the nerves. They observed a large amount of electrical signals. Using a metal device the leg affected by the chemical was scratched, which eased the itch "magically".

"The stimulation at that point stopped the neurons from firing or slowed the activity way down for 20 to 30 to 40 seconds -- the period where you feel relief from scratching. That suggests these neurons in the spinal cord are shut off during scratching," said Glenn.

In order to understand the concept further, the scientists then scratched the leg of a monkey that had not received the chemical injection. They noted that there was a marked increase in the number of neurons, thereby establishing that the nerves react much differently when there isn’t an itch to be scratched.

Giesler commented "It's like there's a little brain" in the spinal cord. We really want to understand that, because then we think we'll understand how to relieve itch.”

The focus of the researchers is to find the same neurons in people, which could lead to new ways of relieving itching from skin problems or other diseases.

Professor Patrick Haggard, of University College London, told the BBC: "We all know that scratching helps alleviate itch, but this elegant study helps to show how this mechanism works.

"It's an interesting illustration of a very general principle of the brain controlling its own inputs, in this case by making movements that triggers an interaction between scratchy touch and itch."

Professor Gil Yosipovitch, an expert on itching from Wake Forest University in North Carolina, felt the study was "potentially significant".

He stated, "Although there is a long way to go, methods that can induce a pleasurable scratch sensation without damaging the skin, via mechanical stimuli or drugs that can inhibit these neurons, could be developed to treat chronic itch."