When you have an itch, what is happening under your skin?

The average human body is covered by about 20 square feet (2 square meters) of skin. Skin is the only organ that is constantly exposed to potential irritation. And, with so many things coming into contact with your skin daily, you're bound to get an itch or two. Serious itching can be caused by allergies, disease, emotions and infections, but let's take a look at what causes the common itches that aggravate you everyday.

Itching, also known as pruritus, starts with some kind of external stimuli, including bugs, dust, clothing fibers and hair. Like tickling, itching is a built-in defense mechanism that alerts your body to the potential of being harmed. In this case, it might be the potential of being bit by a bug.

When the stimuli lands on your skin, it may not bother you at first, but soon it will begin to rub back and forth across your skin. Once the hair or dust scratches your skin's surface layer, receptors in the dermis of the skin will become irritated. In a split second, these receptors send a signal through fibers in the skin to your spinal cord and then up to the cerebral cortex in your brain.

The same fibers that send itching signals are also used to send pain signals to the brain, which once led some scientists to believe that itching was a form of light pain. That notion has since been dispelled by research, which showed that pain and itching elicit opposite responses. Pain causes us to withdraw and itching causes us to scratch.

As soon as we feel an itch, our first natural response is to scratch the spot of the itch with our fingernails. The reason for this response is simple -- we want to remove the irritant as soon as possible. Once you've scratched the area of irritation, you are likely to feel some relief. When your brain realizes that you've scratched away the irritant, the signal being sent to your brain that you have an itch is interrupted and therefore no longer recognized by the brain.

Even if you don't remove the irritant, scratching will at least cause pain and divert your attention away from the itching. The irritant that caused the itching is very small, maybe only a few microns in length, so it disturbs only a few nerve endings. When you use your fingernail to scratch the spot where the irritant is, you not only remove the irritant but you irritate a lot more nerve endings than the irritant.

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