I also find it wonderfully satisfying to get my nails into an itchy patch on my skin.
I've often thought scratching an itch seems a strange thing to do. How can scraping at your skin with sharp nails (the sharper the better, if the itch is really annoying) stop the discomfort? Wouldn't it just make things worse?
An article at Newsweek has some answers, courtesy of dermatologist and researcher Dr. Gil Yosipovitch, of Wake Forest University Baptist Medical Center, in the US. He says:
What we do know is that the itch doesn't stand alone. Rather the itch involves not only the skin, but also the spinal cord and the brain. We used to think that the itch shared the same neurological pathway as pain. But now we know that the itch has its own neural road, if you will. There are actually some nerves in the spinal cord that are itch specific.
He points out what we all know, that scratching an itch takes our minds off the annoyance of the itch and replaces it with a positive, pleasant sensation. Brain imaging has revealed the truth of this common experience.
It's nice to read an article that focuses on the fact that a certain amount of scratching is normal, in contrast to a Google search on 'dogs + itch + scratch', which returns hundreds of hits dealing with problem scratching. Dr Yosipovitch noticed that the areas of the brain that deal with compulsive behaviours are stimulated when we scratch an itch, so if dogs' brains react in the same way, this may explain why they, like humans, can develop obsessive scratching.
I know that when I am overtired I can start scratching at my scalp, so much so that sometimes it alerts me to the fact that I should stop what I'm doing and have a sleep. I wonder if dogs do the same? (On the other hand, they're too sensible to keep slogging away at something when they're overtired. Maybe there's a lesson for the human race there.)
The BBC also has an article about Dr. Yosipovitch's work, and there is a clear, simple explanation of the scratch reflex at Howstuffworks. This one, though, doesn't refer to the pleasure response. It says:
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