So Sophie,

My chin itches. No big deal, ordinarily. The thing is, I got home from the dentist's office a couple of hours ago with enough Novocain in my face to take the spring out of Tigger's tail. I can't feel anything when I scratch and the itch doesn't know that I'm doing anything for it. So it keeps on itching. Why? If it is too numb to feel the scratching, how on earth does it feel the itch?

-Driven Crazy

Hey Driven,

ou'd have to be crazy to resort to horrible comparisons involving "Winnie the Pooh" characters. I'm half tempted to just tell you something snarky and move on, but I won't.

To understand why you're feeling what you do, you need to understand pain. Pain is an important safety feature of the human body because without it, no one would have any warning of injury. Nerves transmit pain messages by a combination of chemistry and electricity. When a nerve receives a pain stimulus over a certain intensity, it "fires" by changing the arrangement of positive and negative charges across its cell membrane. When the message reaches the end of the nerve cell, chemicals known as neurotransmitters spill out into a space (synapse) and stimulate the receiving areas (receptors) on the next nerve cell. The signal passes from nerve to nerve very quickly until it reaches the brain, where the message registers as pain (and you say "Ouch!").

Because nerves send messages by a combination of chemistry and electricity, interference in either area can relieve pain. Traditionally, dentists have used a shot of lidocaine (a substitute for Novocain(tm)) to numb the tooth so the patient can't feel the drill. This is a little alarming in itself when the cavity is in a lower jaw tooth; the only available nerve to numb is way in the back of the mouth, so the needle is several inches long.

The nerves in your mouth are completely numbed after getting the shot, which should explain to you why you can't feel the scratching or any other stimulus in that area for several hours. All of your touch receptors have been turned off.

Now, you need to understand the itching phenomenon. 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. When the stimulus 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 it 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.

So basically, what the answer boils down to is this: your face has been numbed, so you can't feel the scratching. However, there is still some irritant on the surface of your skin that's sending signals to your brain telling you to remove it. So you scratch. Irritant gone. BUT the fact that your skin has been numbed is preventing it from telling your brain that it doesn't need scratching any more, so the brain short circuits and keeps on making you scratch long after the real need for it is gone.