So, when you fall asleep leaning on your arm and roll over, it starts to hurt as the feeling comes back. But if you go to the dentist and they numb your mouth, it doesn’t hurt once that starts coming back?
In: Biology
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The feeling you are describing (often called pins and needles) is the result of a reduced blood flow. Your body weight basically cut off the circulation to some degree. I can’t tell you exactly why it is uncomfortable when it starts to move properly again. Maybe someone else can chime in on that.
A numbing agent dampens the nervous signals, but doesn’t mess with the circulation in the same way.
Edit: some good answers came in while I was typing, refer to those for a better explanation of the why
Because when leaning on your arm, you were most likely squishing your nerves and veins. Doing this causes the blood vessels to constrict, and the nerves begin to “turn off” due to lack of blood supply. When the blood comes back, they begin firing again, and the “pins and needles” sensations is each individual nerve turning back on again, and sending the “SOMETHING BAD HAPPENED” signal. When you go to the dentist, the numbing agent doesn’t “turn off” your nerve cells, the cells are still on and are actually trying to tell your brain that there is pain, but the numbing agent works by interfering with a part of your nerves where the “SOMETHING BAD IS HAPPENING” signal is intercepted and prevented from traveling to your brain. This is why as the numbing agent wears off, you don’t get that “pins and needles” feeling, just the left-over ache from whatever the dentist was doing in your mouth.
Doctor here:
When you lean on your arm you compress the nerves. This irritates them. The pins and needles sensation is not primarily from a lack of blood supply, despite what other have written here, although it can be a contributing factor.
When the compression stops, the nerves begin to work again. They are irritated so they report this to your brain in the form of pain. After a while the irritation ends and things go back to normal.
When a numbing agent is used, it simply blocks the transmission of pain rather than compressing and irritating the nerves.
There’s more to it than this, but this is ELI5.
EDIT: some typos.
Physician here:
The pins and and needles you feel is due to the return of blood supply and electrical activity to your nerve endings happening all at once. With a nerve block there is no loss of blood supply, just a slow return of electrical activity as the drug wears off.
When you squish your arm it makes the little nerves go to sleep. When you stop squishing your arm the nerves wake up and tell your brain “Owie, we got squished!”
Medicine doesn’t make your nerves go to sleep, but it makes them talk quietly and your brain doesnt hear about the owies. When the medicine goes away the nerves think that your brain already heard about the owies and dont tell it again.
Uh…. Do others not have pain when the numbing wears off from dental work?? I feel a pins and needles adjacent, very uncomfortable feeling for awhile until it’s gone. It’s the worst part of getting dental work for me
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