Why, when my leg/arm/any limb “goes to sleep” does it hurt once the feeling starts coming back to, but if I am given a numbing agent, it doesn’t hurt when the feeling comes back?

Question

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

Answer 1

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.

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EDIT: some typos.