system
Given the following post, respond as if you're commenting on Reddit:
human
When you're hand is cold, why does it hurt more when you hit something with it?

A mate of mine asked me this question, he heard it being asked on the radio, and the scientist who usually answers the questions wasn't really sure.

I told him that I guessed that it has to do with the fact that there isn't as much blood closer to the surface of your hand when you are cold, and this possibly exposes the nerves more, but I am not really sure of that. 

Being someone cursed with 'needing to know', any insights would be greatly appreciated :)
gpt
It is about the sensitivity of the nerve that changed with temperature and the degree of coldness.
If it is harshly cold, this will be interpreted as pain sensation and if it lasts a bit longer, nerve will be sensitized that mean it will be sensitive to all things to be pain even non-hurt stimuli.
If it is not that sever cold, the nerve still be more sensitive than when it is too hot, so more nerves are active and send the perception to the brain, so we feel more painful when we hit something.
About vascular, it is not the samething. Someone are more sensitive to cold and their blood vessels tends to be constricted when they exposed to cold that leads to ischemia and pain. It is called raynaud's phenomenon which is pathologic, not physiologic conditions. Cause of this phenomenon can be occupational related such as road digger with the machines, or autoimmune disease like Systemic Sclerosis, SLE, Rheumatoid symndrome or isolated Raynaud's phenomenon.