What's the electrical current messing up that's causing pain?
Care to elaborate? Can't just be nerves, anyway, because I know that sometimes it fries you. And you can't die from a lot of input coming in your nerves. (Can you!?)
Because muscles work by maintaining a potential difference along the muscle which, when discharge, causes your muscles to contract. Therefore, when you get electrocuted the excess current messes up this whole system and your muscles convulse (contractt and extend) wildly. Which can be quite painful.
However, to really answer you question our body DOESN'T detect electric current. The reason is simply because we never evolved to (not much evolutionary advantage in the natural world, the only exposure to electric current would be when we got struck by lightning). The convulsion of the muscles only occurs at a certain current level. However, we conduct current across our skin all the time and don't notice. If you were to take an oscilloscope (or a volt/multi-meter) and grab each terminal with different hands you would find that you have a 60Hz (depending on what region you're in) current conduction across you right this second. It's coming from your electrical sockets and conducting through you (this current is small of course). But such small currents are harmless to us and we don't notice them.
Along with what maverick said, your muscles are controlled by electrical signals, your heart is a muscle. The big danger from AC electricity is that the 60Hz makes your heart try and beat 60 times per second which exceeds the manufacturers recommended specification by quite a large margin - which generally leads to a failure.
Yes, you can. That's what happens when current goes through the brain. It also kills by interfering with the heart's electrochemical triggers, either stopping it completely or causing it to flutter uselessly (a condition called fibrillation).
But the part that hurts is the nerves being electrostimulated.
If I ever have to have surgery I want some those "warranty void if seal broken" and "no user serviceable parts inside" stickers
Cripes, don't get me started on the warranty program either.
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