According to my university
psychology professor, no one really knows exactly why general anaesthesia works. We do know, however, that it produces short term
memory loss apart from its
soporific effects, e.g. if someone is given just a little, they won't be able to remember the next fifteen minutes of what happens to them, even though they didn't go to sleep and might have been having a
conversation and seeming
perfectly normal.
So the really creepy implication of this is that (and apparently there are stress tests that back this up) maybe anaesthesia doesn't actually block your brain from registering pain: you just can't move and can't remember the ordeal afterwards.
The preceeding information is from an undergraduate psychology class taken several years ago. I remembered it because it really scared me, but take it with a grain of salt. And feel free to correct me if I'm remembering things wrong.