In a sci-fi story I'm working on, a group of aliens utilizes a implanted device that induces incredible pain solely by stimulating the nervous system; specifically, the nerves that tell you you're in pain. This is done directly, using a wired connection to the nervous system to send the signals it wants through the nervous system to the brain. Basically, it induces 'phantom pain'; you feel intense pain all over your body without having suffered any sort of injury. Sort of like Fibromyalgia.
As you can probably guess, these aliens are completely and totally morally bankrupt.
And now, the question: What kind of side effects might stimulating the specific pain-detecting nerves have on a human victim? Note that this isn't being done using any sort of toxin or virus; the firing of nerves associated with pain is the only effect.
In case you need to know, the effect's duration can range from a split second to the eventual death of the victim, though the aliens rarely make it last longer than a few minutes.