Pain causes not just symptoms, but also physical signs. Increased blood pressure, for one. These are the physiological consequences of the way pain signalling happens in the body. People can even die from these physiological effects.
The physiological signs of pain occur mostly as a consequence of the way we consciously experience and process pain. (This is why "general anesthetic" works: central processing of pain is required before sensory input is actually registered as painful, and so, without the brain there to decide that a thing is painful, the physiological consequences don't happen. Pain that would give you a heart attack if you were awake doesn't cause the slightest physiological problem when you're unconscious from general anesthesia.)
If you are receiving a lot of "painful-stimulus describing" sensory input, and you're consciously experiencing those stimuli as painful qualia—but your body shows relatively little physiological response—then you have a high tolerance for pain.
This can be measured by using a standardized painful stimulus (e.g. a sub-cutaneous injection of a standardized dose of some insect's venom) together with an fMRI (or just self-reporting) to measure the intensity of the painful qualia, and various physiological monitors (EKG, etc.) to measure physiological response to the pain.
I believe this has been done before specifically in the case of certain sects of monks who like to demonstrate the depth of their meditative ability by tolerating large amounts of pain. They did the experiment to try to figure out whether the monks are tolerating an experienced pain, or merely are somehow not experiencing the painful stimulus as pain-qualia.
Not at the levels we like to use for surgery, certainly. But my understanding is that a “medically-induced coma” (the potentially-irreversible palliative-care kind) does suppress all pain signalling. Likewise, people in a vegetative state have no physiological response to pain. If you completely suppress central functioning, then you completely suppress pain response.
(There might still be a local release of pro-inflammatory cell-danger-response purines from the wound site that do things when they hit various organs/tissues on their way through the circulation, but I believe we don’t tend to call those “pain signals”, for the same reason we don’t call them that in plants or fungi.)
Some anesthesiologists believe that, but there's really no solid evidence for it. The reality is that no one really knows how or why anesthetics work, we just know what generally does work. I was told this by an honest doc, who also told me this scary fact: Because it's best to minimize the drugs that put you under (especially gaseous), and b/c we're getting better at walking that line, it's increasingly common to have patients wake up and freak out at the sight of their chest spread open. That's why the anesthesiologist keeps a couple of loaded syringes on the cart ready for emergency use: one is an instant paralyzing agent, the other a powerful memory blocker.
The physiological signs of pain occur mostly as a consequence of the way we consciously experience and process pain. (This is why "general anesthetic" works: central processing of pain is required before sensory input is actually registered as painful, and so, without the brain there to decide that a thing is painful, the physiological consequences don't happen. Pain that would give you a heart attack if you were awake doesn't cause the slightest physiological problem when you're unconscious from general anesthesia.)
If you are receiving a lot of "painful-stimulus describing" sensory input, and you're consciously experiencing those stimuli as painful qualia—but your body shows relatively little physiological response—then you have a high tolerance for pain.
This can be measured by using a standardized painful stimulus (e.g. a sub-cutaneous injection of a standardized dose of some insect's venom) together with an fMRI (or just self-reporting) to measure the intensity of the painful qualia, and various physiological monitors (EKG, etc.) to measure physiological response to the pain.
I believe this has been done before specifically in the case of certain sects of monks who like to demonstrate the depth of their meditative ability by tolerating large amounts of pain. They did the experiment to try to figure out whether the monks are tolerating an experienced pain, or merely are somehow not experiencing the painful stimulus as pain-qualia.