View Full Version : Why do we have internal pain receptors?

02-28-2001, 09:51 AM
Pain is supposed to be a warning/avoidance system. Thog put hand near fire; Thog hurt. Thog remove hand.
But a good deal of internal injuries can't be avoided, and avoidance of activities doesn't promote healing. Take for example kidney stones. Thog get sharp pain in lower back. Thog roll around floor in agony for a few hours/days. Thog eaten by cave bear while rolling on floor. The same can be said for some sorts of cancers, etc.
Certainly, many internal pain sensations are useful. Pain receptors around broken ribs warn you to lie still for a few more days until you heal. But in general, internal pain doesn't AFAIK give an advantage to the sufferer of the pain. Is there any evolutionary advantage that I'm missing here?


P.S. Yes, I'm aware that ironically, internal pain receptors are much more useful now, when we have medical techniques to deal with many internal problems. But that doesn't explain their evolution.

02-28-2001, 10:08 AM
As marvelous as life is, it is not perfect. Certain physiological events happen that do not help. The symptoms of a cold, for example. The sneezing, etc. are caused by our immune system in action. Well, sure it has to be in action, but does it have to cause such symptoms? And the symptoms continue long after the virus is gone. We injure some soft tissue and develop a painful bruise, with swelling. The pain and swelling ain't helping. That's why we should ice the area to reduce swelling immediately after the incident. Autoimmune diseases are another, but of course, that is an abnormal situation.

We have neurons with both sensory and motor fibers located throughout our body. We feel sensations. We react. We want to move muscles. Need nerves. My WAG is that sometimes the sensory fibers are not helpful. No one said the system is perfect.

02-28-2001, 10:37 AM
Keep in mind the difference between infections and injuries. In the case of infection, symptoms are sometimes the body fighting the parasite (fever, swelling) and sometimes the parasite itself provoking reactions (sneezing, vomiting) in order to spread to new hosts.

Most symptoms can be explained as one of these cases, but I'll admit kidney stones are something else. Cancer is often initially caused by infection, but the parasite isn't present when the cancer starts taking over. Why would the body get so run down in these cases? What does our immune system do when faced with a problem it cannot build antibodies for? How sophisticated can a system ever be, when it's based on protein recognition only? Proteins aren't the whole world.


02-28-2001, 02:18 PM
It's not always true that there's nothing to be done about internal pain. Animals sometimes self-medicate in response to digestive upset, and I've seen some claims that chimps exploit some natural medicines to treat more complex symptoms. Also, pain is not always a symptom of an intractable distress--a stong urge to unrinate or defecate comes to mind.

02-28-2001, 08:10 PM
The problem isn't the mechanism -- as you point out, the body has limits and needs to know when they are exceeded beyond its ability to compensate -- lifting too much weight or being stabbed 23 times by a dagger. The physiological problem is turning off the mechanism once it has started; lots of pain is caused by too much inflammation, etc.