Why does warmth alleviate pain?

When Do Fish SleepA caller on Tom Snyder’s radio show posed this Imponderable. We had no idea of the answer, but it was surprising that so many physicians we spoke to didn’t know the answer either.
We finally got the solution from Daniel N. Hooker, Ph.D., coordinator of Physical Therapy/Athletic Training at the University of North Carolina at Chapel Hill.  His answer included plenty of expressions like “receptors,” “external stimuli,” and “pain sensors.”  So let’s use an analogy to simplify Hooker’s explanation.
If a pneumatic drill is making a ruckus outside your window, you have a few choices.  One is to do nothing, which won’t accomplish mush until the drill stops.  But another option is to go to your stereo and put on a Led Zeppelin record at full blast.  The pneumatic drill is still just as loud- you may still even be able to hear it.  But the music will certainly distract you (and for that matter, your next door neighbors are well), so the drilling doesn’t seem as loud.
Hooker emphasizes that most of us associate warmth with pleasant experiences from our youth.  By placing heat on the part of our body that hurts we stimulate the sensory receptors, which doesn’t eliminate the pain, but the distraction makes us less aware of the pain.  As our body accommodates to the high temperature, we need fresh doses of warmth to dampen the pain.  When we receive the renewed heat treatment, we expect to feel better, so we do.

From: When do fish sleep? by: David Feldman