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A kidney stone has no escape
No way to withdraw
A hot plate triggers a reflex: the hand withdraws. A kidney stone offers nothing to withdraw from. In the middle of the night in a remote little town in Switzerland, a neuroscientist curled into a ball on the floor, breathing loudly and making rocking motions. His wife asked if he could stop and get on a plane to the US for an ultrasound. He could not, any more than he could stop pulling a hand off a hot plate. It is a strongly motivated behavior to withdraw.
Counting the pain
Someone arrived that night and kept asking for a pain rating from zero to ten. His answers went from ten to nine and a half. Eventually he reached a clinic and passed the stone.
The insula at work
This pain differs from the pain in the eye or from a hot stove. It is worse, because it cannot be escaped, rubbed, or fixed. An MRI at that moment would have shown his insula strongly activated. The insula is a part of cortex that receives input from the heart, liver, kidney, and every other organ, and it is the leading candidate for the substrate of consciously feeling bodily states like pain, nausea, and strong emotions.
Why does kidney stone pain come in waves?
A stone lodged in the ureter makes the muscular wall squeeze rhythmically against the blockage, the same motion that normally moves urine along. Those spasms are known as renal colic, and they produce pain that surges and eases. Signals from the ureter enter the spinal cord at the same levels that receive signals from the flank and groin, so the pain is often felt there. The ureter is a tube roughly twenty-five centimeters long that runs from each kidney to the bladder.
What does the insula do in pain?
Neuroscientist A. D. Craig argued in a 2002 paper in Nature Reviews Neuroscience that the insula holds the brain's map of the body's internal condition, a sense called interoception. Pain and temperature signals travel up a dedicated spinal pathway to the thalamus and on to the insula. Craig proposed that the right anterior insula supports the conscious feeling of those states. Brain imaging has found the insula active in many kinds of pain, as well as in nausea and thirst.
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