The itchiness theme will end soon, I promise. But without even searching for it, I happened to come across some more information. I came across it reading Atwul Gawande’s article in the New Yorker, The Itch. Reading it in full is worth it.
Answering a question from my last post, he explains how the nerves that sense an itch are different from those that can sense pain, temperature, proprioception, etc. And, that histamine is what causes these nerve ending to fire, with higher levels of histamine creating a greater itch.
He also details a patient who has an itch so unrelenting that she scratches through her scalp, then through her skull, and eventually reaches her brain. She ends up half paralyzed due to the damage to her cortex. In discussing what may have gone wrong in this patient’s brain, he also brings up some newer ideas about the ways in which our sensory systems work. What Gawande says is that our way of perceiving reality does not happen solely by receiving information from the world. Much of our perception comes from receiving information from our brains, in addition to the rest of the world. The part of the information that we take in from the environment is not complete (perhaps only 20% of the total), and our brains will “guess” to complete the picture.
This theory partially explains phantom limb pain. People who have had limbs amputated more often than not still sense many ordinary sensations in the missing limb.
People report not just pain but also sensations of sweatiness, heat, texture, and movement in a missing limb. There is no experience people have with real limbs that they do not experience with phantom limbs. They feel their phantom leg swinging, water trickling down a phantom arm, a phantom ring becoming too tight for a phantom digit. Children have used phantom fingers to count and solve arithmetic problems.
In addition to these ordinary sensations come excruciating pain and in some cases itching. The theory of the brain making a “best guess” is further supported by the fact that Ramachandran’s mirror box, which is as simple as it sounds– a box that contains a mirror– can relieve this pain. A description of the process:
They put their surviving arm through a hole in the side of a box with a mirror inside, so that, peering through the open top, they would see their arm and its mirror image, as if they had two arms. Ramachandran then asked them to move both their intact arm and, in their mind, their phantom arm—to pretend that they were conducting an orchestra, say. The patients had the sense that they had two arms again. Even though they knew it was an illusion, it provided immediate relief. People who for years had been unable to unclench their phantom fist suddenly felt their hand open; phantom arms in painfully contorted positions could relax. With daily use of the mirror box over weeks, patients sensed their phantom limbs actually shrink into their stumps and, in several instances, completely vanish.
Gawande explains how the brain makes a “best guess” that the amputated limb is still there, but in a painful position, and how looking at it through the mirror box adds new information into the whole picture, allowing more information to be factored into the brain’s “new best guess,” which can cause the pain (or itch) to go away.
Anyway, the article is fascinating and does a much better job of explaining things than I do, so again, I suggest you read it.