Sunday, June 5, 2011

Itchiness and Evolution

Why do we itch?


How does it work?


Should you resist the itch to itch?

Possible reasons that evolutionarily this strategy arose:

  • Remove skin infected with fungi and/or bacteria
  • Remove scabs to make way for fresh skin
  • Remove ticks, lice, misquitoes, etc (which can be disease carrying)
  • System gets highjacked by (conceivably so as to spread itself)
    • Scabies
    • Chickenpox
    • Cutaneous larva migrans
    • Herpes
  • To exfoliate off toxic plant residue
    • Is it our body saying don’t touch this or more likely the plant saying don’t touch me?
  • General maintenance, cleaning grooming
    • For sanitary reasons
    • To look good
  • I wonder if it was more important when we had fur?
    • Ever see a croc scratch like a dog?
    • Maybe fur provides:
      • Protection from too much scratching
      • A reason to clean
        • Think about it, which gets dirtier--tile or carpet?

How it works:
  • Unlike pain which causes a recoil from the impetus we are drawn towards the itch to deal with it (kind of a no-brainer, but worth contrasting with pain to show they work differently)
    • When it the itch is scratched they stop firing
  • Itchiness and pain have considerable crossover
    • You have nerve endings that both feel itchiness and pain
    • Your spinothalamic neurons not only deals with itchiness, but also temperature, pain and crude touch.
    • The more pain you're feeling the less your able to feel itchiness
    • There have been numerous studies that expose people it irrating and itchy substances (chloroquine and hisatmines) and then proceeded to burn, poke, vibrate, and shock them. Turns out you can't itch and hurt all at the same time thus suggesting some significant crossover.
  • Itching also uses the pleasure zones of your brain
  • Itching can be contagious (just like yawning)
    • Have you itched since reading this?
  • It can also be psychosomatic 
    • Don't believe me? Just try thinking about bugs crawling on you and try not itching.
  • Histamines (protein triggers) are released when you itch
    • Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues.
      • They're also involved in digestion by causing smooth muscle contraction and gastric acid release
      • If that wasn't enough they also help regulate sleep (hence why antihistamines, which bind on certain histamine receptors to block them [as an antagonist] cause drowsiness)
    • Histamines are released by
      • Mast cells - tissue cells that both release histamine and heparin 
        • Both regulate blood flow to affected area h
          • Histamine as a vasodilator 
          • Heparin as an anticoagulant
      • Basophils 
        •  Special kind of white blood cell representing only 0.01% to 0.3% of white blood cells in blood

Is itching bad?  Should you not scratch that itch?  Does itching infact, "Just make it worse!"?

  • Yes
    • It could stimulate the release of more histamines which could further the spread of the itch
    • Evolution gave your body reasons to itch that are no longer relevant 
    • Scratching can get so bad that the skin barrier becomes broken
  • No
    • Maybe your body wants you to itch to remove a thin layer of skin (yes, we over do it sometimes which is bad)
    • It does quiet your screaming itch neurons by scratching
  • Maybe
    • Listen to your body's cries, but don't become a slave to outdated evolution
Pictures from here, here, here, here, here.

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