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More specifically, shampoo. What are the mechanics of detecting a noxious chemical stimulus in terms of which receptors recognise what, how do they do it, and how is this information relayed to the nerve cells?

Why is the pain (and so signal, I presume) larger if one is having a hot shower at the time?

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I edited the question a bit, as I don't think the sensation is specific to shampoo, but this is interesting. –  jonsca Dec 9 '12 at 2:17
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I guess the second part has to do with hot water dilating blood vessels, facilitating extravasation –  nico Dec 9 '12 at 13:41
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Shampoo is a detergent. Detergents emulsify lipids, including those in membranes. Skin cells are probably a bit more resistant to detergent than other cells. When it gets in cuts, the shampoo is probably causing rapid cell injury and/or death. I think pain is a perfectly proportional response to that. –  Chinmay Kanchi May 24 '13 at 16:51
    
@ChinmayKanchi I'd be happy to accept that if you post that as an answer I (perhaps with an extra link). –  Alyosha May 24 '13 at 17:02
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@Alyosha There you go... –  Chinmay Kanchi May 24 '13 at 17:39
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Shampoo contains surfactants, chemicals which cause lipids to emulsify. The cell membrane is composed primarily of phospholipids, which are vulnerable to action by surfactants. In fact, sodium dodecyl sulphate (SDS, often labelled SLS on shampoo bottles), an integral component of many shampoos is also used in the lab (albeit at substantially higher concentration) to achieve cell lysis.

The outer layer of skin is composed largely of a very tough type of cells called keratinocytes, which form an impenetrable barrier to potentially harmful chemicals like surfactants. Once the skin is ruptured, the surfactant can act on cells that don't have the protection of keratin, causing damage to cell membranes, and perhaps even cell death. This will obviously lead to an immune response and inflammation, which is likely to be the source of the burning sensation/pain that one feels.

The inflammatory process involves various immune system-related mediators such as bradykinin and prostaglandins, which act to increase the sensitivity of neurons to pain.

With other chemicals that cause a burning sensation, the mechanism of cell injury might be different, but ultimately, the immune mechanisms kick in, causing inflammation and pain.

Kid & Urban (2001) reviews the processes that cause inflammatory pain in great detail (full text is available for free).

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