A positive feedback with a negative outcome
Who remembers or better who wouldn’t remember Billy Wilder’s film “The Seven Year Itch” starring Marilyn Monroe or have memories of movies featuring gangsters with an itchy trigger finger? But those itches are not what I want to cover in this blog. The kinds of itches I mean can be hilarious and funny to watch when it involves animals, e.g. “Does a bear itch in the woods?” and “Watching a Grizzly Bear Scratching against a tree at Knight Inlet BC“. It seems that scratching oneself is something universal, for you can even see fish engaging in what must be a response to an itchy stimulus and I have even seen newts seemingly trying to remove an itchy old skin by deliberately squeezing themselves through dense plant growth. While watching animals solve their itchy problems may be amusing, when the itch involves us, ourselves, it’s much less of a laughing matter.
Once seen as a category of pain, researchers now consider itch to involve a unique sense with specific receptor points and neurons involved. Although progress has been made in recent years to understand the physiology of itch a little better, the phenomenon is difficult to study. Animal models are only partially helpful and animal protection societies will not permit certain itch-inducing experiments to be done while ethics committees watch that observations on humans won’t deviate from their ethical guidelines. What has become clear, nevertheless, is that different stimuli elicit responses in different nerve fibres. Delta fibres, for example, are thicker and conducting information faster than so-called 1 µm thick C-fibres, of which one type is involved in conducting pain responses and another in responding to itch stimuli at speeds of 1 µm/sec. But the causes of itch vary and an itch caused by the irritating hairs of the pods of a tropical plant known as Velvet Bean Vine ( Mucuna pruriens ) involve the faster Delta fibres and is mediated by a cysteine protease and not a histamine, which is a response to insect bites and causes swelling and itching. An anti-histamine may reduce that kind of itch, but not the type caused by the Velvet Bean. Other identified itch mediators are, e.g. serotonin 5-HT, substance P, cytonkines from white blood cells, dust mite allergens, etc.
Four or five categories of pruritus (the scientific name of ‘itch’) are distinguished. The skin-derived itch stems from damage to the skin, be it from an insect bite or chemical exposure. The psychogenic pruritus can be elicited by seeing creepy crawlies or even thinking of them or by nervousness. The neuropathic pruiritus is due to a pathologic change facilitating conduction from skin receptor to the brain, while the neurogenic itch is the result of signals from the central nervous system to an itchy region reducing inhibition from the central nervous system and can accompany diabetes, kidney malfunction, herpes zoster or multiple sklerosis, etc. The final category would be that of the atopic pruritus, and as the name suggests the causes are difficult to identify as skin-derived components may “team up” with neurogenic causes. What all the various origins of itch have in common, however, is that they lead the sufferer to scratch himself or herself to relieve the irritating itchy sensation. Unfortunately, the scratching triggers a positive feedback loop, which means one action leads to more (that’s why it is termed “positive”). In other words, you scratch and instead of reducing the itch, the latter increases; you scratch more and that further increases the itching and so it goes on and on until you are too exhausted to continue to scratch.
The body of a human adult is covered by approx. 1.8-2.2 m2 of skin. An itch makes you want to touch the itchy place, but a painful area is something you want to avoid. Another difference is that exposure to heat will usually aggravate the pain, but heat can lead to an easing of the itch, just like antihistamines and an exposure to UV-radiation can. Obviously, all mammals experience an itch sometimes and then respond with scratching; birds do, and even fish display responses to itchy stimuli. And insects? The rubbing and ‘washing’ that one can see flies and other insects engage in, isn’t that a kind of ‘scratching’, a response to an ‘itch’? Now that’s something for which we really need to scratch our heads, I think.
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