Itching

[Sisyphus]

I've always found that nearly-scalding hot water is brings the best and longest-lasting relief from itching, but I realized my explanation for it (that it "fries the nerves for a while") was unscientific, and I really have no idea how it works. Looking into it, I discovered that the whole phenomenon of itching seems to be very poorly understood. Can anyone give a decent account of the mechanism, and answer why heat (and for that matter, scratching) brings relief?

[SkepticLance]

High standards of hygiene are very recent. Until a few decades ago, the normal state for a human was to carry a large number of parasites, which feed off us in one form or another, inlcuding blood sucking ticks.

 

Obviously this is not healthy. These parasites consume scarce energy, and can carry disease. We, like other animals, have evolved itching as a signal to tell us to remove the parasites. Scratching is at least partially effective at such removal, and evolution has equipped us with a reward for carrying out the act - the feeling of relief having scratched.

 

Hot water, presumably, stimulates the nerves that give the 'scratching' signal, thus stimulating the reward.

 

Of course, today we itch for a variety of reasons which usually have nothing to do with parasites. But the old mechanism stays in place.

[imp]

Some forms of itching, especially whole-body itching, are symptomatic of certain serious diseases, such as biliary cirrhosis, a life threatening illness.

 

Widespread itching not attributable to external cause, such as poison ivy, insect bites, etc., should always be checked out by a medical practitioner.

imp

[carol]

usually, itching happens when a foreign object (irritants) is introduced. its like a signal that tells you to remove something that's not suppose to be there (scratching to relieve onself). i tried to insert a very thin wire in a follicle at the back of my hand and it produced itching.

[stevo247]

Recently, I was hiking in the woods of Vermont and I got poison something. My ankle got a rash that developed very slowly over a few days and then it started to itch. And man o' man did it itch. I was like a drug addict with the scratching. It felt so.. damn... goood. When I stopped scratching, and swore off it for life, my willpower lasted about 10 seconds before I started scratching again. I very easily could have sat in the corner and scratched myself silly for eternity. It felt that good!

 

My question has to do with the sensation of pleasure associated with this scratching of the itch. Normally, the nerves in my ankle don't provide much along the lines of pleasurable stimuli. What is it about the repeated scratching of a rash or an itch that causes a nerve ending to suddenly transmit such an intense pleasurable sensation?

[doG]

OTOH, my Goliath Birdeater recently molted and I soaked the molt in some water to straighten out the legs. In the process the water got saturated with urticating hairs from the molt which got all over my hands, causing them to itch for days. This itching got much worse every time I washed my hands with hot water.

[Monomer]

I've always found that nearly-scalding hot water is brings the best and longest-lasting relief from itching, but I realized my explanation for it (that it "fries the nerves for a while") was unscientific, and I really have no idea how it works. Looking into it, I discovered that the whole phenomenon of itching seems to be very poorly understood. Can anyone give a decent account of the mechanism, and answer why heat (and for that matter, scratching) brings relief?

 

 

However, different temperature ranges have different effects on itch;

moderately warm temperatures (as in hot ambient temperature) increases it, while temperatures around 40oC (hot shower) relieve it by stimulating pain fibres, thus creating a block effect as explained by the gate theory.

http://www.nsc.gov.sg/content/613/NSC%20Bulletin%20Vol_17_2.pdf#page=15

 

http://en.wikipedia.org/wiki/Gate_control_theory_of_pain

 

 

I've also read that hot water draws out the histamine that's causing the itchiness, so you could look into that.

 

 

As for why scratching brings relief, take a look at these:

http://www.newsweek.com/id/106588

http://www.sciencedaily.com/releases/2008/01/080131121804.htm

[big314mp]

There was an article fairly recently in scientific american about itching, and if I recall, it said that the "itch signal" traveled along the same pathway as pain signals. Therefore, pain tends to mask itching, whereas pain killers, such as opiates, tend to cause (a better word may be unmask) itching.

 

I should mention that the itching they were referring to was independent of histamine (i.e. not allergic reaction/poison ivy type).

 

I'll go look for the article. If I can find it, I'll post specifics.

[Monomer]

The scientific american article and another article I found, basically state that scratching provides a painful distraction, taking the mind off the itch.

 

The general theory is that scratch provides a counterirritation: a slight pain that functions as a kind of transient neurologic distraction for the brain. Focusing on the discomfort of the scratch, the brain eventually forgets the itch.

http://query.nytimes.com/gst/fullpage.html?res=9E00E3D6143AF932A35754C0A9659C8B63&sec=health&spon=&pagewanted=2

 

 

In 1997, however, neurophysiologist Martin Schmelz, then at the University of Erlangen-Nuremberg in Germany, proved that the need to scratch reaches the spinal cord from the skin via independent nerve fibers called polymodal C-fibers. These C-fibers seem to be identical to those that signal pain, but they transmit only itching sensations. Signals conveying skin irritation travel down the nerve fiber to the spinal cord and then on to the brain. Scratching and rubbing may interfere with these nerve endings by stimulating pain and touch receptors in the same areas, thus inhibiting the surrounding itch receptors, called pruriceptors.

 

In addition, Schmelz’s team, together with Hermann Handwerker, also at the University of Erlangen-Nuremberg, discovered connections between the itch-mediating C-fibers and pain C-fibers. This finding of possible communication between signaling fibers adds a further mechanism by which pain relieves itching.

http://www.sciam.com/article.cfm?id=chronic-itching

 

 

So the scalding water is having the same effect as scratching because it's activated the pain C fibres, and maybe it's having the best, longest lasting effect because it's causing more pain, and perhaps more damage...

[ishmael]

I think the itch is the body's reaction to whatever the irritant is. The body produces histamines locally, and that is what causes the itchy feeling, not the insect bite or poison ivy or whatever. You can fool your body into producing excess histamines at the site of the itch by applying heat. But the body can only produce a certain amount of histamines in any limited amount of time. By applying the heat, you actually cause your body to use up all the histamines in that area for a while, and the itch stops. When the body has time to produce more histamines in that area, the itch returns. This is very useful information if you want some temporary relief, to get to sleep, say.

[blazarwolf]

Yep, an itch is regulated by mast cells that release hitamine. The over the counter medication Benadryl (an anti-histimine) is often perscribed to people that have to wear a casts for a while.

Not sure about the hot water. A massive systemic release of histamine is the primary moderator of Anaphylatic Shock. It is called Shock becuase histamine is a very potent Vasodilator. So potent that not only does this dangerously decrease a persons blood pressure, but actually causes leaking (thirdspacing) into the suronding tissues (this is why they appear so swollen)

Knowing that its a vasodilator, so is hot water (or just heat for that matter). There may be some physiological connection, im not sure what, other than the hot water is doing alot of what the histamine wants to do.

-next time pop a benadryl (i think there are some non-drowsey anti-histamine varients on the shelfs these days)

[big314mp]

Benadryl is actually sold as a topical gel in pharmacies (OTC).

[Avalanche3319]

Interesting. I remember as a child my parents answer to an itchy mosquito bite was to hold a lit cigarette a few cm from the bite for a few seconds. Worked pretty well for temporary relief.