Science Questions

Why do healing wounds itch?

Sun, 13th Mar 2011

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Question

Toby Tennent asked:

We are told that if a cut is itching it is healing, and that scratching itches is bad for healing, why have we evolved this tendency to cause the healing cut to itch?

Answer

Chris -   There are special nerves in the skin, and this has only been discovered in fairly recent times as in the last few years, but there are itch specific nerve fibres in the skin, and their job solely is to signal to the A comparative picture of untreated 7-days-healed road rash in the form of a scar one year later. Fresh wound is on the right, and scar is on the left.spinal cord that an area of the skin is being irritated in a way that we would perceive as itchy.

Those nerve cells are activated in a number of ways.  One of them is mechanically.  So, if you have an insect crawling on you for example, the insect crawling over your skin elicits the right kind of stimulus that those nerve cells are interested in, and they are triggered.  So you are therefore paying attention to that bit of skin because there might be a bug which is biting you, which might be about to give you malaria or something.  So that's the first point, its mechanical.

The nerve cells are also sensitive to chemicals and there are certain chemicals which when you put them in your skin, they're irritants.  As a result, they make you think, Oh!  I've got an itchy patch of skin again.  You pay attention and you brush away the irritant chemical.  

Now when you have a wound, the wound closes by cells around the margins of the wound, proliferating, in other words, growing and they then migrate from the margins of the wound, down into the base of the wound.  They actually follow the electrical gradient, a guy in Aberdeen discovered in the last 5 years or so that the inside of the wound is at a different voltage than the margin, and the cells flow down this electrical gradient, so they know where the base of the wound is.  They then unite with their cellular counterparts, and stitch themselves into place.  Then they start to contract, contractile filaments which pull the wound closed.  

So as they do that, they're eliciting a mechanical stress which the itch sensitive nerves will respond to and at the same time, there are various other factors which get released in a healing wound, chemicals which provoke healing in the wound, but also, upregulate the activity of these itch sensitive nerves.

So therefore, a wound that's closing up will feel itchy for mechanical and chemical reasons which are precisely the reasons why those nerve cells get stimulated in the first place.  So that's the reason.

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Wounds heal as new cells born at the wound margin migrate into the base of the injury, unite with their counterparts and then draw the wound closed by activating contractile elements that apply tension.

This shrinkage and tension activates the mechanoreceptive nerves that signal itch sensation, creating the urge to scratch...

Chris chris, Fri, 11th Mar 2011

I quote 'upregulate the activity of these itch sensitive nerves'. The writer must be American. Love 'em like brothers but maybe this is the reason for the longevity of Latin as the accepted language of learning...clarity and certainty are priceless, especially when communicating complex or novel ideas. david, Tue, 12th Mar 2013

David...how would you have stated it then??? CWigs, Thu, 16th May 2013

http://www.merriam-webster.com/medical/upregulation I know this is an ancient conversation, but this person David has got his panties in a knot. In attempting to discredit someone, he managed to make himself look like a fool. European, I suspect. Self, Wed, 16th Apr 2014

Chris you didn't answer the question. The guy is asking - if scratching a healing wound is NOT RECOMMENDED, then why have we evolved to have the irrepressible urge to SCRATCH IT? Because in our evolution, if a cave man kept scratched his healing wounds and reinfecting them, then that trait would have been lost. Josh, Tue, 6th May 2014

I'm guessing that the cavemen failed to realise that their constant stupid scratching was keeping their wound from healing perished; while those who exercised a modicum of self-discipline and self-denial (arts that seem to have been lost to modern youth as Western civilisation declines ever faster) survived and reproduced. Based upon this hypothesis, I predict that modern humans have an alert system that causes them to pay attention to any wound they have just scratched and cease scratching it if it becomes clear that the constant scratching is not helping the wound heal one little bit. But all this is merely lay logic; something anyone could do, who has paid a modicum of attention to learning how natural selection works. I reckon dr Chris is smart enough to know this, which is why as a challenge to the intellectually lazy he has refused to address this no-brainer and instead chosen to give us the much more interesting benefit of his specialist knowledge as a Doctor; something we our GPs never have time to explain to us. Thank you Doctor Chris; the neurobiology, microbiology, mechanics and chemistry of wound healing are fascinating and draw to our attention the miracle that is the human body, hopefully reinforcing the tragedies that are the modern cultures of violence and making some of us resolve to eschew violence and cultivate our compassion. Greg, Fri, 7th Nov 2014

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