Dr. Andy

Reflections on medicine and biology among other things

Monday, May 15, 2006

Why we scratch

Why do we scratch?

A simple answer would be because we itch, but how does scratching make itching (or pruritis in doctor lingo) better. But the real answer is
the most characteristic response to itching is the scratch reflex: a more or less voluntary, often subconscious motoric activity to counteract the itch by slightly painful stimuli. This itch reduction is based on a spinal antagonism between pain- and itch-processing neurons
In other words scratching causes a mild, painful sensation that inhibits transmission of the "itch" signal:

I never knew that!

Evolutionarily this makes sense: noxious (e.g. painful stimuli) cause withdrawl, but itching usually signifies something on or under the skin. The pain from scratching prompts closer inspection.

The linked article has everything you'd ever want to know about itching and more.

7 Comments:

At 6:20 PM, Blogger ollie said...

Hmmm, so scratching works on a similar principle as things like ben-gay?
(I am not refering as to the evolutionary reason)

But as to the evolutionary reason: why do animals such as dogs, cats, rabbits and squirrles (sp) scratch?

I don't think that the latter gets all that much from inspection, do they? In fact, they scratch areas that they can't come close to seeing.

 
At 3:14 PM, Blogger Dr. Andy said...

Good question, Ollie, especially for a mathematician.

I'd say that scratching could dislodge irritants or parasites and that might be beneficial.

There is an even more interesting possibility and that is that is the damage done by itching could trigger an immune response. I'll try to post more about that in the near future.

 
At 4:33 PM, Anonymous Jason said...

Capsacian cream has been used to treat itch. I have actually recommended to patients and used personally ice baths for localized areas for this very reason. When I have a flare of dyshidrotic eczema on my hands where the itch can be unbearable, I soak then in water and ice for 10-15 second cycles (maybe 4 or 5 times) (The point is to hold them in the water until they HURT). The therapy seems to interfere with the itch signal for a number of hours. Of course it only works if you have a localized itch and it's on an area that can be dipped. I doubt people are gonna want to jump in a tub of ice.

 
At 2:24 AM, Anonymous Jon Mikel, M.D. said...

Did you know that pruritus (itch in latin) means "little pain"?

 
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