The Seven Day Itch

By: Rachel Wilson

With my recent moderately severe poison oak reaction, and my subsequent transformation into the feminine counterpart of the Elephant man, itching and scratching have been on my mind. I’ve either been thinking about NOT scratching, or mercilessly partaking in the activity when I thought no one was watching. With this experience, I’ve come to two conclusions. One, scratching itches appears to be very taboo in our culture. I think this may be rooted in our history when itching was tightly linked to disease. But alas, I won't be addressing this question. I am more interested in the observation my friend Manny made, “Scratching is better than sex.” To sound a little less definitive, and some-what close minded, I’ll say that scratching an itch is arguably better than sex. Never the less, I couldn’t help but wonder why scratching is so very satisfying?

In order to ultimately feel good from scratching your itch, you need to first have an itch. As it turns out there are four current theories for the itch response mechanism with specific regards to receptors (Nakagawa and Hiura 2013). In other words there seems to be controversy in what kind of receptor(s) elicit an itch response. Without going into too much detail, receptors are responsible for the way in which we respond to and perceive the environment and ensure our body conditions are within an acceptable range. The most relatable receptors, I think, are taste buds. For specific flavors, we have differing receptors, or taste buds, that respond to specific taste chemicals. Activating specific taste buds tells our brain what flavor we are eating. For instance eating a lemon activates sour receptors:

So with regards to itching, controversy exists as to how to classify itch receptors. The first theory is that we have specific itch receptors. Another theory suggests that weak activation of pain receptors (think about when you get a cut, pain receptors tell your brain IT HURTS!) is responsible for the sensation of itch. Going along with the pain receptors, perhaps itch receptors are a specific kind of pain receptor. Like sour taste receptors are a subset of chemoreceptors. And lastly differing activation patterns or timing of receptors causes itching (Nakagawa and Hiura 2013). So hopefully I haven’t activated pain receptors in your brain like Butters:

So activation of those ‘itch’ receptors ultimately sends a message to your brain that you have an itch. I don't know about you but I am no good at ignoring that signal. 

Regardless of the classification of the itch receptor, scratching an itch kind of makes your brain go crazy:

The orange and yellow coloring indicates higher activity as measured by blood flow. Focusing in on the last row (C), we can see that there is more activation from scratching at the site of itchiness than scratching in close proximity to the itchy site. These images were made by simply subtracting row A (scratching the itch) from row B (scratching near the itch site). Now let’s focus in on those scary names. The striatum and midbrain regions (boxed in white above) are regions of the brain associated with the reward system. So every time you scratch an itch, your brain gives you a nice little treat. This is partly why scratching an itch is so satisfying. So why the heck do other areas of the brain become activated from scratching? The authors of this study suggest the regions in the brain boxed in yellow may be involved in processing the feeling of pleasure associated with itching, while the blue-boxed brain regions are involved in movement or the actual act of itching. The movement or motor control region activation suggests reinforcement of scratching behaviors (Hideki et al. 2014). 

One caveat of this study is that the subjects did not perform the actually itching themselves. Another study found that active or self-itching activates the associated brain areas seen above more than mechanical or passive itching (Papoiu et al. 2013). So the lighting up of the brain seen above may be even more pronounced. 

In addition to activation of the brain, scratching an itch also causes deactivation of some regions of the brain (Hideki et al 2014, Papoiu et al. 2013). However, this deactivation seemed unlikely to be the reason why scratching is so satisfying. This is because scratching the itchy spot and scratching a spot in close proximity to the itch elicited the same response in deactivation of brain regions, seen in the image below (Hideki et al. 2014). In other words, the ‘itch’ receptor is turned off due to the act of directly itching the site or itching near the site. This means that the pleasantness associated with scratching an itch is caused by the activation of certain brain areas, and not the deactivation of brain regions.

In addition to feelings of pleasantness associated with itching, scratching relieves that itch. In the below image, the pink color indicates the activation of brain regions associated with the pleasantness of scratching while the blue color indicates itch relief activated brain regions. While these two responses are in the same region of the brain, there is very little overlap. This suggests that there are two differing pathways involved in both these responses (Papoiu et al. 2013). So the pleasantness you’re feeling when you do itch is not associated with the relief of that itchiness.

It turns out to get relief from itching you don’t even have to be scratching the itching area. I know I said earlier scratching an itch in close proximity to the itch site doesn’t elicit the same response as scratching the itch site with regards to brain activation. That still holds true, but if your brain perceives to be scratching the itch site then itch relief is achieved (Helmchen et al. 2013). You’re probably wondering how they did this. The scientists used mirror images of arms to give the illusion that the itch site was being scratched. And the subject’s brains perceived that the itch site was being scratched even when it was actually the non-itchy site on the opposite arm being scratched. Pretty nifty stuff. What’s more nifty is that this suggests your brain responds as if the itch was actually scratched, and perhaps causing the same brain activations. Crazy brain…

I don't know about you reading, but writing this blog has made me somewhat itchy. As it turns out just seeing someone else itch is enough to cause a contagious itch (Papoiu et al. 2011), much like the commonly know social cue of contagious yawning. So when you simply see someone yawn or itch that causes you to display the same behavior. Even more interesting is that this same contagious itchiness is seen in a nonhuman primate, rhesus macaques (Feneran et al. 2013). Perhaps there is an evolutionary advantage to itch when observing your cohorts itching.

So…sorry about that. If you did become itchy, you’ve activated some ‘itch’ receptor that sent a signal to your brain perceiving the itchiness. If you lack the self-control I do, then you’ve starting scratching that itch. Doing so not only deactivated certain brain areas, but also activated brain regions associated with reward, motor-control, the processing of pleasurable feelings, and lastly but certainly not least, itch relief.

References

Feneran, A.N., R. O’Donnell, A. Press, G. Yosipovitch, M. Cline, G. Dugan, A.D.P Papoiu, L.A. Nattkemper, Y.H. Chan, C.A. Shively. 2013. Monkey see, monkey do: Contagious itch in nonhuman primates. Acta Derm Venereol 93: 27-29.

Helmchen, C., C. Palzer, T.F. Munte, S. Anders, and A. Sprenger. 2013. Itch relief by mirror scratching. A psychophysical study. Plos One 8(12): e82756.

Hideki, M., S. Tanaka, T. Morita, T. Wasaka, N. Sadato, and R. Kakigi. (2014). The cerebral representation of scratching-induced pleastness. J Neurophysiol 111: 488-498.

Nakagawa, H., and A. Hiura. 2013. Four possible itching pathways related to the TRPV1 channgel, histamine, PAR-2 and serotonin. Malays J Med Sci 20(4): 5-12.

Papoiu, A.D.P., L.A. Nattkemper, K.M. Sanders, R.A. Kraft, Y-H. Chan, R.C. Coghill, and G. Yosipovitch. 2013. Brain’s reward circuits mediate itch relief. A functional MRI study of active scratching. Plos One 8(12): e82389.

Papoiu, A.D.P., H. Wang, R.C. Coghill, Y-H. Chan, G. Yosipovitch. 2011. Contagious itch in humans: a study of visual ‘transmission’ of itch in atopic dermatitis and healthy subjects. Brit J Derm 164(4): 1299-1303.  

Image and Video Sources

http://upload.wikimedia.org/wikipedia/commons/b/be/Joseph_Merrick_carte_de_visite_photo,_c._1889.jpg

http://www.passyunkpost.com/wp-content/uploads/2013/08/Sour-Face-past-the-moon.jpg

https://www.youtube.com/watch?v=svYQG_BnAlsThe S
https://www.youtube.com/watch?v=9KHw7kBr43U

Hideki et al. 2014  Figure 3

http://hamstertails.com/wp-content/comics/2011-05-23-Blinded-by-the-Light.jpg

Hideki et al. 2014 Figure 5

Papoiu et al. 2013 Figure 5

http://lh3.ggpht.com/_kZXbL4O94IA/Snw5LTcB7QI/AAAAAAAAAfM/ABrjJgrhgr8/s800/The%20Great       %20Mental%20Power%20of%20the%20Brain-Bat.jpg

http://youtu.be/n2n8Jv-LYrQ