Why we itch has always been very mysterious to me. I mean, what causes my nose to itch right when I’m slid into an MRI machine for a required exam and must remain perfectly still? And why does my back seem to itch right where I can’t scratch it, even by reaching down over my shoulder with either hand or by twisting my arm behind my back from the side and reaching up. Truly that itch seems to move and stay just out of reach. Or perhaps I’m lying down, seizing a moment of silence or inactivity to drift off into a pleasant and restful ten or twenty minute nap and there’s a tiny itch on my nose which I dispel with a swipe of my forefinger. Wait, now my earlobe itches so I dispatch that itch with another quick touch. Once that’s gone, there’s another irritating itch in my scalp so a quick fingernail scratch is necessary. What’s going on here pray tell? What is causing these irritating little itches that are preventing me from drifting off to sleep?

We all know the cure for an itch – that’s the scratch. That little nose itch can be addressed with a swipe of a fingertip. And that back itch responds positively to your fingernails it you ever reach it. If you can not, use your trusty wood back scratcher. But the real mystery to me has always been – how did the itch get there, what caused it and how did my brain sense the itch and locate it perfectly so I could try to scratch it? And why does that scratch feel so good?

The answers to these questions are not simple. An itch can occur for many reasons. There’s the terrible itch that occurs after touching poison ivy, poison oak or poison sumac. And fully 80 to 90 percent of us are allergic to urushiol, the oil on the leaves of these plants. Then there are insect bites which cause an almost instantaneous allergic reaction and itching, caused by the histamine released by our immune system. And all of us have experienced a healing skin wound which, when a scab appears and begins the healing process, can itch badly and sometimes causes us to scratch the scab off and have to start the healing all over again. And certainly diseases like eczema or psoriasis can cause incredible itching over generalized areas of the body. Or an itch can be caused by something as innocuous as bits of hair or dust which scratch the skin’s surface layer.

While definitely an annoyance, itching and other little skin irritations do play an important role in keeping us safe and healthy, just like other sensations such as heat, cold, and pain. After all, an insect lighting on my arm produces a slight itch which when I unconsciously reach for it, chases that potentially harmful insect away. Other such irritations may signify that a bug of some kind, perhaps a dangerous spider or tick, has already begun its violation of the skin surface, and to reach for it and scratch, thus dislodging it, also helps protect our health. 

But exactly what is an itch? One source (https://www.scientificamerican.com/article/experts-why-we-itch-and-scratch/) asserts that pruritus, the scientific name for itching, occurs due to stimulation of itch sensing nerve endings, called “pruriceptors”, by certain agents – “chemicals from immune response (histamines) and pain relief (opiods); neuropeptides, which include pain-regulating messengers released within the brain, such as endorphins; or the neurotransmitters acetylcholine and serotonin; or prostaglandins that cause inflammation or other damage to the skin that produce an itching sensation”, for example the saliva left by a mosquito producing a “mosquito bite” or that oil on a poison ivy leaf.

An itch is very similar to pain. However, while a painful sensation on the skin causes a withdrawal or avoidance reaction, an itch causes a “need to scratch” sensation. These itch sensations are transferred along the spinal cord to the brain by specialized nerve fibers, which constitute about five percent of the C-fibers that normally transmit the sensations of touch or pain. Scratching or rubbing relieves the itch by arresting the transmission of the itch sensation by stimulation of various pain and touch receptors in the same area of the skin. 

Additional research has determined that an itch is quite unique. Apparently when we itch, a molecule released by the heart, called natriuretic polypeptide B, or “Nppb”, sends a message to the spinal cord that causes us to feel that itch. This was demonstrated in research on mice when Nppb was injected into their skin and no itching or scratching resulted. But when injected into a spot in their spinal cords that communicates with other nerves, the mice started scratching. In addition, when mice specially bred without the Nppb molecule were exposed to a variety of compounds that normally induce itching, they did not scratch at all. So itching not only has its very own communication channel directly to the brain but one which is facilitated by Nppb.

So that’s what an itch is and how its sensation gets to our brain and persuades us to scratch. But why does scratching an itch feel so particularly good? Well one reason it feels good is that scratching an itch stimulates the itching area and interrupts its trip to the brain so it no longer itches. Also, scratching actually may cause pain sufficiently intense to temporarily distract the brain from sensing the itch. But at times relieving a serious itch produces such a good feeling that it has to be more than this. Well actually, the evocation of pleasant feelings caused by scratching an itch can be traced to activation of regions of the brain involved in reward, or positive stimulation that reinforces certain behaviors. And additional research has shown that scratching an itch also dumps a bit of dopamine – yes, that same miracle neurotransmitter that produces the good feeling from alcohol, drugs or sex – into the brain. And if that was not enough, the pain caused by scratching also causes the brain to release the feel-good neurotransmitter serotonin to relieve the pain but which at the same time may intensify the itch.

So to summarize the science and physiology behind itching and scratching, an itch sensation which reaches our brain through specialized nerve fibers, can result from a variety of causes. And scratching that itch causes some pain and distraction that not only relieves that condition but also makes us feel good. But while this knowledge might help us visualize and analyze why we itch, it unfortunately does not help us reduce the sensation, or its frequency or intensity. So now, when I unconsciously reach for that little itch on my ear or my nose or strive mightily to alleviate that persistent itch on my back, I know that it’s more than just an inconvenience – a signal has gone to my brain from an area on my skin and made me scratch that area.

And some additional details about itching and scratching: When the stimuli lands on your skin, it may not bother you at first, but soon it will begin to rub back and forth across your skin. Once the hair or dust scratches your skin’s surface layer, receptors in the dermis of the skin will become irritated. In a split second, these receptors send a signal through fibers in the skin to your spinal cord and then up to the cerebral cortex in your brain.

And your brain, amazingly, when receiving these itch signals, can tell exactly from what part of your body they are coming from. As I outlined in the second to the last paragraph of my article about memory, the brain maintains a very detailed and complete “map” of the body. If the body senses an intense sensation like heat, cold or pain, it can locate it perfectly so that it can be addressed and relieved. And it’s the same with an itch: once it receives an itch signal, the brain can guide your finger, fingernails or wood back scratcher to the exact place the itch is located.

The same fibers that send itching signals are also used to send pain signals to the brain, which once led some scientists to believe that itching was a form of light pain. That notion has since been dispelled by research, which showed that pain and itching elicit opposite responses. Pain causes us to withdraw and itching causes us to scratch.

As soon as we feel an itch, our first natural response is to scratch the spot of the itch with our fingernails. The reason for this response is simple — we want to remove the irritant as soon as possible. Once you’ve scratched the area of irritation, you are likely to feel some relief. When your brain realizes that you’ve scratched away the irritant, the signal being sent to your brain that you have an itch is interrupted and therefore no longer recognized by the brain.

Even if you don’t remove the irritant, scratching will at least cause pain and divert your attention away from the itching. The irritant that caused the itching is very small, maybe only a few microns in length, so it disturbs only a few nerve endings. When you use your fingernail to scratch the spot where the irritant is, you not only remove the irritant but you irritate a lot more nerve endings than the irritant.

Itching probably evolved as a way of coping with biting insects and other ectoparasites. The itch encourages us to swat mosquitoes, and pick lice and ticks off ourselves and each other. Itches can be triggered by the central nervous system as well, even if nothing is touching your skin. This is why itches can be contagious like yawns. It might have warned our ancestors to start swatting flies proactively, or encouraged mutual grooming sessions.

Addenda

This article has been stuck in my computer files for a very long time. Somehow I was never quite satisfied with it so it was never published on my blog. However, news of two researchers being honored by earning the Nobel Prize in Medicine who did their work on “receptor molecules”, discovering the molecular bases of how nerves convert stimuli—the burn of a chili pepper, or the soft pressure of a hug—into signals that can be sensed by the brain, refocused my attention on the article, which deals with a more casual, personal and certainly more unscientific consideration of similar receptors for itch sensations. 

Drs. David Julius and Ardem Patapoutian, both based in the US, have produced crucial research discoveries  related to how the human body senses temperature and touch and how these sensations reach and are processed by the brain. The importance of these sensations to our survival are obvious. We must be able to feel and process the sensation of touching something hot or cold. Also feeling and processing the sensation of pressure is supremely important. Thankfully we feel the pressure of a full bladder motivating us to relieve ourselves. The pressure of a hug or of a hand once conveyed to and processed by the brain produces sensations of pleasure or companionship or of caution. 

So an itch seems to be but one more very important sensation that is picked up by molecular receptors on the surface of the skin and transferred to the brain along specific nerve channels, just as are those of heat, cold, pain or pressure. Thus this article on itching and scratching has acquired additional relevance to the science of how conditions or trauma are sensed and then how the information gets to the brain so that we might react in the right way.

The itch sensation can be somewhat innocuous and a mere minor annoyance or it can be serious, when caused by a dangerous insect on the skin or a disease that causes debilitating pruritus or itching which must be treated medically. But feeling an itch, transporting this sensation to the brain so we can treat it by swiping, scratching or with drugs is very important.

Yet another personal incident has refocused my attention on the article and added some impetus to its publication – my wife Bobbie being afflicted a couple of summers ago with a serious and potentially fatal tick-borne disease – babesiosis. This disease, in which babesia microti parasites attack the red blood cells, is spread by the deer tick right here at our summer residence in her native state of Vermont. And she was unfortunately a victim of a bite from this tiny, dangerous insect whose presence on her skin she obviously did not feel or sense in time to remove it, underscoring again the importance of these remarkable molecular receptors conveying sensations to the brain so that we can react appropriately.

So we need to do everything we can to prevent itching and perhaps enjoy our analysis when it occurs, but we need to keep those fingernails sharp, our arms and wrists flexible and that wooden back scratcher handy!

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