Scratching the Surface of an Itch
Ally Thayer
Illustrations by: Alex Tansey
You know the feeling. That irritating, all-consuming, and often hard-to-reach sensation: an itch. Have you ever thought about why you itch? Or, what an itch even is? As you read this, you will probably start to feel itchy. Why is that? Were your parents right when they told you that scratching an itch will only make it worse? All of these questions seem very simple, but neuroscientists and doctors have been studying itching, medically known as pruritus, and its causes for centuries now to understand how and why we itch.
Nearly everyone has experienced an itch in their lifetime. But what causes itchiness? Itches have many causes, a few of which you may have experienced. Maybe you have experienced an itch from an allergy, dandruff, lice, dry skin, or an unfortunate encounter with poison ivy. Maybe you have a skin condition that causes a more painful and chronic itch, such as eczema or psoriasis. Maybe you have an experience with some sort of psychosis that can cause an itchy sensation with no physical cause. Or maybe you have a very specific condition, such as brachioradial pruritus, in which a pinched nerve in the neck causes the outer arm to itch [4]. Because itching has so many causes, it is difficult to study and understand all kinds of itches as one condition. Regardless of the cause, the experience of an itch ranges from a brief annoying sensation to a relentless pain that has even led people to commit suicide [4]. By understanding an itch and how to treat it, we can not only better understand the human body and nervous system, but also improve and save lives.
In order to understand the science of itching, it is important to define what exactly an itch is. The widely accepted definition of an itch has not changed since it was first postulated in the 17th century by German physician Samuel Hafenreffer, who defined an itch as an “unpleasant sensation that elicits the desire or reflex to scratch” [3]. Although itching may seem easily defined, it is not yet completely understood. Itching can be classified into two categories: acute and chronic [2]. Acute itching refers to a severe and sudden onset of an itchy sensation that is not long-lasting, while a chronic itch refers to a persistent, debilitating itch that often has few treatment options [2].
With this knowledge of what itches are and how they are classified, we can begin to think about what happens neurologically when we itch. In general, the nervous system receives sensory information through neurological pathways. Sensory receptors in the skin collect sensory information, which is then relayed to the spinal cord, which sends a message to a region in the top of the brain called the sensory cortex. This area is where the conscious experience of physical sensation occurs, and it is where your brain alerts you to the itch. The sensory cortex then communicates with the motor cortex, another region of the brain, to move the body to scratch, ignore the sensation, or move away from it [1]. This may seem like a fairly simple pathway, but it quickly becomes more complex upon close examination.
Scientists used to believe that itching was a lesser form of pain and, conversely, that pain was an extreme form of itch. They believed that the sensory neurons on the skin responding to information for itching were the same ones that responded to information for pain [5]. However, a 2013 study disproved this theory and confirmed that the neurons that respond to itchy stimuli are different than those that relay pain signals to the brain, demonstrated by the discovery of a protein present in only some sensory neuron receptors: Nppb. Receptors without Nppb did not respond to itch-inducing stimuli but did respond to other sensory stimuli, such as pain or heat. Once the protein was reintroduced, scratching began [5]. This research has been pivotal to the conceptualization of an itch and how itchiness is currently treated and has led to the development of drugs to target this itch receptor. However, scientists still understand little about itching at the molecular and cellular level.
With all this discussion of itching, you may be feeling rather itchy yourself. In neuroscience, the compulsion of one individual to copy a certain behavior of others is known as behavioral contagion and is often subconscious. Yawning and contagious itch are both examples of behavioral contagion. Recent research in mice has identified the neural mechanisms in the brain that explain this phenomenon. When mice were shown videos of another mouse scratching, they began to scratch themselves. Researchers noticed increased activity in the suprachiasmatic nucleus (SCN), an area in the brain that is involved in the regulation of the sleep-wake cycle, when mice viewed videos of other mice who were experiencing an itch [10]. After further analysis, researchers found that the SCN releases a substance called gastrin-releasing peptide (GRP), which is involved in the transmission of itch signals between the brain and the spinal cord. [10]. When researchers then blocked GRP in a mouse’s brain, the mouse did not scratch while watching videos of other mice but would still scratch if exposed to an itch-inducing stimulus. The mouse could still itch, but because of GRP inhibition, it was not “catching” a contagious itch; this led researchers to the conclusion that itching is both an innate and instinctive behavior. As said by Dr. Zhou-Feng Chen of the Washington University School of Medicine in St. Louis research team, the “next time you scratch or yawn in response to someone else doing it, remember it’s really not a choice or a psychological response; it’s hardwired into your brain” [10].
Itchiness is undoubtedly an uncomfortable sensation, and when experiencing an itch, the immediate solution usually seems to be to scratch it. However, you have probably been told at some point not to scratch an itch, since it will only make the feeling worse. The Buddhist philosopher Nāgārjuna described how “there is pleasure when an itch is scratched. But to be without an itch is more pleasurable still” [2]. So, what does science tell us about scratching an itch? Don’t do it. But why? The answer lies within a mechanism called the Itch Scratch Cycle [9]. Scratching gives short term relief, as when we scratch our itches, we overwhelm the body’s itch signals with pain signals. The itch does not stop, but rather the induced pain from the scratching of our nails on our skin is felt instead. While this may feel like relief, it is merely a distraction from the real discomfort: the itch. Because you have generated a pain sensation, the body responds by releasing serotonin, a chemical in the nervous system that helps deal with pain [9]. This release then causes itch receptors to become extra sensitive when responding to a stimulus. So, when scratching stops, the induced pain disappears, but the itch remains. The itch receptors are more sensitive now, so the next itch will be perceived as worse. You will then want to scratch again, and the cycle continues [9]. Whether you listen to science, Nāgārjuna, or your parents, it seems the consensus is to avoid scratching your itches. They will only feel worse.
While scratching an itch will just make it feel worse, an anti-itch cream can often lead to relief. Because itching is so diverse in its causes, it is also very diverse in its treatments. For example, in patients with mild eczema, a common treatment is to encourage moisturizing. Dry skin becomes flaky, and the flakes can act as irritants that cause the body to itch [7]. A common treatment for itching is hydrocortisone skin cream, which treats itches associated with allergic reactions or irritation. During these allergic reactions or irritations, various chemicals in the skin are released that cause blood vessels to open and the skin to become red, swollen, and itchy. Hydrocortisone cream acts inside the skin cells to stop the releasing of these chemicals and prevents the itchy situation from beginning [7].
New information in itch science brings us closer to understanding and treating itchiness. In April of 2020, periostin, a new protein, was found in the skin and is believed to directly activate the itch receptors [6]. In September of 2020, a study was released arguing that rubbing of the skin, rather than scratching, could potentially activate an anti-itch pathway that travels through the spinal cord [8]. Perhaps we should be rubbing our itches rather than scratching them? Hopefully, one day soon we will be able to completely understand what an itch really is and how to best treat them. But for now, to the best of your ability, do not scratch your itches!
REFERENCES
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