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Humans and other animals itch for a variety of reasons. Scientists believe the underlying purpose of the annoying sensation (called pruritus) is so we can remove parasites and irritants and protect our skin. However, other things can lead to itching, including drugs, diseases, and even a psychosomatic response.
Key Takeaways: Science of Itching
- Itching is a sensation the produces a desire to scratch. The technical name for an itch is pruritus.
- Itching and pain use the same unmyelinated nerve fibers in the skin, but pain causes a withdrawal reflex rather than a scratching reflex. However, itching can originate in the central nervous system as well as the peripheral nervous system (skin).
- Itch receptors only occur in the top two skin layers. Neuropathic itching can result from damage anywhere in the nervous system.
- Scratching an itch feels pleasurable because the scratch fires pain receptors, causing the brain to release the feel-good neurotransmitter serotonin.
How Itching Works
While drugs and disease typically stimulate itching because of a chemical response, most of the time the sensation is a result of skin irritation. Whether the irritation starts from dry skin, a parasite, an insect bite, or chemical exposure, the itch-sensing nerve fibers (called pruriceptors) become activated. Chemicals that activate the fibers may be histamine from inflammation, opioids, endorphins, or the neurotransmitters acetylcholine and serotonin. These nerve cells are a special type of C-fiber, structurally like the C-fibers that transmit pain, except they send a different signal. Only about 5% of C-fibers are pruriceptors. When stimulated, pruriceptor neurons fire a signal to the spinal cord and the brain, which stimulates a rubbing or scratching reflex. In contrast, the response to the signal from pain receptors is an avoidance reflex. Scratching or rubbing an itch stops the signal by stimulating pain receptors and touch receptors in the same region.
Drugs and Diseases That Make You Itch
Since the nerve fibers for itching are in the skin, it makes sense most itching starts there. Psoriasis, shingles, ringworm, and chicken pox are conditions or infections that affect the skin. However, some drugs and illnesses can cause itching without underlying skin irritation. The antimalarial drug chloroquine is known to cause severe itching as a common side effect. Morphine is another drug known to cause itching. Chronic itching can result from multiple sclerosis, certain cancers, and liver disease. The ingredient that makes peppers hot, capsaicin, can cause itching as well as pain.
Why Scratching an Itch Feels Good (But Isn't)
The most satisfying relief for an itch is to scratch it. When you scratch, neurons fire pain signals to your brain, which temporarily overrides the itching sensation. The feel-good neurotransmitter serotonin is released to provide relief from the pain. Essentially, your brain rewards you for scratching.
However, a study conducted at Washington University School of Medicine in St. Louis indicates scratching ultimately intensifies the itch because serotonin binds 5HT1A receptors in the spinal cord that activate GRPR neurons that stimulate more itchiness. Blocking serotonin isn't a good solution for people suffering chronic itching because the molecule is also responsible for growth, bone metabolism, and other key processes.
How to Stop Itching
So, scratching an itch, while pleasurable, is not a good way to stop itching. Getting relief depends on the cause of the pruritis. If the issue is skin irritation, it may help to cleanse the area with a gentle soap and apply an unscented lotion. If inflammation is present, an antihistamine (e.g., Benadryl), calamine, or hydrocortisone may help. Most pain relievers don't diminish itchiness, but opioid antagonists offer relief to some people. Another option is to expose skin to sunlight or ultraviolet light (UV) therapy, apply a cold pack, or apply a few electrical zaps. If itching persists, it's a good idea to see a doctor to check for underlying medical conditions or itching in response to a drug. If you absolutely can't resist the urge to scratch, try rubbing the area rather than scratching it. If all else fails, a German study indicates you can lessen itching by looking into a mirror and scratching the corresponding non-itching body part.
Itching Is Contagious
Are you getting itchy reading this article? If so, it's a completely normal reaction. Itching, like yawning, is contagious. Doctors who treat itchy patients often find themselves scratching as well. Writing about itching leads to itchiness (trust me on this). Researchers have found people attending lectures on itching scratch themselves a lot more often than if they were learning about a different topic. There may be an evolutionary advantage to scratching when you see another person or animal do it. It's likely a good indicator you might want to check for biting insects, parasites, or irritating plants.
- Andersen, H.H.; Elberling, J.; Arendt-Nielsen, L. (2015). "Human surrogate models of histaminergic and non-histaminergic itch." Acta Dermato-Venereologica. 95 (7): 771-7. doi:10.2340/00015555-2146
- Ikoma, A.; Steinhoff, M.; Ständer, S.; Yosipovitch, G.; Schmelz, M. (2006). "The Neurobiology of Itch." Nat. Rev. Neurosci. 7 (7): 535-47. doi:10.1038/nrn1950