Why do people itch?
Compared to cold, heat, pain and other sensations, “itch” is an indescribable sensation. So far, scientists have not discovered the receptors of itchiness from histology, but itchiness is objective. Scientists have found that the sensation of itch is related to histamine, a protein secreted by the body during allergic reactions, and this substance is fed back to the brain, the body will have the sensation of itch. As to whether the sensation of itch has a special receptor like other sensations, there are two possibilities: one is that it may share a receptor with other sensations, and the other is that itch and pain are actually one sensation, with light stimuli being itch and heavy stimuli being pain.
The skin is a complex “receptor”
The skin is a sensory organ for all types of touch in the human body, and in the general impression, the skin seems to be a layer of tissue that is a few millimeters thick and somewhat elastic. In fact, the structure of the skin is much more complex and sophisticated than imagined.
The skin has three layers: epidermis, dermis, and subcutaneous tissue. The epidermis contains the stratum corneum, which can be very effective in resisting external influences. The inner layer of the epidermis, the dermis, is rich in connective tissue and blood vessels, sweat glands, and also contains receptors and sebaceous glands. The sebaceous glands secrete oil with special ingredients that lubricate the skin and are waterproof, and to some extent, protect against bacteria. The innermost layer of skin is the subcutaneous tissue.
How is “sensation” created?
As a natural barrier between the human body and the outside world, the skin receives stimuli from the outside world all the time and generates excitement, which is transmitted to the nerve center to form various sensations. The human skin has many dotted receptors, which are divided into four main types: pain, temperature, cold and pressure. These receptors are mixed together and distributed in different degrees of sparseness. They feel the degree of sensitivity, depending on the density of receptor distribution on the skin, such as lips, fingertips are most sensitive to touch.
Cold receptors are nerve endings in the skin with connective tissue capsule, heat receptors are a free nerve endings, they feel the stimulus below or above the skin temperature, respectively. Tactile pressure receptors can feel mechanical stimuli such as contact and squeeze, so that people can feel touch and pressure, so that they can know the shape, size and some physical properties of objects. And any kind of excessive physical or chemical stimulation can cause nociception, which is a protective sensation.
The causes of itchy skin are very complex. Some skin diseases such as mosquito bites, drug allergies, dermatitis, eczema, fungal infections and some chronic diseases such as indigestion, anemia, diabetes, liver and gallbladder diseases, kidney disease and cancer can all cause symptoms of itchy skin. Therefore, if you feel inexplicable itching of the skin, you must find the right cause and not use drugs indiscriminately.
For the elderly, usually should pay attention to reasonable skin care, clothing should be wide, loose, do not wear chemical fiber and woolen fabrics, underwear use cotton or silk fabrics. Do not use too strong alkaline soap or rub too much in the bath, avoid scratching as much as possible, and the temperature of bath water should be 35-37℃. Bedding should not be too warm. In winter, the appropriate amount of emollient cream should be applied to protect the skin.
Why does the armpit itch when others scratch it, but not when you scratch yourself?
There are certain parts of the human body that are usually less disturbed, such as the armpits, which are usually less exposed, it is more sensitive parts, which are usually covered more places it is more sensitive. When others scratch you, itch is a feedback mechanism between fear and kind expression.
Itchy, sore, numb and other sensations are actually a stored format of the human brain to external stimuli, the brain can make different reflections on different stimuli according to the above categories to prevent the human flesh from being more harmed. Almost everyone’s body has a particular fear of itching, when being tickled by others to the heart of the feet, armpits and other locations, many people will feel itchy and laugh out loud. However, when they scratch themselves, they often do not feel anything. Why is this?
British scientists have experimented with this phenomenon and found that when people tickle themselves, the human cerebellum will send a signal, when the brain feels that they are “teasing” themselves, with no precautions and “fear”, they do not feel the itch. However, when tickled by others, even if people know in advance, but the cerebellum does not send a warning signal, the brain will immediately respond to external stimuli, people will feel particularly itchy, usually, our armpits, groin, soles of the feet and other places are most sensitive to itch. The reason is, these places are “non-exposed areas”, usually by scratching stimulation opportunities are few, plus these parts of the skin receptors and richer, the combination of the two, so the feeling of itch is much more acute. However, there are some places such as nose hairs, eyelashes and lips that are most itchy, followed by armpits and feet.
Diseases related to itch
Simple skin problems. “There are two types of itchiness felt because of problems with the skin itself.” One type is when there is no primary skin damage and it is mostly caused by a lack of moisture in the skin. Like now in the dry winter months, many people will feel dry, itchy, peeling skin. The other kind is the skin itself, dermatitis, eczema and other skin diseases are mostly accompanied by itching.
Skin degeneration in middle-aged and elderly people. Middle-aged and elderly people appear dry because their skin atrophies and thins, water content decreases, sebaceous glands and sweat glands secrete less, making the skin lose its lubricating and protective effect. In the cold season, the dry climate also makes the skin rough and even the epidermis falls off, making the nerve endings in the skin more easily stimulated and itchy. The itchy body of the elderly is mostly due to this reason.
Allergy. One third of the patients who come to dermatology clinics have symptoms of itchy skin. Allergies to diet, pollen, dust mites, etc., and exposure to certain chemical agents can cause allergies, which can make the skin feel itchy.
Gynecological inflammation. Women who find itching in the vulva should be especially checked for gynecological inflammation such as trichomonas and Candida albicans infections. Surveys show that 73% of women feel localized itchy skin during their periods, which may be related to poor quality sanitary napkins or clothing that is too tight.
Emotional tension. Depression, nervousness, irritability and other bad emotions may cause local or generalized itchy skin. deficiency of B vitamins also tend to cause itchy skin.
Liver and biliary diseases. Many systemic diseases can also cause itchy skin. Some studies show that about 40-60% of patients with liver and biliary diseases develop itchy skin when liver discomfort, abnormal liver function, and jaundice are present.
Diabetes. The incidence of pruritus in diabetic patients is as high as 15-35%, and about 10% of early patients develop generalized or localized pruritus.
Abnormal thyroid function. Itchy skin may occur whether the thyroid is hyper- or hypothyroid, but itching due to hyperthyroidism tends to develop slowly, the skin tends to be dry, and it gets worse in the winter.
Malignant tumors. Certain tumors occur with itchy skin, such as lymphatic system, stomach, intestine, liver, ovarian and prostate cancer.
Itching research progress
Researchers used certain techniques to make volunteers itch like crazy on one arm without allowing them to scratch it in any way. Then they were sent to a magnetic resonance imaging (MRI) scanner to observe which parts of their brains were active when they felt the itch, when the researchers helped them scratch, and when they were finally able to scratch themselves.
The scientific question this study is trying to address is: Why does tickling feel so good?
Contrary to expectations, itching and scratching are not only related to sensory areas of the brain, but also involve certain psychological processes, such as motivation and reward, pleasure, craving and even addiction, which explains to some extent why people like to scratch. Scratching turns off brain activity that is activated by itching, and scratching yourself is often better than doing it for someone else. These findings were published in the journal Public Library of Science: Omnibus (PLOSOne). For a long time, both in the field of research and treatment, itching as a subject has always been stolen by another subject – pain, and even, for a time, it was thought to be just a milder form of pain. However, with millions of patients suffering from itch worldwide, times have changed. Research has found that itch has its own specific neural, molecular and cellular receptors that distinguish it from pain. The medical community has recognized that itch can also be debilitating and has begun to take it seriously as a problem worthy of research and treatment. A series of studies have emerged in the past decade that have explored what causes itch and what can be done to stop it. With the help of brain imaging technology, research is now increasingly focusing on the activity of the genes involved and trying to capture and characterize the signals that flow between skin cells, the immune system, and the spinal cord and brain. The research focuses not on fleeting nuisances like mosquito bites and poison ivy, but on the unrelenting pain caused by chronic, long-term itching – which lingers, torments people day and night, and is often resistant to treatments such as antihistamines and cortisone ointments.
The Temple University Pruritus Center, which opened in Philadelphia in September 2013, and Washington University, which opened in St. Louis in 2011, were the first pruritus research and treatment centers to open in the United States. “Today, itch is valued to a similar degree as pain was valued 20 years ago.” says LynnCornelius, MD, chair of the Department of Dermatology at Washington University School of Medicine. “In the past, people have always confused it with pain.” But now, she continues, people are more interested in itch and its classification, and are willing to invest large sums of money in research in this area. “I firmly believe that scientific developments will lead to advances in the field of treatment.” If that day comes,” Cornelius said, “the results of scientific research will translate into better and more targeted treatments. That way, doctors won’t have to prescribe antihistamines to patients who suffer from itching.”
Scratching behavior is prevalent in the animal world. Although no one can know exactly why animals scratch, bite or peck themselves, or rub their bodies against trees or fences, it is speculated that it is likely because they too feel itchy. “Even lowly creatures like fruit flies, when they are infected with mites, exhibit a vigorous grooming behavior that looks quite similar to scratching.” says Diana Bautista, an assistant professor of cell and developmental biology at the University of California, Berkeley. In addition, her study included multiple strains of itchy mice, all of which are established model mice for different human diseases. “I also collected a lot of videos describing scratching behavior in different animals,” Dr. Bautista said, “and I hope they will help me determine if there are differences between scratching behavior caused by itching and rubbing and other behaviors in different species.” One of her favorite videos shows the sight of a seal lying on the beach, gently rubbing its head with its flippers.
Among human behaviors, there are many different types of itching. The most familiar one is caused by insect bites or hives, when histamine is released from skin cells, causing nerves in the skin to produce signals that are transmitted to the spinal cord and brain. Antihistamine tablets or creams can often relieve its symptoms. However, these medications are often of little help to patients with chronic itching, which can be caused by skin conditions such as eczema or psoriasis (psoriasis), kidney or liver failure, dry skin, hyperthyroidism, certain cancers, and pinched or damaged nerves. And almost certainly, pruritus due to psoriasis has a different mechanism than pruritus due to nerve compression. “This is a very hot area,” Dr. Cornelius said, “and it is both a major clinical problem and provides a huge market with a far unmet need.” Dr. Bautista explained that recent studies have shown that, in addition to histamine, inflammatory cells release a number of other substances associated with chronic itch, and that three different types of nerve cells are also involved in the process. Pharmaceutical companies are working to find ways in which these substances can be blocked. “Previously, their research and development focus has been on the next generation of antihistamines,” Dr. Bautista said, “but now they are targeting new molecular and cellular targets to develop new therapies. The pharmaceutical industry has recognized that they can no longer limit themselves to antihistamines and must go beyond that.” But finding new targets will require dissecting every tiny detail of the pain pathway.
Zuofeng Chen, now director of the University of Washington’s Pruritus Center, led a team that used mice to study cell surface receptors and molecules that can respond to specific chemical signals and alter cell behavior. the 2007 study, published in Nature, is agreed by many researchers to be one of the most important advances in the field.
The group identified the first pruritus-specific receptor, called the gastrin-releasing peptide receptor, or GRPR, in the spinal cord, a finding that helped demonstrate that the signaling pathways for pruritus and pain are not the same. In an interview, Dr. Chen said that mice that did not have the receptor, or whose receptor was blocked by drugs, did not experience itching, and that mice in the receptor-free group were not impaired by it. “Simply blocking the receptor’s function was almost enough to put an end to chronic itching,” he said. The receptor is also present in humans, and Dr. Chen sees promise in developing a drug that can block it in the future. But for most patients, new treatments aren’t coming fast enough. Chronic itching is becoming more common in the population as we age. One of the reasons for this is the dry skin that often exists in older adults. But Dr. Yancey Spovich believes that damage to certain nerves in the skin may also cause itching – these nerves are originally responsible for transmitting pain and suppressing itching, and when they go awry, “the itching pops up,” he says.
Aging monkeys provide some clues to this. Back when Dr. Yancey Spovich was at Wake Forest University, he and his colleagues found that older female macaques scratched their backs and lower limbs – the same areas that often itch in older people. They sent samples of these monkeys to Dr. Chen’s lab. Dr. Chen and his team then found that the gene expressing the itch receptor GRPR was overactive in the skin and spinal cord of these monkeys. Dr. Chen said it is not clear why the gene tends to become active with aging, but this finding in primates supports the idea that the receptor is a good target for developing new drugs to treat itch in humans.
Many older adults have trouble with the itchy area located just below or between the shoulder blades on their backs, making it difficult for their hands to reach. “It’s driving them crazy,” Dr. Cornelius said while at the University of Washington. They have to rub their bodies against door frames, acquire itchy scratches in bulk and ask others to help them itch.
The disease is called sensory abnormal back pain (totalgiaparesthetica), and it is usually associated with problems in the spine and intervertebral discs that compress or damage nerves. There is a chance that the skin tone at the itchy spots may darken. “Some neurologists, and arguably most of them, don’t know this,” says Dr. Jan Spovich. He and other doctors have prescribed a variety of treatments: anti-itch patches, sometimes supplemented with capsaicin, a component of chili peppers; injections of botulinum toxin (Botox); agents that affect nerve conduction, such as gabapentin; and physical therapy, in an attempt to improve itching symptoms. But basically none of them work.
Dr. Yancey Spovich said many patients have come to them after visiting numerous doctors who said they couldn’t help (some of them even misdiagnosed physical itching as a mental illness). “They’re certainly not brain-dead,” he says. One of the patients, a boy, scratched his arms and legs until they broke the skin. Doctors could not find the cause of the disease, and there is no effective treatment, so he was referred to a psychiatrist. The patient named Joshua Riegel (JoshuaRiegel) is now 18 years old, he said in an interview: “They all say I do this on purpose, in order to make fun of my parents.” From that point on, what he calls “one of the most bizarre experiences of my life as a psychopath” began. When he was 12 or 13 years old, his psychiatrist kept prescribing him antidepressants, which he obediently took for two or three years. But these drugs had terrible side effects: at one point he had to be hospitalized because of suicidal thoughts. With a last hope, his parents took him away from his home in Hillsville, Virginia, to visit Dr. Spovich Young, who was working there at the time, at Wake Forest University. “He seemed to know intuitively what was wrong,” Mr. Riegel said.
After tests, researchers found he had a very rare genetic condition called epidermolysis bullosa, which can cause a devastating array of symptoms: intense itching and skin so extremely fragile that even scratching can cut the skin.
Mr. Riegel recalls that after stopping his antidepressants, his spirits were revived and he returned to normal. Since then, his doctor has prescribed other medications to treat the itching, with mixed results. Although he never really got rid of the itching, Mr. Riegel was able to distract himself with the help of computer games or his cell phone to avoid scratching himself. For patients suffering from other types of chronic itching, Dr. Yancey Spovich says, “This great era is just kicking off. Targeted drugs specifically for pruritus are expected to be available within the next five years. We are standing on the tip of the iceberg.”