When the gentle breeze blows across our cheeks, we can feel coziness and comfort; when the frigid wind blows across our skin, we can feel the chill and the cold again. When the raindrops fall on our hair, we can feel the footsteps of spring; when the snowflakes fall on our palms, we can feel the sincerity of winter.
Our ability to sense temperature and touch is vital to survival, and it underpins our interactions with the world around us. For thousands of years, one of the big puzzles facing humanity has been how we perceive our environment. In the 17th century, the philosopher Rene Descartes envisioned wires connecting different parts of the skin to the brain, a mechanism through which feet touching an open flame would send mechanical signals to the brain. Later discoveries showed that the human body has specific sensory neurons that sense changes in our surroundings.
Scientists Joseph Erlanger and Herbert Gasser discovered that there are different types of sensory nerve fibers in the body that respond to different stimuli, such as pain and non-painful touch. Nobel Prize in Physiology or Medicine. From a neural level, receptors that perceive external stimuli are the biological basis of the five human sensory systems. Its operation process can be colloquially understood as “receiving stimuli → transmitting signals → the brain receives signals and responds”. This process requires the help of a class of structures called “receptors.” They are activated under certain circumstances, as if a lock requires a certain key to open.
However, until the discoveries of the 2021 Nobel Prize winners in physiology or medicine, David Julius and Arden Pataptien, there is still a gap in our understanding of how the nervous system perceives the environment: how temperature and touch Converted to electrical impulses in the nervous system?
There is a complex interplay between the senses and the environment around them, and David Julius and Arden Pataputian have identified the crucial missing link in understanding this interaction. Their groundbreaking discovery has sparked research that has led to rapid advances in our understanding of the mechanisms by which the nervous system senses heat, cold, and mechanical stimuli. Of course, many studies based on this discovery are still underway, and the researchers hope that these research results can be applied to the treatment of many diseases in the future.
The pain of itching and the joy of scratching
”The pain of itching, the joy of scratching” has vivid examples in ancient and modern China and abroad. Du Mu, a great poet in the late Tang Dynasty, had a profound experience of scratching, and there is a poem to prove it: “Du Shihan’s pen is worried to read, like Qian Ma Gu scratching at itching.” The general idea of this poem is: reading beautiful articles is like beautiful The fairy, Magu, was as happy as tickling herself with her nimble, slender hands. According to legend, the fingers of the fairy Magu are as slender as chicken feet, also known as “Magu’s claws”. According to historical records: “When the back is very itchy, it is good to have this claw to climb the back.”
And the great French writer Montaigne believed that scratching the itch is one of the sweetest actions that can easily make people feel the sweetest satisfaction. That said, what else can be compared to a good thing like tickle, which can be done at any time, to satisfy oneself instantly?
However, the persistent and unstoppable itching is unbearable, even more uncomfortable than pain. As the saying goes: “The pain is unbearable, the itch is unbearable; the bitterness is unbearable, the acid is unbearable.” The great Renaissance poet Dante wrote in “The Divine Comedy Hell”: “The best way to punish forgers in hell is , that is, let them have scabies from head to toe, so itchy, they can only pinch their nails deeply into the flesh, but they can’t stop the itching!”
The French Emperor Napoleon in many portraits has an unusual and classic pose, which has attracted the interest of many researchers. His right hand is habitually inserted into his jacket, while his left is behind his back. After research, some researchers believe that Napoleon suffered from some kind of skin disease, such as scabies, dry, peeling and itchy skin, so his hands should be placed on his chest to facilitate scratching. This may be the origin of the famous “Napoleon’s Itch”.
meaning of itching
I itch, I scratch, therefore I am. But why is it itchy? What is itching? Why does it get more itchy? These are not only confusions for you and me, but also for scientists.
Animal skin is the body’s first barrier against external damage. Pain, touch and temperature are the most basic senses of animals. Because of pain, so hide; because of touch, so warmth; because of temperature sense, so know hot and cold.
With pain and touch, why is there still itching? From an evolutionary point of view, it is because of the need to survive. The ancient ancestors were naked and lived in the virgin forests full of dangers. Every day they faced threats ranging from tigers, wolves and leopards to small mosquito bites. So, how should humans prevent mosquito bites? There must be an early warning method to remind the body that the body has been violated, which may be the origin of the itch. From this point of view, itch is a special alert and protective mechanism.
The relationship between itching and pain
Many people think that itching is not separate, that itching is just a mild form of pain. Chinese medicine interprets itching as a qi that roams the skin but is not strong enough to become pain. Studies have shown that people who are ticklish are also more afraid of pain. When it is itchy, I can’t help scratching it; when it hurts, I will shrink and avoid it conditioned reflexively. When it hurts, you dare not touch it. When you touch the sore spot, you jump your feet, but when you scratch the itch, the pleasure is beyond words.
Obviously, itching and pain are very different sensations. The sensory information of pain and itching is transmitted by roughly the same sensory nerve pathways. Itch receptors are found in nerve endings in the skin. The itch signal is transmitted to the central spinal cord through the dorsal root ganglia in the peripheral nervous system, and then from the spinal cord to the brain. But receptors on the skin that transmit both itchy and painful sensations have sparked debate. How do nerve cells and neural circuits differentiate which is itchy and which is pain? Are there any dedicated nerve cells in the nervous system that transmit itch? If so, what “signals” do these nerve cells use to exchange and transmit information?
itch gene and itch neurons discovered
For nearly a hundred years, scientists have been trying to find the genes and nerves that specifically transmit itch information, and there is a lot of debate about this. Many scientists do not believe that there are specific itch genes or nerve cells.
In 2007, this question finally had an unexpected answer. In an experiment, a research team from the University of Washington led by Professor Chen Zhoufeng found that among many genes, the expression of a gene called gastrointestinal releasing hormone receptor (GRPR) is very important for transmitting sensory information. At first, the researchers assumed that the gene was important for pain perception, but by removing the gene to knock out GRPR, the mice responded to pain stimuli completely normal, which overturned this assumption. In spite of their disappointment, the researchers were not discouraged. After they injected gastrointestinal-releasing hormone (GRP) into the spinal cords of mice, the mice immediately scratched. This unexpected surprise revealed the tip of the iceberg of the secret of itch. It can be seen that GRP or GRPR has nothing to do with pain, but is the first polypeptide and its receptor that specifically transmits “itch”.
After further experiments, the researchers found that killing the neurons in the spinal cord of mice that released GRPR, no matter what kind of itching substance was injected into the mice, the mice did not scratch it. This shows that mice without GRPR neurons completely lost the sense of itch. The mice that lost the sense of itch, however, responded completely normal to various painful stimuli. These findings, for the first time, demonstrate that pain and itch can be separated at the molecular and cellular levels.
It can be seen that the mechanism of itching is as follows: when the skin comes into contact with a certain itching substance, it will cause a series of inflammatory reactions, resulting in the rapid release of GRP and activation of its receptor GRPR at the central terminals of dorsal root neurons. The information is quickly transmitted to the brain through the itch neurons in the spinal cord, and the brain sends corresponding commands to guide the hand to scratch the itch where it is felt.
Is it drinking poison to quench thirst with pain and itching?
Although itch and pain are transmitted by different spinal nerve cells, pain can inhibit itch. Usually when acute itching occurs, the mild pain caused by a little scratching is enough to relieve the itch. That is to say, the purpose of scratching is to create pain and relieve itching with pain. In fact, everyone has a lot of similar experiences in life. Being bitten by a mosquito makes it unbearably itchy, but it doesn’t itch when it hurts. It is difficult to obtain the pleasure of “relieving itching with pain” by “scratching the itch across the boot”.
As for chronic itching, such as skin allergies, psoriasis, eczema under the crotch, and athlete’s foot, although scratching can temporarily relieve the itching, it will soon become more itchy, and the more itchy, the harder it is to scratch. In this case, pain relief can only have the opposite effect. One possible explanation for this is that the pain caused by scratching will activate the brain’s analgesic system, but the neurotransmitters that inhibit pain may also stimulate the transmission of itching information at the same time. The more itching, the more vicious circle is formed.
Itching is a mental illness
Itching, the body will feel uncomfortable. This discomfort is not only physical, but also psychological. The so-called “temporary itchiness” or “temporary itchiness” both describe the intolerable and torment of itching.
Some people scratch their scalp when they can’t answer a question; when someone they know tickles your armpit, you ticklish and laugh; when you touch the sole of your foot, you ticklish and laugh; Scratching with your own hands is much more enjoyable… These are all due to psychological factors at work.
Tickling is as contagious as yawning. When you see someone yawning, you yawn too, and when you see someone tickling, you can’t help but scratch. The influence of chronic itching and psychological factors is even greater. Often such patients will toss and turn, sleepless at night, and extremely painful.
The discovery of itch genes and itch neurons does not mean that all the secrets of itch have been solved. On the contrary, this is just the first step in a long march. There are more questions that need to be investigated in more depth, such as, how did the itch signal form in the first place? How is the itch message transmitted from the spinal cord to the brain? How does the brain differentiate between itching and pain? Are there neurons in the brain that are specifically responsible for transmitting itch information? How do different diseases activate itch receptors? Cold water can relieve itching, but hot water can make it worse, why?
Pain can make you unforgettable; itch can make you think about it. Itching and pain are important indicators to feel the existence of life, and it is fascinating and interesting to study their occurrence and transmission. Our scientific understanding of itch is just beginning…