Why is tickling contagious?

Produced by: Science Popularization China

Author: Ishimiya

Producer: China Science Expo

In daily life, when we see others yawning or laughing, we may unconsciously be "infected" and start doing the same actions.

Scratching is more contagious than the above situations. In daily life, more than 60% of people will scratch if they see others scratching.

Are you really itchy at this time?

Why does it itch?

Itch is a feeling that needs to be relieved by scratching after the skin is irritated. On average, each person will feel itchy more than ten times a day. There are different classifications according to the different causes of itching.

Chemical Itch

Chemical itch is the itchy feeling caused by external stimuli on the skin, such as mosquito bites, skin allergies, etc. To be precise, when chemical itch occurs, the body will have corresponding inflammation and immune responses, which are mediated by a variety of itch-related receptors in the body.

When we are bitten by mosquitoes, they leave their "saliva" on the skin. The main component is formic acid, which is an anticoagulant that prevents blood clotting. Formic acid can cause allergies in the human body. At this time, the human immune system will secrete histamine, which will promote capillary dilation and increase local blood flow. Therefore, red and swollen papules will appear after being bitten by mosquitoes. At the same time, histamine will also stimulate itching-related neurons, causing itching. The medicine we apply after the mosquito bite causes an allergic reaction is to relieve the itching of the skin through antihistamines.

Arm after mosquito bite

(Photo source: veer photo gallery)

However, the itching caused by allergic dermatitis caused by wearing new clothes, applying cosmetics, or contacting pollen, catkins, etc. has nothing to do with the production of histamine. This type of itching is caused by the activation of mast cells due to the specifically expressed G protein-coupled receptor (MrgprX2) in the body. When this type of allergic reaction occurs, taking antihistamines may not help.

Allergy to cosmetics

(Photo source: veer photo gallery)

Mechanical itch

Mechanical itch is an itch caused by external stimuli, such as scratching, feathers brushing across the soles of feet, etc. When the skin is slightly mechanically stimulated, the skin transmits this stimulation signal to the brain through neurons, and we will feel itchy.

Tickle

(Photo source: veer photo gallery)

Contagious Itch

When you see others scratching or see a caterpillar crawling on your body, you will also feel itchy for no reason. The brain regulates itself to produce itch signals. This situation is contagious itching, which is why we will scratch when we see others scratching.

Caterpillar

(Photo source: veer photo gallery)

How did this "contagion" occur?

Scientists have given a reasonable explanation for the "contagious" behavior of tickling.

British researchers first asked volunteers to fill out a personality questionnaire, then watch a video of someone scratching themselves, and then observed the volunteers' behavior. In the end, they found that more than two-thirds of the people followed the video and scratched themselves. Comparing the personality questionnaire, they found that those who were infected by scratching had more negative emotions. Finally, the researchers studied the connections between different parts of the brain and found that infectious itching is closely related to neuroticism, which controls one's own emotional stability. People with high neuroticism are more likely to have high emotional reactions, more likely to have negative emotions, and more likely to perceive stress, so they are more likely to be infected.

Negative emotions

(Photo source: veer photo gallery)

Recently, researchers used mice to further explore the cause of contagious itch, and the relevant results were published in the journal Cell Communications on October 4. The researchers found that when mice were tickled by other mice, the suprachiasmatic nucleus (SCN) in the brain would light up, and the SCN happened to be the midpoint between the cells in the retina that achieve light capture and the paraventricular nucleus of the thalamus in the center of the brain. In order to explore this mechanism, the researchers injected a virus into the SCN in the mouse brain, which can infect target neurons in the SCN and inhibit neurons in the retina. Then, the infected behavior of the mice injected with the virus was observed again. The results showed that when the neurons in the retina were inhibited, the behavior of the mice scratching when infected was significantly inhibited. It can be seen that the neurons in the retina that achieve light capture are involved in the process of contagious itch. After light exposure, the photosensitive ganglion releases a chemical (pituitary adenylate cyclase activating peptide) to the SCN, which triggers the behavior of scratching. It can be seen that contagious itch is regulated by unconscious behavior and is more like a conditioned reflex for animals to protect themselves from harm. However, human brain circuits are more complex than those of mice, and whether this explanation of behavioral mechanism is applicable to humans requires further research.

Mouse scratching

(Photo source: veer photo gallery)

How to control itching?

There are many ways to control itching, and we will mainly discuss a few of them below.

Pain can suppress itching

Although "pain" and "itch" are two completely different feelings, they have many of the same neuronal activation areas in the brain. So when the skin itches, we can suppress it by producing pain. This is why pinching the mosquito bite with your fingernails can relieve the itch.

pinch

(Photo source: veer photo gallery)

Cold Control Itch

When the skin is red, swollen and itchy, you can also use a cold towel to apply cold compresses to relieve the itching.

Cold compress

(Photo source: veer photo gallery)

Apply medication to relieve itching

If you are allergic or your skin itches are unbearable, you can use anti-inflammatory treatments such as medication or anti-itch sprays.

Taking medicine

(Photo source: veer photo gallery)

Conclusion

Itch is a self-protection mechanism for us and an indispensable feeling. When you face inexplicable itching, remember to go to the hospital to see a doctor!

Editor: Sun Chenyu

References:

Chen, Zhou-Feng. A neuropeptide code for itch. Nat Rev Neurosci. 2021;22(12):758-776.

Meixiong, James; Anderson, Michael; Limjunyawong, Nathachit, et al. Activation of Mast-Cell-Expressed Mas-Related G-Protein-Coupled Receptors Drives Non-histaminergic Itch. Immunity, 2019, 50, 1163.

Yu, Yao-Qing; Barry, Devin M.; Hao, Yan, et al. Molecular and neural basis of contagious itch behavior in mice. Science, 2017, 355, 6329.

Gao, Fang; Ma, Jun; Yu, Yao-Qing, et al. A non-canonical retina-ipRGCs-SCN-PVT visual pathway for mediating contagious itch behavior. Cell Reports, 2022, 41, 111444.

Steele, Haley R.; Xing, Yanyan; Zhu, Yuyan, et al. MrgprC11(+) sensory neurons mediate glabrous skin itch. PNAS, 2021, 118, e2022874118.