Which part of our body is the "itchy spot"? Why did evolution leave this human weakness?

Perhaps, people in the world can be classified like this: those with "itchy spots" and those without "itchy spots".

If you just poke the waist of the former or touch the soles of their feet with a feather, they will roll over with laughter and beg for mercy; as for the latter, you can use all your tricks and they will remain calm and unmoved. So, which category do you belong to? Is the ticklish spot really a piece of meat?

(Photo source: Internet)

Before we figure out what ticklish skin is, we have to mention the two brothers, itch and pain. These two senses of touch are actually very similar, and both serve as warnings to the human body , thereby avoiding further harassment and harm from the outside world; there are also many overlaps in the brain areas activated by itch and pain ; many neuroactive substances involved in the transmission of pain information are also involved in the itch response (for example, calcitonin gene-related peptide-positive neurons); some patients who are congenitally lacking in pain cannot feel itchy . Therefore, for a long time, people believed that itch was a type of pain. It was not until 2007 that Sun Yangang's team discovered that gastrin-releasing peptide (GRP) was a specific receptor for itch based on the scratching behavior of mice after injection of gastrin-releasing peptide (GRP).

Later, they knocked out the specific receptor gene in the mice and found that no matter what kind of itching drug was injected into the mice lacking this gene, it would not cause a scratching reaction. The mice that lost the sense of itch responded normally to painful stimuli. This was the first time that it was proved at the cellular and molecular level that pain and itch are two different things and that itch has its own signal transduction pathway .

The team dug deeper and deeper into the study of itch and found that itch is highly correlated with the human brain's perception of emotions , which further explains why when we feel itchy, we also feel a bad mood.

In 2019, the team used mice as a model, used histamine or chloroquine to create an itchy feeling in the mice, and recorded the changes in neuronal activity in the gray matter area around the aqueduct of their brains during the "scratching behavior". The study found that a group of neurons in the gray matter area around the aqueduct of the mouse brain is the key to regulating the spontaneous behavior of "itch-scratching" .

When scratching, the scratching of nails and skin will make our skin feel pain below the pain threshold. This mild pain can temporarily distract the brain from the itch stimulation by inhibiting the transmission of surrounding itch signals . Distracting attention alone is far from enough. In 2014, a Japanese research team used magnetic resonance imaging technology to observe the brain activity of people when they were scratching, and found that when people are scratching, the brain will also release serotonin, known as the "happy hormone", to respond, making people feel happy .
However, the appearance of serotonin will reduce the slight pain sensation that has an antipruritic effect, and the brain will feel the itching stimulation again, which will trigger a new round of scratching and pain, producing more serotonin, and the cycle will repeat. Therefore, although scratching is comfortable, it not only cannot stop the itching, but also makes us more itchy . This is why there is only one itchy place on the body, but after scratching twice, the skin all over the body begins to itch. In daily life, when someone touches certain parts of our body, such as the neck, armpits, back, soles of the feet, or even the waist, it will stimulate obvious and strong itching, and then we will laugh. These parts are affectionately called " itchy meat " by us. The location of our "itchy meat" is mostly located in the main distribution area of ​​the human body's lymph and nerves. These nerves are directly connected to our brain nerves, so they are particularly sensitive to external stimuli . Our body is always predicting what will happen next, and adjusting our own senses accordingly according to what is predicted to happen. The "itchy meat" formed in the long process of evolution is a good helper for the body to predict danger, which can help humans survive tenaciously in a complex environment.

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Why do we not feel itchy when we tickle our own "ticklish spots", but react strongly when others tickle us? Sarah-Jayne Blakemore, a researcher at the Institute of Cognitive Neuroscience at University College London, conducted an experiment on five volunteer subjects. She first used a machine to tickle the palms of the subjects' hands and used a magnetic resonance imaging device to record the changes in brain currents. Then she asked the subjects to tickle themselves and also recorded the changes in brain currents. The study found that if we scratch the ticklish spots on our bodies, the cerebellum will perceive that this is our "spontaneous behavior" and subconsciously dull the sensory stimulation imposed by ourselves, so that the body ignores this stimulation . This reaction was later called sensory attenuation by neuroscientists. If the person on the other side of the tickle is someone else (a machine in the experiment), even if we know in advance that we will be tickled, the cerebellum will not send a dulling signal, so people will feel extremely itchy. Some people are ticklish, while others are naturally ticklish. This may be because their nerve receptors are insensitive , or because the cerebellum sends out deceptive signals , suggesting that the external stimulus is actually caused by themselves, thereby raising their threshold to external stimuli. So why does tickling cause people to laugh? Scientists believe that laughter may be a form of self-protection designed for humans during the evolutionary process .

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That is, when the sensitive parts of the body are stimulated, the nervous system responsible for laughter will be activated, thereby releasing a "friendly signal" to the other party in order to reduce the risk of the other party hurting us. Those with very good itch reflexes can become nature's survivors , while those who have no reaction to itch or are slow to react are gradually eliminated. So over time, itch has become a human reaction.