Why do you keep getting hotter when you pick your feet? Do you want to smell it after you pick your feet?

A few days ago, the news that a certain celebrity "picked his toes and then smelled his hands" became a hot topic, triggering widespread attention and discussion.

Many netizens said in their comments that it was no big deal, as they often pick their feet in private and can't help but smell it.

Indeed, when you are at home wearing slippers and lying on a chair, feeling free as if the world is at your fingertips, you will start picking your feet unconsciously, and the more you pick, the more pleasurable it becomes, and you can't stop.

When they are excited, not only do they put their hands under their noses to smell it, but sometimes they even lie down on their feet to smell it.

Why is there such a magical power in picking your feet? Why can't you resist smelling them after picking them? Let's talk about it in detail today.

01

Why does picking your feet feel so good?

This actually has to do with the brain's reward mechanism.

We know that the richer the nerves in a part and the more nerve endings it has, the more stimulation signals it can receive and transmit these signals to the brain, making us more sensitive to these stimulations.

The nerve distribution in the feet is very dense, with more than 200,000 nerve endings on the sole of the foot alone. The brain can quickly sense any disturbance, such as itching, pain, or soreness, and make judgments and take action.

When the feet feel itchy, the brain will first quickly feel discomfort and disgust, and then control the body to react, such as scratching with the hands.

Every time you grab, scratch, pick, or pinch your feet with your hands, the nerve endings in your feet will be rubbed and stimulated at the same time. Your brain will immediately feel a comfortable and pleasant feeling, and before you know it, you will get addicted and fall into a cycle of the more you scratch, the more itchy it becomes - and the more itchy it becomes, the more you want to scratch.

A study published in Neuron in 2019 described this process in detail: when people scratch, dopaminergic neurons in the ventral tegmental area of ​​the midbrain are activated, releasing dopamine, thereby activating the brain's reward mechanism.

The brain's reward system is mainly composed of the ventral tegmental area, nucleus accumbens and amygdala in the midbrain. It also involves the prefrontal cortex, hippocampus and other brain areas closely related to emotions, learning and memory.

Usually when the actual reward or reward is higher than the psychological expectation, dopamine will be secreted in large quantities.

For example, behaviors such as eating melon seeds, admiring handsome men and beautiful women, and eating sweets, because the brain expects that they can easily get "instant satisfaction", give the brain a "reward" beyond expectations, and promote the secretion of dopamine.

The further secretion of dopamine drives us to continue doing the same thing and continue to get satisfaction.

Back to the behavior of "picking our feet", when we pick our feet, we actually start the positive feedback mechanism of "scratching - comfortable - continue scratching", so we can't help but want to scratch our feet constantly, and the more we pick, the more comfortable it feels.

Some people say that when they pick their feet, they also peel off some dead skin, and they can't stop. This is the same principle as "the more you pick your feet, the more pleasurable it is."

As for "you don't feel any pain when peeling the skin off your feet", it may be because peeling the skin off your feet will further stimulate nerve endings and trigger the release of endorphins.

Endorphins have analgesic, antidepressant and pleasurable effects, and can also bind to opioid receptors in the brain to reduce pain perception.

02

Why do you want to smell your feet after picking them?

At this point, many people may ask: It is understandable that picking your feet feels so good and you can’t stop, but your feet are so smelly, why can’t you help but want to smell them after picking them ?

Indeed, most people's feet are smelly. There are more than 250,000 sweat glands on our feet. They are covered and can produce a lot of sweat every day. Coupled with some nutritious dandruff, the feet are very suitable for various microorganisms to survive and reproduce.

These bacteria, fungi, etc. decompose sweat or dandruff on the feet, releasing some pungent gases in the process, such as methanethiol, isovaleric acid, etc.

Although this smell is not very pleasant, it usually smells very special and may be slightly different each time, such as the smell of expired cheese, sour smell, rotten smell, etc.

These strange smells can easily arouse people's curiosity . Of course, human curiosity also has its limits. For example, almost no one would be happy to smell someone else's stinky feet.

But what’s interesting is that people’s tolerance for their own smelly feet has greatly increased, and they may not even feel that they stink .

This is because the human sense of smell is highly adaptable, and when one smells a certain odor frequently, one may not be able to sense it due to adaptation or tolerance.

An article published in Neuron in 2020 pointed out that peripheral glial cells can directly sense environmental odor stimuli and inhibit the activity of olfactory neurons in real time through GABA neurotransmitters, resulting in a period of "olfactory failure."

This is also why people with smelly feet often fail to realize that their feet stink, even though they make others feel sick with the stink. They even think aggrievedly, “ My feet are not that smelly, don’t exaggerate .”

Finally, I would like to remind you that although picking your feet feels good for a while, the following two groups of people should try not to do it:

1. People suffering from foot diseases such as athlete's foot and onychomycosis

2. People whose feet sweat a lot and don’t like to wash their feet

After all, this irresistible smell is mostly produced by the decomposition of sweat, dandruff, etc. by bacteria and fungi. If you pick your hands too much, it is inevitable that some of it will be contaminated on your hands. If you don't pay special attention to hygiene, there is still a risk of infection.

References:

[1] Su, X.-Y., et al., Central Processing of Itch in the Midbrain Reward Center. Neuron, 2019.

[2] Kanda F.,et al.Elucidation of chemical compounds responsible for foot malodour. Br J Dermatol.1990 Jun;122(6):771-6.

[3] Hughes RN. Neoticpreferences in laboratory rodents: issues, assessment and substrates. NeurosciBiobehav Rev. 2007;31(3):441-64.

[4] Duan D., et al.,Sensory Glia Detect Repulsive Odorants and Drive Olfactory Adaptation. Neuron.2020 Nov 25;108(4):707-721.e8.

Author: Ding Yu, PhD in Neurobiology, Zhejiang University

Review | Tao Ning Associate Researcher, Institute of Biophysics, Chinese Academy of Sciences