1. Why Is Cutting Hair Painless but Touching It Noticeable?
When getting a haircut, you can hear the scissors, but there is no pain. However, if your hair is gently stroked with a finger or blown by the wind, you are aware of it. If hair were a part that felt nothing, you would not notice being touched. Conversely, if hair were a part that felt things, cutting it would hurt. You cannot explain both facts with just one description. The answer is that the "place being cut" and the "place feeling" are in different locations. The part being cut is the hair outside the skin, while the part feeling is the hair root inside the skin. First, let us look at what the cut part of the hair is made of.
2. What Are the Hair Shaft and Hair Root Made Of?
Hair is divided into the part outside the skin and the part inside the skin. The outside part is called the hair shaft, and the inside part is the hair root [1]. Scissors at a salon cut the hair shaft. Looking at a cross-section of the shaft, it has three layers from the inside out: the medulla, cortex, and cuticle. The cortex undergoes keratinization and becomes hard keratin [1]. Anatomy textbooks explain that visible hair is made of dead, keratinized cells, so it has no sensation [2]. Since you are cutting something with no sensation, it is natural that no pain is produced. However, this is only half the story of "hair feeling nothing." The mechanism for feeling is inside the skin. To find that location, we must look at where the dead cells of the hair are made.
3. How Does the Hair Root Make Dead Hair Cells?
At the base of the hair, there is a small part called the dermal papilla. Surrounding it in a single row are the matrix cells. When these matrix cells divide and multiply, new hair and the cells covering it are born and pushed upward [1]. As they are pushed up, the cells undergo keratinization, becoming hard and dead. The hair visible from the outside is a column of these "pushed-out, dead cells." Hair grows as living cells divide below and dead cells build up above. Head hair in the growth phase is said to grow 0.3 to 0.5 mm per day [1]. If it grows every day without rest, that is 9 to 15 mm in one month (30 days) and about 11 to 18 cm in one year. There are said to be about 100,000 head hairs [1]. Here we understand one thing: scissors only reach the part that is already dead. The cells that are alive and dividing are inside the skin, so they are not cut during a haircut. So, what else is happening at the root inside the skin?
4. How Do Nerve Endings at the Hair Root Sense Hair Movement?
At the base of the hair, there are endings of sensory nerves, which are described in textbooks as being very sensitive to air movement and the feeling of objects touching them [2]. Japanese research points in the same direction. Human body hair follicles have a sensory mechanism called the follicle receptor, and its nerves are classified as rapidly adapting, meaning they react quickly to changes. There are reports that stimulating a single follicle receptor with electricity produced a sensation of vibration [3]. This study also measured the minimum strength of vibration applied directly to hair that humans could feel [3]. The sensation is detected at the hair root. You can think of the hair as a "lever" that transmits the force of touch to the root, and the root as a "switch" that receives it. However, this is just an analogy; there is no mechanism inside the hair itself. In reality, nerves around the follicle receive the movement of the hair. Returning to the haircut, if the hair is pulled or shaken and that movement reaches the root, you might feel something. However, this article does not discuss experiments that directly investigated the act of cutting with scissors.
5. Besides Nerves, What Else Helps the Hair Root Detect Touch?
The job of hair is not just to report touch. According to dermatology textbooks, the hair organ helps sensory nerves detect touch, but also protects the head from external force and light, and helps regulate temperature [1]. Regarding touch, in 2018, RIKEN and others announced that epidermal stem cells in the follicle surrounding the hair adjust the shape and function of sensory nerve endings connected to tactile receptors [4]. Epidermal stem cells were known for supporting skin regeneration, but this discovery shows they are also involved in touch. However, this was research on animals like mice and cannot be directly applied to humans [4]. Animal whiskers have a different structure from human body hair. Whisker roots have many nerve fibers and blood vessels, and cat whiskers are said to work as tactile sensors [3]. Do not think they work the same way as human hair. So, how much do human hair sensors work in daily life? Let us check with our own bodies.
6. How Can You Find the Hair Root's Touch Sensor by Blowing on Arm Hair?
There is an experiment you can do with things you have at home. Try this: gently blow air from your mouth onto the hair on your arm. Closing your eyes makes the difference easier to notice. If you feel "something touching" only while the hair is moving, the mechanism at the root may be receiving the hair's movement. However, since the breath also hits the skin, this method alone does not tell you how much the hair root is involved. Try it on different parts of your arm. Does it feel different in places with more or less hair? The palm also feels things through the skin, so it is not a perfect comparison for hair presence alone, but observing differences in feeling is one method. Results vary by person and location, so write down your own feelings as they are. If you want to know more, read the textbook diagrams from Hokkaido University Dermatology and the RIKEN press release listed in the sources below to learn more about the relationship between hair and touch.