Why does the hair stand on end when people are afraid?

Because this is caused by a central nervous reflex phenomenon, the nerves in the brain are stimulated and transmitted to the skin and hair surface through nerves.

This is a "stress reaction"! In human body, there is a stress axis: hypothalamus-pituitary-adrenal cortex axis. When you encounter cold and fear, it begins! Release a lot of adrenocortical hormone. This hormone has a "permissive effect" on adrenomedullin, which can increase adrenoceptor and enhance its sensitivity! There are "mane muscles" around human hair to receive adrenaline. Control, let the "vertical hair muscles" get excited and stand up your hair!

This is an experience that everyone has had. When the weather is cold and the clothes are not warm enough, you will not only feel cold, but also tremble all over. There is a piece of goose bumps on the skin. This is because human skin can not only protect our internal organs and sweat, but also regulate and maintain body temperature. When the skin is stimulated by cold, the sensory cells under the skin will immediately inform the brain that people feel cold, and at the same time, the mane muscles under the hair will contract, making the hair vertical. At this time, the skin surface becomes very tight, forming a protective wall to prevent heat loss in the body.

When the vertical hair muscle contracts, it will pull the hair roots and make the hair stand upright. If you want to straighten your hair, you will pick out a piece of skin and form a small pimple, which looks like goose bumps without hair, so it is called goose bumps. Goose bumps are a sign of skin self-defense. It reminds people to "keep warm and cool". At this time, warm measures should be taken immediately to avoid catching a cold.

In short, when the skin is stimulated or frightened by cold, the sensory cells under the skin will immediately inform the brain, making people feel cold. At the same time, they also make the mane muscles under the hair contract and make the hair vertical. When the vertical hair muscle contracts, it will pull the hair roots and make the hair stand upright.