You most likely have noticed this phenomenon. When someone you know gets critically injured, experiences a serious illness or a major stressor, they soon look older. Sound familiar? Next time you see your friend, their hair is gray or even white. Have you ever wondered why that happens?

A new study from scientists at Harvard now reveals our sympathetic nervous system likely plays a major role in this process. The sympathetic nervous system is also called the “fight or flight” system. When you’re in a stressful or dangerous situation, the sympathetic nervous system releases hormones to increase your alert level and heart rate to prepare you to act.

It all starts in a small area of the brain called the amygdala, and it eventually activates adrenal glands in the kidneys, which release adrenaline to coordinate the response of your internal organs and heighten your mental focus.

We have stem cells in the bottom of each hair follicle. These stem cells mature into cells called melanocytes, which produce pigment for our hair and skin color. Using mice, researchers found stressful events can lead to damage in the stem cells in our hair follicles.

Stress caused these stem cells to mature much faster than normal, which meant the supply of stem cells in the hair follicles eventually ran out. Without the stem cells producing melanocytes, the hair in the follicle lost its color and looked gray.

But how did this happen? The researchers thought maybe the immune system attacked these melanocytes resulting in the graying or that a hormone called cortisol produced by the adrenal glands might be responsible. Neither of those turned out to be the cause of the graying hair.

In some intricate experiments, the scientists discovered sympathetic nerve cells associated with hair follicles were the key. These nerve cells released a hormone called noradrenaline into the melanocyte stem cells, which caused them to mature and eventually disappear from the base of the follicle and caused the graying of the hair.

In other experiments, noradrenaline treatment of human melanocytes in lab cultures lead to rapid growth, suggesting this same process likely occurs in humans leading to the graying of hair.

Other scientists also have shown depleting melanocytes leads to a progression of the first gray hairs to gray and then to white fur in dark-colored mice, which sounds like what we see in humans.

So here’s a question: If you have rapid growth of these melanocytes, can you prevent the hair from graying? The answer appears to be yes, based on the treatment of these same mice with drugs that block the release of noradrenaline.

This treatment couldn’t be used in humans because blocking the “flight or flight” response would have severe consequences in other areas of our lives. But this new information could be used in the future for more specific drugs that would just work on the hair follicles. This would be a great benefit to me — if I only had enough hair.

Medical Discovery News is hosted by professors Norbert Herzog at Quinnipiac University, and David Niesel of the University of Texas Medical Branch. Learn more at

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