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While chronic stress is bullying the hippocampus—pruning its dendrites, killing its neurons, and preventing neurogenesis—the amygdala is having a field day. The stress overload creates more connections in the amygdala, which keeps firing and calling for cortisol, even though there’s plenty of the hormone available, and the negative situation feeds on itself. The more the amygdala fires, the stronger it gets. Eventually the amygdala takes control of its partnership with the hippocampus, repressing the context—and thus the connection to reality—and branding the memory with fear. The stress becomes generalized, and the feeling becomes a free-floating sense of fear that morphs into anxiety. It’s as if everything is a stressor, and this colors perception and leads to even more stress.
Regular aerobic activity calms the body, so that it can handle more stress before the serious response involving heart rate and stress hormones kicks in. It raises the trigger point of the physical reaction. In the brain, the mild stress of exercise fortifies the infrastructure of our nerve cells by activating genes to produce certain proteins that protect the cells against damage and disease. So it also raises our neurons’ stress threshold.
“The mind is so powerful that we can set off the [stress] response just by imagining ourselves in a threatening situation,” writes Rockefeller University neuroscientist Bruce McEwen in his book The End of Stress as We Know It. In other words, we can think ourselves into a frenzy.
This is where the ripple effects of the body’s stress response can lead to full-blown mental disorders such as anxiety and depression, as well as high blood pressure, heart problems, and cancer. Chronic stress can even tear at the architecture of the brain.