Spark: The Revolutionary New Science of Exercise and the Brain

by John J. Ratey

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. “The animal becomes more anxious even while its cognitive skills are being eroded,” says McEwen.