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In November 2002, Michelle “Micki” Glenn was snorkeling off French Cay, about 160 miles northeast of Cuba, when she noticed a seven-foot female reef shark just beneath her fins. It glided upward, brushed gently against her leg, and aligned its body with hers until the two were side by side, staring at each other. “The hair was standing up on the back of my neck,” says Glenn, 50, an avid diver and lifelong lover of animals. “I thought I was the luckiest person ever.” The shark flicked its tail and glided away.

Just as she let out a breath of relief, Glenn felt a powerful surge of water hit her side as the shark flipped around. Then it took her in its mouth, its upper jaw slicing into her back like a razor; the lower jaw ripped away half of her armpit. Just as suddenly, the shark planed away into the darkness.

As Glenn began thrashing toward the boat, the only thing she could see in the billowing red cloud of blood trailing behind her was a white, chalky creature jerking along. It was her hand.

Six surgeries and 14 units of blood later, doctors had rebuilt Glenn’s upper arm and back. But the physical recovery turned out to be the least harrowing part of her ordeal. While in the hospital, Glenn kept witnessing the walls transform into a bloody sea. For months on end, she suffered recurring, full-blown flashbacks, hallucinating about sharks. Most nights she woke up screaming. “I was reliving the whole thing over and over,” she says.

Her experience was not uncommon. The years following a survival episode—an animal attack or, say, a devastating tornado—can be immensely damaging to survivors, who suffer the same post-traumatic stress symptoms that afflicts some soldiers. And new research is revealing fascinating truths about how we endure, showing that extreme trauma actually changes the physical makeup of the brain.

It turns out that when a person survives an extreme situation, his or her brain draws an instantaneous link between circumstances surrounding that event and the emotional stimuli that trigger the fear response. “Evolutionarily speaking, these kinds of associations help us anticipate harm,” says Joseph LeDoux, a neuroscientist at New York University. “But sometimes they also end up tying us up in knots.”

HERE’S HOW IT WORKS: The brain constantly records memories on several different tracks. The conscious track allows you to remember episodic and semantic memories—those you can picture (like your bedroom) and those you have learned (the U.S. has 50 states). But there are also unconscious memories, which LeDoux calls emotional learning. Those memories might give you a feeling of discomfort when you remember the sound of the dentist’s drill. When you survive a traumatic event, this emotional learning kicks into overdrive—your memories tell you to respond in the future the way you responded in the past. You’re alive, therefore whatever you did during the event worked.

The other effect of a traumatic event is that the amygdala—the part of your brain that controls your emotional responses—is triggered. Whenever something stress­ful happens, from a stubbed toe to a ­minor earthquake, your amygdala activates your auto­­nomic and endocrine systems, releas­­ing stress hormones and neurotransmitters such as norepinephrine and glucocorticoids. Normally this is a good thing: those chemicals usually persist in your body for minutes at a time, allowing you to react quickly to harm. But extreme events can leave you permanently primed, with raging stress hormones. High levels of norepinephrine and glucocorticoids can con­tribute to PTSD and depression.

The question is, how do you unprime your brain? “Work, work, work,” writes Richard Mollica, a professor of psychiatry who runs the Harvard Program in Refugee Trauma. “This is the single most important goal of traumatized people throughout the world.” The brain acts like a seesaw: if emotion is too high, you can’t think straight, and vice versa. Working the logical part of the brain—the frontal lobes of the cerebral cortex—takes the pressure off the amygdala. Performing a logical task puts the brakes on your emotions, and the satisfaction from completing those tasks releases dopamine, the neurotransmitter responsible for feelings of motivation and reward. Don McCall, 86, who survived eight battles in the Pacific while serving in the U.S. Navy, as well as the sinking of the USS Indianapolis in 1945, worked virtually nonstop as a bricklayer for more than 40 years to cope with his PTSD. Today he says, “I enjoy my life. I really do.”

For Glenn, images of sharks set off panic attacks. LeDoux likes to use the example of an auto accident: survivors of bad wrecks often freak out when they hear car horns. For survivors of the recent tsunami in Japan, a big wave might cause some serious inner turmoil. For the residents of Joplin, Missouri, it could be a windy day. Psychologists call these triggers retrieval cues. But they can be overcome. Glenn’s husband, Mike, put a photograph of a shark on her computer so that she saw it every day. Gradually, by repetition, the image became attached not to panic but to the feeling of entering an office. Glenn extinguished her response to the shark by writing a new memory track.

Another key to recovering from trauma is having fun. For Glenn, an avid equestrian, this meant literally getting back in the saddle. Three weeks after the attack, she sat on a horse. “That did more for me emotionally than anything at that time,” she said. By holding the reins, Glenn was triggering joyous memories. It’s difficult to feel joy and panic at the same time: horses were in, so sharks were out.

Competition helps, too. It’s a well-established fact that humans, being hardwired to scrap for everything from food to mates, get an emotional high anytime they can whip someone else. About four months after the attack, Glenn competed in a horse show. “We smoked our competition,” she told me. But you don’t have to win. Testing yourself physically and achieving a personal goal releases dopamine. It was no mere publicity stunt when Edison Peña, one of the men who were trapped in Chile’s San Jose gold-and- copper mine for 69 days, ran the New York City Marathon last November.

WHILE THERE’S NO foolproof recipe for over­coming trauma, most experts agree that the majority of survivors can live productive and satisfying lives by facing their fears, having fun, laughing whenever possible (a proven stress reducer), and helping ­others. In fact, many do so without ever seeing a therapist or taking medication. George Bonanno, a psychologist at Columbia University and one of the leading investigators of resiliency, estimates that 85 percent of survivors do just fine without any professional help.

As for Micki Glenn? Two years after the shark attack, she returned to the Caribbean. For two days she sat on the shore of Dominica, practicing putting her face in the water. It wasn't easy. “The first day that I actually went to dive, I threw up my breakfast,” she said. But eventually, she was able to work through the fear. The last time I spoke with her, she was considering a scuba trip to Australia.