Watch Dr. Oz's anatomy lesson.
It starts in the pituitary gland and hypothalamus, which he calls the mind-body connection, or where the brain and body intersect. When the hypothalamus and pituitary are stimulated, they activate the adrenal gland, which releases chemicals into the body that rev the body for a "fight or flight" action.
How does the body calm down after that? "The body always has an antidote to a problem," Dr. Oz says. The way the body balances stress is with the vagus nerve. "This nerve sends branches to the heart, the lungs, the intestinal tract. And what it's saying is it can calm you down. We never knew until recently that three-fourths of what the vagus does is bring information back to you."
When there is too much information coming back along the vagus nerve, you can become ineffective at relaxation. "Your brain short circuits," Dr. Oz says. "It can't cope with all the information flowing back."