The new blueprint of how we respond to stress also may explain why people have different tolerances for stress. In the past stress tolerance may have been chalked up to mental fortitude: "He's a rock," or "She's really bearing up under pressure." Now it's clear that our ability to withstand stress has less to do with whether we are strong-willed than with how much and what kind of stress we encountered in the past.
Whether we end up stressed-out executives or laid-back surfers, we all start out with the same biological machinery for responding to stress. Stress activates primitive regions of the brain, the same areas that control eating, aggression, and immune response. It switches on nerve circuits that ignite the body's fight-or-flight response as if there were a life-threatening danger.
From this evidence researchers have concluded that the stress response is "wired" into the brain, that we inherit the same ancient reactions that jump-started hunter-gatherers to escape a charging saber-tooth tiger without having to give their actions time-consuming thought. Only this same life-or-death reaction is now called into play largely by non-life-threatening situations. Studies have found the same fight-or-flight circuits all working overtime in response to such varied stressors as extreme exercise, the death of a loved one, an approaching deadline.
One conclusion from the evidence is that we may be victims of evolution, hard-wired with a stress response system that's better suited to a life filled with occasional life-threatening events than one filled with everyday irritations like failing a test or blowing a sales call. Unfortunately, when stresses become routine, the constant biochemical pounding takes its toll on the body; the system starts to wear out at an accelerated rate.
By responding to the stress of everyday life with the same surge of biochemicals released during major threats, the body is slowly killing itself. The biochemical onslaught chips away at the immune system, opening the way to cancer, infection, and disease. Hormones unleashed by stress eat at the digestive tract and lungs, promoting ulcers and asthma. Or they may weaken the heart, leading to strokes and heart disease. "Chronic stress is like slow poison," King observes. "It is a fact of modern life that even people who are not sensitized to stress are adversely affected by everything that can go wrong in the day."
If stress has a central command post, it is the hypothalamus, a primitive area of the brain located near where the spine runs into the skull. By way of a dazzling array of hormonal signals, the hypothalamus is closely connected with the nearby pituitary gland and the distant adrenal glands, perched atop the kidneys. The so-called hypothalamicpituitary axis (HPA) has a virtual monopoly on basic body functions. It regulates blood pressure, heart rate, body temperature, sleep patterns, hunger and thirst, and reproductive functions, among many other activities.
About the size of a grape, the hypothalamus does its work by releasing two types of signaling hormones; those that stimulate glands to release other hormones and those that inhibit the glands from performing their job. Among the best known of these hormones are follicle-stimulating and luteinizing hormones, which, dispatched on a strict schedule from the pituitary, begin the monthly process that prepares women for pregnancy or menstruation.
Like a cherry attached by its stem, the pituitary gland hangs off the hypothalamus waiting to receive instructions on which of its many hormones to release and in what quantity. In hormonal terms it is the little gland that could. The pituitary releases substances that regulate growth, sex, skin color, bone length, and muscle strength. It also releases adrenocorticotropin, a hormone that activates the third part of the body's stress system, the adrenal glands.
When stress sets off the usual ferocious communication between the hypothalamus and the pituitary, the buck stops at the adrenal glands. They manufacture and release the true stress hormones—dopamine, epinephrine (also known as adrenaline), norepinephrine (noradrenaline), and especially cortisol. So responsive to the adrenal hormones are basic body functions like blood flow and breathing that even minute changes in levels of these substances can significantly affect health.
Slight overproduction of dopamine can constrict blood vessels and raise blood pressure; a shift in epinephrine could precipitate diabetes, or asthma, by constricting tiny airways in the lungs. If the adrenal gland slacks off on cortisol production the result may be obesity, heart disease, or osteoporosis; too much of the hormone can cause women to take on masculine traits like hair growth and muscle development and lead to one of the greatest fears of all for aging men—baldness. High levels of cortisol also may kill off brain cells crucial for memory.
The adrenal gland is also home of the grand daddy of all stress reactions, the fight-or-flight response. Sensing impending danger the hypothalamus presses out cortisol-releasing factor, a hormone that prompts the pituitary gland to release adrenocorticotropin (ACTH). Carried in the bloodstream to the adrenal glands, ACTH triggers production of cortisol and epinephrine. The end result of this hormonal relay is a sudden surge in blood sugar, heart rate, and blood pressure—everything the body needs to flee or confront the imminent danger.
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