This Is Your Body on Stress

Stress is just your body's natural response to perceived danger. But what happens when your stress reaction never shuts off?

By Roger Cole

But when Sally thought an intruder was entering her kitchen, her cortex and limbic system screamed "Danger!" at the top of their neural lungs. The posterior hypothalamus got the message loud and clear. In a flash, this little complex of brain cells turned on all the physiological systems she needed to get her muscles and mind going at full power, and turned off everything that might interfere. It told her pituitary gland to send out a chemical messenger to her adrenal cortex, the outer layer of her adrenal glands, stimulating it to release the stress hormone cortisol into the bloodstream. It told her brain's sleep centers to shut off and its wakefulness centers to kick into their highest gear. It activated brain centers that control muscle tone, increasing tension everywhere in her body. It told breathing centers at the base of Sally's brain to increase respiration to provide oxygen for all the extra muscle and brain activity about to occur. And, most important of all, it cranked her entire sympathetic nervous system up to full throttle.

All Revved Up, No Place to Go

The sympathetic nervous system is a network of nerve cells that extends throughout the body. It helps support our normal activities; for example, it makes our heart beat faster when we climb stairs. In an emergency, though, it goes into overdrive —and Sally felt the results. To get more blood to her heart, skeletal muscles, and brain, the sympathetic nervous system widened arteries in those places, narrowed them in others, and started her heart racing and pounding. That's why she felt a throbbing in her temples. In her digestive tract, Sally's sympathetic system narrowed arteries and inhibited other functions. That's why she felt a dry mouth and a knot in her stomach. To help her get more oxygen, sympathetic nerves opened up her air passages. That's why her nostrils flared, her nose cleared, and her voice faltered when she first saw Sam.

Other sympathetic nerves worked to make sure Sally could see everything going on around her. They dilated her pupils and opened her eyelids so wide that Sam could see the whites all the way around. To keep her from overheating, still other sympathetic nerves activated sweat glands.

The sympathetic nervous system triggered most of these responses by releasing a major chemical messenger called norepinephrine (or noradrenaline) at nerve endings on target tissues like blood vessels and sweat glands. It also stimulated the adrenal medulla (the core of the adrenal glands) to flood the bloodstream with more norepinephrine plus a second essential chemical, epinephrine (also called adrenaline). These chemicals not only intensified the stimulation of organs directly targeted by sympathetic nerves, they also acted on parts of the body that don't have these nerve connections. For example, they made Sally's blood clot faster (so her cut did not bleed much), made her muscle fibers contract more strongly (so she could easily lift an iron table), and made her brain activity speed up (so the world around her seemed to slow down).

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