When you encounter a perceived threat — a large dog barks at you during your morning walk, for instance — your hypothalamus, a tiny region at the base of your brain, sets off an
alarm system in your body. Through a combination of nerve and hormonal signals, this system prompts your adrenal glands, located atop your kidneys, to release a surge of hormones, including
adrenaline and cortisol.
Adrenaline increases your heart rate, elevates your blood pressure and boosts energy supplies. Cortisol, the primary stress hormone, increases sugars (glucose) in the
bloodstream, enhances your brain's use of glucose and increases the availability of substances that repair tissues.
Cortisol also curbs functions that would be nonessential or detrimental in a fight-or-flight situation. It alters immune system responses and suppresses the digestive system,
the reproductive system and growth processes. This complex natural alarm system also communicates with regions of your brain that control mood, motivation and fear.
Experiencing stressors over a prolonged period of time, can result in a long-term drain on the body. As the SNS continues to trigger physical reactions, it causes a wear-and-tear on the body. It's not so much what chronic stress does to the nervous system, but what continuous activation of the nervous system does to other bodily systems that become problematic.