The Body's Response
The best way to envision the effect of acute stress is to imagine oneself in a primitive situation, such as being chased by a bear.
The Brain's Response to Acute Stress
In response to seeing the bear, a part of the brain called the hypothalamic-pituitary-adrenal (HPA) system is activated.
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Release of Steroid Hormones and the Stress Hormone Cortisol. The HPA systems trigger the production and release of steroid hormones (glucocorticoids), including the primary stress hormone cortisol. Cortisol is very important in marshaling systems throughout the body (including the heart, lungs, circulation, metabolism, immune systems, and skin) to deal quickly with the bear.
Release of Catecholamines. The HPA system also releases certain neurotransmitters (chemical messengers) called catecholamines, particularly those known as known as dopamine, norepinephrine, and epinephrine (also called adrenaline).
Catecholamines activate an area inside the brain called the amygdala, which apparently triggers an emotional response to a stressful event. (In the case of the bear, this emotion is most likely fear.)
Effects on Long- and Short Term Memory. During the stressful event, catecholamines also suppress activity in areas at the front of the brain concerned with short-term memory, concentration, inhibition, and rational thought. This sequence of mental events allows a person to react quickly to the bear, either to fight or to flee from it. (It also hinders the ability to handle complex social or intellectual tasks and behaviors during that time.)
On the other hand, neurotransmitters at the same time signal the hippocampus (a nearby area in the brain) to store the emotionally loaded experience in long-term memory. In primitive times, this brain action would have been essential for survival, since long-lasting memories of dangerous stimuli (such as the large bear) would be critical for avoiding such threats in the future.
Response by the Heart, Lungs, and Circulation to Acute Stress
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