The 3 stages of the “General Adaptation Syndrome”: Alarm Reaction, Resistance/Adaptation, and Exhaustion.
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Stress is the body’s response to a physical pressure such as illness or injury; or a psychological pressure such as fear or anxiety.
In the short-term, the stress response is adaptive. It helps the body fight, cope with, or flee the stressor. Chronic stress, however, is maladaptive, and may become detrimental to the body’s normal functions.
Stress response is mediated mainly by the sympathetic nervous system and the endocrine system. It occurs in 3 stages of the so called “General Adaptation Syndrome”.
Stage 1 - Alarm reaction:
Upon perceiving the stressor, the body produces a fast, “fight or flight” response via the activation of the sympathetic nervous system, the SNS.
SNS activation results in an immediate release of the neurotransmitter norepinephrine from sympathetic nerves; and shortly after, the secretion of epinephrine and norepinephrine, as hormones, from the adrenal medulla.
These substances increase cardiac output, accelerate respiratory rate, release glucose from stored glycogen, and direct blood flow to the heart and skeletal muscles. They also dilate pupils, enhance alertness and cognition, and reduce pain sensation. At the same time, SNS activation also inhibits body processes that are less important in emergencies, such as digestion and urination.
Stage 2 - Resistance or Adaptation.
This stage is due mainly to the slow response - the activation of the hypothalamus-pituitary-adrenal (HPA) axis.
In response to stressors, the hypothalamus produces corticotropin-releasing hormone (CRH). CRH promotes the secretion of adrenocorticotropic hormone, ACTH, from the anterior pituitary. ACTH then stimulates the production of cortisol by the adrenal cortex.
Cortisol, known as the stress hormone, helps the body stay on high alert to deal with long-term stress. Cortisol overall effect is to increase energy supply:
- In the liver, it promotes catabolism of glycogen to glucose.
- In the muscles, it acts to reduce glucose uptake and consumption, and increase degradation of muscle
proteins into amino acids, which can then be used to synthesize glucose, fats, or generate energy.
- In adipose tissue, cortisol promotes lipid breakdown for additional energy.
- In the pancreas, it acts to decrease insulin and increase glucagon secretion, which together leads to increased blood glucose.
In addition, cortisol maintains higher blood pressure by increasing the sensitivity of vascular smooth muscle to vasoconstrictors and inhibiting the release of vasodilators.
At the same time, cortisol also reduces inflammation and suppresses the immune system.
Stage 3 - Exhaustion:
Stress response is meant to restore the body’s homeostasis, but a chronic or repeated stressor will eventually exhaust the body’s resources, causing a progressive wasting of muscles. The HPA axis may become dysregulated. A sustained elevation of blood pressure and heart rate may lead to cardiovascular diseases. Prolonged inhibition of digestive and urinary functions may cause gastrointestinal and renal disorders. Chronic stress also weakens the immune system, making the body more susceptible to infections.
