Stress and General Adaptation Syndrome (GAS)

Stress and the General Adaptation Syndrome (GAS)

• Homeostasis is the balanced state of the body's systems.
• Allostasis is the process of adapting to maintain survival and well-being when homeostasis is disrupted.
• Stress is the sum of biological reactions to disruptions in homeostasis.
• Stressors are agents or conditions that cause stress and disrupt homeostasis, triggering allostatic processes.

General Adaptation Syndrome (GAS)

• Hans Selye proposed GAS, a model of the body's response to stress.
• The GAS model has three stages:
  • Alarm stage
  • Resistance stage
  • Exhaustion stage

Alarm Stage

• The "fight-or-flight" response is activated.
• The hypothalamus senses a stressor (internal or external, physical or emotional).
• The SNS (sympathetic nervous system) is activated.
• The adrenal medulla releases catecholamines (epinephrine and norepinephrine).
• The body prepares for quick action.
• The hypothalamic-pituitary-adrenal (HPA) axis is activated, with the hypothalamus releasing CRH, which stimulates the anterior pituitary gland to release ACTH.
• ACTH stimulates the adrenal cortex to release glucocorticoids (especially cortisol).
• This whole cascade is known as the HPA axis.

Alarm Stage (continued)

• Prolonged alarm stage results in permanent damage.
• SNS, adrenal medulla and cortex function at full force.
• If the alarm stage continues for extended periods, the body experiences significant damage.

Resistance Stage

• The body attempts to return to homeostasis.
• Levels of SNS activity, catecholamines, and glucocorticoids are normalized.
• If stressors are adequately addressed, the body returns to a steady state showing signs of adaptation.
• Increasing resistance to the stressor.

Exhaustion Stage

• The body is unable to return to homeostasis.
• Adaptive energy stores are depleted.
• The body fails to adapt.
• Physiological effects of excessive stress become apparent.
• A "pathologic triad" might occur:
  • Hypertrophy of adrenal glands
  • Atrophy of lymphoid tissues
  • Bleeding GI ulcers
• Death is possible if homeostasis is not restored.

Problems with the GAS Theory

• The GAS theory fails to fully explain that stress is often psychosocial.
• The concept of allostasis enables inclusion of psychosocial stress-related illness.
• Allostasis is the process of achieving stability or homeostasis through physiological and/or behavioral changes.
• Allostatic load is the overall effect of chronic stress on the organism resulting in wear and tear.
• Allostatic overload occurs when the allostatic load is excessive or ineffectively regulated.

Stressors

• Stressors can be internal or external, physical or emotional, biological, chemical, or social.
• May be low-intensity but cause significant impact over time.
• Risk factors increase the likelihood of experiencing a stressor.

Neurohormones of Stress and Adaptation

• Catecholamines (norepinephrine and epinephrine) are neurotransmitters.
• Released from the CNS in response to stressors.
• Norepinephrine constricts blood vessels and maintains blood pressure.
• Epinephrine increases heart rate, cardiac output, and blood pressure.

More Neurohormones

• Adrenocortical steroids (cortisol and aldosterone) help regulate cardiovascular system, fluid volume, and metabolism.
• Glucocorticoids, like cortisol, have a slower onset but longer lasting effect.
• Cortisol increases protein synthesis in the liver and stimulates gluconeogenesis. It suppresses immune and inflammatory responses.
• Glucocorticoids also support catecholamines and control blood pressure and cardiac output.
• Aldosterone regulates sodium, fluid volume, and blood pressure.

More Neurohormones

• Endorphins and enkephalins are peptides that reduce pain.
• Released in response to stressors, by certain foods, laughter, massage, and acupuncture.
• Endorphins raise pain thresholds.

Sex Hormones and Stress

• Cortisol has a negative effect on the release of estrogen and progesterone leading to possible reproductive issues.
• Testosterone and other androgens help counter cortisol's catabolic effects.
• Stress can lower testosterone levels.

Other Stress Hormones

• Growth hormone is released in response to physical or psychological stimuli.
• It increases protein synthesis and fat mobilization, and reduces carbohydrate utilization.
• Prolactin is also released in response to stress, sexual activity, and breastfeeding.
• It has similar effects to growth hormone.

Allostatic Overload Effects

• Many physiological and psychological problems can result from excessive or chronic stress.

• Effects vary by affected systems.