Physiological Stress and the General Adaptation Syndrome

Stress is defined in medical terms as a wide range of strong external stimuli that can cause a physiological response within the body.
Three specific concepts are bound to physiological stress:
- The stressor: The external stimulus causing the state of stress.
- The effects: The physical and chemical changes that occur in the body.
- The body's response: The systemic reaction to the stressor.
If left unchecked, the stress response can lead to negative outcomes, including an internal chemical dependency on endorphins to maintain normal bodily functions.
Elevated levels of endorphins and catecholamines are associated with severe medical risks, including:
- Heart attacks.
- Strokes.
- Depression (often triggered by the dumping of dopamine and melatonin).
- Headaches.
- Severe abdominal pain.

The hypothalamic-pituitary-adrenal axis is a major part of the neuroendocrine system that controls reactions to stress and mediates the General Adaptation Syndrome.
It involves a set of interactions among the glands and hormones of the midbrain.
The Hypothalamus and Pituitary Gland function together in the following sequence:
- The hypothalamus stimulates the pituitary gland, specifically the anterior pituitary gland.
- The pituitary gland releases a chemical called Adrenocorticotropic Hormone ( $\text{ACTH}$ ).
The Brain-Pituitary-Adrenal Cortex pathway involves:
- Brain stimulation of the pituitary gland to produce $\text{ACTH}$ .
- $\text{ACTH}$ stimulates the cortex of the adrenal glands to produce glucocorticoids.
- Glucocorticoids are derived from cholesterol.
- Glucocorticoids function to increase access to energy storage and speed up the metabolism.

In this pathway, the brain sends a nerve signal downward to the adrenal glands.
This signal causes the medulla of the adrenal gland to dump the hormone adrenaline (epinephrine).
Adrenaline release leads to several physiological increases:
- Increased heart rate.
- Increased breathing rate.
- Increased blood glucose levels.
This process also involves the dumping of mineralocorticoids and glucocorticoids.

The Alarm Stage is the initial reaction where the body responds to the release of catecholamines.
Key catecholamines involved include:
- Epinephrine (Adrenaline).
- Norepinephrine.
- Dopamine.
Catecholamines function both as neurotransmitters in the Central Nervous System (CNS) and as hormones in the blood, depending on whether they are released by the brain or the adrenal glands.
Physiological effects of the Alarm Stage include:
- Increased respiratory rate.
- Decreased blood flow to the skin due to vasoconstriction.
- Smooth muscle contraction.
- Effects on the liver that can lead to adverse health consequences.

Adrenaline activates the sympathetic nervous system by binding to specific receptor sites:
- Alpha ( $\alpha$ ) Receptors:
  - Alpha 1 ( $\alpha_1$ ): These receptors primary cause vasoconstriction.
  - Alpha 2 ( $\alpha_2$ ): These receptors function to regulate Alpha 1 receptors.
- Beta ( $\beta$ ) Receptors:
  - Beta 1 ( $\beta_1$ ): Primarily located in the heart. Stimulation results in increased chronotropy (heart rate) and increased inotropy (cardiac contraction/strength of contraction).
  - Beta 2 ( $\beta_2$ ): Primarily located in the bronchial muscles (lungs) and epithelial cells. Stimulation results in bronchodilation.
A useful mnemonic for locating Beta receptors is: "One heart ( $\beta_1$ ), two lungs ( $\beta_2$ )."
The combined systemic effects of epinephrine on these receptors constitute the "fight or flight" response, characterized by increased heart and respiratory rates and the release of glucose from the liver.

During the Resistance stage, the body's glands release hormones to maintain blood pressure and blood glucose levels.
Two primary types of hormones are involved:
- Glucocorticoids: Generally derived from cholesterol.
- Mineralocorticoids: Derived from other tissues in the body.
The Role of Cortisol:
- Released by the adrenal cortex via $\text{ACTH}$ stimulation from the anterior pituitary.
- Increases blood concentration of glucose by speeding up a process called gluconeogenesis.
- Gluconeogenesis is defined as the creation of new glucose.
These chemicals work together to regulate and keep the body's sugar levels up to sustain the stress response.

The Exhaustion stage occurs when the adrenal glands are depleted and have no more hormones to provide.
The body can no longer produce epinephrine or cortisol.
Glucose and mineralocorticoid levels are exhausted, leading to a decrease in glucose levels because the body cannot keep up with demand.
Consequences of the Exhaustion stage include:
- Decreased stress tolerance.
- Progressive mental and physical exhaustion.
- Illness.
- Body collapse.
Long-term overuse of the stress response system increases risks for:
- Depression.
- Headaches.
- Insomnia.
- Ulcers.
- Acne.
- Diabetes.
- Asthma.

The body can only compensate for stress for a limited duration.
Healthy individuals manage stress with minimal impact on the immune system.
Ineffective coping mechanisms can lead to a defective immune system.
Chronic stress management is essential to prevent physiological damage and ensure the immune system remains functional.