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Hypothalamic-Pituitary-Adrenal (HPA) Axis and Stress | BA Psychology Notes

Study material and explanations for Unit 3

Hypothalamic-Pituitary-Adrenal (HPA) Axis and Stress

When human beings encounter physical danger or intense psychological distress (such as financial trauma, bereavement, or preparing for high-stakes university exams), the body mobilizes a coordinated neuroendocrine defense known as the Hypothalamic-Pituitary-Adrenal (HPA) Axis.

While the sympathetic nervous system triggers the immediate split-second “fight-or-flight” sprint, the HPA axis manages the sustained, medium-to-long-term physiological mobilization of bodily resources.

1. The Two Stress Pathways: SAM vs HPA

The brain responds to stress through two distinct temporal pathways coordinated by the Hypothalamus:

The Fast SAM Pathway vs The Sustained HPA Axis
1. SAM Pathway (Sympathomedullary Axis)
  • Speed: Instantaneous (split seconds).
  • Mechanism: Neural impulses travel from hypothalamus down the spinal cord to the Adrenal Medulla.
  • Hormones Released: Epinephrine and Norepinephrine (Adrenaline).
  • Outcome: Heart races, pupils dilate, breathing accelerates.
2. HPA Axis (Neuroendocrine Cascade)
  • Speed: Slower (takes several minutes to hours).
  • Mechanism: Hormonal cascade through bloodstream to the Adrenal Cortex.
  • Hormones Released: Cortisol (Glucocorticoids).
  • Outcome: Elevates blood glucose, blunts inflammation, sustains energy.

2. The Sequential Steps of the HPA Axis Cascade

The Step-by-Step Neuroendocrine Cascade of the HPA Axis
01
Hypothalamus
Amygdala signals threat; PVN of hypothalamus secretes CRH (Corticotropin-Releasing Hormone).
→ Hypophyseal Portal Blood →
02
Pituitary Gland
Anterior pituitary receives CRH and secretes ACTH (Adrenocorticotropic Hormone) into systemic circulation.
→ Systemic Bloodstream →
03
Adrenal Cortex
Adrenal cortex receives ACTH and synthesizes and pumps Cortisol throughout the entire body.

3. Physiological Actions of Cortisol (The Primary Stress Hormone)

Cortisol ensures the body has sufficient metabolic resources to survive a crisis by temporarily altering bodily priorities:

  1. Glucose Mobilization: Stimulates gluconeogenesis in the liver and inhibits peripheral glucose uptake, ensuring a constant flood of sugar to the brain and working muscles.
  2. Lipolysis and Proteolysis: Breaks down stored fats and proteins into fatty acids and amino acids for emergency cellular energy.
  3. Anti-Inflammatory Action: Suppresses the immune response, dampening inflammation to prevent runaway tissue swelling during injury.
  4. Temporary Shutdown of Non-Essentials: Pauses long-term projects that can wait until the danger passes: cellular tissue repair, bone growth, digestive enzyme synthesis, and reproductive libido.

4. The Self-Regulating Negative Feedback Loop

Under normal, healthy conditions, the HPA axis includes an automated “off switch”:

The HPA Axis Negative Feedback Mechanism
01
Rising Cortisol
Circulating cortisol levels reach a critical concentration threshold in the blood.
→
02
Receptor Binding
Cortisol binds to glucocorticoid receptors in the Hippocampus and Hypothalamus.
→
03
Inhibition
Hippocampus signals the hypothalamus to halt CRH secretion, restoring baseline calm.

5. Chronic Stress and Neurotoxicity

While short-term cortisol secretion is lifesaving, Chronic Stress (prolonged financial distress, unresolved grief, toxic relationships, academic burnout) causes the negative feedback system to collapse:

Damaging Effects of Sustained Chronic Hypercortisolemia
1. Hippocampal Atrophy (Brain Damage) Sustained high cortisol causes dendritic retraction, reduces neurogenesis, and destroys hippocampal neurons, leading to severe memory loss and cognitive brain fog.
2. Amygdala Hypertrophy (Hyper-Anxiety) Unlike the hippocampus, the amygdala grows more dendritic branches under stress, locking the individual into chronic hypervigilance, paranoia, and panic attacks.
3. Immunosuppression (Immune Collapse) Prolonged suppression of white blood cells (lymphocytes) leaves the body highly susceptible to viral infections, slow wound healing, and chronic inflammation.
4. Clinical Depression (Mood Pathology) Exhaustion of brain monoamine neurotransmitters (serotonin, dopamine) and glucocorticoid receptor resistance directly produce depressive symptoms (Hans Selye's Exhaustion Stage).