Stress & the Human Stress Response – Comprehensive Study Notes

Learning Objectives

• Outline definitions and mechanisms for:
  • Stress
  • The stress response
  • Stressors (biological, chemical, physical, social, psychological)
• Understand the physiological / psychological adaptations that re-establish balance (homeostasis & allostasis).

Core Definitions & Conceptual Distinctions

• Stress
  • Body’s psychological & physiological reaction to a perceived threat.
  • Threat may be real (e.g.
  • Mugger wielding a knife)
    or implied / anticipated (e.g.
  • Large dog running toward you).
• Stress response: Coordinated multi-system actions aimed at restoring equilibrium.
• Homeostasis
  • Ongoing automatic adjustments that keep internal variables within a narrow health-supporting range.
• Allostasis
  • Short-term, adaptive stability via change.
  • Focuses on rapid adjustments to acute stress.
• Allostatic load
  • Cumulative burden produced when multiple / chronic stressors push systems beyond adaptive capacity.

Real vs. Implied Threat – Example Analysis

• Real: Knife-wielding mugger demands wallet.
• Implied: Dog sprinting toward you; prior dog-bite memory magnifies perceived danger.
• Take-home: Past experiences mold current threat appraisal → divergent responses between individuals.

Neuroendocrine Integration of the Stress Response

1. Central Nervous System (CNS) – Bidirectional Signalling

• Continuous two-way traffic among:
  • Brainstem ↔ Cerebral cortex
  • Cerebral cortex ↔ Thalamus
  • Thalamus ↔ Limbic system
• Outcome: Simultaneous feedback, rapid situational assessment, decision making.

2. Autonomic Nervous System (ANS)

• Sympathetic division “fight/flight” spikes within seconds:
  • ↑ Heart rate, blood pressure, respiratory rate
  • Pupil dilation, sweating
  • Redirected blood flow → skeletal muscle, heart, lungs
  • Overall "overdrive" mode

3. Reticular Activating System (RAS)

• Heightens alertness & baseline muscle tone for immediate action.

4. Limbic System

• Intensifies & integrates emotional tone (fear, anger, anxiety) → influences behavioural choice (fight vs flight).

5. Thalamus & Cortex

• Thalamus: Amplifies sensory input (vision, hearing, smell) for threat localization & escape-route planning.
• Cerebral cortex: Focused attention, planning, goal-directed persistence.

Hormonal Cascade – Timing & Functions

1. Immediate (seconds)
   • Surge of adrenaline (epinephrine) & noradrenaline (norepinephrine) from adrenal medulla:
     • Bronchodilation
     • ↑ Cardiac output (force & rate)
     • Vasoconstriction → maintain BP
2. Secondary (minutes–hours)
   • Glucocorticoids (chiefly cortisol) via HPA axis → sustain energy production.
3. Supporting hormones & modulations
   • Glucagon (pancreas) + glucocorticoids + sympathetic innervation → ↑ blood glucose ≈ readily available fuel.
   • Prolactin → Suppresses reproductive drive.
   • Endorphins / Enkephalins → Analgesia (pain blunting).
   • Vasopressin (ADH) → Anti-diuretic, cardiovascular support.
   • Growth hormone → variable (stimulated or inhibited depending on context & phase).
   • Inhibition of: $\text{Oestrogen},\ \text{Progesterone},\ \text{Testosterone},\ \text{Insulin}$ (preserves glucose for CNS & muscle use).

Hypothalamus–Pituitary–Adrenal (HPA) Axis – Step-wise Flow

1. Stimulus (physical / psychological) → Perceived threat
2. Hypothalamus releases Corticotropin-Releasing Hormone (CRH).
3. Anterior pituitary secretes Adrenocorticotropic Hormone (ACTH).
4. Adrenal cortex emits cortisol →
   • ↑ Blood glucose & lipids → ATP synthesis for muscles & brain.
   • Modulates immune and metabolic functions.
5. Negative feedback: Elevated cortisol inhibits further CRH & ACTH release to prevent overshoot.

Integrated Organism-Wide Effects (Selected Highlights)

• Cardiovascular: Vasoconstriction (norepinephrine), ↑ cardiac output (adrenaline).
• Respiratory: Bronchodilation (adrenaline) → ↑ O$_2$ uptake.
• Metabolic: Glycogenolysis, lipolysis, gluconeogenesis ≈ rapid fuel mobilization.
• Reproductive / Digestive: Temporarily down-regulated (energy diverted to survival systems).
• Immune: Short-term enhancement (adrenaline), prolonged stress → suppression (cortisol).

Behavioural & Emotional Spectrum

• Stress moves individuals along a continuum of behavioural, emotional & physiological states.
• Extremes at either end often coincide with psychological disorders (e.g., anxiety, depression).
• Coping strategies vary:
  • Eating ↑ / ↓
  • Alcohol consumption
  • Exercise / avoidance
• Individual differences (genetics, history, learning) shape choice & effectiveness.

Stressor Taxonomy (expanded from prescribed text)

• Remember: $\text{Stressor}<em>{\text{Person A}} \neq \text{Stressor}</em>{\text{Person B}}$ – perception & prior experience govern impact.

Ethical & Practical Implications

• Chronic allostatic load is linked to cardiovascular disease, metabolic disorders, mental health conditions.
• Designing environments (workplaces, schools) that minimise chronic stressors is a public-health priority.
• Clinicians must account for individual histories when assessing stress vulnerability & resilience.

Key Take-Home Messages / Revision Cues

• Stress = response, stressor = trigger.
• Homeostasis (steady state) vs Allostasis (adaptive change).
• Allostatic load marks point where adaptation fails → health risks escalate.
• Rapid phase: ANS + catecholamines; Sustained phase: HPA + cortisol.
• Multiple brain regions coordinate via bidirectional loops for real-time evaluation & reaction.
• Individual perception re-writes the script: same stimulus, different stress outcome.