Welcome to AS Unit 4: Brain Science – The Physiology and Psychology of Stress
Have you ever felt your heart pounding, your palms sweating, and your breathing quicken right before an exam or presentation? That is your brain and body jumping into action. In this chapter of AS 4: Brain Science, we will explore the biological machinery behind stress. We will break down how your central nervous system and endocrine system communicate, how your body responds to immediate versus long-term challenges, and what happens when the stress response does not switch off.
Don't worry if the biological pathways feel complex at first! We will walk through each pathway step by step with clear analogies and memory aids.
---1. Defining Stress: The Basics
Before looking at the biological wiring, let's clarify the key scientific definitions used in CCEA Life and Health Sciences:
• Stress: A state of mental tension, physiological arousal, and bodily disruption caused by internal or external demands that challenge or exceed an organism's homeostatic balance and coping ability.
• Stressor: Any biological, physical, environmental, or psychological stimulus that disrupts homeostasis (the body's stable internal balance) and triggers a stress response.
• Eustress: Positive, motivating stress that enhances alertness, cognitive sharpness, and physical performance (e.g., the positive excitement before an athletic race).
• Distress: Negative, overwhelming stress that exceeds our ability to cope, impairing function and potentially damaging health.
• Acute Stress: An immediate, short-term activation of survival systems in response to a sudden, urgent threat (e.g., slamming on the brakes to avoid an accident).
• Chronic Stress: Prolonged, ongoing exposure to stressors where the stress response fails to shut off, leading to cumulative somatic (bodily) and psychological wear and tear.
Did You Know?
Stress is not inherently bad! In acute bursts, eustress sharpens your senses and primes your muscles to help you react quickly. Problems arise when stress shifts into long-term, unmanaged distress.
Key Takeaway: A stressor is the trigger, while stress is the bodily and mental response. Acute stress is rapid and short-lived, whereas chronic stress is persistent and draining.
---2. The Physiological Pathways of the Stress Response
When your brain perceives a threat, the limbic system (specifically the amygdala, the emotional threat detector) sends an alert signal to the hypothalamus. The hypothalamus acts as the central control tower, coordinating two distinct biological pathways:
1. The SAM Axis (Acute / Immediate Response)
2. The HPA Axis (Chronic / Long-Term Response)
Pathway 1: The Sympathomedullary Pathway (SAM Axis)
The SAM axis is your body's fast-acting, electrical-and-chemical emergency response (the classic fight-or-flight response).
Step-by-Step Mechanism:
1. Perception: The amygdala identifies a threat and alerts the hypothalamus.
2. Nerve Transmission: The hypothalamus activates the sympathetic nervous system (SNS), sending nerve impulses down the spinal cord.
3. Target Organ: These impulses directly stimulate the adrenal medulla (the inner core of the adrenal glands located above the kidneys).
4. Hormone Release: The adrenal medulla rapidly secretes catecholamines into the bloodstream: primarily adrenaline (epinephrine) and noradrenaline (norepinephrine).
5. Physiological Effects:
• Increased heart rate and blood pressure: Pumps oxygenated blood faster to vital organs.
• Bronchodilation: Airways widen to take in more oxygen.
• Pupil dilation: Lets in more light to improve visual awareness.
• Blood redirection: Vasoconstriction diverts blood away from non-essential systems (skin and digestive tract) and vasodilation directs blood straight to skeletal muscles.
• Glycogenolysis: The liver converts stored glycogen into glucose, releasing it into the blood to supply immediate fuel for cellular respiration and ATP generation.
Analogy for SAM: Think of the SAM axis as an alarm bell wired directly to an emergency siren. It uses direct neural wiring for instant, explosive action.
Pathway 2: The Hypothalamic-Pituitary-Adrenal Axis (HPA Axis)
If the stressor persists beyond a few minutes, the body cannot sustain SAM activation alone. The HPA axis takes over to provide sustained biochemical support.
Step-by-Step Hormone Cascade:
1. Hypothalamus: Releases Corticotropin-Releasing Hormone (CRH).
2. Pituitary Gland: CRH travels via local blood vessels to the anterior pituitary gland, triggering the synthesis and secretion of Adrenocorticotropic Hormone (ACTH) into the general bloodstream.
3. Adrenal Cortex: ACTH travels through the blood to stimulate the adrenal cortex (the outer layer of the adrenal gland).
4. Cortisol Release: The adrenal cortex produces and secretes glucocorticoids, predominantly cortisol.
5. Physiological Effects of Cortisol:
• Gluconeogenesis: Stimulates the liver to convert non-carbohydrates (fats and amino acids) into glucose, maintaining a steady, elevated blood sugar supply.
• Energy Conservation: Temporarily downregulates energy-expensive, non-essential processes such as digestion, reproductive function, tissue growth, and inflammatory/immune responses.
The Cortisol Negative Feedback Loop
In a healthy system, cortisol controls its own production. Elevated circulating cortisol binds to specific glucocorticoid receptors in the hippocampus and hypothalamus. This binding sends an inhibitory signal to halt further secretion of CRH and ACTH, returning the body to homeostasis.
Memory Trick to Keep Them Straight:
• SAM = Medulla = Adrenaline (Think: Inside the core, fast like a motor).
• HPA = Cortex = Cortisol (Think: Outer Crust releases Cortisol).
Common Examiner Pitfall: Students often state that the adrenal medulla secretes cortisol. Remember: Medulla = Adrenaline/Noradrenaline (SAM), while Cortex = Cortisol (HPA)!
Key Takeaway: SAM produces a rapid, electrical-to-chemical rush of adrenaline for instant fight-or-flight, whereas HPA produces a hormonal cascade ending in cortisol to sustain energy over time.
---3. Hans Selye’s General Adaptation Syndrome (GAS)
Hans Selye discovered that the body shows a universal, non-specific physiological pattern when exposed to chronic, prolonged stressors. He called this the General Adaptation Syndrome (GAS), which consists of three distinct stages:
Stage 1: Alarm Reaction
• The body's immediate response to the sudden shock of a stressor.
• The SAM axis is instantly activated, flooding the bloodstream with adrenaline and noradrenaline.
• Heart rate, respiration, and alertness surge as energy reserves are mobilised for fight-or-flight.
Stage 2: Resistance
• If the stressor continues, the body attempts to adapt and resist.
• The acute symptoms of the alarm stage subside, but the HPA axis remains highly active.
• Sustained levels of cortisol and elevated blood glucose keep the body on high alert while the parasympathetic nervous system attempts partial regulation.
• Important note: Even though the individual may look outwardly composed, the body is consuming internal metabolic resources at an unsustainable rate.
Stage 3: Exhaustion
• Prolonged, unremitting exposure to the stressor completely depletes the body's energy stores and hormonal reserves.
• The physiological mechanisms of resistance collapse.
• The immune system becomes severely suppressed, leaving tissues vulnerable to damage, chronic illness, extreme fatigue, and clinical burnout.
Common Examiner Pitfall: Do not confuse the Resistance stage with full recovery! During Resistance, the body is working overtime at high physiological cost to keep coping with the stressor.
Key Takeaway: GAS describes three universal phases: Alarm (mobilisation), Resistance (costly coping), and Exhaustion (depletion and system failure).
---4. Somatic and Psychological Effects of Prolonged Stress
When the stress response is triggered repeatedly over months or years, high levels of catecholamines and cortisol damage both physical body systems and brain structures.
Cardiovascular System Damage
• Hypertension (High Blood Pressure): Constant sympathetic arousal and vasoconstriction keep blood vessels constricted and cardiac output high.
• Endothelial Damage: High pressure damages the delicate inner lining (endothelium) of blood vessels.
• Atherosclerosis: Lipid plaques accumulate at sites of endothelial damage, narrowing the arteries.
• Clinical Risks: Severely increased vulnerability to myocardial infarction (heart attack) and stroke.
Immune System Suppression
• Inhibition of Immune Chemicals: Sustained hypercortisolaemia (high cortisol) inhibits the production of pro-inflammatory cytokines.
• Cellular Depletion: Suppresses the activity and production of lymphocytes (white blood cells) and natural killer (NK) cells.
• Consequences: Increased susceptibility to bacterial and viral infections, slower wound healing, and impaired cellular repair.
Brain Structure and Neurobiology
High, persistent cortisol levels cross the blood-brain barrier and directly alter brain anatomy:
• Hippocampus: Chronic cortisol causes dendritic atrophy (shrinkage of neural branches) and inhibits neurogenesis in the hippocampus. This impairs declarative and working memory and weakens the negative feedback loop that shuts off stress.
• Prefrontal Cortex: High cortisol leads to loss of dendritic spines in the prefrontal cortex, impairing executive functioning, concentration, impulse control, and emotional regulation.
• Amygdala: In contrast to the hippocampus, the amygdala undergoes hypertrophy (growth) and hyper-reactivity, locking the brain into an exaggerated state of fear, anxiety, and threat detection.
Psychological and Behavioural Manifestations
• Emotional & Mood Disorders: Chronic anxiety, emotional exhaustion, and major depressive episodes.
• Sleep Architecture Disruption: Hyperarousal and high nocturnal cortisol disrupt circadian rhythms, causing chronic insomnia and poor sleep quality.
• Cognitive Decline: Indecisiveness, brain fog, and reduced problem-solving capabilities.
• Maladaptive Coping Behaviours: Appetite changes (under- or over-eating), increased alcohol intake, substance abuse, and social withdrawal.
Key Takeaway: Chronic stress leads to physical breakdown (cardiovascular disease and immunosuppression), structural brain remodeling (hippocampal/prefrontal shrinkage alongside amygdala enlargement), and psychological distress.
---5. Quick Reference Revision Summary
Pathway Comparison Table
• SAM Axis: Activated in acute stress | Mediated by nervous system impulses to adrenal medulla | Secretes adrenaline & noradrenaline | Produces instant fight-or-flight, increased heart rate, glycogenolysis.
• HPA Axis: Activated in chronic/prolonged stress | Mediated by hormones (CRH -> ACTH -> Adrenal cortex) | Secretes cortisol | Produces gluconeogenesis, immunosuppression, energy redistribution.
Self-Check Questions for Portfolio and Exam Prep:
1. Can you trace the exact journey of a nerve impulse from the amygdala to the release of adrenaline?
2. What role do CRH and ACTH play in the endocrine cascade of the HPA axis?
3. How does elevated cortisol normally shut down its own release via negative feedback?
4. Why does the Resistance stage in Selye's GAS eventually lead to Exhaustion if a stressor is not removed?
5. What structural changes occur in the hippocampus and amygdala under prolonged hypercortisolaemia?