Physiology of stress and the HPA axis

Introduction to the Physiology of Stress

Welcome to this chapter on the biological foundations of stress! In Health Psychology (Topic G), understanding how the human body reacts to pressure, challenge, and perceived threat is essential. Have you ever felt your heart pound, your stomach churn, and your focus sharpen before an exam, a sports match, or a difficult conversation? That physical sensation is your body's physiological stress response at work.

Stress is fundamentally a biological survival mechanism. It prepares an organism to confront danger or run away to safety. However, while this system evolved to protect us from immediate physical predators, in modern life it often responds to ongoing, psychological challenges. In this chapter, we will break down:

• The key brain regions that perceive and process stress: the amygdala, prefrontal cortex, and hippocampus.
• The long-term chemical response known as the Hypothalamic-Pituitary-Adrenal (HPA) axis and its key hormone, cortisol.
• Hans Selye's classic biological model: the General Adaptation Syndrome (GAS), including the alarm reaction.

Note: Factors such as life events, daily hassles, and personality traits are explored in the "Factors affecting stress" chapter, while clinical interventions are detailed in "Treatment and therapy for anxiety".

Brain Regions Involved in the Stress Response

When you face an unexpected event, your brain acts as the central command station. It evaluates whether the situation is dangerous and decides how your body should react. Three key brain regions work together to manage this response.

1. The Amygdala (The Threat Detector)

The amygdala is an almond-shaped structure nestled deep within the brain's temporal lobes, forming part of the limbic system. You can think of the amygdala as your brain's emotional fire alarm.

• Function: It rapidly processes emotional sensory input—especially cues related to fear, threat, and danger.
• Role in stress: Before you even consciously understand what is happening, the amygdala sends an immediate distress signal to the hypothalamus to trigger physiological defence mechanisms.
• Everyday analogy: Imagine hearing a loud bang outside your window. The amygdala makes you jump and freeze before you even have time to think about what made the noise.

2. The Prefrontal Cortex (The Rational Control Centre)

The prefrontal cortex (PFC) is located at the very front of the frontal lobe. It is responsible for higher-level executive functions, including reasoning, decision-making, emotional regulation, and cognitive appraisal.

• Function: It assesses situations rationally, weighing up evidence and evaluating whether a situation is truly dangerous or merely a false alarm.
• Role in stress: The PFC can send inhibitory (calming) signals to the amygdala if it determines that a situation is safe. However, under intense or chronic stress, the functioning of the prefrontal cortex can be impaired, making rational decision-making and emotional control much harder.
• Everyday analogy: Following that loud bang, your prefrontal cortex evaluates the context, notices that your roommate dropped a heavy textbook, and consciously tells your amygdala to stand down.

3. The Hippocampus (The Memory and Context Specialist)

The hippocampus is located close to the amygdala in the limbic system and is best known for its vital role in consolidating short-term memories into long-term memories.

• Function: It provides situational context to current experiences by comparing them with past memories.
• Role in stress: The hippocampus helps you recognise whether you have faced this specific stressor before and what happened. Crucially, the hippocampus has a high concentration of glucocorticoid receptors, which means it helps regulate the shutdown of the stress response via a negative feedback loop.
• Impact of chronic stress: Sustained exposure to excessive cortisol levels over extended periods can damage hippocampal neurons, impairing memory retrieval and weakening the brain's ability to turn off the stress response.

Quick Review of Brain Regions:

• Amygdala: Senses threat \(\rightarrow\) fires the alarm.
• Prefrontal cortex: Evaluates threat rationally \(\rightarrow\) regulates emotional output.
• Hippocampus: Recalls memory and context \(\rightarrow\) assists in switching off the stress response.

The HPA Axis and Cortisol

While the body's immediate, short-term emergency response acts through rapid autonomic pathways, the body also requires a sustained system to deal with stressors that last longer than a few seconds. This neuroendocrine pathway is the Hypothalamic-Pituitary-Adrenal (HPA) axis.

Step-by-Step: How the HPA Axis Operates

Don't worry if these chemical names look daunting at first! Just remember the sequence: Hypothalamus \(\rightarrow\) Pituitary \(\rightarrow\) Adrenal.

Step 1: The Hypothalamus initiates the response
Once the amygdala registers a continuing stressor, it signals the hypothalamus. The hypothalamus releases a chemical messenger called corticotropin-releasing hormone (CRH) into the bloodstream leading to the pituitary gland.

Step 2: The Pituitary Gland secretes ACTH
CRH arrives at the pituitary gland (often called the master gland, located just beneath the base of the brain). In response to CRH, the pituitary gland releases adrenocorticotropic hormone (ACTH) into the general bloodstream, which carries it down through the body.

Step 3: The Adrenal Glands produce Cortisol
ACTH reaches the adrenal glands, which sit on top of both kidneys. Specifically, ACTH stimulates the outer layer of these glands, known as the adrenal cortex. The adrenal cortex then synthesises and releases steroid hormones known as glucocorticoids, primarily cortisol.

What Does Cortisol Do?

Cortisol is widely recognised as the primary biological "stress hormone". Its main objective is to keep the body fuelled and focused during extended periods of hardship.

• Mobilising Energy: Cortisol stimulates the release of stored glucose (blood sugar) and enhances fat and protein metabolism to supply muscles and vital organs with continuous energy.
• Suppressing Non-Essential Systems: Digestion, growth, reproduction, and certain inflammatory and immune system functions are temporarily dampened down so the body can redirect all energetic resources toward dealing with the immediate challenge.
• Pain Sensitivity: It can temporarily reduce pain sensitivity, allowing an organism to persevere through injury during an emergency.

The Negative Feedback Loop

Under healthy conditions, the HPA axis is self-regulating. As cortisol levels rise in the bloodstream, specialised receptors in both the hypothalamus and the hippocampus detect the elevated hormone levels. These structures then signal the system to reduce the release of CRH and ACTH. This negative feedback loop acts like a household thermostat: once the room is warm enough, the boiler switches off.

Consequences of Prolonged (Chronic) HPA Activation: If a stressor remains present for days, weeks, or months, cortisol remains persistently elevated. This can lead to:

• Suppressed immune system function, leaving the individual more vulnerable to infections and illness.
• Increased blood pressure and cardiovascular strain.
• Impairment of memory and cognitive function due to cellular damage within the hippocampus.

Selye's General Adaptation Syndrome (GAS)

In the 1930s and 1950s, researcher Hans Selye noticed that when animals were subjected to various physical stressors (such as extreme cold, physical trauma, or toxic substances), they consistently showed the same generalised, non-specific biological pattern of physiological damage, including enlarged adrenal glands, shrunken immune tissues, and stomach ulcers.

Selye proposed the General Adaptation Syndrome (GAS), which states that the body responds to any ongoing stressor in three progressive, predictable stages:

1. The Alarm Reaction
2. The Resistance Stage
3. The Exhaustion Stage

Stage 1: The Alarm Reaction

The alarm reaction is the immediate physiological response upon encountering a stressor.

• Shock Phase: The initial shock briefly depresses physiological resistance below normal levels (e.g., a momentary drop in blood pressure or body temperature).
• Counter-shock Phase: The body rebounds rapidly. The brain identifies the threat, triggering emergency hormonal and physiological mechanisms. Heart rate, blood pressure, respiration, and energy mobilisation surge to prepare the individual for defensive action.

Stage 2: The Resistance Stage

If the stressor persists, the body cannot sustain maximum alarm activation without collapsing, so it enters the resistance stage.

• The body attempts to adapt and cope with the continuous demands of the stressor.
• Physiological arousal levels drop slightly compared to the intense alarm reaction, but remain well above baseline levels.
• The HPA axis plays a central role here: cortisol levels remain elevated to keep glucose available for prolonged physiological demand.
• Because energy is constantly funnelled toward maintaining resistance, the body's overall reserve capacity declines, and non-essential activities (such as tissue repair and immune defence) remain compromised.

Stage 3: The Exhaustion Stage

If the stressor continues unabated over a prolonged period, the body reaches the exhaustion stage.

• The body's biological and physiological reserves become depleted.
• Hormonal secretion falters, and tissues can no longer sustain their adapted resistance.
• The immune system becomes severely suppressed, leaving the individual vulnerable to physical illnesses (what Selye termed "diseases of adaptation"), such as cardiovascular disorders, severe infections, chronic fatigue, and organ damage.

Summary of GAS Stages:
• Alarm reaction: Initial shock followed by rapid physiological mobilisation.
• Resistance: Sustained coping, high cortisol, suppressed immune function.
• Exhaustion: Biological resource depletion, physiological breakdown, increased illness risk.

Evaluation of the Physiological Model of Stress (AO3)

When answering 8-mark extended open-response questions in Unit 3, you need balanced analysis and evaluation.

Strengths

• Scientific Credibility and Objective Measurement: The physiological explanations of stress rely on clear, measurable biological variables. Levels of cortisol can be collected via saliva, blood, or urine samples, and brain activity can be observed using neuroimaging technologies. This gives the biological account high scientific validity and objectivity.
• Understanding Health Consequences: Selye's GAS and the HPA axis model provide a direct biological mechanism explaining why prolonged life stress correlates with physical disease (e.g., immune deficiency, cardiovascular issues). This has enabled medical and psychological practitioners to appreciate the profound physical toll of chronic psychological strain.

Weaknesses and Limitations

• Biological Reductionism: The physiological approach reduces a complex human experience down to hormones, glands, and brain circuits. It often overlooks individual psychological differences, such as cognitive appraisal (how a person interprets a situation), personality type, and social support.
• The "Non-Specific" Assumption Challenged: Selye claimed that the body responds in the exact same non-specific way to every stressor. However, modern research shows that different stressors can produce distinct hormonal and emotional profiles; for instance, social evaluation threats often trigger different biological patterns than direct physical danger.
• Generalisability from Animal Research: Much of the early experimental research underpinning GAS was conducted on laboratory animals (such as rats exposed to physical extremes). Humans have a vastly more developed prefrontal cortex, meaning cognitive processes, expectations, and emotional perceptions influence our stress responses far more than in non-human animals.

Exam Practice and Study Tips

• Terminology Check: Always spell out key terms accurately: Hypothalamic-Pituitary-Adrenal axis, corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and General Adaptation Syndrome.
• Understand the Negative Feedback Loop: In short-answer questions, students often forget to mention how cortisol regulates itself. Remember: high cortisol levels instruct the hypothalamus and hippocampus to turn off CRH and ACTH release.
• Distinguish the Stages of GAS: Clearly contrast the initial acute surge of the alarm reaction from the extended metabolic effort of the resistance stage and the final collapse in the exhaustion stage.