Physiological Response to Stress (AQA A-Level Psychology): Revision Notes
Physiological Response to Stress
The body's physiological response to stress involves complex biological systems that prepare us to deal with challenging situations. Understanding these responses helps explain how our bodies react to both short-term and long-term stressful experiences.
Understanding the physiological basis of stress is crucial because it helps explain not only how we respond to immediate threats, but also why prolonged stress can lead to serious health problems. This knowledge forms the foundation for developing effective stress management strategies.
Understanding stress and stressors
Stress refers to the lack of balance between what we perceive as the demands of a situation and our perceived ability to cope with those demands. Stressors are the internal and external sources that trigger this stress response.
Stressors can be categorised as:
- Acute stressors: Short-term challenges like being confronted by a snarling dog
- Chronic stressors: Long-term pressures such as ongoing work demands
The body responds differently to these two types of stressors through distinct biological pathways.
General adaptation syndrome
Selye (1936) developed the general adaptation syndrome (GAS) to describe the body's physiological reactions when experiencing stress. This model outlines three distinct stages that occur in response to stressful situations.
The three stages of GAS
The Three Stages in Action: Academic Stress
Stage 1 - Alarm: Student receives notification of a major exam tomorrow they forgot about. Heart rate increases, adrenaline surges, immediate panic response.
Stage 2 - Resistance: Student begins intensive studying, body adapts to sustained work, cortisol maintains energy levels through the night.
Stage 3 - Exhaustion: After multiple all-nighters during exam period, student becomes physically ill, immune system compromised, performance deteriorates.
Stage 1: Alarm reaction The initial stage involves physiological changes linked to emotional reactions to stressors. The hypothalamus signals the sympathetic nervous system, which activates the adrenal glands to release adrenaline and noradrenaline. This creates the well-known fight-or-flight response, increasing heart rate, blood flow, and blood sugar levels to prepare the body for immediate action.
Stage 2: Resistance If the stressor persists, the body attempts to recover from the initial alarm and begins coping with the situation. Sympathetic nervous system activity decreases, reducing adrenaline and noradrenaline production. However, the adrenal cortex increases activity, releasing glucocorticoid hormones (primarily cortisol) controlled by ACTH from the hypothalamus. These hormones provide energy through increased blood glucose to help resist the ongoing stress.
Stage 3: Exhaustion When stress continues for extended periods, the body's resources become depleted. The adrenal glands can no longer function effectively, blood sugar levels drop, and physical health deteriorates. This can lead to serious health problems including high blood pressure, heart disease, and ulcers.
Research support for GAS
Selye's Original Research (1950)
Selye exposed rats to various stressors including extreme temperatures, loud sounds, intense light, forced exercise, and organ extracts. Regardless of the type of stressor, he found identical stress reactions: enlarged adrenal glands, shrunken thymus and lymph glands, and stomach ulcers. This consistency supported his theory of a universal bodily stress response.
Timio et al. (1988) conducted a 20-year study comparing nuns living in protected environments with working women exposed to everyday stressors. The nuns maintained unchanged blood pressure whilst the working women showed increased blood pressure, supporting the idea that long-term stress negatively affects physical health in line with GAS predictions.
Leshem and Kuiper (1996) applied different stressors to plants, including heat, cold, drought, and salt. The plants showed similar stress responses with reduced growth and lower yields, suggesting that GAS principles apply across different species and demonstrate the biological basis of stress reactions.
Evaluation of GAS
Strengths:
- GAS was the first comprehensive theory to explain stress's physiological effects and significantly influenced subsequent stress research
- The model has been particularly valuable for understanding the negative health impacts of prolonged stress
Weaknesses:
- Much early research used rats, making it difficult to generalise findings to humans who have more complex emotional and cognitive responses to stress
- Individual differences exist in stress responses - Mason (1995) showed that stressors produce varying amounts of stress hormones depending on the level of fear and anger they create
- Selye's experiments involved severe, painful stressors that may be considered ethically questionable, though he believed they were justified for potential therapeutic benefits
The sympathomedullary pathway
The sympathomedullary pathway (SMP) consists of the sympathetic nervous system (SNS) and the sympathetic adrenal medullary system (SAM). This system primarily handles acute, short-term stressors.
How the SMP works
The autonomic nervous system has two divisions that work in opposition:
- The sympathetic nervous system (SNS): Acts as the body's 'troubleshooter', being highly responsive to stimuli and responsible for emotional states and heightened arousal
- The parasympathetic nervous system (PSNS): Functions as the 'housekeeper', maintaining equilibrium and calming bodily processes
When exposed to an acute stressor, the SNS activates simultaneously with the SAM system. This stimulates the release of adrenaline from the adrenal glands into the bloodstream. The hormone prepares the body for fight-or-flight responses by increasing oxygen and glucose supply to the brain and muscles while suppressing non-essential processes like digestion.
Research on the SMP
Gender Differences in Stress Response (Taylor et al., 2000)
Fight-or-Flight (typically male): Confronted by a threat, males show increased aggression or escape behaviours, with high adrenaline and cortisol release.
Tend-and-Befriend (typically female): Confronted by the same threat, females show increased nurturing behaviours and seek social support, with higher oxytocin production promoting relaxation and bonding.
This demonstrates evolutionary adaptations where women's responses better ensured offspring survival through community protection.
McCarty (1981) found that blood levels of adrenaline and noradrenaline were similar in rats of different ages before stress exposure. However, after electric shocks, older rats showed lower hormone levels than younger rats. This indicates that the sympathomedullary pathway becomes less responsive with age, reducing older animals' capacity to adapt to stressful situations.
Horwatt et al. (1988) exposed animals to identical stressful stimuli daily for several weeks, leading to adaptive changes in the sympathomedullary pathway. These included increased production and storage of stress hormones. When the animals were then exposed to new stressful stimuli, they showed exaggerated responses compared to animals experiencing stress for the first time.
Evaluation of the SMP
Strengths:
- Gender differences in stress activation may reflect evolutionary adaptations where women's 'tend-and-befriend' response better ensured offspring survival
- Research provides clear biological explanations for observable stress responses
Weaknesses:
- Early human stress research focused mainly on men, as researchers believed women's hormonal fluctuations during menstrual cycles would create too much variability for valid results
- Much research relies on animal studies, creating problems for generalisation since human stress responses likely involve more cognitive elements than animal responses
The hypothalamic-pituitary-adrenal system
The hypothalamic-pituitary-adrenal system (HPA) responds to chronic, long-term stressors. This system takes longer to activate than the SMP but provides sustained responses to ongoing stress.
How the HPA system works
Prolonged, chronic stress triggers the HPA system through the following sequence:
- Continuous stressors alert the hypothalamus brain region
- The hypothalamus releases corticotropin-releasing hormone (CRH) into the bloodstream
- CRH stimulates the pituitary gland to release adrenocorticotropic hormone (ACTH)
- ACTH travels through the bloodstream to the adrenal glands above the kidneys
- The adrenal cortex releases stress-related hormones, most importantly cortisol
The role of cortisol
Cortisol is a glucocorticoid hormone that serves several important functions during chronic stress:
- Provides a steady energy supply by maintaining blood sugar levels
- Allows the body to cope with ongoing stressors by supplying constant energy
- Increases pain tolerance beyond normal levels
- However, prolonged cortisol release impairs cognitive ability and reduces immune system performance
The Double-Edged Nature of Cortisol
While cortisol is essential for managing chronic stress, prolonged elevation can lead to serious health consequences including memory problems, weakened immune function, and increased susceptibility to illness. This explains why chronic stress is so damaging to long-term health.
Research on the HPA system
Heim et al. (2000) studied women who experienced sexual abuse during childhood, comparing them with women who had not been abused. The abused women showed increased pituitary-adrenal and autonomic responses to stress, measured through ACTH and cortisol levels. This suggests that childhood abuse leads to hyperactivity in the HPA system, with researchers proposing that treatments targeting stress hormone receptors could help address early-life stress effects.
Newcomer et al. (1999) gave participants different levels of cortisol - some received amounts high enough to simulate major stress (like surgery), whilst others received lower levels similar to minor stress responses. Participants with higher cortisol levels performed worse on memory tasks involving prose recall compared to those with lower cortisol levels. This demonstrates that excessive HPA system activation negatively affects memory function.
Watson et al. (2004) compared HPA system functioning in 26 people with bipolar disorder, 27 people with major depression, and 28 healthy controls. Both patient groups showed elevated cortisol levels, including those currently in remission from bipolar disorder. This suggests that HPA system dysfunction contributes to the underlying disease processes in mood disorders.
Evaluation of the HPA system
Strengths:
- Individual differences in stress hormone production help explain why people respond differently to similar stressors
- The biological approach allows for objective, measurable assessments of stress responses
- Research has identified important links between chronic stress and serious health conditions
Weaknesses:
- Prolonged HPA activation can lead to serious health problems including Cushing's syndrome, characterised by weight gain, memory problems, and attention difficulties
- People respond more actively to stressors involving cognitive and emotional factors, which the purely biological model doesn't fully capture
- Some individuals who have had their adrenal glands surgically removed still require hormonal support to manage stressors, highlighting the crucial role of both the HPA system and SMP in stress management
Key Points to Remember:
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The body has two main stress response systems: the sympathomedullary pathway handles acute stress through fight-or-flight responses, whilst the HPA system manages chronic stress through cortisol release
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Selye's General Adaptation Syndrome describes three stages: alarm reaction (immediate response), resistance (coping phase), and exhaustion (resource depletion)
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Gender differences exist in stress responses: men typically show fight-or-flight whilst women often display tend-and-befriend behaviours
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Chronic stress has serious health consequences: prolonged cortisol release can impair memory, reduce immune function, and contribute to various physical and mental health problems
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Research evidence supports biological stress models: studies across different species and populations consistently demonstrate the physiological basis of stress responses