The Physiological Response to Stress

• Selye (1956) developed the GAS model of stress, arguing the physiological effect of stressors was to leave humans and animals vulnerable to illness
• Selye developed his model after measuring the effects of stress on rats suffering various stressful conditions, such as extreme cold, electric shocks and invasive surgery, and generalising the findings to humans 

• The GAS model comprises three stages:

Stage 1 - Alarm	Stimulated by the hypothalamus, the autonomic nervous system (ANS) releases adrenaline and noradrenaline (stress hormones) into the bloodstream: heart rate and blood pressure both increase in a fight or flight response
Stage 2 - Resistance	If the stressor continues, the fight-or-flight response ceases but stress hormone output from the adrenal cortex continues and the adrenal glands may become enlarged
Stage 3 - Exhaustion	If the stressor continues for a long time, the body’s resources are reduced,the immune system may be damaged and stress-related diseases such as high blood pressure and cardiovascular disorders are likely to occur

• The sympathomedullary pathway (SMP) is made up of the sympathetic nervous system (SNS) and the sympathetic adrenal medullary system (SAM) 
• The SNS is the division of the autonomic nervous system that produces localised adjustments (such as sweating as a response to an increase in temperature) and automatic adjustments of the cardiovascular system and a state of heightened emotion and arousal
  • The SAM stimulates the release of adrenaline and noradrenaline into the bloodstream from the adrenal glands in the adrenal medulla 
  • The SNS and the SAM prepare for ‘fight-or-flight’  by boosting the supply of oxygen and glucose to the brain and muscles through raising the heart rate and blood pressure
  • Thus the SMP is the physiological system that responds to acute short-term stressors
• After the acute stressor is dealt with, the body returns to homeostasis through activation of the parasympathetic branch of the nervous system, conserving energy and resting 

Alarm stage of GAS showing the role of the SMP

• The Hypothalamic Pituitary-Adrenal system (HPA) is responsible for arousing the ANS in response to a stressor 
• The HPA response happens at the same time as the SMP is activated, but the whole process takes much longer and is longer-lasting
• The hypothalamus produces a chemical (corticotropin-releasing factor, CRF) which causes the pituitary gland to release adrenocorticotropic hormone (ACTH)  into the bloodstream 
• Once the adrenal cortex detects the ACTH in the bloodstream, it releases cortisol, another stress hormone cortisol, which restores energy to help the body cope with the stress
• High cortisol levels trigger a reduction in CRF and ACTH, and once the stressor is removed, the cortisol levels also drop

HPA and cortisol release

• Because of its immunosuppressive properties , over a period of time cortisol can damage the immune system
• This damage happens if stress passes from the acute stage to a longer-lasting chronic stage, such as stage 3 of Selye’s GAS, when increased levels of cortisol reduce the production of antibodies to fight infection
• Prolonged stress has also been linked to heart disease and strokes

Prolonged exposure to cortisol can cause many health problems

Research which investigates the physiological response to stress

• Kiecolt-Glaser et al (1984) measured natural killer cells (that protect against infection) in blood samples from American college students one month before their exams and then during the exams and found lower levels of these immunity-protecting cells in the second sample taken in a stressful situation
• Timio et al. (1988) conducted a longitudinal study comparing nuns (who are protected from chronic stress by their lifestyle) with working women, and found the nuns’ blood pressure was unchanged while the working women had raised blood pressure, suggesting long-term stress has a physical effect on health
• Taylor et al. (2000) found that acute stress produces the fight-or-flight response in men but a different 'Tend-and-befriend' response in women,demonstrating a gender difference in the activation of the SMP

Evaluation of the physiological response to stress

Strengths

• GAS was the first theory to explain the physiological effects of stress, influencing many later theories and a lot of research, especially into the negative effects of stress upon health.
• Research into the physiological response to stress has had positive implications for helping people cope with stress, especially with its contribution to research into and the development of medicines to reduce the physiological response to stress.

Weaknesses

• Research fails to consider the effect of psychological processes on how we physically respond to stress, as it could be that those with more resilience respond in a different physiological way
• Most of Selye’s research was on rats and his theory assumes that the response remains the same to all stressors, which is not the case with humans, who have individual responses, depending on the stressor

Link to Issues and Debates:

The GAS model, in its universal approach and failure to account for psychological and gender differences in responses to stress, is reductionist and also takes a nomothetic approach by ignoring the complexity of the stress responses in different circumstances. 

Beta bias is the attempt to downplay the differences between the genders and Selye’s argument that the fight-or-flight response is universal ignores the fact that according to Taylor et al (see above) and Lee and Harley (2012) that females ‘tend and befriend’ which makes Selye’s theory beta-biased.

Link to Approaches:

The physiological response to stress takes a Biological Approach in its suggestion of a universal animal and human response to any stressor. Selye’s research on rats suggested that there is one bodily stress response, supporting the notion of the GAS. This universal argument is characteristic of the biological approach.