The Physiology of Stress including: general adaptation syndrome (GAS), the hypothalamic pituitary-adrenal system (HPA) and the sympathomedullary pathway (SAM) and the role of cortisol.

AO1, Description:

Definition of Stress: “When an imbalance or discrepancy exists between perceived demands and perceived coping resources, then a state of stress exists.” Cox and Mackay (1978).

How does the body respond to stress?

The stress response is important for survival in animals because the physiological changes associated with stress are essential in conditions of fight or flight (i.e. attacking or running away).

The stress response is therefore thought to be:

  • An innate, defensive and adaptive response that should promote survival.
  • A bodily response which enables an animal to react quickly to potentially dangerous stimuli.

Anything that prompts the body to respond in this biological way by becoming stressed is known as a ‘stressor’.

Definition of a stressor: A stressor refers to any event that causes a stress reaction in the body. Stressors include environmental stressors (such as the workplace). Stressors also include life events (such as illness or divorce).

For an examination question in this area, you will need to know what happens in the body when we are stressed. You will need to know:

1. The Sympathetic-Adrenal-Medullary Pathway (SAM) ~ response to acute stress

2. The Hypothalamic-Pituitary-Adrenal System (HPA) ~ response to chronic stress

3. General Adaptation Syndrome –GAS, Selye

The Structure of the Nervous System

Nervous System

These parts of the nervous system work in opposition to each other and so only one part of the system can be activated at any one time. That is, they work antagonistically.

AO1, Description, The Body’s Response to Stress – The Fight or Flight Response

(1) The Sympathetic-Adrenal-Medullary Pathway (SAM):

For any event to cause a ‘stress response’ in the body, that event has got to be appraised by an individual as being stressful (i.e. not all individuals will find the same situations stressful).

Stress Response SAM 1Stress Response SAM 2Stress Response SAM 3

Evaluation, AO3 of the SAM (Acute Stress) Response


(1) Point: Research has suggested that there are gender differences when it comes to the way individual’s respond to acute stressors. Example: For example, Taylor et al (2000) found that acute stress produces the fight-or-flight response in men but the tend-to-befriend response in women. This different effect occurs as women produce more oxytocin, a chemical promoting relaxation and nurturing. Evaluation: This therefore suggests that the acute stress response has a different outcome depending on the individual – it should not be assumed that all individuals will react in the same physiological way to an acute stressor.


(2) Point: Research has suggested that age also affects an individual’s physiological response to stress. Example: For example, McCarty (1981) found that blood plasma levels of adrenaline and noradrenaline were equal in rats of varying ages before subjecting them to one minute of inescapable electric shocks to the feet (acute stressor), however, older rats had lower levels of adrenaline and noradrenaline than younger rats after being shocked. Evaluation: This therefore suggests that diminished responsiveness of the sympathomedullary pathway clearly explains the reduced capacity of aged animals to adapt to stressful situations – further highlighting that individual differences play a part in the physiological response to stress.


(3) Point: However, lots of research into the acute stress response has focused on the use of animals in research. Example: For example, McCarty (1981) used rats in his study in order to investigate the individual differences in the sympathomedullary stress response. Humans and rats are physiologically different and therefore it can be considered that their response to stress will not be comparable (humans are more likely to have a cognitive element also to their stress response). Evaluation: This is a weakness because it means that the findings from animal research (such as McCarthy) cannot be generalised or extrapolated to humans meaning the findings surrounding the human acute stress response is limited.


(2) The Hypothalamic-Pituitary-Adrenal System (HPA)

Stress HPA 1Stress HPA 2

It takes about 20 minutes for this process to be complete, with cortisol levels typically rising sharply 20 minutes after the initial perception of a stressor.


Evaluation, AO3 – Points to Consider in Relation to the HPA Response to Stress:


(1) Point: Scientific and objective research has highlighted the importance of the HPA stress response in adaptive human behaviour. Example: For example, research has suggested that individuals who had to have their adrenal glands surgically removed had to be given hormonal supplements in order to combat stressors. Evaluation: This is a strength because such research illustrates the crucial role of the HPA system in dealing with the physiological effects of stress and remoting a response that is adaptive and aids individual survival.


(1) Point: Research has suggested that there are individual differences in a person’s physiological response to a chronic stressor. Example: For example, Mason (1975) found that different individuals produce different levels of stress hormones when exposed to the same stressor. Evaluation: This therefore suggests that individuals appraise stressful situations in different ways. As a result, researchers shouldn’t assume that there is a universal response to stress and should recognise that individual cognitive processing can affect to what degree an individual will experience the chronic stress response.

(2) Point: Many researchers have questioned the idea that the Hypothalamic-Pituitary Adrenal response to stress is adaptive and aids our survival. Example: For example, research has suggested that prolonged activation of the HPA system can lead to Cushing’s syndrome which produces weight gain, memory and attention lapses as well as mood disruption. It is caused by external exposure to high levels of cortisol, often resulting from lengthy periods of stress. Evaluation: This is a weakness because, the body’s stress response is seen as an adaptive process that allows individuals to deal with potential threatening situations effectively thus aiding individual survival. The fact that prolonged exposure can lead to a decrease in immune functioning and can impact negatively on an individual’s health (e.g. weight gain, memory lapses etc…) suggests that such physiological response is in fact hindering individual survival suggesting the response to be maladaptive.

(3) The General Adaptation Syndrome (GAS)

The General Adaptation Syndrome (GAS) describes the physiological reactions that occur in response to stress. There are three stages to GAS:

(1) The Alarm Reaction Stage:

Involves physiological changes associated with emotional reactions to stressors. The hypothalamus signals the sympathetic nervous system, which activates and stimulates the adrenal medulla to secrete adrenaline and noradrenaline, which helps to increase; heart rate, blood flow and sugar-levels (this is commonly known as the fight or flight response).

(2) The Resistance Stage:

If the stressor continues the body recovers from the initial alarm and starts to cope with the situation. Sympathetic nervous system activity declines, thus adrenaline and noradrenaline secretion also declines but increases from another part of the adrenal gland – the adrenal cortex. The adrenal cortex is controlled by the amount of ACTH in the bloody, which in turn is controlled by the hypothalamus. Levels of glucocorticoid hormones (mainly cortisol) are increased in the blood sugar and these provide the energy (in the form of glucose) to resist the stress.

(3) The Exhaustion Stage:

If the stress continues, bodily resources become depleted so that the adrenal gland cannot function efficiently, blood-sugar levels drop and physical health is affected. E.g. high blood pressure, heart disease, ulcers etc can develop.


Evaluation, AO3 – Points to Consider in Relation to the SAM Response to Stress


(1) Point: A strength of the General Adaptation Syndrome model is that there is research to support its assumptions. Example: For example, Selye (1936) experimented with rats, subjecting them to various stressors including extreme cold, excessive muscular exercise and surgical injury. He found that the same typical collections of responses occurred (a syndrome) regardless of the stressor. This was a general response of the body that could not be attributed to any specific injury, a bodily reaction to damage that appeared after 6 to 48 hours. Selye was then able to track the rats’ response to a continuing stressor through the resistance and exhaustion stages. Evaluation: This is a strength because such findings provide evidence of the three stages of GAS, the research shows that they body’s general response to a stressor is as physiological reality (at least in rats).


(1) Point: However, a weakness of research aiming to support GAS is that much of the research is conducted on animals (e.g. rats). Example: For example, Selye (1963) has used rats to investigate the different stages of the stress response. Rats are physiologically different to humans, the types of stressors that rats are exposed to in experiments (e.g. exposure to extreme temperatures) may not cause the same levels of stress/a stress response in humans (differences in emotional intelligence between rats and humans). Evaluation: This is a weakness because research that is carried out on on-human animals (rats) cannot be generalised to humans as humans and animals are physiological different, the way a rat responds to stress may be very different to the way a human responds to stress.

(2) Point: Another weakness is that research has challenged the GAS assumption that the stress response is non-specific (i.e. the response is always the same regardless of the stressor). Example: For example, Mason (1971) replicated Seyle’s research with monkeys. Mason measured the monkeys’ levels of urinary cortisol and found varied effects dependent on the stressor (e.g. extreme cold increased cortisol levels, heat reduced them and excessive exercise produced no change). Evaluation: This is a weakness because, these findings question the validity of the key assumptions of the GAS model. They challenge a central concept of Selye’s theory by showing that specific stressors can produce specific patterns of responses.