Physiology of Stress

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The Stress Response

Stress is a response to a threatening stimulus that we perceive is greater than the resources we have to deal with it. These resources may be within the environment or related to our own abilities.

Own judgement

A key thing to remember is that the stress response is triggered by a judgement which we make and we often incorrectly estimate the demands of a situation.
Regardless of whether we actually have the resources to cope with a situation or not, if we perceive we cannot, then the stress response will be induced.

Brain regions

Judgements about whether a stimulus is a stressor or not, are made in the cerebral cortex.
If a stimulus is judged to be a stressor, the cerebral cortex sends signals to the hypothlamus which triggers the stress response.

Sympathomedullary pathway

First described by Cannon (1932) as the ‘fight or flight’ response, this is the first stage of the stress response.
The sympathetic branch of the autonomic nervous system is activated. This causes the adrenal medulla (the central part of the adrenal gland) to release adrenaline and noradrenaline into the blood.

Effects of sympathetic arousal

The arousal of the sympathetic nervous system causes physiological changes that help the body prepare to deal with threat and prepare to either fight or flee. These changes are:
- Pupils dilate.
- Heart and breathing rate increase.
- Blood is diverted from digestion in the stomach to the muscles in the legs and arms.

HPA system

If the stress persists over a long period of time, the Hypothalamic Pituitary-Adrenal (HPA) System is activated.
As the sympathomedullary pathway is very intense, it uses a lot of bodily resources. So the HPA system works to support this by producing more fuel for the body.

General adaptation syndrome

Hans Selye first proposed the general adaptation syndrome in the 1930s. The general adaptation syndrome describes the body’s nonspecific physiological response to stress - that regardless of the type of stressor, the same pattern of reactions happens.

Hypothalamic Pituitary-Adrenal System and Stress

The HPA System is activated with the sympathomedullary pathway and produces fuel to support the ‘fight or flight’ response. But extended activation of this system has negative effects on the body.

Mechanism of the HPA

In response to stress, the hypothalamus releases corticotrophin-releasing hormone (CRH).
This stimulates the anterior pituitary gland to release adrenocorticotropic hormone (ACTH).
ACTH then activates the adrenal cortex (the outer part of the adrenal glands) to secrete a number of hormones into the bloodstream, including cortisol.

Cortisol

Cortisol is commonly known as the stress hormone, and helps provide that boost of energy when we first encounter a stressor, preparing us to run away or fight.
Cortisol converts protein to glucose, which provides this extra energy.
Cortisol also improves immune system functioning temporarily and decreases pain sensitivity.
In short bursts, this process is very beneficial.

Negative effects of cortisol

Prolonged, chronic stress leads to prolonged HPA system activation and so extended release of cortisol.
High levels of cortisol have been shown to produce a number of harmful effects, including weakening of the immune system.
High levels of cortisol have been regularly observed in individuals with psychological disorders such as depression.

Evolution

In the early years of humanity, most stressors would have been physical threats. So the physiological stress response would have been beneficial for survival.
But in today’s society, most stressors are psychological and long term. So the physiological stress response is often activated to no real gain, and may be doing us more harm than good.

The General Adaptation Syndrome and Stress

Hans Selye proposed the general adaption syndrome in the 1930s. The general adaptation syndrome is the body’s nonspecific physiological response to stress - regardless of the type of stressor, the same pattern of reactions happens.

Selye's observations

In the 1930s, Selye was researching sex hormones in rats.
Selye observed that rats became ill when exposed to prolonged negative stimulation (stressors) such as extreme cold, surgical injury, excessive muscular exercise and shock.
Selye referred to this as the general adaptations syndrome and proposed that there were three distinct steps.

Stage 1 - alarm reaction

The alarm reaction describes the body’s immediate reaction upon facing a threatening situation or emergency. It is roughly analogous to the fight-or-flight response that Cannon described.
During an alarm reaction, you are alerted to a stressor and your body alarms you with a cascade of physiological reactions that provide you with the energy to manage the situation.

Stage 2 - stage of resistance

If exposure to a stressor is prolonged, the organism will enter the stage of resistance.
During this stage, the initial shock of the alarm reaction has worn off and the body has adapted to the stressor.
Nevertheless, the body also remains on alert and is prepared to respond as it did during the alarm reaction, although with less intensity.

Stage 3 - stage of exhaustion

If exposure to a stressor continues over a longer period of time, the stage of exhaustion comes on.
At this stage, the person is no longer able to adapt to the stressor. The body’s ability to resist becomes depleted as physical wear takes its toll on the body’s tissues and organs.
Illness, disease and other permanent damage to the body - even death - may happen as a result.

Physical focus

You should be aware that this model is a response-based conceptualisation of stress that focuses exclusively on the body’s physical responses, while largely ignoring psychological factors, such as appraisal and interpretation of threats.
The model offers a general explanation for how stress can lead to physical damage and disease, but it's likely that there are psychological mediators at play too.