The Transactional Model
Stress is a term borrowed from engineering where it refers to tension placed on a metal by a load. Humans describe stress as both something in the environment which makes them feel under pressure. Something they feel as a result of that pressure.
The most modern psychological view of human stress sees it as a mismatch between the perceived demands of a situation and ones perceived ability to cope.
The word perceived is important here since everyones idea of what is stressful is different. The view of stress is called the Transactional Model it recognises the role of cognition in the experience of stress and accept that there are individual differences in how people perceive and react to stressful situation. It also offers cognitive strategies to help people cope.
Some people thrive on being busy with many projects on the go at a time. Others would find this extremely stressful, preferring to finish one project before beginning another.
The General Adaptation Syndrome
Psychologists have not always recognised the role of individual differences in the experience of stress.
In the 1930s and 40s Hungarian endocrinologist, Hans Selye 1956- injected mice with extracts of various organs looking for a new hormone. He soon found that every irritating substance he injected produced the same symptoms. Swelling of the adrenal cortex, shrinking of the thymus gland, gastric and duodenal ulcers. He also noted that people with difference diseased exhibit similar symptoms.
This led to his description of the effects of 'noxious agents' as he at first called it. He later coined the term 'stress' and came up with a theory called the general adaptation syndrome GAS. To explain how animals coped with stressors on a physiological level.
Selye Believed that this response was the same in animals and humans and that it was universal, the same response no matter what the stressor.
The GAS has three distinct parts.
When we are surprised or threatened we have an immediate physical reaction, often called the Fight or Flight reaction.
This prepares the body for life threatening situations, channeling away resources from organs such as the digestive system towards more immediate muscular and emotional needs.
As levels of stress related hormones rise, the body is placed on high alert. Heart and breathing rate increase, blood pressure increases and energy reserves are mobilised. This reaction was useful during out evolutionary past when we had to run away from or fight a predator.
In the modern world, preparation for fight or flight does little to help us cope with a driving test or with the stress of having to meet a deadline.
The alarm phase is often maladaptive since the body is prepared by there is no subsequent physical activity to reduce the arousal.
Resistance and Exhaustion
As we become used to the stress levels, we initially become more resistant to stress and to illness, which leads us to believe we can easily adapt to these more stressful situations.
Stress hormones and bodily arousal remain high.
Eventually our bodies literally run out of stress hormones. During this phase, we are no longer able to resist stress. Parts of the body can start to break down and we can suffer from stress related illness.
SAM Sympathetic Adrenomedullary Pathway
The GAS is mediated by two different body systems both involved the adrenal glands and both controlled by a brain structure called the hypothalamus.
SAM Sympathetic Adrenomedullary Pathway-
Hypothalamus activates sympathetic branch of autonomic nervous system ANS
Sympathetic arousal increases heart rate and blood pressure slows digestion and mobilises fats and sugars.
Adrenal medulla releases adrenaline an noradrenaline hormones which maintain sympathetic arousal.
HPA Hypothalamic Pituitary Adrenal Axis Response.
The second is a slower back up response known as HPA Hypothalamic Pituitary Adrenal Axis Response-
Hypothalamus stimulates pituitary gland
Pituitary gland releases adrenocorticotrophic troph to grow and adrenocortico cortex or outside of adrenal gland hormone or ACTH
Adrenal cortex releases corticosteroids into bloodstream.
Liver releases energy and immune system is suppressed.
Selye's work has been influential in developing research into stress. He describes the SAM pathway and HPA axis and demonstrated the link between chronic stress and illness.
This work was carried out on animals. Unlike humans, animals lack cognitive awareness of their situation and this makes it very difficult to generalise.
He also believed that the GAS was a universal response to all stressors which is now known to be untrue.
Stress related illnesses are now thought to be caused by a long term increase in, rather than a using up of, stress hormones such as cortisol which can damage the body.
Stress Related Illness and the Immune System
Selye believed that in the exhaustion phase of his GAS, the body literally ran out of hormone and couldn't cope with further stress.
We now know it's not running out of hormones, but prolonged exposure to high levels of stress hormones, such as cortisol, which can damage the immune system and leads to illness.
The immune system functions to defend us against viral or bacterial attack. Key players in the immune system are the white blood cells - leukocytes made in the bone marrow and circulating in the bloodstream.
Some of these white blood cells provide leukocytes made in the bone marrow and circulating in the bloodstream. Some of these white blood cells provide non specific or natural immunity by attacking and ingesting any invading bacteria or viruses they happen to come across.
Cells which carry out natural immunity include phagocytes, macrophages and natural killer NK cells. Other white lymphocytes are responsible for mounting a specific immune response to a foreign particle antigen.
Lymphocytes can be divided into two classes, T cells and B cells. T cells mature in the thymus gland and are responsible for cell based immunity, while B cells mature in the bone marrow and are responsible for antibody based, blood stream humoral immunity. To help them perform their functions T cells come in different forms.
Killer T cells seek out and destroy cells recognised as foreign in transported tissues and cells infected with antigens such as viruses and bacteria.
Memory T cells have a system that remembers the chemical characteristics of antigens. If encountered again, the response to that antigen is faster and more effective.
Helper T cells respond to infection by stimulating increased production of both T and B lymphocytes.
T cells attack antigens which are in the body's cells but B cells destroy invading agents while they're still in the blood stream and before they enter the body's tissues. They do this by producing larger proteins called antibodies which attach themselves to the viruses or bacteria, slow them down and make it easier for them to be destroyed by other immune cells.
Stress and Immune Systems
The immune system is very complicated and stress response hormones can affect it directly.
Willis et al 1987- High levels of corticosteroids can shrink the thymus gland, preventing the growth of T cells. The immune system is also more sensitive to stress than was thought, Short lasting life events, such as brief marital strife, can cause immonosuppression while long term life stress causes long term reductions in immune function.
Sapolsky 1994- Exercise, diet and social support can improve immune function. The immune system is self regulating and will recover once the stressor is removed.
The effects of stress on the immune system: Infection-physiological changes during a stress reaction, act to direct resources away from the immune system making it function less effectively. The person is therefore more likely to pick up infection and take longer to recover from it. Indirect effects-high levels of corticosteroids during periods of chronic stress inhibit immune cell functioning. Inflammatory Disorders-chronic stress affects the immune system's ability to deal with pre existing inflammatory disorders such as eczema and psoriasis. These conditions get worse.
Research studies into stress and the immune system have focused on attempting to measure the effects of stress directly, by measuring levels of immune cells in the blood stream, or indirectly by assessing physical illness or wound healing in stressed individuals.
Laudenslager et al 1983- conducted laboratory studies in which rats were given series of escapable shocks, identical inescapable shocks or no shock. The rats were re exposed to a small amount of shock 24 hours later, after which their immune systems were challenged with an antigen and their responses noted. The number of lymphocytes produced in response to the test was reduced in the inescapable shock group but not in the escapable shock group. This suggests that stress does suppress the immune system in rats especially when it can't be controlled.
Attempting to research the effects of stress on the immune system in humans can be difficult. It's not ethical to deliberately stress humans measure the effects. Psychologists have to find situations where stress is occurring naturally and monitor the effects, or rely on people providing an accurate record of all stressful events they've experienced during a given period of time usually a year.
Measuring immune functioning is also problematic. Researchers can measure physical illness such as cold symptoms or time taken for a wound to heal but this doesn't directly relate to the state or a person's immune system. Alternatively, they can measure the number of T cells in a sample of blood by reduction in T cells doesn't necessarily mean that the person will be more susceptible to illness.
Cohen et al 1993- to see if general life stress affects vulnerability to the common cold virus, 394 participants general population. Stress measure, number of stressful events in previous year, personal rating of stress, level of negative emotions. Immune system measure, development of clinical cold after 7 days post infection. Failure to fight off cold virus correlated with stress scores. Life stress and negative emotions reduce the effectiveness of the immune systems.
Kiecolt Glaser et al 1984- studied the immune function of medical students during their exams, taking blood samples a month before and on the day of their first exam.Levels of virus-fighting 'killer-t' leucocytes (white blood cells) was significantly reduced in the second sample. Participants also completed psychological questionnaires, and the difference in blood counts was greatest for those who reported highest levels of anxienty and social isolation. The main conclusion seems to be that the processes involved in stress have an effect on the body which can be harmful to health. In the fight-or-flight response, hormones are released which boost an individual's chances of short-term survival in a fight or pursuit. The long-term effect of this may be harmful. But in our evolutionary past, most stressors were short-lived - most unlike a university exam diet.
Although Cohen et al's study demonstrated a correlation between stress and ability to resist illness, the researchers didn't measure the immune system directly so they can't say that stress reduces its function. However Keicolt Glaser et al 1984 have shown reduction of natural killer cell activity in medical students experiencing exam stress.
These effects were high in those students who also reported feelings of isolation, or in other words were already stressed. Another problem with Cohens study is that it used correlational analysis and therefore, it's difficult to determine cause and effect. It's also difficult to know whether one part of the stress measure was more important than another in affecting ability to resist illness.
Keicolt Glaser et al 1995 small wounds take longer to heal in carers of Alzheimers patients.
Cohen et al 1995 blister wounds on the arm of married couples heal more slowly after conflictive rather than supportive discussions.
Factors Mediating the Effects of Stress
Much research has been carried out to try to understand how stress affects the immune system and psychologists now realise that the relationship between stress and the immune system is a complex one. Some types of short term stress can improve the immune system and different types of stressor can have different effects. In addition, there are age and gender differences in the body's ability to manage stress.
An American National Consumer League survey 2003- found that people under the age of 65 were more likely to report being stressed than older people.
Segerstrom and Miller 2004- claim that elderly people may be more vulnerable to stress related illness because age makes it harder for the immune system to regulate itself.
The National Consumer League Survey 2003- in terms of gender found that women were more likely to report problems and feelings of stress.Kiecolt Glaser et al 2001 women show greater immunosuppression following marital conflict.
Segerstrom and Miller 2004- have carried out a meta review of over 293 studies into the effects of stress on the immune system. Type of Stressor-Acute short lasting. Effect- Upregulation. Part involved- Natural immunity. Type of stressor - Brief Naturalistic. Effect- no overall ill effects. Part involved- shift from cellular to humoral immunity. Type of stressor- chronic long lasting. Effect- downregulation. Part involved- natural and specific immunity. Type of stressor- non specific life events. Effect- decrease in production. Part involved- natural killer cells, over 55s only.