Physiological or biological stress is an organism’s response to a stressor such as an environmental condition. Stress is the body’s method of reacting to a condition such as a challenge or physical and psychological barrier. Stimuli that alter an organism’s environment are responded to by multiple systems in the body. The autonomic nervous system and hypothalamic-pituitary-adrenal (HPA) axis are two major systems that respond to stress. The sympathoadrenal medullary (SAM) axis may activate the fight-or-flight response through the sympathetic nervous system, which dedicates energy to more relevant bodily systems to acute adaption to stress, while the parasympathetic nervous system returns the body to homeostasis. The second major physiological stress, the HPA axis regulates the release of cortisol, which influences many bodily functions such as metabolic, psychological and immunological functions. The SAM and HPA axes are regulated by a wide variety of brain regions, including the limbic system, prefrontal cortex, amygdala, hypothalamus, and stria terminalis. Through these mechanisms, stress can alter memory functions, reward, immune function, metabolism and susceptibility to diseases. Definitions of stress differ; however, one system proposed by Elliot and Eisdorfer suggests five types of stress. The five types of stress are labeled “acute time-limited stressors”, “brief naturalistic stressors”, “stressful event sequences”, “chronic stressors”, and “distant stressors”. An acute time-limited stressor involves a short term challenge, while a brief naturalistic stressor involves an event that is normal but nevertheless challenging. A stressful event sequence is a stressor that occurs, and then continues to yield stress into the immediate future. A chronic stressor involves exposure to a long-term stressor, and a distant stressor is a stressor that isn’t immediate.
There likely is a connection between stress and illness. Several studies have suggested such a link, theories of the stress–illness link suggest that both acute and chronic stress can cause illness. According to these studies, both acute and chronic stress can lead to changes in behavior and in physiology. Behavioral changes can include smoking, changes in eating habits and physical activity. Physiological changes can include changes in sympathetic activation or HPA activity, and immunological function. However, there is much variability in the link between stress and illness.
The HPA axis regulates many bodily functions, both behavioral and physiological, through the release of glucocorticoid hormones. The HPA axis activity varies according to the circadian rhythm, with a spike in the morning. The axis involves the release of corticotropin releasing hormone and vasopressin from the hypothalamus which stimulates the pituitary to secrete ACTH. ACTH may then stimulate the adrenal glands to secrete cortisol. The HPA axis is subject to negative feedback regulation as well. The release of CRH and VP are regulated by descending glutaminergic and GABAergic pathways from the amygdala, as well as noradrenergic projections. Increased cortisol usually acts to increase blood glucose, blood pressure, and surpasses lysosomal, and immunological activity. Under other circumstances, however, the activity may differ. Increased cortisol also favors habit based learning, by favoring memory consolidation of emotional memories.
Selye demonstrated that stress decreases adaptability of an organism and proposed to describe the adaptability as a special resource, adaptation energy. In recent works, it is considered as an internal coordinate on the “dominant path” in the model of adaptation. Stress can make the individual more susceptible to physical illnesses like the common cold. Stressful events, such as job changes, may result in insomnia, impaired sleeping, and physical and psychological health complaints. Research indicates the type of stressor (whether it’s acute or chronic) and individual characteristics such as age and physical well-being before the onset of the stressor can combine to determine the effect of stress on an individual. An individual’s personality characteristics (such as level of neuroticism), genetics, and childhood experiences with major stressors and traumas may also dictate their response to stressors.
Chronic stress and a lack of coping resources available or used by an individual can often lead to the development of psychological issues such as delusions, depression and anxiety (see below for further information). This is particularly true regarding chronic stressors. These are stressors that may not be as intense as an acute stressor like a natural disaster or a major accident, but they persist over longer periods of time. These types of stressors tend to have a more negative effect on health because they are sustained and thus require the body’s physiological response to occur daily. This depletes the body’s energy more quickly and usually occurs over long periods of time, especially when these microstressors cannot be avoided (i.e. stress of living in a dangerous neighborhood). See allostatic load for further discussion of the biological process by which chronic stress may affect the body. For example, studies have found that caregivers, particularly those of dementia patients, have higher levels of depression and slightly worse physical health than noncaregivers.
It has long been believed that negative affective states, such as feelings of anxiety and depression, could influence the pathogenesis of physical disease. However recent studies done by the University of Wisconsin-Madison and other places have shown this to be untrue, it isn’t stress itself that causes the increased risk of illness or death, it is actually the perception that stress is harmful. For example, when humans are under chronic stress, permanent changes in their physiological, emotional, and behavioral responses are most likely to occur and such changes could lead to disease. Chronic stress can include events such as caring for a spouse with dementia, or it results from brief focal events that have long term effects, such as experiencing a sexual assault. Studies have also shown that psychological stress may directly contribute to the disproportionately high rates of coronary heart disease morbidity and mortality and its etiologic risk factors. Specifically, acute and chronic stress have been shown to raise serum lipids and are associated with clinical coronary events.
However, it is possible for individuals to exhibit hardiness—a term referring to the ability to be both chronically stressed and healthy. Even though psychological stress is often connected with illness or disease, most healthy individuals can still remain disease-free after being confronted with chronic stressful events. This suggests that there are individual differences in vulnerability to the potential pathogenic effects of stress; individual differences in vulnerability arise due to both genetic and psychological factors. In addition, the age at which the stress is experienced can dictate its effect on health. Research suggests chronic stress at a young age can have lifelong effects on the biological, psychological, and behavioral responses to stress later in life. Recent studies have shown that severe psychological stress resulting in PTSD can also significantly affect parenting perception, behavior, neural activity and HPA-axis physiology in response to stressful parent-infant interactions. These recent studies support the existence of intergenerational effects of early chronic psychological stress.
In animals, stress contributes to the initiation, growth, and metastasis of select tumors, but studies that try to link stress and cancer incidence in humans have had mixed results. This can be due to practical difficulties in designing and implementing adequate studies.