The Biology of Stress
Stress, either physiological, biological, or psychological 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 threat, challenge or physical and psychological barrier. Stimuli that alter an organism’s environment are responded to by multiple systems in the body. In humans and most mammals, the autonomic nervous system and hypothalamic-pituitary-adrenal (HPA) axis are the 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 adaptation to stress, while the parasympathetic nervous system returns the body to homeostasis. The second major physiological stress-response center, 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 several 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. Disease risk is particularly pertinent to mental illnesses, whereby chronic or severe stress remains a common risk factor for several mental illnesses. One system suggests there are five types of stress 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.
- A Brief Natural 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 is not immediate.
Chronic Stress & The Psychology of Stress
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, such as Gang Stalking. 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 micro-stressors 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 non-caregivers.
When humans are under chronic stress, permanent changes in their physiological, emotional, and behavioral responses may occur. Chronic stress can include events such as caring for a spouse with dementia, or may result 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.
Chronic stress is seen to affect the parts of the brain where memories are processed through and stored. When people feel stressed, stress hormones get over-secreted, which affects the brain. This secretion is made up of glucocorticoids, including cortisol, which are steroid hormones that the adrenal gland releases, although this can increase storage of flashbulb memories it decreases long-term potentiation (LTP). The hippocampus is important in the brain for storing certain kinds of memories and damage to the hippocampus can cause trouble in storing new memories but old memories, memories stored before the damage, are not lost. Also high cortisol levels can be tied to the deterioration of the hippocampus and decline of memory that many older adults start to experience with age. These mechanisms and processes may therefore contribute to age-related disease, or originate risk for earlier-onset disorders. For instance, extreme stress (e.g., trauma) is a requisite factor to produce stress-related disorders such as post-traumatic stress disorder (PTSD).
Chronic stress also shifts learning, forming a preference for habit based learning, and decreased task flexibility and spatial working memory, probably through alterations of the dopaminergic systems. Stress may also increase reward associated with food, leading to weight gain and further changes in eating habits. Stress may contribute to various disorders, such as fibromyalgia, chronic fatigue syndrome, depression, and functional somatic syndromes.
Long-Term Effects of Stress on Health
Coping with the impact of chronic stress can be challenging. Because the source of long-term stress is more constant than acute stress, the body never receives a clear signal to return to normal functioning. With chronic stress, those same lifesaving reactions in the body can disturb the immune, digestive, cardiovascular, sleep, and reproductive systems. Some people may experience mainly digestive symptoms, while others may have headaches, sleeplessness, sadness, anger, or irritability.
Over time, continued strain on your body from stress may contribute to serious health problems, such as heart disease, high blood pressure, diabetes, and other illnesses, including mental disorders such as depression or anxiety.
Allostatic load is “the wear and tear on the body” which accumulates as an individual is exposed to repeated or chronic stress. The term was coined by Bruce McEwen and Stellar in 1993. It represents the physiological consequences of chronic exposure to fluctuating or heightened neural or neuroendocrine response which results from repeated or prolonged chronic stress.
The term is part of the regulatory model of allostasis, where the predictive regulation or stabilization of internal sensations in response to stimuli is ascribed to the brain. Allostasis involves the regulation of homeostasis in the body to decrease physiological consequences on the body. Predictive regulation refers to the brain’s ability to anticipate needs and prepare to fulfill them before they arise.