Stress

Stress may be defined as a threat, real or implied, to the psychological or physiological integrity of an individual. Al-though stress can be assessed as subjective experience, it is the behavioral and physiological responses to stress that are the most closely linked to measurable health outcomes.

Stress involves a stressor and a stress response. Stressors include trauma or injury, physical exertion at the limit of the body's capacity, and environmental factors like noise, overcrowding, and excessive heat or cold. Major life events are stressful, as are daily hassles in the family and workplace. Social isolation is stressful, and supportive social contact reduces the physiological stress response.

Stress responses may be both behavioral and physiological. Behavioral responses to stress may avoid trouble, but they may also exacerbate the consequences of stress: for instance, confrontational behaviors that exacerbate the stress, as well as self-damaging behaviors like smoking, drinking, and driving an automobile recklessly. Physiological stress responses involve the activation of the autonomic nervous system and the hypothalamo-pituitary-adrenal (HPA) axis.

The brain and behavior play an important role in determining what is stressful. The brain interprets what is stressful on the basis of past experience of the individual, and then determines the behavioral response; and the brain regulates the physiological stress response. The brain is also a target of stress, which increases activity of systems that subserve fear (the AMYGDALA) and impairs systems that subserve declarative, episodic, spatial, and contextual MEMORY (the HIPPOCAMPUS; see also NEUROENDOCRINOLOGY).

Hans Selye described the "general adaptation syndrome" in response to stressors, which consists of the outpouring of catecholamines and cortisol as a common response to many noxious situations. There are two important features of the physiological stress response: (1) turning it on in amounts that are adequate to the challenge, and (2) turning it off when it is no longer needed. The physiological mediators of the stress response, namely, the catecholamines of the sympathetic nervous system and the glucocorticoids from the adrenal cortex, initiate cellular events that promote adaptive changes in cells and tissues throughout the body, which in turn protect the organism and promote survival.

The physiological systems that react to stress are important protectors of the body in the short run but cause damage and accelerate disease over long periods of time. Thus "good stress" involves the adaptive response to acute stress, and "bad stress" involves chronic stress, with sustained physiological responses that produce wear and tear on the body and brain over months and years. Stressful experiences can often be exhilarating to some individuals, whereas prolonged stress is generally not beneficial. "Bad stress" occurs when the stress response is stimulated frequently or when it does not shut off when not needed. The price of adapation, involving wear and tear on the body, has been called "allostatic load." Allostasis, meaning "achieving stability through change," describes the process of physiological adaptation, and allostatic load refers to a gradual process of wear and tear on the body. Examples of allostatic load include the exacerbation of atherosclerosis by psychosocial stress; stress-induced acceleration of abdominal obesity; hypertension and coronary heart disease resulting from job strain; bone calcium loss in depressive illness and as a result of intensive athletic training; and atrophy and damage to nerve cells in the hippocampus with accompanying memory impairment.

The vulnerability of many systems of the body to stress is influenced by experiences early in life. In animal models, unpredictable prenatal stress causes increased emotionality and increased reactivity of the HPA axis and autonomic nervous system, and these effects last throughout the lifespan. Postnatal handling in rats, a mild stress involving brief daily separation from the mother, counteracts the effects of prenatal stress and results in reduced emotionality and reduced reactivity of the HPA axis and autonomic nervous system, and these effects also last throughout the lifespan. The vulnerability of the hippocampus to age-related loss of function parallels these effects -- prenatal stress increasing and postnatal handling decreasing the rate of brain aging.

Stress and stress-related disorders represent one of the most common complaints that physicians encounter, and these are estimated to cause a loss of $300 billion annually in lost productivity and medical expenses. Some examples show what stress means for health. Social conflict and the formation of hierarchies of dominance produce measurable effects in the body and brain: for example, changes in brain structure and function, and acceleration of coronary artery atherosclerosis. Stressful experiences and lack of social support have been reported to increase susceptibility to the common cold, and social support is a protective factor.

In Russia, there has been an increase of almost 40 percent in the mortality rate among men since the fall of communism. Less dramatic but still very meaningful are the gradients of health across socioeconomic status, as exemplified by the British civil service system. The lowest employment grades have increased overall mortality and increased rates of cardiovascular disease and abdominal obesity.

One of the most important aspects of stress related to disease is the sense of control. Learned helplessness is a condition that has been described in animals and humans and represents one type of coping mechanism. Less extreme, the lack of control on the job has been shown to have adverse health consequences, affecting rates of cardiovascular disease. Interventions that have increased the sense of control and reduced time pressures have increased physical and mental health.

Stress is a highly individualized experience: events that are stressful for one individual may not be stressful for others; and stress may be exhilarating and even beneficial in the short run. Individual differences in stress responses are traceable to life experiences during development and adult life, as well as to individual differences in physiological vulnerability based on genetic background and physical and mental health. Most individuals cope with stressful situations and adapt, as long as the stress does not continue for long periods. Nevertheless, prolonged stressful experiences and more subtle aspects of stress physiology captured under the term allostatic load do produce a gradual wear and tear on the body and brain that can accelerate the onset and severity of diseases.

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-- Bruce S. McEwen

Stress

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