Understanding Stress

Characteristics and Caveats
by National Institute of Health

Exposure to stressful situations is among the most common human experiences. These types of situations can range from unexpected calamities to routine daily annoyances. In response to stressors, a series of behavioral, neurochemical, and immunological changes occur that ought to serve in an adaptive capacity. However, if those systems become overly taxed, the organism may become vulnerable to pathology. Likewise, the biological changes, if sufficiently sustained, may themselves adversely affect the organism's well-being. Several factors may dictate an individual's response to environmental stressors, including characteristics of the stressor (type of stressor and its controllability, predictability, and chronicity); biological factors (age, gender, and genetics); and the subject's previous stressor history and early life experiences. Research on the physiological and psychological responses to different types of stressful stimuli is presented, focusing particularly on processes that may be relevant to the development of alcohol use disorders. Stressful events may profoundly influence the use of alcohol or other drugs. For example, the resumption of AOD use after a lengthy period of abstinence may reflect a person's attempt to self-medicate to attenuate the adverse psychological consequences of stressors (anxiety). Alternatively, stress may increase the reinforcing effects of AODs.

Regardless of their degree of severity, however, stressors may promote physiological and behavioral disturbances, ranging from psychiatric disorders to immune system dysfunction. Stressful events also may profoundly influence the use of alcohol or other drugs. For example, the resumption of AOD use after a lengthy period of abstinence may reflect a person's attempt to self-medicate to attenuate the adverse psychological consequences of stressors (anxiety). Alternatively, stress may increase the reinforcing effects of AODs.

This article provides a working definition of stress and describes research on the physiological and psychological responses to different types of stressful stimuli, focusing particularly on processes that may be relevant to the development of alcohol use disorders.

Stress: A Working Definition

As commonly used, the term "stressor" indicates a situation or event appraised as being aversive in that it elicits a stress response which taxes a person's physiological or psychological resources as well as possibly provokes a subjective state of physical or mental tension. As relevant scientific data have accumulated, however, a simple, universally accepted definition of stress has become increasingly elusive.

This article focuses on some of the factors that may influence the mechanisms by which a person responds to stressful situations (stressors). Much of the information presented here is based on animal research, which can provide essential information not obtainable from human studies. However, the human stress response is influenced by a host of personality characteristics and life experiences that cannot be duplicated in animal studies. Other articles in this issue provide more specific information on possible interactions between stress and human behavioral responses, such as alcohol consumption.

Many researchers view the stress response as an adaptive mechanism designed to maintain the relative stability of the body's overall physiological functioning (homeostasis) in response to a challenge. However, not all stress responses are clearly adaptive. Some physiological reactions to stress that appear to confer short-term benefits are followed by adverse long-term repercussions. In other instances, changes that appear to have adverse consequences may, on closer examination, turn out to be beneficial. Finally, some changes that may have little positive value and no adaptive significance may yet comprise part of the overall stress response.

The ambiguity of the stress response can be illustrated by examining the functions of cortisol, a hormone released by the adrenal glands in response to stressful stimuli. Among other functions, cortisol helps promote the release of energy stores essential for coping with stress. Yet, cortisol may suppress the normal functioning of the immune system, a response that could theoretically render the body more susceptible to infectious diseases. However, cortisol-induced immune suppression also may serve a protective function, preventing the development of illnesses characterized by immune attack on the body's own tissues (rheumatoid arthritis). Even when cortisol release has adaptive consequences, the elevated cortisol levels persist for an extended period, then the adaptive nature of the response may be lost and adverse effects may ensue. Thus, what we consider to be an adaptive short-term response may subsequently provoke long-term pathophysiological consequences.

Because stressful stimuli often elicit cortisol secretion, some researchers have proposed the use of cortisol levels as an index of the stress response. However, not all events perceived as stressful lead to the release of hormones specifically associated with stress. Indeed, several other hormones and similar chemical messengers are extremely responsive to stressful stimuli and may influence the cascade of events activated by stress. Furthermore, positive stimuli may elicit physiological responses comparable in many respects to those provoked by adverse events, and increased cortisol release is not uniquely provoked by events perceived as stressful. For example, rats that were offered food in the laboratory exhibited activation of the hypothalamicpituitary- adrenal axis (HPA) identical to that elicited by stressful stimuli, such as physical restraint. HPA activation could arguably represent an anticipatory response to any strong stimulus, preparing the animal to respond appropriately. Alternatively, the presentation of food, at least in animals, may actually threaten to disrupt homeostasis. In that case, the stress response may help mobilize the body's physiological response to the potential onslaught of nutrients, which require digestion and absorption. In addition, food may naturally contain or be contaminated by any number of toxic compounds that must be eliminated or destroyed (by immune system activity or enzymatic degradation in the liver). Furthermore, in the wild, an animal approaching a food source may experience some risk from either predators or competitors. The evaluation of these hypotheses is complicated by individual differences in the perception and appraisal of a stimulus as stressful.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.