Understanding Stress

Characteristics of the Stressor
by National Institute of Health

(Page 2 of 6)

Several factors serve a fundamental role in determining the nature and consequences of the stress response. These factors include inherent features of a given type of stressor as well as the conditions under which the stressor is encountered.

Evaluating the Stress Response

In general, stressors may be psychogenic and/or neurogenic. Psychogenic stressors are purely of psychological origin (anticipating an adverse event, experiencing the death of a loved one, or caring for a chronically ill person). Neurogenic stressors involve a physical stimulus (a headache, bodily injury, or recovery from surgery).

In addition, environmental stressors can be classified as either processive or systemic. Processive stressors are those that require appraisal of a situation or involve high-level cognitive processing of incoming sensory information. Examples of processive stressors among animals include exposure to new environments, predators, or situations that trigger fear because of previous association with unpleasant stimuli. In contrast, systemic stressors are of physiological origin (disturbances of normal bodily metabolism resulting from bacterial or viral infection).

Herman and Cullinan have suggested that both processive and systemic stressors might activate the HPA axis through distinct but converging neurological circuits. Specifically, processive stressors may primarily activate the limbic system, a region of the brain comprising interconnected structures that are associated with arousal, emotion, and goal-directed behavior. Conversely, systemic stressors may more directly influence the hypothalamus, a brain structure with multiple regulatory functions that interacts extensively with the limbic system. In the absence of experimental evidence, it seems reasonable to speculate that processive stressors might be more closely associated with increased alcohol consumption than would systemic stressors.

When evaluating the impact of adverse events on an individual, a researcher or health professional must consider the specific nature of the stressor involved. Although most stressors elicit some common neurochemical and behavioral effects, their responses are not always identical.

In animal studies, researchers have employed a wide range of stressors to assess behavioral and biological outcomes. Some of these stressors are ethologically sound (they represent situations that the animal would ordinarily encounter in its natural environment and for which it may have developed natural, evolutionary defenses). Ethological stressors may include the sight or odor of predators, confrontation with unfamiliar members of the same species, or fear cues.

Other commonly employed experimental stressors include exposure to cold air, immersion in cold water, and mild electric shocks administered to the animal's foot or tail. In various studies, investigators have administered footshock and tailshock at varying intensities, thereby obtaining information on the effects of controllable (escapable) versus uncontrollable (inescapable) stressors. However, the generalizability of experimental results involving some stressors is limited. For example, the effects of exposure to cold air or cold water may reflect physiological processes specific to the generation of body heat rather than the psychological consequences of stress.

The nature of the stress response varies depending on the nature of the stressor and the stressor regimen. This fact is illustrated by the phenomenon of adaptation (a diminished response after prolonged or repeated exposure to a stressor). For example, within a single experimental session, the brain's chemical response exhibits adaptation to some stressors (restraint) but is less likely to occur in response to others (footshock or tailshock). Although the reason for this finding is unknown, one possibility is that restraint is continuous, whereas footshock is intermittent. Thus, drawing firm conclusions about the adverse effects of a stressor based on a specific stressor regimen can sometimes be difficult. The variability of the stress response may provide important clues to the identification of the psychological and physical processes that govern voluntary alcohol consumption.

Stressor effects in humans are more complex than in animals. Some investigations of the human stress response have been conducted under contrived conditions in the laboratory, and the meaningfulness of such studies may be limited. Studies that attempt to simulate natural conditions are more likely to produce realistic outcomes. Some of the latter studies rely on a person's recollection of past events (retrospective studies). The disadvantage of retrospective studies, however, includes the potential distortion of recall resulting from subsequent experience or the subject's current mental state. Prospective studies, which are less commonly employed, involve an initial baseline examination of the subject with subsequent followup evaluations.

Irrespective of the experimental approach, research clearly indicates that stressors, which are usually multidimensional, produce not only immediate actions but also protracted effects secondary to the primary stressor. For instance, stressful experiences are often followed by persistent brooding (rumination) that may in itself be stressful, and some events (bereavement) may have secondary effects (financial burden and loss of social support). Whereas some stressor effects may diminish over time (sadness, remorse, or guilt), the effects of other stressors may increase (financial burden and loss of social support). In addition, the stress response itself may function as a stressor. For example, symptoms of depression induced by stress may lead to interpersonal conflict or, conversely, social withdrawal, further exacerbating depression.

With respect to behavioral outcomes, some stressors (loss of social support) are more likely than others to provoke depressive symptoms, whereas other stressors (threats or impending stress) are more closely associated with anxiety symptoms. Surprisingly, stress-induced psychiatric pathology is often elicited not by a major adverse life event but by a series of relatively mild stressors. Furthermore, the effects of the minor stressors may be especially profound if they occur following a major stressful event.

The severity of stress-induced effects may be related to characteristics of the individual coupled with the nature of the stressor. Relevant stressor characteristics include the following: 1. the degree to which stress can be mitigated or eliminated by an appropriate response (controllability), 2. the predictability of onset of the stressor, 3. the duration or chronicity of exposure (either acute or over a relatively protracted period), and 4. the timing and frequency of exposure (intermittent).

Controllability and Coping

Perceived controllability clearly influences some stress responses. For example, uncontrollable stressors provoke behavioral disturbances in animals that are not induced by controllable stressors of comparable severity. Some investigators interpret these differences as the consequences of "learned helplessness". Other researchers interpret these findings in terms of the strain that such events place on the neurotransmitter systems in the brain. The excessive strain on, or the resulting variations of, neurotransmitters may increase an individual's vulnerability to pathological states. Indeed, studies in rodents have indicated that in some brain regions (the hypothalamus), the response of the neurotransmitter norepinephrine to uncontrollable stressors is more profound than that provoked by controllable stressors. Likewise, the controllability of stressors may differentially influence the functions of the neurotransmitters serotonin and dopamine in specific brain regions. In addition, some behavioral disturbances evoked by uncontrollable stressors can be mimicked by drugs that disrupt the functioning of these neurotransmitters. Conversely, treatments that attenuate the neurochemical alterations elicited by stressors limit such behavioral disturbances. In effect, an individual's response to a stressor may be dictated by the availability of appropriate coping strategies, and certain behavioral disturbances may be most pronounced under conditions where stressor controllability is not possible or where coping responses are ineffective.

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.