Theoretical Framework For Understanding Stress Responsive Systems

On some level, we all have an intuitive understanding of stress as a psychological concept. Most people would define stress as a sense of internal pressure, nervous tension, anxiety, strain, or even a state of constant worry. While these terms are perfectly accurate and sufficient for communicating about stress in a colloquial setting, they are insufficient when we try to operationalize stress in a scientific manner. Indeed, scientists have argued for the better part of the last century about what would be an appropriate scientific definition for the term stress. In 1946, Hans Selye published a seminal paper describing a nonspecific biological response to physical stressors (Selye, 1946). His approach was based on the observation that the bodily consequences of physical trauma were independent of the nature of the precipitating insult. His ultimate synthesis was encapsulated as the general adaptation syndrome (GAS), which remains one of the guiding theories for stress research today. According to Selye, the GAS consisted of three successive stages of adaptation to insult: (1) the alarm reaction, (2) the stage of resistance, and (3) the stage of exhaustion. During the alarm reaction, or acute, phase of the GAS, the organism's general resistance to the stressor falls below normal. Then, as adaptation is acquired in the stage of resistance, the capacity to resist rises above normal. Small, repeated, or modest but continuous exposure to the precipitating stressor will be adequately handled by the organism during the stage of resistance. Eventually, continuous exposure to the stressor will lead to a state of exhaustion, in which organisms' defenses against further challenges systematically erode. This usually occurs when the agent or insult begins to overwhelm the capacity of the physiological systems to effectively respond. The exhaustion phase is where pathological processes begin to emerge.

While these stages have proven applicable to psychological stressors, it is important to note that Selye was trained as a physician. Thus, the basis of his original theory rested primarily within the bounds of physiological insult, such as pharmacological challenges, exposure to a cold environment, infections, and surgery. Nevertheless, the original framework provided by Selye continues to be a guiding light for stress researchers to this day and provides an effective springboard toward a more modern synthesis of stress as a psychological concept.

With this in mind, there are several key considerations regarding stress that must be conveyed from the outset. The first consideration is that of stimulus versus state. Irrespective of how we define stress, it is imperative that we distinguish between the stimulus that induces stress (referred to as the stressor) and the ultimate state that is produced in the individual by that stimulus (referred to as stress). Consider a deer meandering down to a stream for a drink of water, with a mountain lion poised ready to attack. Upon detection of the predator, the deer experiences an immediate rush of physiological, affective, and cognitive alterations that may be globally characterized as a heightened state of arousal. In this scenario, exposure to a predator is the precipitating stimulus, or stressor, that clearly elicits an internal state of stress in the host organism. This nomenclature, originally developed by Selye (1946, 1956), remains relevant for a proper discussion of the relationship between stress and major psychiatric illness.

In the above example, the presence of a predator is clearly identifiable as the environmental event that elicited the deer's stress reaction. Predator-prey interactions fit conveniently into the common parlance of stress terminology. However, if the deer had encountered a pathogen such as a virus or bacteria in the water rather than a mountain lion, a very similar complement of physiological, affective, and even cognitive alterations would likely ensue, although on a slightly delayed time course. Thus, the second consideration that we must take into account is that psychologically undetected physiological challenges representing threats to survival can elicit a stress reaction comparable to overt threats from the environment.

Perhaps the most important distinction between environmental and physiological stressors is that physiological stressors do not necessarily require cognitive appraisal, emotional evaluation, or conscious awareness to exert their effects. For instance, exposure to infectious agents (bacteria, viruses, toxins, etc.), hypoxia, hypoglycemia, and hypothermia are all examples of physiological stressors that elicit a stress reaction. Clearly, once the seriousness of the physiological challenge passes some identifiable threshold, the individual would normally develop a subjective experience of stress, which may then further activate stress responsive systems. Nevertheless, psychic stressors are qualitatively different than environmental ones and may activate stress responses via distinct pathways (Herman et al., 1996; Herman and Cullinan, 1997).

The final consideration is one of perception. It is common to view stress as a maladaptive, debilitating state that is best avoided. However, when we delve into the basics of stress physiology, it becomes clear that physiological responses to stress represent positive evolutionary adaptations. Most components of an organism's response to stress have evolved in such a manner as to promote an adaptive outcome (i.e., survival) under the given circumstances. However, this may not be the case when an organism is exposed to chronic stressors above and beyond those encountered in ancestral environments of evolutionary adaptation; repeated and sustained exposures to stressors eventually exhaust resources that are normally available for contending with more modest challenges, which in turn produce various adverse health outcomes [see McEwen (2000) for a superb review].

One conclusion that can be drawn from the discussion thus far is that regardless of the type of stressor an organism encounters (i.e., environmental, psychological, or physiological), there are several underlying characteristics that help us define a given event as a stressor: (i) It represents an immediate (real or perceived) threat to the individual, (ii) requires mobilization and coordination of multiple physiological systems (usually accompanied by increased metabolic demand), and (iii) necessitates behavioral adjustments that typically represent deviations from the prestressor agenda. Importantly, the successful implementation of these changes would normally promote survival, solidifying the adaptive nature of the organism's response to the stressor.

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