GENERAL ADAPTATION SYNDROME

General Adaptation Syndrome

General Adaptation Syndrome

Introduction

Modern ideas regarding stress and its effects on the body date from the classic work of physiologist Walter Cannon (1929). Cannon studied the physiological processes involved in the maintenance of homeostasis and observed what he called the fight-or-flight response (Sukiasyan et al., 2003). The fight-or-flight response involves a chain of biochemical reactions that begin with the stimulation of the cerebral cortex once the threat has been perceived. Among other responses, the cerebral cortex triggers the hypothalamus, which activates the sympathetic nervous system, which, in turn, stimulates the adrenal cortex to secrete cortisol and the adrenal medulla to secrete the hormones epinephrine and norepinephrine (catecholamines). The circulation of these hormones triggers the physiological processes associated with the fight-or-flight response, including increases in heart rate, respiration, blood pressure, blood volume, blood sugar, and blood flow to the muscles and brain, and inhibition of major bodily systems such as digestion and reproduction.

Together, these physiological changes act to increase oxygen to the brain and muscles to mobilize the system for action. These processes can provide a temporary means of survival for the organism; however, they may have undesirable physiological consequences in the event of prolonged activation.

In medical and psychological sciences, stress is a term used to describe a person's physiological, behavioral, cognitive, and emotional responses to events that are evaluated and appraised as threatening and challenging. Physiologically, stress is the nonspecific response of the body to any demand made on it. Stress causes a psychophysiological response called the general adaptation syndrome (GAS)—first described in 1936 by Hans Selye in the journal Nature.

The Work of Hans Selye

Hans Selye (1956) was the first to advance the notion of stress as a stimulus that elicits physiological responses to threat. Selye exposed laboratory rats to a variety of stimuli including toxins, insulin injections, electric shocks, and extreme temperatures (Selye, 1976). Extending Cannon's observation of catecholamines release, he noted that regardless of the stimulus, prolonged exposure activated the secretion of adrenocorticotropic hormone (ACTH), which, in turn, stimulated the secretion of corticosteroids. In the short term, corticosteroids serve to protect the body from harm by increasing energy levels through increases in blood sugar levels, reductions in immune response and inflammation of body tissue, and enhancement of muscle tone in the heart and blood vessels. However, prolonged secretion of these hormones may have the effect of increasing susceptibility to illness via increases in blood pressure, suppression of immune function, damage to muscle tissue, and weakening of the body's ability to fight infection. The observation of these nonspecific physiological processes in response to a broad range of stimuli led Selye to the formulation of the general adaptation syndrome (GAS). Thus, the physiological responses triggered by different stressful stimuli may be mediated through a common set of pathways.

Three Phases of General Adaptation Syndrome

Selye hypothesized that these responses occur in three progressive phases (Selye, 1976). The first phase, Alarm, is triggered by a threatening encounter, during which the organism experiences the physiological responses involving the concurrent release of cortisol, ...