The General Adaptation Syndrome and Allostasis
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The General Adaptation Syndrome and Allostasis Components of the GAS can be subdivided into three unique, largely physiologic stages (Table 2.1). Examining the stages separately is the best way to understand the entire GAS. The specific chemicals involved are among those seen today as integral to the broader view of allostatic responses to stress in the maintenance of homeostasis. All will be discussed later in the chapter.
Alarm Stage The alarm stage has been called the fight-or-flight response, derived from Cannon’s work, because it provides a surge of energy and physical alterations to either evade or confront danger (Fig. 2.1). This stage begins when the hypothalamus, as it monitors the internal and external environment, senses a need to activate the GAS in response to a stimulus, a stressor placing the balance of homeostasis at risk. The stressor might be physical or emotional, positive or negative—arguing with a friend, having an upper respiratory tract infection, running to catch a bus, or winning the lottery. The hypothalamus then secretes corticotropin-releasing hormone (CRH) to activate the sympathetic nervous system (SNS), which in turn also stimulates the adrenal medulla (the inner portion of the adrenal gland) to release the catecholamines— norepinephrine and epinephrine. The increased levels of catecholamines