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Allostasis, Homeostats, and the Nature of Stress Stress, 2002 Vol. 5 (1), pp. 55–58 Allostasis, Homeostats, and the Nature of Stress DAVID S. GOLDSTEINa,* and BRUCE MCEWENb aClinical Neurocardiology Section, National Institute of Neurological Disorders and Stroke, NIH, 10/6N252, 10 Center Drive MSC-1620, Bethesda, MD 20892-1620, USA; bRockefeller University, New York, NY, USA (Received 2 February 2001; Revised 12 July 2001; In final form 22 August 2001) This essay continues discussion of a new formulation of homeostasis that uses the concepts of allostasis and homeostats. The new formulation moves beyond Cannon’s concept of “homeostasis,” which posits an ideal set of conditions for maintenance of the internal environment. The notion of allostasis recognizes that there is no single ideal set of steady-state conditions in life, and different stressors elicit different patterns of activation of the sympathetic nervous and adrenomedullary hormonal systems. Allostasis reflects active, adaptive processes that maintain apparent steady states, via multiple, interacting effectors regulated by homeostatic comparators—“homeostats.” “Allostatic load” refers to the consequences of sustained or repeated activation of mediators of allostasis. From the analogy of a home temperature control system, the temperature can be maintained at any of a variety of levels (allostatic states) by multiple means (effectors), regulated by the thermostat (homeostat). Allostatic load and risks of system breakdown increase when, for example, the front door is left open in the winter. Applying these notions can aid in understanding how acute and chronic stress can exert adverse health consequences via allostatic load. Keywords: Stress; Distress; Allostasis; Homeostasis; Homeostat CANNON AND “HOMEOSTASIS” of emergencies, activation of the sympathico-adrenal system would enhance survival. It is by now clear that the For personal use only. Walter B. Cannon, extending Claude Bernard’s concept of sympathico-adrenal system is active tonically and the internal environment, introduced the term, “homeo- contributes to “basal” levels of key internal variables stasis,” to describe the product of the “coordinated such as blood pressure and glucose. Moreover, activities of physiological processes which maintain most of the steady daily life, such as meal ingestion, speaking, changing states in the organism” (Cannon, 1929a,b; 1939). posture, and movement—i.e., not only emergencies—are According to Cannon, rapid activation of homeostatic associated with continual alterations in sympathetic systems—especially of what he called the “sympathico- nervous system outflows, maintaining appropriate body adrenal system”—preserves the internal environment, by temperature, delivery of metabolic fuel to body organs, producing compensatory and anticipatory adjustments that and so forth. Stress Downloaded from informahealthcare.com by Monash University on 07/01/13 enhance the likelihood of survival. Each of these activities is associated with a somewhat Cannon rarely used the term “stress” and never defined different set of “normal” apparent steady-states, directed it. One may infer that he viewed stress as a noxious aspect by the brain and determined by coordinated actions of a of the environment: variety of effector systems. This principle leads directly to the concept of “allostasis,” as will be discussed below. It is Perhaps a comparative study would show that every complex organism must have more or less effective self-right adjustments in also by now clear the different everyday challenges, as order to prevent a check on its functions or a rapid disintegration of well as rarer emergencies, can alter sympathetic nervous its parts when it is subjected to stress... (Cannon, 1939, pp. 23– and adrenomedullary hormonal activities relatively 25). differentially. Thus, sympathetic nervous system acti- vation is especially prominent during exposure to cold, According to Cannon, coordinated body processes orthostasis, and mild exercise, whereas adrenomedullary would work toward the goal of an ideal set of steady- hormonal system activation is especially prominent during states—“homeostasis.” In emergencies and in anticipation *Corresponding author. Tel.: +1-301-496-2103. Fax: +1-301-402-0180. E-mail: [email protected] ISSN 1025-3890 q 2002 Taylor & Francis Ltd DOI: 10.1080/10253890290012345 56 D.S. GOLDSTEIN AND B. MCEWEN acute glucoprivation and emotional distress (Goldstein, circumstances, do not match the current or anticipated 2001). perceptions of the internal or external environment, and this discrepancy between what is observed or sensed and what is expected or programmed elicits patterned, THE HOME HEATING ANALOGY compensatory responses. One can assimilate many of the concepts presented here from the analogy of a home temperature control system. In “ALLOSTASIS” AND “ALLOSTATIC LOAD” such a system, the thermostat plays a central role, by sensing discrepancy between the setpoint, determined by a Levels of physiological activity that are required to re- regulator, and the temperature, which produces differen- establish or maintain homeostasis differ, depending on tial bending of metal bands in the thermostat. This type of continually changing conditions in which the organism system is a classical example of regulation by negative finds itself—e.g., running vs. standing vs. lying down. feedback. Analogously, as discussed below, the body has “Allostasis” refers to levels of activity required for the many comparators—including an actual thermostat. individual to “maintain stability through change”—i.e., to Home temperature control systems usually include adapt (Sterling and Eyer, 1988; McEwen and Stellar, multiple effectors. The redundancy comes at relatively 1993; McEwen, 1998; 2000; Schulkin et al., 1998). In little cost, compared with the economies afforded. The terms of the homeostatic definition of stress, “allostasis” availability of multiple effectors for heating, ventilation, refers to the set of apparent steady-states maintained by and air conditioning has three advantages. First, the multiple effectors. In the home temperature control system, multiplicity extends the range of external temperatures one can regulate temperature at different levels, by where the internal temperature can be maintained. Second, appropriate use of effectors. Analogously, among individ- when a single effector fails to function, the other are uals, levels of glucose, blood pressure, body temperature, activated compensatorily, and the compensatory acti- metabolism, and so forth are normally held stable at different vation can help to maintain the temperature at about the levels, with different patterns of effector activation. set level. Third, the use of the effectors can be patterned as “Allostatic load” refers to effects of prolonged appropriate to maximize economy and efficiency. continuous or intermittent activation of effectors involved in allostasis. In the analogy of the home temperature control system, allostatic load would increase if a window “HOMEOSTATS” or door were left open. In this situation, one or more effectors might be activated frequently or even continu- For personal use only. Stress occurs when the organism senses a disruption or a ously. An even more extreme example would be having threat of disruption of homeostasis, leading to a the air conditioner and the furnace on at the same time, as compensatory reaction. The sensation requires a com- is often the case in an overheated apartment in the spring parative process, where the brain compares available when there is a warm day before the boilers have been shut information with setpoints for responding. Central to the down. Continued use of the furnace and air conditioner in present theory is that the body possesses numerous opposition to one another, an example of an inefficient homeostatic comparators. We call them “homeostats.” “allostatic state,” consumes fuel and contributes to wear- Each homeostat compares a specific category of and-tear on both pieces of equipment. Long-term allostatic information with a setpoint for responding, determined load—the wear and tear cost of adaptation—provides Stress Downloaded from informahealthcare.com by Monash University on 07/01/13 by a regulator. Homeostatic systems typically use multiple a conceptual basis for studying long-term health effectors to change values for the controlled variable. The consequences of stress. loop is closed by monitoring changes in the levels of the controlled variable, via one or more monitored variables. A tremendous array of homeostatic systems detect PRINCIPLES OF HOMEOSTAT OPERATION perturbations of monitored variables. In particular, in line with the home heating analogy, afferent information to the Homeostatic systems operate according to a few principles, brain about cutaneous and blood temperature determines which, despite their simplicity, can explain complex activities of cholinergic and noradrenergic nerve fibers in physiological phenomena and help to resolve persistently the skin that regulate sweating and vasomotor tone. controversial issues in the area of stress and disease. According to the current concept, even a simple Homeostatic systems always include regulation by homeostatic reflex reflects “stress” when a perceived negative feedback. Increases in values of the monitored discrepancy between a setpoint for a monitored variable variable result in changes in effector activity that oppose and information about the actual level of that variable and thereby “buffer” changes in that variable. This
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