A new stress model, the Allostatic Load Model, refers to the ability to achieve stability through change. The various biological functions activated during stress serve an important role in the organism's adaptation to the environment by protecting and restoring the body but may, under certain conditions, also have health damaging consequences. Two different psychoneuroendocrine stress systems are of particular interest: (1) the sympathetic adrenal medullary (SAM) and (2) the hypothalamic pituitary adrenocortical (HPA) systems. Sustained activation of the SAM system with overexposure to epinephrine (adrenaline) and norepinephrine (noradrenaline) is considered to contribute to the development of cardiovascular disease (CVD). Chronic stress exposure influencing the HPA-axis is associated with metabolic changes which also increase the risk of CVD but, in addition, also contribute to impaired immune function, diabetes, depressive symptoms and cognitive disturbances. The present paper is focused on the possible biological pathways between environmental stress and somatic illness, including the role of environmental stress for the development of musculoskeletal disorders. It is concluded that the SAM and the HPA systems play an important role in linking environmental stress to various negative health outcomes and that knowledge about these psychobiological pathways is of considerable importance for the possibilities to prevent and treat environmentally induced ill health.
Keywords: Stress Model, Neuroendocrine reactions, coping, HPA axis
|How to cite this article:|
Lundberg U. Coping with stress: neuroendocrine reactions and implications for health. Noise Health 1999;1:67-74
| Introduction|| |
The major somatic health problems today, such as cardiovascular disease, musculoskeletal disorders and cancer, all seem to have a multifactorial etiology, where the physical conditions interact with psychosocial factors, such as stress, and the individuallEs behavior and genetic background.
Stress is generally defined in terms of the interaction between the individual and the environment, such as an imbalance between environmental demands and the individuallEs resources to meet these demands (Frankenhaeuser, 1989), between demands and control (Karasek, 1979) or between effort and reward (Siegrist, 1996).
According to a new stress model, the Allostatic Load Model (McEwen, 1997; McEwen & Stellar, 1993), which refers to the ability to achieve stability through change, a normal and "economical" response to stress means activation of physiological systems in order first, to cope with the stressor and then, to shut off the allostatic response as soon as the stress is terminated. Activation of the various biological functions, such as the sympathetic adrenal medullary (SAM), the hypothalamic pituitaryadrenocortical (HPA) and the cardiovascular, metabolic, and immune systems, serve an important role in the organism's adaptation to the environment by protecting and restoring the body. However, over- or underactivity of the allostatic systems may add to the wear-and-tear of the organism. Examples are:
1. Too frequent and intensive activation of physiological systems, which do not allow enough rest and restitution and which may increase the risk of atherosclerosis and, consequently, of myocardial infarction (Kaplan et al., 1991; Muller et al., 1989).
2. An inability to shut off the stress response and return to baseline after the stress exposure or lack of adaptation to an environmental situation, which may cause overactivity and exhaustion of the systems. Over-exposure to stress hormones increases the risk of cardiovascular illness, compromised immune functions, and cognitive impairment (Brunner, 1997).
3. Lack of adequate response in one system, due to for example, exhaustion of the system, which through feed-back mechanisms may cause compensatory overactivation of other systems.
Responses to stress and health risks
Physiological recordings, such as elevated blood pressure, heart rate and stress hormone levels, may serve as "objective" indicators of the stress that an individual is exposed to but, at the same time, these bodily reactions are assumed to create a link to somatic symptoms and illness.
Henry (1992) integrated findings from animal and human research into a model describing the biological effects and health risks associated with different coping styles. In response to acute stress or threat, the individuallEs earlier experiences, personality characteristics, and genetic makeup determine his or her way of coping. Two different types of coping mechanisms can be distinguished:
1.The aim of the active "defence mechanism" is to try to control the stressor by "fight-or-flight". It activates mainly the SAM system with the secretion of the catecholamines epinephrine (adrenaline) and norepinephrine (noradrenaline). If the individual is successful in his or her coping efforts and attains control over the stressor, the arousal level may return to its normal basal level. However, if the individual has to keep fighting the stressor or is exposed to frequent or chronic stress, physiological arousal is sustained.
2. The passive "defeat reaction" is associated with a lack of control, helplessness, subordination, and feelings of distress, anxiety, and depression. The concomitant physiological reactions involve mainly activation of the HPAaxis and secretion of glucocorticoids (e.g., cortisol). At the same time, reproductive functions are downregulated and the secretion of steroid sex hormone levels reduced. Under conditions where control is impossible, the "defeat reaction" is the only choice. However, repeated experiences of uncontrollable environmental conditions may lead the individual to believe that control is not possible, even when it is, and thus he or she will become more likely to assume a passive coping style when exposed to new stressors. This phenomenon has been described as "learned helplessness" (Seligman, 1975).
Henry's model (1992) is strongly supported by animal research (cf. Bjorntorp, 1996; Folkow, 1993; Sapolsky, 1990; Shively, in press). Dominant male animals on top of the social hierarchy usually have elevated catecholamine levels and low cortisol secretion, whereas subordinate animals show the opposite hormonal pattern with increased activity of the HPAsystem caused by altered feed-back mechanisms (Sapolsky, 1990). Although the studies on humans are less conclusive, a similar pattern of responses has been noticed (Henry & Stephens, 1977) with, for example, elevated cortisol levels associated with anxiety, depression and low socioeconomic status (Checkley, 1996; Sachar, 1970; Seeman & McEwen, in press).
The sympathetic adrenal-medullary (SAM) system
Research on the SAM system has its roots in the work of Walter B. Cannon at the beginning of this century (Cannon, 1914). On the basis of animal experiments, he described the fight-orflight response and the emergency function of the adrenal medulla. The SAM system is activated when the individual is challenged in its control of the environment. Via hypothalamus and the sympathetic nervous system, psychological stress stimulates the adrenal medulla to secrete the two catecholamines, epinephrine, and norepinephrine, into the blood stream. This defense reaction prepares the body for battle.
The cardiovascular and neuroendocrine functions activated by the SAM system mobilize energy to the muscles, the heart, and the brain and, at the same time, reduce blood flow to the internal organs and the gastro-intestinal system. Increasing the organism's capacity for fight-orflight in response to physical threat is an efficient means for survival. Today, however, the SAM system is probably more often challenged by threats of a social or mental nature.
Cognitive factors are also known to interact with physical exposure (e.g., Glass & Singer, 1972). For example, it has been found that individuals exposed to noise during performance have the alternative options to reduce their performance level in order to keep the physiological arousal constant, or to keep the performance level constant by increasing their physiological activation (Frankenhaeuser & Lundberg, 1974, 1977). Environmental conditions inducing a constant performance level (e.g., machine paced work) despite increased noise load, may thus be associated with a greater physiological ocosto (Frankenhaeuser & Lundberg, 1977). The effects of increased noise load may also be reflected after the acute exposure as demonstrated by Glass and Singer (1972).
To identify the health consequences of intense, repeated, and sustained activation of bodily systems is a major objective for stress research. Numerous studies from laboratory experiments as well as from natural settings illustrate the sensitivity of the SAM system to various environmental conditions (see reviews by Axelrode and Reisine, 1984; Frankenhaeuser 1979; Henry & Stephens, 1977; Levi, 1972; Lundberg, 1984; Mason, 1968; Ursin et al., 1978; 1983; Usdin et al., 1980).
Lasting elevated catecholamine levels are considered to contribute to the development of atherosclerosis and predispose to myocardial ischemia (Karasek et al., 1982; Krantz & Manuck, 1984; Rozanski et al., 1988). The elevated catecholamine levels also make the blood more prone to clotting, thus reducing the risk of heavy bleeding in case of tissue damage but, at the same time, increasing the risk of arterial obstruction and myocardial infarction. The role of the catecholamines in hypertension is also of great interest (e.g., Nelesen & Dimsdale, 1994).
Whereas epinephrine output is mainly influenced by mental demands, norepinephrine, which to a large extent is produced by sympathetic nerve endings, is more sensitive to physical activity and body posture. Comparisons of catecholamine responses among blue- and white-collar workers are consistent with experimental findings. Data from a series of real life studies (Lundberg & Johansson, in press) show that, in general, white collar workers increase their epinephrine but not their norepinephrine levels at work, whereas blue collar workers increase both their epinephrine and norepinephrine levels compared with their normal resting levels. The physical activity of white-collar workers is probably too low to influence their norepinephrine output.
Considering the various cardiovascular and metabolic functions influenced by the catecholamines, blue-collar workers tend to be exposed to a greater total physiological load than white-collar workers. Adverse physical conditions, such as noise, vibration, heat and cold etc. are likely to contribute to contribute to these responses. In addition, blue collar workers in repetitive assembly line work seem to have elevated stress levels also after work. Their physiological arousal remains elevated at least 1u2 hours after work compared with the more rapid "unwinding" of workers in flexible jobs (Johansson et al., 1978; Melin et al., 1997). This means that workers in simple, monotonous, and repetitive jobs not only suffer a greater physiological toll at work but also have less chance for relaxation and recuperation off the job (Melin & Lundberg, 1997).
The hypothalamic pituitary adrenocortical (HPA) axis
Activation of the HPA-axis and secretion of glucocorticoids such as cortisol was a central part of Hans Selye's formulations on stress and the General Adaptation Syndrome (Selye, 1956). Selye considered this the body's nonspecific response to stress and described three different phases of the stress response: (1) The alarm reaction, when the threat is noticed, (2) The resistance phase, when the organism mobilizes resources to overcome the stressor, and in case adaptation to the stressor does not occur, this is followed by (3) Exhaustion of the system and death of the organism.
As described by Bjorntorp (1996), sustained activity of the HPA-system is related to suppressed steroid sex and growth hormone levels and to insulin resistance. Cortisol influences the metabolism in the fat cells and causes increased storage of fat in the abdominal region due to the higher density of corticosteroid receptors on the fat cells there. The amount of central fat accumulation can be used as an indicator of long term overactivity of the HPA system. A useful epidemiological measure of HPA activity is the relationship between the waist and the hip circumference (i.e., the waist/hip ratio, WHR), which has been linked to increased risk of CHD (Bjorntorp, 1990) and to low socioeconomic status (Seeman & McEwen, in press).
Through its anti-inflammatory effects, cortisol has an important role for the immune system (McEwen, Biron et al., 1997). Sustained activation of the HPA-axis is related to impaired immune function, whereas short term stress may contribute to an enhanced immune response (Dhabhar & McEwen, 1997). High cortisol levels may also cause degenerative processes in hyppocampus leading to a decline in explicit memory functions (e.g., Lupien et al., 1997).
More extensive reviews of the regulation of the HPA-axis in animal and human adaptation and the somatic, psychiatric and cognitive health implications have been presented by for example Chrousos (1992).
In summary, the pattern of results suggests that individuals exposed to negative psychosocial conditions (unemployment, low income, low education, living alone etc.), individuals with psychiatric disturbances (anxiety, distress, depression) and individuals with a negative life style (smoking, alcohol abuse, anti-depressant treatment) are more likely to show elevated HPA activity, lower levels of steroid sex and growth hormones, impaired immune functions, cognitive deficiency and more central fat accumulation (Bjorntorp, 1996). However, the mechanisms relating to the HPA system are complex, and conditions such as chronic fatigue syndrome and post-traumatic stress disorder may be associated with normal or even below normal cortisol levels (Yehuda et al., 1996) and with elevated responsivity, or with elevated baseline cortisol levels and blunted responsiveness.
In a recent study, Kristenson et al. (1998) compared matched groups of men from Sweden and Lithuania and found that Lithuanien men, compared with Swedish men, had elevated baseline levels of cortisol but blunted responses to experimental stress. The chronic psychosocial stress that people in Lithuania has been exposed to during recent years was assumed to have contributed to this "inadequate" response pattern according to the Allostatic Load Model. It is also of interest to note that men from Lithuania were found have a fourfold higher incidence of myocardial infarction, compared with Swedish men, but did not differ markedly in traditional CHD risk factors, such as blood lipids, smoking, overweight and hypertension.
Environmental Stress and Musculoskeletal Disorders
Although people do not die from pain syndromes in the neck, shoulders, and lower back, the musculoskeletal disorders constitute a major health problem in terms of costs, prevalence and suffering. Musculoskeletal disorders differ from many other major health problems, such as cardiovascular disease and cancer, in that symptoms often appear very early in life and after a relatively short exposure to adverse environmental conditions.
So far it has been difficult to explain the high incidence of musculoskeletal disorders in light physical work. However, it has been suggested that sustained low-level muscle activity may initiate a pathogenetic mechanism that results in muscle pain. New theories have been proposed to explain the development of musculoskeletal disorder symptoms in psychologically stressful jobs with a moderate or low physical load (Hagg, 1991; Johansson & Sojka, 1991; Schleifer & Ley, 1994).
Recent findings from laboratory experiments (e.g., Larsson et al., 1995; Lundberg et al., 1994; Waersted et al., 1991, 1996) show that not only biomechanical exposure but also cognitive factors and mental stress may induce muscle tension and thus create a link to musculoskeletal disorders. As psychosocial stress usually lasts longer than biomechanical load and cannot be avoided by, e.g., taking a break or leaving the job, this will reduce rest and restitution and thus contribute to sustained activity in low-threshold motor units. Thus, a stressful work situation may prevent the individual from shutting off the physiological arousal and return to baseline.
Additional factors contributing to sustained muscle tension could be exposure to low levels of noise, e.g., from ventilation and airconditioning systems. This means that ongoing stress may keep low-threshold motor units active more or less continuously. Although these small motor units are assumed to be fatigue resistant, there is likely to be an upper limit for continuous activation (Kadefors et al., 1998; Waersted, 1997).
A possible pathogenic mechanism for muscle pain is that nociceptors are sensitized due to local metabolic changes in fatigued muscle fibers (Sejersted & Vollestad, 1993). In the modern work environment it is possible that stress-induced muscle tension is an even more important health problem than the absolute level of contraction or the frequency of muscular activation.
Numerous studies suggest an interaction between physical, psychosocial and behavioral factors in the development of the major health problems today. Several plausible psychobiological mechanisms have been proposed and received considerable empirical support. The two major stress systems, i.e., the SAM and the HPA systems, seem to play an important role in influencing cardiovascular, metabolic and immune functions.
Information on psychobiological pathways between environmental conditions, symptoms, and illness is of great importance in relation to the possibilities to prevent and treat environmentally induced ill health. Such knowledge could also help individuals to better understand their own health problems and find ways of breaking vicious circles between behavior and symptoms. It may also help to create a more understanding attitude in general towards people who, under negative environmental conditions, have developed somatic symptoms. However, most important is that knowledge about these mechanisms is a strong incentive for implementing structural and organizational changes, in order to prevent people from being exposed to adverse environmental conditions contributing to mental and physical disorders. The psychobiological parameters may be used as "warning signals", but they may also offer new possibilities to evaluate changes aimed at contributing to a better environment before those conditions are reflected in somatic illness patterns - a process that usually (but not always) takes a long time.
Recently, Ryff and Singer (1998) have proposed a model for analysing "the contours of positive human health", emphasizing the key role of psychological factors such as "a life of purpose", "quality connections to others", "positive selfregard" and "mastery". This model could stimulate research into the possibilities of promoting factors contributing to "a good health", in addition to the interventions aimed at reducing environmental health risks.
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Department of Psychology, Stockholm University
Source of Support: None, Conflict of Interest: None