Human Biology: General Adaptation Syndrome - Term Paper Example

The term "stress" or "general adaptation syndrome" was introduced into medicine and psychology by Canadian physician and endocrinologyst, Hans Selye.He defined general adaptation syndrome as "the sum of all non-specific, systemic reactions of the body which ensue upon long continued exposure to stress" (Selye H., 1976). Nowadays many researchers discuss the role of stress in the pathogenesis of various diseases, e.g. hypertension, coronary disease, depression, stomach ulcer etc. But all these conditions resulted the complicate chain of physiological reactions, which are crucial for the survival of alive organism. Selye and his followers divided the response of the body functional systems into three stages: alarm, resistance, and exhaustion. The first stage of general adaptation (or alarm stage) is triggered by the threat, which is recognized by the analysers (vision, hearing, olfaction; tactile, dolorous and temperature sensitivity etc). The irritation of the peripheral nervous terminals results in the activation of reticular formation and other structural elements of suprasegmental vegetative nervous system. These elements through the complicate system of mediator processes activate excretory function of the hypothalamus. The releasing hormones of hypothalamus, for its part, stimulate the secretory function of hypophysis. This process is followed by biosynthesis and release of adrenocorticotropic hormone (ACTH) as well as other tropic hormones (thyrotropic hormone, somatotropic (or growth) hormone etc). ACTH, particularly, stimulates the secretion of the steroid hormones (corticosteroids) in the cortical layer of the adrenal glands (see fig. 1 in Appendix). But the main hormone of alarm phase of stress reaction is ADRENALINE - catecholamine which is synthesised in the medullar layer of the adrenal glands. This hormone has powerful sympathomymetic effect and can work also as an excitatory neurotransmitter for nervous system. Thus alarm phase is characterised by tachicardia, tachipnoe, hypertension, immunosupression, hyperglycaemia and other effects. All these reaction can be helpful for avoiding contact to the threat, its removing or physical destroying. If this is impossible and afferent stimulation is continued than other "stress hormones" go on. Both glucorticoids and mineralocorticoids participate in this process. Glucocorticoids are important for developing the resistance to action of stress factor. No organism can exist for a lonf time in the forced regimen of "alarm stage" of general adaptation reaction. Consequently the stage of resistance changes it. In the resistance stage body remains alert and physiological arousal is preserved. But in this stage the catabolism is decreased and the intensity of physiological processes is lower than in intact state. Selye wrote: " it is not necessary for all three stages to develop before we can speak of G.A.S. [general adaptation syndrome]. Only the most severe stress leads rapidly to the stage of exhaustion and death." But exhaustion stage can occur if stressful stimulation is continued. In this stage all functional mechanisms work ineffectively - instead adaptation the organism gets desadaptation. The risk of diseases and their severest manifestations is increased. In other words, only short-term stress can be adaptive but not prolonged. Great Arabic physician, Avicenna, many years before Selye's investigations have done an experiment which is a good example of chronic stress and its consequences. He put young lamb in the cage and placed near this cage another one with a wolf. In this way a lamb should see his enemy every moment in his life. Another lamb taken from the same brood was placed in safe conditions - no wolfs in the close environment. Avicenna found that the first lamb died significantly earlier than second one (Lagerlund H., 2005). But stress kills much rarely than different diseases, which could be related to prolonged stress impact. Because the stress hormones suppress immune response an organism became more vulnerable to the infections and infestations. Other stress related diseases include irritable bowel syndrome, peptic ulcer, Crohn's disease and non-specific chronic ulcerative colitis, migraine, coronary diseases, diabetes, depression, hypertension, chronic fatigue syndrome etc (McPhee et al., 2002). The stress response is a complex mechanism. As it was stated above the central components of the stress system include the neurons of the hypothalamus, which produce releasing hormones (see fig. 2 in Appendix) as well as other groups of the neurons of the brainstem including some nuclei of the medulla and pons. These central components of stress system are added with the hypothalamic-pituitary-adrenal axis. Tsigos C. et al. (2004) wrote: "The integrity of the Hypothalamic-Pituitary-Adrenal (HPA) axis and the precise regulation of its function are critical for the successful response to any stressor, since this axis is a vital component of both the central and the peripheral limb of the stress system". Because of existing negative and positive feedbacks between the different components of hypothalamic-pituitary-adrenal axis this system is able to self-regulation. But the system response of an organism is not limited only to the functional changes in hypothalamic-pituitary-adrenal axis (see fig. 2 in Appendix) but also related to the dynamical and functional changes in thyroid axis, reproductive axis and growth development effects. The plenty and variability of the manifestations of chronic stress syndrome can be explained by the complexity of the reactions involved in the process of adaptation. Thus many reproductive problems can be related to chronic stress. For example the synthesis of steroid sex hormones can be inhibited by high concentrations of the glucocorticoids and the activation of cytokine system. Such pathology as endometriosis, inflammatory diseases of genitals, osteoporosis and menstrual disorders can be related to chronic stress. Growth development of children and adolescents is related to the mechanisms of stress response also. It was proved that prolonged activation of hypothalamic-pituitary-adrenal axis could lead to the suppression of growth hormone secretion as well as to the inhibition of the synthesis of thyrotropic and thyroid hormones (Stratakis & Chrousos, 1995)). The gastrointestinal system is one of the most sensitive targets in chronic stress. During acute stress the motor function of stomach is inhibited but colonic motor function is stimulated. This paradoxical reaction can be explained by the inhibition of the parasympathetic (vagal) influence on the gastric motility and by the simultaneous activation of the sacral parasympathetic centres. Stress-induced colonic hypermotility is one of the crucial causes of the irritable bowel syndrome, which is considered to be a classic example of stress-induced diseases. Another widely spread stress related health problem is peptic ulcer. The hypothesis about the exclusive role of stress in peptic ulcer pathogenesis was disproved by the findings of recent researches but there are no doubts that stress can aggravate the course of the diseases. Tsigos C. et al. (2004) discuss the intimate relationship between the components of hypothalamic-pituitary-adrenal axis and other structure of the endocrine system as well as neural structures of central nervous system (e.g. hippocampus-amygdale area, dopaminergic nuclei of the brainstem etc). These interactions are important for regulating such important physiological functions like appetite or body temperature. Thus, some eating disorders like anorexia nervosa or bulimia can have stress origin. Because stress hormones activate catabolic processes the risk of obesity during chronic stress is increased significantly. There was proved that chronic activation of hypothalamic-pituitary-adrenal axis could result in increase of the adiposity, decrease muscle component of lean body weight and insulin resistance. The severest manifestations of such changes can be described as pseudo-Cushing's state which is characterised "by abdominal and trunk fat accumulation and decreased lean body mass, in combination with manifestations of the metabolic syndrome (visceral adiposity, insulin resistance, dyslipidemia, hypercoagulability, hypercytokinemia, hypertension)" (cited by Tsigos et al., 2004). Generally the symptoms of chronic stress syndrome usually includes the manifestations of various endocrine, metabolic, immune and circulatory components - due to the multisystemic character of stress response. But stress is not only threat for health and well-being of an organism. It's universal mechanism of adaptation and improvement. Consequently, the understanding the nature of stress response is crucial for professional development of every biologist, physician or psychologist. References. 1. Lagerlund H. (2005) Mental Representation in Medieval Philosophy. Retrieved on 29.05.2005 from web-site 2. Mc Phee et al. (2002) Pathophysiology of Disease McGraw-Hill Medical; 4th edition 760 p. 3. Selye H. (1974) The nature of stress. Retrieved on 29.05.2005 from web-site 4. Stratakis C. A. & Chrousos G. P. (1995) Neuroendocrinology and pathophysiology of the stress system Ann NY Acad Sci Vol. 771 No. 1 pp. 1-18 5. Tsigos C. et al. (2004) Stress, endocrine physiology and pathophysiology Chapter 8. Retrieved on 29.05.2005 from web-site Appendix Figure 1. Stress pathways. Figure 2 Stress regulation Read More