Stress syndrome

Stress, either physiological or biological, is an organism's response to a stressor such as an environmental condition. Stress is the body's method of reacting to a condition such as a threat, challenge or physical and psychological barrier. Stimuli that alter an organism's environment are responded to by multiple systems in the body. The autonomic nervous system and hypothalamic-pituitary-adrenal (HPA) axis are two major systems that respond to stress. The sympathoadrenal medullary (SAM) axis may activate the fight-or-flight response through the sympathetic nervous system, which dedicates energy to more relevant bodily systems to acute adaptation to stress, while the parasympathetic nervous system returns the body to homeostasis. The second major physiological stress, the HPA axis regulates the release of cortisol, which influences many bodily functions such as metabolic, psychological and immunological functions. The SAM and HPA axes are regulated by several brain regions, including the limbic system, prefrontal cortex, amygdala, hypothalamus, and stria terminalis. Through these mechanisms, stress can alter memory functions, reward, immune function, metabolism and susceptibility to diseases. Definitions of stress differ. One system suggests there are five types of stress labeled 'acute time-limited stressors', 'brief naturalistic stressors', 'stressful event sequences', 'chronic stressors', and 'distant stressors'. An acute time-limited stressor involves a short-term challenge, while a brief natural stressor involves an event that is normal but nevertheless challenging. A stressful event sequence is a stressor that occurs, and then continues to yield stress into the immediate future. A chronic stressor involves exposure to a long-term stressor, and a distant stressor is a stressor that is not immediate. Stress and illness may have intersecting components. Several studies indicate such a link, while theories of the stress–illness link suggest that both acute and chronic stress can cause illness, and lead to changes in behavior and in physiology. Behavioral changes can include smoking, and changes in eating habits and physical activity. Physiological changes can include changes in sympathetic activation or HPA activity, and immunological function. However, there is much variability in the link between stress and illness. The HPA axis regulates many bodily functions, both behavioral and physiological, through the release of glucocorticoid hormones. The HPA axis activity varies according to the circadian rhythm, with a spike in the morning. The axis involves the release of corticotropin releasing hormone and vasopressin from the hypothalamus which stimulates the pituitary to secrete ACTH. ACTH may then stimulate the adrenal glands to secrete cortisol. The HPA axis is subject to negative feedback regulation as well. The release of CRH and VP are regulated by descending glutaminergic and GABAergic pathways from the amygdala, as well as noradrenergic projections. Increased cortisol usually acts to increase blood glucose, blood pressure, and surpasses lysosomal, and immunological activity. Under other circumstances the activity may differ. Increased cortisol also favors habit based learning, by favoring memory consolidation of emotional memories. Selye demonstrated that stress decreases adaptability of an organism and proposed to describe the adaptability as a special resource, adaptation energy. One study considered adaptation energy as an internal coordinate on the 'dominant path' in the model of adaptation. Stress can make the individual more susceptible to physical illnesses like the common cold. Stressful events, such as job changes, may result in insomnia, impaired sleeping, and physical and psychological health complaints. Research indicates the type of stressor (whether it is acute or chronic) and individual characteristics such as age and physical well-being before the onset of the stressor can combine to determine the effect of stress on an individual. An individual's personality characteristics (such as level of neuroticism), genetics, and childhood experiences with major stressors and traumas may also dictate their response to stressors.