stress (biology) stress (biology) stress, either physiological, biological or psychological, 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. there are two hormones that an individual produces during a stressful situation, these are well known as adrenaline and cortisol. there are two kinds of stress hormone levels. resting (basal) cortisol levels are normal everyday quantities that are essential for standard functioning. reactive cortisol levels are increases in cortisol in response to stressors. stimuli that alter an organism's environment are responded to by multiple systems in the body. in humans and most mammals, the autonomic nervous system and hypothalamic-pituitary-adrenal (hpa) axis are the two major systems that respond to stress. schematic overview of the classes of stresses in plants neurohormonal response 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-response center, 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. disease risk is particularly pertinent to mental illnesses, whereby chronic or severe stress remains a common risk factor for several mental illnesses. psychology main article: psychological stress acute stressful situations where the stress experienced is severe is a cause of change psychologically to the detriment of the well-being of the individual, such that symptomatic derealization and depersonalization, and anxiety and hyperarousal, are experienced. the international classification of diseases includes a group of mental and behavioral disorders which have their aetiology in reaction to severe stress and the consequent adaptive response. chronic stress, and a lack of coping resources available, or used by an individual, can often lead to the development of psychological issues such as delusions, depression and anxiety (see below for further information). chronic stress also causes brain atrophy, which is the loss of neurons and the connections between them. it affects the part of the brain that is important for learning, responding to the stressors and cognitive flexibility. chronic stressors may not be as intense as acute stressors such as natural disaster or a major accident, but persist over longer periods of time, tend to have a more negative effect on health because they are sustained and thus require the body's physiological response to occur daily. this depletes the body's energy more quickly and usually occurs over long periods of time, especially when these microstressors cannot be avoided (i.e. stress of living in a dangerous neighborhood). see allostatic load for further discussion of the biological process by which chronic stress may affect the body. for example, studies have found that caregivers, particularly those of dementia patients, have higher levels of depression and slightly worse physical health than non-caregivers. when humans are under chronic stress, permanent changes in their physiological, emotional, and behavioral responses may occur. chronic stress can include events such as caring for a spouse with dementia, or may result from brief focal events that have long term effects, such as experiencing a sexual assault. studies have also shown that psychological stress may directly contribute to the disproportionately high rates of coronary heart disease morbidity and mortality and its etiologic risk factors. specifically, acute and chronic stress have been shown to raise serum lipids and are associated with clinical coronary events. however, it is possible for individuals to exhibit hardiness—a term referring to the ability to be both chronically stressed and healthy. even though psychological stress is often connected with illness or disease, most healthy individuals can still remain disease-free after being confronted with chronic stressful events. this suggests that there are individual differences in vulnerability to the potential pathogenic effects of stress; individual differences in vulnerability arise due to both genetic and psychological factors. in addition, the age at which the stress is experienced can dictate its effect on health. research suggests chronic stress at a young age can have lifelong effects on the biological, psychological, and behavioral responses to stress later in life. etymology and historical usage the term "stress" had none of its contemporary connotations before the 1920s. it is a form of the middle english destresse, derived via old french from the latin stringere, "to draw tight". the word had long been in use in physics to refer to the internal distribution of a force exerted on a material body, resulting in strain. in the 1920s and '30s, biological and psychological circles occasionally used the term to refer to a mental strain or to a harmful environmental agent that could cause illness. walter cannon used it in 1926 to refer to external factors that disrupted what he called homeostasis. but "...stress as an explanation of lived experience is absent from both lay and expert life narratives before the 1930s". physiological stress represents a wide range of physical responses that occur as a direct effect of a stressor causing an upset in the homeostasis of the body. upon immediate disruption of either psychological or physical equilibrium the body responds by stimulating the nervous, endocrine, and immune systems. the reaction of these systems causes a number of physical changes that have both short- and long-term effects on the body. the holmes and rahe stress scale was developed as a method of assessing the risk of disease from life changes. the scale lists both positive and negative changes that elicit stress. these include things such as a major holiday or marriage, or death of a spouse and firing from a job. biological need for equilibrium homeostasis is a concept central to the idea of stress. in biology, most biochemical processes strive to maintain equilibrium (homeostasis), a steady state that exists more as an ideal and less as an achievable condition. environmental factors, internal or external stimuli, continually disrupt homeostasis; an organism's present condition is a state of constant flux moving about a homeostatic point that is that organism's optimal condition for living. factors causing an organism's condition to diverge too far from homeostasis can be experienced as stress. a life-threatening situation such as a major physical trauma or prolonged starvation can greatly disrupt homeostasis. on the other hand, an organism's attempt at restoring conditions back to or near homeostasis, often consuming energy and natural resources, can also be interpreted as stress. the brain cannot sustain a concentrated equilibrium under chronic stress, overtime if you constantly struggle in a simmering sea of stress, and you body budget accrued an ever-deepening deficit, that's called chronic stress and it does more than just make you miserable in that moment, it can and will gradually eat away at your brain and cause illness in your body. the ambiguity in defining this phenomenon was first recognized by hans selye (1907–1982) in 1926. in 1951 a commentator loosely summarized selye's view of stress as something that "...in addition to being itself, was also the cause of itself, and the result of itself". first to use the term in a biological context, selye continued to define stress as "the non-specific response of the body to any demand placed upon it". neuroscientists such as bruce mcewen and jaap koolhaas believe that stress, based on years of empirical research, "should be restricted to conditions where an environmental demand exceeds the natural regulatory capacity of an organism". the brain cannot live in an harsh family environment, it needs some sort of stability between another brain. people who have reported being raised in harsh environments such as verbal and physical aggression have showed a more immune dysfunction and more metabolic dysfunction. indeed, in 1995 toates already defined stress as a "chronic state that arises only when defense mechanisms are either being chronically stretched or are actually failing," while according to ursin (1988) stress results from an inconsistency between expected events ("set value") and perceived events ("actual value") that cannot be resolved satisfactorily, which also puts stress into the broader context of cognitive-consistency theory. biological background stress can have many profound effects on the human biological systems. biology primarily attempts to explain major concepts of stress using a stimulus-response paradigm, broadly comparable to how a psychobiological sensory system operates. the central nervous system (brain and spinal cord) plays a crucial role in the body's stress-related mechanisms. whether one should interpret these mechanisms as the body's response to a stressor or embody the act of stress itself is part of the ambiguity in defining what exactly stress is. the central nervous system works closely with the body's endocrine system to regulate these mechanisms. the sympathetic nervous system becomes primarily active during a stress response, regulating many of the body's physiological functions in ways that ought to make an organism more adaptive to its environment. below there follows a brief biological background of neuroanatomy and neurochemistry and how they relate to stress. stress, either severe, acute stress or chronic low-grade stress may induce abnormalities in three principal regulatory systems in the body: serotonin systems, catecholamine systems, and the hypothalamic-pituitary-adrenocortical axis. aggressive behavior has also been associated with abnormalities in these systems. biology of stress human brain: hypothalamus = amygdala = hippocampus/fornix = pons= pituitary gland= the brain endocrine interactions are relevant in the translation of stress into physiological and psychological changes. the autonomic nervous system (ans), as mentioned above, plays an important role in translating stress into a response. the ans responds reflexively to both physical stressors (for example baroreception), and to higher level inputs from the brain. the ans is composed of the parasympathetic nervous system and sympathetic nervous system, two branches that are both tonically active with opposing activities. the ans directly innervates tissue through the postganglionic nerves, which is controlled by preganglionic neurons originating in the intermediolateral cell column. the ans receives inputs from the medulla, hypothalamus, limbic system, prefrontal cortex, midbrain and monoamine nuclei. the activity of the sympathetic nervous system drives what is called the "fight or flight" response. the fight or flight response to emergency or stress involves mydriasis, increased heart rate and force contraction, vasoconstriction, bronchodilation, glycogenolysis, gluconeogenesis, lipolysis, sweating, decreased motility of the digestive system, secretion of the epinephrine and cortisol from the adrenal medulla, and relaxation of the bladder wall. the parasympathetic nervous response, "rest and digest", involves return to maintaining homeostasis, and involves miosis, bronchoconstriction, increased activity of the digestive system, and contraction of the bladder walls. complex relationships between protective and vulnerability factors on the effect of childhood home stress on psychological illness, cardiovascular illness and adaption have been observed. ans related mechanisms are thought to contribute to increased risk of cardiovascular disease after major stressful events. the hpa axis is a neuroendocrine system that mediates a stress response. neurons in the hypothalamus, particularly the paraventricular nucleus, release vasopressin and corticotropin releasing hormone, which travel through the hypophysial portal vessel where they travel to and bind to the corticotropin-releasing hormone receptor on the anterior pituitary gland. multiple crh peptides have been identified, and receptors have been identified on multiple areas of the brain, including the amygdala. crh is the main regulatory molecule of the release of acth. the secretion of acth into systemic circulation allows it to bind to and activate melanocortin receptor, where it stimulates the release of steroid hormones. steroid hormones bind to glucocorticoid receptors in the brain, providing negative feedback by reducing acth release. some evidence supports a second long term feedback that is non-sensitive to cortisol secretion. the pvn of the hypothalamus receives inputs from the nucleus of the solitary tract, and lamina terminalis. through these inputs, it receives and can respond to changes in blood. the pvn innervation from the brain stem nuclei, particularly the noradrenergic nuclei stimulate crh release. other regions of the hypothalamus both directly and indirectly inhibit hpa axis activity. hypothalamic neurons involved in regulating energy balance also influence hpa axis activity through the release of neurotransmitters such as neuropeptide y, which stimulates hpa axis activity. generally, the amygdala stimulates, and the