Schematic overview of the classes of<unk> s that plants are exposed to Neuro-hormonal response to<unk> Stress, either physiological, biological, or psychological is an organism's response to a<unk> r such as an environmental condition.<unk> 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. In humans and most mammals, the autonomic nervous system and hypothalamic-pituitary-adrenal (HPA) axis are the two major systems that respond to<unk>  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<unk>  while the parasympathetic nervous system returns the body to homeostasis. The second major physiological<unk> 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,<unk> can alter memory functions, reward, immune function, metabolism and susceptibility to diseases. Disease risk is particularly pertinent to mental illnesses, whereby chronic or severe<unk> remains a common risk factor for several mental illnesses. One system suggests there are five types of<unk> labeled "acute time-limited<unk> rs", "brief naturalistic<unk> rs", "stressful event sequences", "chronic<unk> rs", and "distant<unk> rs". An acute time-limited<unk> r involves a short-term challenge, while a brief natural<unk> r involves an event that is normal but nevertheless challenging. A<unk> ul event sequence is a<unk> r that occurs, and then continues to yield<unk> into the immediate future. A chronic<unk> r involves exposure to a long-term<unk> r, and a distant<unk> r is a<unk> r that is not immediate.
Stress 
Social stress
Allostatic load
Surgical stress
Incident stress
Chronic stress
Psychological stress
Drug withdrawal