Stress (biology)

Stress, whether physiological, biological or psychological, is an organism's response to a stressor, such as an environmental condition or change in life circumstances.^[1][2] When stressed by stimuli that alter an organism's environment, multiple systems respond across the body.^[1] In humans and most mammals, the autonomic nervous system and hypothalamic-pituitary-adrenal (HPA) axis are the two major systems that respond to stress.^[3][4] Two well-known hormones that humans produce during stressful situations are adrenaline and cortisol.^[3][4]

The sympathoadrenal medullary axis (SAM) 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.^[3][4]

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.^[4] The SAM and HPA axes are regulated by several brain regions, including the limbic system, prefrontal cortex, amygdala, hypothalamus, and stria terminalis.^[3] Through these mechanisms, stress can alter memory functions, reward, immune function, metabolism, and susceptibility to diseases.^[3][4]

Disease risk is particularly pertinent to mental illnesses, whereby chronic or severe stress remains a common risk factor for several mental illnesses.^[1][5]