Stressors can be conveniently divided into physical stressors, social stressors resulting from the interactions with individuals of the same species and stressors related to handling by humans. To a great extent the stress response is mediated by the corticotropin releasing factor (CRF) that is secreted mainly by the paraventricular nucleus of the hypothalamus. Neurons in the hypothalamus, for example, are sensitive to internal physicochemical stimuli and to external physical and psychosocial stimuli. Several areas of the brain are involved in the organization of responses to aversive or threatening stimuli, and these areas interact extensively. As a result, it has been suggested that the term "stress" should be restricted to conditions where an environmental demand exceeds the regulatory capacity of the organism, in particular when such conditions include unpredictability and uncontrollability.Ĭurrent research on stress biology has addressed the role of the brain. On the other hand, there is now sufficient evidence showing that it is not the physical nature of an aversive stimulus that has negative consequences on the animal but rather the degree to which the stimulus can be predicted and controlled. Therefore, if stress is perceived as potentially negative, it may be misleading to consider stress as a synonymous of the HPA axis activation. play and mating) may elicit a similar physiological stress response. The problem with this approach, however, is that the HPA axis and the SAM system have a crucial function in energy mobilization and redistribution of nutrients to active tissues and both aversive (e.g.
Pathways involved in stressīoth the HPA axis and the SAM system are generally considered to be the two main elements of the stress response and plasma levels of glucocorticoids have been widely used as measures of stress. Later on, Selye conducted some of his classic studies on the response of the hypothalamic-pituitary-adrenal (HPA) axis to noxious stimuli and suggested that the organism reacted in a non-specific manner to a wide variety of aversive stimuli, mainly with an increase in the HPA axis activity.
In 1929, Cannon described stress as the sympatho-adrenomedullary (SAM) system's attempt to regulate homeostasis when threatened by a variety of aversive stimuli or stressors.
The term “stress” has been widely used in biology to describe a set of physiological and behavioural changes elicited by aversive stimuli.