tHE strEss rEsPonsE. The term ‘stress’ is frequently used to describe the negative emotional state experi- enced when a person perceives that the demands (e.g. resulting from work or social engagements) exceed the resources the individual is able to mobilise. Therefore, stress has a negative connotation, as it is associated with a reduced feeling of well being and, after prolonged periods, with the development of stress-related disease (e.g. anxiety and depression). In the present society most, if not all, individuals feel that they have experienced stress, however there is not a clear-cut definition for this phenomenon. From the scientific point of view however, the stress response is part of an organisms natural defence mechanism to demanding situations. The internal environment of all living organisms is regulated in such a way that a dynamic equilibrium, called homeostasis, is maintained. Changes in the internal or external environment that threaten homeostasis (stressors) turn on a spectrum of physiological and behavioural responses aimed at restoring homeostatic balance 146. This process of adaptation is also known as ‘allostasis’, meaning ‘achieving stability through change’. However, when exposed to stressful situations repeatedly, or when the allostatic mechanisms remain activated when no longer needed, the price that has to be paid for maintain- ing stability (allostatic load) may become too high, resulting in the development of stress-related pathology 433. The concept of allostasis is however a matter of debate as, in contrast to a physiological adaptive process as proposed by XxXxxx, it has also been suggested to represent a pathological maladaptive process (see x.x. Xxxx and Le Moal 361,433). In 1936, Xxxxx first defined the concept of stress as ‘the non-specific response of the body to any demand’ 615. Non-specific indicates that the stress response is comprised of a fixed set of neuroendocrine adaptations, irrespective of the nature of the stressor. These include activation of the autonomic sympathetic nervous system (ANS) and the hypothalamic-pituitary-adrenal (HPA) axis (figure 3), which are described in detail in the following sections. Nowadays, it is known that the degree to which the neuroendocrine cascades are activated depends on the severity of the stressor but can also show considerable individual variation due to genetic background and life history 146.
