Definition of stress and stress models

Animals are capable of reacting to environmental changes by adapting body functions. This is necessary to keep the body’s inner stability (homeostasis) constant.

The situations (internal and external) causing a process of adaptation are called

‘stress’ which is often referred to as the counterpart of well-being. In general ‘stress’

challenges the homeostasis of an organism and evokes a physiological response.

Nowadays the term ‘stress’ is broadly used in both scientific and colloquial parlance.

A universally valid and clear definition could not be established although many authors intended to do so (e.g. BROOM & JOHNSON 1993; FRASER et al. 1975;

LADEWIG 1994; MOBERG 2000). The major reason for this is that ‘stress’ refers to different aspects of the stress process. For example the effect of an (environmental) incident (stress stimulus / stressor) on an organism as well as the corresponding body response (stress reaction) are described by this term (LADEWIG 1987;

BROOM & JOHNSON 1993). In addition to that the assumption that there is only one stress response is incorrect because individuals use a variety of different responses in order to face a challenging or potentially aversive situation (BROOM 2001). As a result its usage has become ambiguous. Today, for scientific use the most appropriate definition of stress has been given by BROOM (2001). He states that

“stress is an environmental effect on an individual which overtaxes its control systems and results in adverse consequences, eventually reduced fitness”.

CANNON (1929) and SELYE (1936, 1950) are the founders of the traditional stress model. According to this model, stress mainly is a physiological state, induced by external -usually aversive- stimuli. It is defined by a cascade of physiological reactions such as the activation of the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system. The organism’s responses when encountered by a threat were first described by CANNON (1929). This emergency reaction mobilizes the body’s resources for immediate action (normally for “flight or fight”) and therefore enhances the chances of survival. Its characteristic is an increased activity of the

allocation of the central nervous system and muscles helping the body to survive.

SELYE (1936, 1950) defined stress to be the ‘non-specific response of the body to any demand’. Thus any stimulus affecting an organism will evoke a complex and non-specific response that bears the possibility to induce adrenal hypertrophy, thymicolymphatic involution and gastrointestinal ulcers (the so-called ‘stress triad’).

SELYE developed the General Adaptation Syndrome (GAS), also known as the stress syndrome, where he postulated that the body passes through three progressive stages of coping when confronted with a stimulus. First there is an ‘alarm reaction’, characterized by surprise and anxiety when exposed to a new situation.

Here the body prepares itself for ‘fight or flight’ by producing epinephrine and norepinephrine. Additionally, the adrenal cortex is stimulated to produce additional hydrocortisone and related hormones. No organism can sustain this condition of excitement, however, and a second stage of adaptation ensues (provided the organism survives the first stage). In the second stage, a ‘resistance’ to the stress is built where the body learns to efficiently cope with the situation. Ideally, this adaptive stage continues until the stressful situation is resolved, leading to a rapid return to the resting state. In the presence of long-term exposure to the same stressor, we enter the third stage of GAS: exhaustion. Exhaustion is characterized by a depletion of energy reserves and loss of adaptational ability, leading to fatigue or other symptoms or diseases. The third stage is sometimes referred to as the adrenal maladaptation, or hyperadaptosis (DILMAN & DEAN 1992). Adrenal dysfunction may be manifested by (1) an excess or inadequacy of cortisone, DHEA, ACTH and/or CRF, (2) imbalances of these hormones and releasing factors, and (3) loss of sensitivity of the hypothalamus and pituitary gland to the normal inhibiting effects of these hormones.

MASON (1971), BURCHFIELD (1979) and VON HOLST & SCHERER (1987) state that stimuli evoking an emotional excitement such as fear can be regarded as stressors and that a stimulus can even become a stressor because of its novelty (WIEPKEMA & KOOLHAAS 1993). Therefore the traditional stress model is supplemented by a concept considering the interactions between the individual and the strain as well as the participation of the cognitive CNS, the so-called cognitive mediation theory (LADEWIG 1994). In accordance with this theory MASON (1975) showed that the physiological reactions to a stressor are more variable than postulated by SELYE (1936, 1950), and that a stressor only provokes a stress response if the consciousness is influenced. The cognitive mediator theory also takes into consideration that the stress response depends on the individual’s possibility and ability to develop coping strategies and therefore it is not a non-specific response but moreover a specific and stressor-dependant response to a psychological state (DANTZER 1994, DE BOER et al. 1990, HENRY & STEPHENS 1977, LAZARUS &

FOLKMANN 1984, LEVINE et al. 1989, MASON 1975, WEISS 1972). Furthermore, the effects of a stressor on an animal do not necessarily depend on its physical nature, but above all on the animal’s possibility to have some behavioral control over the stressor as demonstrated by WEISS (1971a,b). The predictability of a situation’s ending is important in order to control a stressor (ARTHUR 1987, HENRY &

STEPHENS 1977, WEISS 1971, WIEPKEMA & KOOLHAAS 1992) whereas the confrontation with uncontrollable and/or unpredictable stressors may lead to an overtaxing called ‘learned helplessness’. Therefore ‘coping’ is the animal’s ability to control its mental and bodily stability (FRASER & BROOM 1990). Housing conditions lacking stimuli but having a high predictability, however, may cause damage to the animals (WIEPKEMA & KOOLHAAS 1993). The coping-predictability concept by HENRY & STEPHENS (1977) is based on the cognitive mediation theory emphasizing the role of coping in the entire stress process. If an animal actively tries to control a stressful situation the sympathetic nervous system will be activated which results in an increased release of catecholamines. On the other hand situations in which the animal looses control over a stressor activate the HPA and consequently lead to a depression of behavior (DE BOER 1990).

process by means of the ‘Stundenglasmodell’ [hourglass concept] developed by LADEWIG (1994) according to VEITH-FLANIGAN & SANDMAN (1985), the currently accepted stress model.

In conclusion, the crucial measurement of stress may not be the physical reaction per se, but rather how the animal perceives and is able to cope with the stimulus, but of course physiological measures are essential parts of an overall stress assessment, providing indications of, for example, the intensity of a response.

Figure 1: The ‘Stundenglasmodell’ [hourglass concept] (LADEWIG 1994) according to Veith- Flanigan & Sandman (1985) resumes the effects of all possible external incidents on an individual’s response after passing through a complex filter of individual differences.