The physiology of stress
The general adaptation syndrome (GAS) describes in three stages the body’s physiological reaction to stress:
1 Alarm reaction — the body responds to emotional reactions.
2 Resistance — the body fights the stressor.
3 Exhaustion — bodily resources become depleted.
The sympathomedullary pathway (SMP) comprises the sympathetic nervous system (SNS) and the sympathetic adrenal medullary system (SAM). Acute stressors activate the two divisions of the autonomic nervous system (ANS): the SNS, which responds to stimuli and is responsible for emotional states and elevated arousal, and the parasympathetic nervous system (PSNS), which maintains equilibrium and reduces bodily processes.
On exposure to acute stressors, the SNS becomes activated, while simultaneously the SAM system stimulates production of the hormone adrenaline into the bloodstream from the adrenal glands in the adrenal medulla, preparing the body for ’fight or flight’ by increasing available oxygen and glucose to the brain and muscles, while suppressing non-emergency processes, like digestion.
The hypothalamic pituitary-adrenal system (PAS) is activated by chronic stress, prompting the hypothalamus to stimulate production of corticotrophin-releasing hormone (CRH) into the bloodstream. This activates the pituitary gland to produce adrenocorticotropic hormone (ACTH), which goes to the adrenal glands, stimulating the production of stress-related hormones, like cortisol, allowing a steady supply of energy to permit individuals to deal with stressors. Cortisol helps greater pain toleration, but also incurs diminished cognitive ability and immune system performance.
Fig 14.1 The hypothalamic—pituitary—adrenal axis
Horwatt et al. (1988) assessed the effects of stress on the SMP by subjecting rats to the same stressor, being placed in water, each day for several weeks, finding that several adaptive changes occurred in the SMP, including the increased production and storage of catecholamines, flight or fight hormones that are produced in response to stress. When these animals were then subjected to a new stressful stimulus, they displayed an exaggerated response of the SMP compared with animals that were exposed to the same stressful situation for the first time but had not experienced the original stressor. The conclusion reached was that acute stress responses develop differently, due to previous stress experiences.
• Leshem & Kuiper (1996) found that applying different stressors to plants, like heat and drought, made them produce a similar stress response of retarded growth and lower yields, which suggests that GAS can be applied to plants as well as animals, illustrating the biological nature of stress responses.
• Taylor et al. (2000) found that acute stressors produce a ’flight or fight response’ in males, but a ’tend and befriend’ response in females, arguably because females produce more oxytocin, a chemical that promotes relaxation and nurturing, suggesting a gender difference in the workings of the SMP.
• McCarty (1981) found that older and younger rats had equal blood plasma levels of stress hormones before being subjected to stress, but that older rats had lower levels after being stressed, which implies that the SMP has diminished responsiveness with age.
• Heim et al. (2000) found elevated PAS responses to stress in females who had endured sexual abuse in childhood, suggesting that PAS hyper-sensitivity, due to corticotropin-releasing factor (CRF) hypersecretion, results from childhood abuse, but that CRF-receptor antagonist drugs could be used to treat conditions related to such early-life stress.
GAS was the first explanation of the physiological reaction to stress and inspired interest and research that led to later theories and therapies to counter the negative consequences of stress.
Although much research into stress is carried out on animals and invariably causes harm, it is justified by some in terms of a cost—benefit analysis, where the costs of the research, in terms of animal suffering, is seen as outweighed by the benefits of such research, such as in developing effective therapies to counteract stressful effects.
A lot of research into stress systems involves the use of animals, but presents a problem of extrapolation, because the stress responses of animals do not necessarily correspond to those of humans. For instance, humans are considered to have more of a cognitive element to their stress responses than animals.
Research findings often cannot be generalised to females, as they were excluded from experiments on the basis that monthly fluctuations in female hormone levels would produce stress responses that varied too considerably to generate useful, valid data.
There are individual differences in response to stressors, which means there is no universal explanation for physiological stress reactions.
The main practical application of research into stress responses is in health. Advice based on evidence can be given as to how to reduce and control stress levels, along with therapies being developed, like CBT, which address the negative consequences of stress.