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Essentially, stress occurs when an animal (human or otherwise) cannot
react to remove a stressor (which could be real, potential, or perceived
threat to well-being).
This intrusion and its perception as a threat results in behaviour,
which serves to remove the stressor - i.e. to close the loop , negative
feedback.
If, however, the situation is such that the stressor cannot be removed
by behaviour (i.e. open-loop situation), then a physiological condition
results which is termed STRESS.
An essential feature of this is departure from the body's normal homeostatic
levels.
Increased risk of pathological conditions, e.g. cardiovascular problems, increase incidence of URTIs.
Abnormal behaviour, e.g. stereotypies.
This may be adaptive as enabling animal to react effectively to
a threatening situation; this is so in the short term - the General Adaptive
Syndrome.
However, if the stressful situation persists, long-term stress resu1ts
in several physiological conditions which are potentially harmful.
has either a direct effect using noradrenalin as neurotransmiuer
at neuromuscular junctions to affect heart rate etc.
or indirect, by its action on the adrenal medulla to release adrenalin and noradrenalin.
Adrenalin and noradrenalin are both catecholamines.
HPA axis
CRH = corticotropin-releasing hormone - secreted by the hypothalamus
ACTH = adrenocorticotropic hormone - secreted by the pituitary - resulting
in increased corticosteroid levels in the blood
The diagram below shows changes in hormonal levels with time after the
onset of a stressor.
- as shown in the next diagram.
Find out more about the immune system and immunity.