Animals have a stress response system and they use it to adjust to changes in the world around them. This system is called the HPA axis, and it works in the following cascade: perceiving a stressor (for our purposes in training: a dog, a stranger trainers nickname these stimuli a "trigger"), sending a messenger (corticotropin-releasing hormone) from a part of the brain (hypothalamus) to info receiving part of the brain (anterior pituitary gland) and this then sends another messenger (adrenocorticotropic hormone or ACTH) into the blood and now receptors on the adrenal cortex pick the message up and that and stimulates adrenal cortisol to release. The change in arousal and readiness hormones changes behavior thresholds for animals, a stressed dog will react faster, and longer. The systems seems too complex at first glance, way too many delivery guys when one fella would complete the chain, however those other stops in the chain serve a purpose. The systems needs brakes. So high levels of cortisol also has a dampening effect on the hypothalamus and the pituitary, using this built in negative feedback animals can return to a normal level in a few hours.
So what happens when there is more than one stressor in a short time?
With repeated activations of the HPA axis, the dampening negative feedback loop weakens and stress levels increase faster than the animal can lower them. This can have many consequences, one being that the behavior can change in terms of what an animal's response to stimuli (trigger) that might normally cause no reaction. With our dogs these elevated hormone levels start producing reactive behavior from minor triggers that normally would not produce enough stress hormone to elicit a response. We call this trigger stacking.
Big common trigger stacking events: holidays, separation anxiety, pain, etc. Knowing all this, we can just adjust our situations to help provide relief from the stressors on these days.
Author: Kate Wilson, BS, CBCC-KA, CPDT-KA