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physiology of Reactivity

 To fully understand why a dog is showing reactive behavior. It’s important to understand the physiology behind this behaviour, to comprehend the anatomical components and the involved chemical processes.

Whenever a dog is exposed to a trigger, a part of the brain, the amygdala, is activated. The amygdala is a section of the limbic system, located in the forebrain, that receives information from all the sensory systems in the body. This sensory information is translated into emotional responses and other parts of the brain are activated by it autonomously.  


So when a dog perceives danger, this sensory information is dealt with directly by the amygdala without going to the cerebral cortex first. The relationship between the “thinking brain” and the limbic system is very important to understand. Once the latter is activated, the cerebrum is by-passed. The dog will no longer “think” on how to handle the situation but will react in an impulsive, automated way.


If the amygdala is activated it will trigger the release of stress hormones and activate the sympathetic nervous system.

The body tries to re-find it's balance by switching between the sympathetic  (fight or flight) and parasympathetic (rest and digest) nervous systems and strives continuously towards an equilibrium. This is called homeostasis.


Thus homeostasis is the ability or tendency of an organism to maintain its internal equilibrium by adjusting its physiological processes. The brain is the organ primarily responsible for maintaining this, it does this by balancing both the chemicals inside the brain (neurotransmitters) and all the other chemicals in the body (hormones).

Stress in Dogs

Stress is a collective term used for being in a state of high alert. It’s the body’s response to danger or a stress-provoking event. It is an evolutionary survival mechanism ensuring the body is able to perform in an optimal way. Stress itself isn’t bad for the dog, but an excess can harm the body.

A stress reaction in the body is triggered in case of:

  • (Perceived) danger

  • Physical activities

  • Being excited

  • Feeling pain

  • Frustration of goals

Stress, thus can be divided in “physical stress” and “psychological stress” both types work in a similar way and have the same effect on a dog. Due to their effect they will make it harder for the body to reach and remain in homeostasis



If an organism is exposed to a stressful situation, the level of hormones will get out of balance. Once the situation has passed, the body will strive to return to a balanced state. The level of adrenaline may be cleared out of the system swiftly, often within 15 minutes. The effects of an increased cortisol level on the other hand can take from 48 hours up to 6 days, depending on the intensity of the reaction, to return to its normal state.

If the dog would be exposed to another stressful situation during this recovery period, the cortisol level will not be able to return to the normal level and the dog will eventually become chronically stressed. As a result of this constant increased cortisol level, the body will undergo certain changes, common side effects of ongoing stress are a reduction of “feel good” brain chemicals, resulting in a tendency to be more irritable, become more reactive and have less self-control.

Self-SuStaining Cycle

Showing reactive behavior is often a self-sustaining cycle. As described above: if a dog feels threatened by a certain trigger his brain, nervous and endocrine system kicks-in to anticipate on this perceived threat. The “thinking brain” (cerebral cortex) is by-passed and the sensory information is send directly to the amygdala, which in its turn activates the brain stern for one of the 4F responses. All this happens in the blink of an eye.

The dog is ready to react to the threat.


In the end the dog will form a “neural pathway” telling him how to react when being exposed to a certain trigger. Every time the dog is confronted with the same situation, the same neural pathway will be activated, strengthening the neural connection. This is called “long term potentiation”. If on the other hand, this neural pathway is used less the neural connections will become weaker, being “long term depression”. An organism will always choose the path of least resistance, it is thus our task as owners of a reactive dog to “weaken” the reactive brain path and strengthen the new “normal behavior” one.

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