I watched a National Geographic program on stress and how it is a physical response in all species and an especial problem in primates who live in hierarchical societies similar to industrial culture.
Here is a link to information on the program. If you have Netflix, you can watch it online.
http://www.pbs.org/stress/
Freydis
I have to agree it's physical response that is normal to the experiences and enviromental suroundings. Every experience you have comes from the environment around you…the good, the bad and the ugly.
The Brain is a Physical response...
You learn early not to do things, because it causes injury or pain like putting your hand in fire to get burnt you just don’t want to do that again. Dogs are restrained by invisible electric fences. Despite there being no power to enforce the authority of the fences, the dogs continued to behave as if it were on. (A past experience)Can we all associate our brain with something...the root of depression.
Nature's way... to responses is the perceptual framework of a habitual way of thinking or interpreting that what is natural, for survival and protection.