The momenta of individual objects changes. The total momentum remains constant. I have to disagree. If you have two cars that collide head on, the momentum of both vehicles stops. The ENERGY created by the impact causes usually, some reverse momentum but the momentum is lost.
As the SNS continues to trigger physical reactions, it causes a wear-and-tear on the body. It's not so much what chronic stress does to the nervous system, but what continuous activation of the nervous system does to other bodily systems that become problematic.
effects of sympathetic nervous system. increased HR, dilated pupils, constricted arterioles in skin and intestine, dilated muscular arterioles, high BP, pale face, dry mouth, hair stands up, blood rushes to brain heart and skel muscle preferentially, closed alimentary and urinary tract sphincters, bronchi and bronchioles dilated. Sympathetic chain.