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.
These and other body actions are controlled by the autonomic nervous system. The autonomic nervous system also has two divisions: the sympathetic division and the parasympathetic division. These two divisions have antagonistic (opposing) effects on the internal organs they innervate (send nerves to = act on). The sympathetic division, shown at the left, is the emergency system. It prepares the body to put out energy and to protect it from effects of injury.