Nervous System
The nervous system has several divisions: the central division involving the brain and spinal cord and the peripheral division consisting of the autonomic and somatic nervous systems. The autonomic nervous system (ANS) has a direct role in physical response to stress and is divided into the sympathetic nervous system (SNS), and the parasympathetic nervous system (PNS).
When the body is stressed, the SNS generates what is known as the "fight or flight" response. The body shifts all of its energy resources toward fighting off a life threat, or fleeing from an enemy. The SNS signals the adrenal glands to release hormones called adrenalin and cortisol. These hormones cause the heart to beat faster, respiration rate to increase, blood vessels in the arms and legs to dilate, digestive process to change and glucose levels (sugar energy) in the bloodstream to increase to deal with the emergency.
The SNS response is fairly sudden in order to prepare the body to respond to an emergency situation or acute stress, short term stressors. Once the crisis is over, the body usually returns to the pre-emergency, unstressed state.
Chronic stress, experiencing stressors over a prolonged period of time, can result in a long-term drain on the body. 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.