Let’s start with the big picture. The nervous system is a mechanism of delivering information to and from the brain via the brainstem, the spinal cord, and the nerves that run out to the peripheries, the organs, the glands, and the muscles. The central nervous system is the brain and spinal cord. It is also a system of homeostasis that works to maintain a certain balance.
Motor neurons tell the body what to do. This happens in two ways. Firstly, there is the somatic nervous system, which is what one uses to pick up a pencil and write a sentence. Secondly, and more importantly for our purposes, is the autonomic nervous system. Whereas picking up a pencil and writing is a self-initiated activity, the activity of the autonomic nervous system seems to happen automatically, or without conscious control. The ANS controls whether or not one breaks into a sweat or has a racy heart when getting unexpected news.
The autonomic nervous system (ANS) is a way of regulating the body’s functions in light of external circumstances. The system is actually two mutually exclusive “drives” that act as a sort of gas pedal and a break. Just as in a car, the driver’s foot alternates between the gas and the break, the human body also alternates between sympathetic nervous system as the dominant drive or the parasympathetic nervous system as the dominant drive.
The sympathetic nervous system kicks in with the perception of danger. Via the “fear” reaction of the amygdala, in the limbic system, the hypothalamus then carries out four functions, which if you're interested in the neurochemistry... look it up!
The release of these hormones into the blood provides a burst of energy to deal with a stressful situation. These hormones increase blood glucose levels (to prepare to run, for instance). They also increase oxygen available to cells by increasing the heart rate and dilating the bronchioles. In addition, they increase blood supply to essential organs such as the heart, brain, and skeletal muscles and they divert blood away from nonessential organs such as the digestive system.
Short term stress is designed to help a human escape a threatening situation. It is designed to be short term, and followed by a “fight or flight” reaction. The stress reaction helps a human to deal with a stressful situation like getting chased by a bear quite efficiently. However, the system is not meant to deal with long-term stresses like losing a job while trying to pay a mortgage, which can stretch months into years in duration.
Chronic stress can contribute to or cause cardiovascular disease. The blood volume increases, its force and speed increase. Vasopressin causes the kidneys to absorb less water. There is considerable wear and tear on the system. Atherosclerosis, the accumulation of plaque, is also a risk.
Robert Sapolsky notes in Why Zebras Don't Get Ulcers that the inhale is more associated with sympathetic activation (the heart rate speeds slightly on inhale), while the exhale is more associated with parasympathetic activation (the heart rate slows slightly on exhale). People with hypertension sometimes cannot “slow down” naturally with the exhale. This can be a marker for trouble.
Stress (sympathetic activation due to a psychological or physical stimulus) is particularly bad for metabolic issues like Type-2 Diabetes. The stress response dumps energy resources out into the blood stream. It liquidates the stores thinking a person needs to run from a lion. Not only does it dump the stores, but it stops any future storage projects in anticipation of a physical event. However, sitting on a stressful conference call is not exactly charging across the savannah. This dipping into the bank account can be quite disruptive to feeling adequately nourished and rested.
The immune system has an interesting reaction to sympathetic activation. Upon short term activation, the immune system’s function is actually heightened. However, that comes at a cost. And after a short term gain in immunity, the body slips to a lowered state of immunity, and stays there. This second state is lower than the starting state. This means that long-term sympathetic nervous system activation (long-term stress, like a mortgage) may lead to more incidents of getting sick. There are many caveats and this is not a blanket statement. For instance, cancer has not shown to be affected by stress. But many other types of sickness or disease process are affected. The common cold has been shown to be more common for subordinate, stressed mice.
There is a strong link in extreme psychological stress and depression. Stress brings about some of the typical endocrine changes of depression. Genes that predispose to depression only do so in a stressful environment. Glucocorticoids, the central hormone of the stress response, can bring about depression-like states in an animal, and can cause depression in humans.
All of this has come together to make me realize how important it is to recognize and live as though every day is NOT an emergency.