The Anatomy in a Nutshell
The human nervous system is divided into three parts: the CNS, or central nervous system, comprised of the brain and the spinal cord; the PNS, or peripheral nervous system, made up of all the nerves which leave the brain and spinal cord to supply the muscles (making them contract) and the skin (allowing us to feel); and the ANS, or autonomic nervous system, comprised of tiny spider web-like nerves that stimulate or sedate the activity of internal organs and wrap around blood vessels, enabling them to constrict and dilate. In this discussion, we will concern ourselves primarily with the ANS because it is the part of the nervous system which mediates the physiology of emotion and stress.
The ANS is not Conscious
Autonomic nerves are not under control of the conscious or thinking part of the brain (the cortex). That means you can’t will autonomic nerve activity. Instead, autonomic nerves carry out the directives of a part of the brain called the limbic system, a collection of evolutionarily primitive nuclei similar in function to the analogous parts of the brains of lower mammals like cats or dogs. The nuclei of the limbic system form the emotional part of the brain, enabling us to to have feelings, and are responsible for the phenomenon known as the ‘Fight-or-Flight’ response, which is the body’s way of reacting to stress. In stressful situations heart rate and breathing become more rapid, palms sweat, and muscles (especially in the back and neck) tense up in preparation for defending ourselves.
The stress response happens to our bodies automatically–whether we want it to or not. Constriction and dilation of blood vessels, speeding up and slowing down of heart rate, movement of food through our intestines, sweating, temperature regulation, and a host of other important physiological functions are controlled by the ANS, which carries out the orders of the emotional part of the brain, the limbic system, and none of these physiological changes are under volitional control. We can move, speak, and think through conscious intention, but we cannot change the physiology of our bodies in this way. We can control the way we behave while under stress but we cannot control how we feel in our bodies when our physiology changes during the stress response. You can act surprised (or scared, or happy, or brave, etc.) but you cannot make yourself feel or not feel the bodily sensations which accompany these emotions. It is often said that the heart wants what the heart wants, which is to say that our feelings, unlike our thoughts, are not changeable through debate or other rational processes.
The ANS Has a Brake and an Accelerator
The ANS is divided into two parts: the sympathetic division which speeds and tightens things up, and the parasympathetic division which relaxes and slows things down. In this way, the sympathetic division, usually referred to as the sympathetic nervous system or SNS is like the accelerator pedal of a car, while the parasympathetic division or PSNS acts a little like a brake.