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.
It is commonly assumed that emotions are a function of thought, but that isn’t quite right. Let’s take fear as an example. Fear is an emotion which begins when nerve cells in the limbic system are aroused in response to some threatening stimulus. If a person has a fear of heights then riding in an elevator to the top of the Empire State Building is a stimulus that will arouse the limbic system to initiate the physiological responses we experience as feeling scared. Constriction of blood vessels in the gut (making the stomach feel sick or tight) and in our legs (making them feel weak and wobbly); increased cardiac and respiratory rates (making our hearts beat fast and our breathing rapid and shallow); activation of sweat glands (making our palms and underarms become moist), and dilation of the pupils of the eyes are all part of the fear response.
When our hearts start to pound, our palms become sweaty, our breathing becomes rapid, and our stomachs go tight or sick, that’s how we know that we are feeling scared. Sometimes when we are scared, we behave as though we are feeling excited, glad, angry, or nothing at all. How we choose to show our emotions is not always an accurate or complete revelation of our true feelings. Let us take a moment to consider the case of Mark, a patient of mine who reported that a co-worker of his was going through a particularly painful divorce. Mark felt sorry for his co-worker. But he also resented having to pick up the slack since she had stopped doing significant portions of her job. It had been several months and her lack of focus, frequent taking of sick days, and inability to complete vital tasks assigned to her was getting worse, not better. Mark felt trapped. On the one hand, he had sympathy for his colleague. On the other hand, he felt taken advantage of. He had personal problems of his own but did not bring them with him to the workplace. He feared confronting her as she seemed emotionally fragile to him and did not want to be labeled as uncaring. At the same time he resented coming in early, staying late, and using his weekends to do someone else’s work. He was trapped, emotionally speaking, in a state which we sometimes refer to as cognitive dissonance. Fear of hurting his emotoinally fragile colleague and looking bad in the eyes of coworkers was preventing his confronting her about her not doing her work; without confronter her he
Variations in the relative intensities of each of these physiological responses are what account for the varying shades of the primary emotion of fear that we all feel at certain times: anxiety, nervousness, panic, terror, etc. For example, if we start with fear, then decrease the heart and respiratory rates a little, while increasing the intensity of the stomach discomfort, we are now experiencing anxiety. If we start with anxiety and then strongly increase the cardiac and respiratory rates (tachycardia and hyperventilation) we are now experiencing panic. Prolonged panic can make us feel light-headed and may even cause us to pass out. This experience has been well described as what is known as a panic attack. Our bodies, through activation of the SNS by limbic system, create the physiological experiences we know as emotions. The variations of the fear response represents the body’s way of responding to specific types and levels of perceived threat.
Perhaps most interesting is the fact that we can experience an emotion (such as fear) without consciously registering that we are, in fact, afraid. Going back to the earlier example in the elevator, if the person with fear of heights rides to the top of the Empire State Building he may still be entirely unaware of the fact that he is afraid, even as his body is feeling strong fear-feelings in his chest, stomach, and legs. He may say to himself, “This is safe. A billion people have done this and no one has ever been injured. It is more dangerous to cross a busy street or drive a car than it is to go to the top of this building and I am not afraid of doing those other things, therefore, I am certainly not going to be afraid of this.” Though the logic of such thinking is hard to refute, emotions, quite famously, are not logical, and a person with fear of heights cannot avoid the physiology of fear: rapid breathing and heart rate, wobbly legs, sweating, dilation of the pupils. If he gets to the 110th floor and tries to walk out onto the observation deck, he may in fact experience a panic attack and faint. Knowing that we are safe does not mean that we are therefore not feeling scared.