Polyvagal Theory Part 1: The Wandering Nerve
The polyvagal theory is a neurobiological theory relating social engagement, physiology, and developmental outcomes. When I was first learning the theory, I struggled to understand some of the theory’s basic terms and could not find a resource that simplified it. Therefore, this three–part series of articles is intended to serve as an introduction to the theory. In this article I will introduce the physiology behind stress. In the second I’ll discuss specific stress responses, and in the third I’ll mention some of the theory’s research and clinical implications. The Nervous System
Your body has two systems—the central nervous system (CNS) and the peripheral nervous system (PNS). Your CNS consists of the brain and spinal cord and controls thoughts and sensations, while the PNS contains all of the other nerves of the body and ganglia, which regulate muscles, organs, and limbs. The PNS contains the somatic nervous system and the autonomic nervous system (ANS), which are responsible for voluntary (i.e. looking, speaking) and involuntary functions (i.e. breathing, digesting) respectively. The ANS is then further divided into the sympathetic nervous system (‘fight-or-flight’ responses) and the parasympathetic system (‘rest and digest’). When you’re at home and relaxed, you’ll engage your parasympathetic system. However, if someone breaks into your house, you hear a loud noise, or have to watch the presidential debate, you might get stressed out and have a physiological response. This means your body is mobilizing in order to deal with a threat—whether you realize it or not—by pumping your heart faster, dilating your pupils, producing saliva, and increasing your blood sugar.
This seems like a straightforward model; the body gets ready when there’s a threat present. This does, however, leave a lot of questions. Are these systems reciprocal, such that one is always on and the other is always off? What happens in chronic stress? Why would I have a fight-or-flight response from watching a debate on TV? Are emotions important for this response? Why didn’t Inside Out talk about anything outside the brain?
As social creatures, our physiological responses integrated with social engagement. Therefore, we have evolved to cope with urgent stressors as well as social stressors, and we have different systems to do this effectively. The polyvagal theory ties together the evolutionary, physiological, and social factors related to these responses. First, I’ll describe the role of the vagus nerve in stress regulation.
The Vagus Nerve
The tenth cranial nerve is the vagus nerve, which is a system of fibers connecting the throat, heart and brain. The name derives from the Latin root vagus which means wandering because the nerve is able to communicate signals between such different parts of the body. About 80% of the vagus nerve fibers are afferent, meaning these fibers send signals from the organs to the brain. These afferent fibers relay sensory information about the states of the organs (how fast your heart is beating, how much you’re digesting, how large your pupils are). The remaining fibers are efferent, such that the brain can relay signals to these organs and alter function (we’ll discuss these fibers more in the next articles). The role of the vagus nerve in the nervous system is to maintain a state of balance among the different internal organs, including the heart. This state of balance is called homeostasis and can be with respect to temperature, activation, chemistry, and other factors. This nerve is especially important because the equilibrium state of an organ can vary with context.
For example, if it’s warm outside, your blood will be circulating throughout your body and disperse heat. However, if it’s cold outside, then your body will keep your blood circulating by your major organs to keep them warm. In fact, the arteries in your extremities may actually constrict to limit the blood flow. You may notice that your fingers and toes will be especially cold in cold weather since they are the furthest parts of your body from your heart, and the blood is not circulating to them. You may feel frustrated that your fingers are cold, but your body is maintaining homeostasis with respect to temperature for your major organs, so be grateful for your vagus and make sure to wear a jacket and gloves.
Homeostasis can also change because of social conditions, such as the presence of a stressor. Your heart should naturally be relaxed and beating 60-80 times per minute. However, if you’re being chased by a lion or are midway through a marathon, your heart should be beating faster and pumping more oxygen. In both cases your goal is to go further, so oxygen and blood are circulating to help you reach this goal. Alternatively, your heart also tends to race when you are nervous, such as when you have to take an important exam or you are getting ready to talk to that special someone. In these cases, it may seem less advantageous for your heart to race. You are not actually getting ready to fight or flee, but your body is displaying a stress response as if there is a real threat present. The different types of stress responses are addressed by the polyvagal theory.
The term polyvagal refers to the fact that the vagus has a myelinated and an unmyelinated branch.1 Myelin sheath is the fatty substance that lines some nerves and enables for signals to be sent more quickly and accurately. The unmyelinated branch is more primitive and lacks this myelination, and it is also consequently less rapid and organized than the myelinated branch. You can picture the myelinated branch like a street with stop lights—the street is paved and you can drive quickly, but the street is still organized so cars can travel safely. The unmyelinated branch is more like a bumpy dirt road. It is not nicely paved, so it is hard to drive quickly. It also is not regulated, so it can be a little haphazard and you might risk getting into an accident.
These branches maintain homeostasis among different internal organs, including the heart. They are able to do this by means of three different stages of neural control. These stages, in order from least to most evolved, are the unmyelinated vagus, the sympathetic-adrenal system, and the myelinated vagus.1 Each stage has different effects on the body and operates at different times. In the next article, I’ll go through what each of these stages do, when they are activated, and what this means for us each day.
1 Porges, S. W. (2009). The polyvagal theory: New insights into adaptive reactions of the autonomic nervous system. Cleveland Clinic Journal of Medicine,76(Suppl_2). doi:10.3949/ccjm.76.s2.17