Physical calm can be frustratingly elusive after trauma. We receive messages that we “should” be over it, “just calm down,” or suggestions to try yoga or meditation. When we’re already doing our best to calm an irritated nervous system, it can help to understand the underlying physical mechanisms in our bodies.
Stephen Porges has done decades of careful research into the parts of the nervous system that control activation and settling: the sympathetic and parasympathetic nervous systems, which together make up the autonomic nervous system. He found that the parasympathetic system has two parts, creating a hierarchy of three systems that respond to danger and also function in safety.
Action: sympathetic nervous system
The sympathetic nervous system helps you take action. It powers fight-or-flight reactions, as well as any muscular effort. It raises your heart rate and breathing rate, increases blood flow to large muscles, and dilates the pupils of the eyes. To balance this use of energy and resources, blood flow decreases to digestive organs, and immune system function decreases.
The sympathetic nervous system is activated by hormones secreted by the adrenals, located on top of the kidneys. Once the hormones are circulating, it takes a few minutes for them to clear from the body, which is why an adrenaline rush does not immediately subside, even when we realize there is no danger or the danger is over.
Rest: parasympathetic nervous system
The parasympathetic nervous system helps you rest, digest, and repair. It decreases your heart rate and breathing rate, increases blood flow to digestive organs, and increases immune system function.
Did you ever wonder how someone with a paralyzing spinal cord injury can still breathe and digest food? While sympathetic nerves originate from the spinal cord at the middle and lower spine, parasympathetic nerves originate directly from the brain. Parasympathetic nerves are also called vagus nerves, or the tenth cranial nerves.
Many vagus nerve fibers
The Polyvagal Theory says that there are older and newer parts of the vagus (“wandering”) nerves, which are long nerves that run directly from the brain stem to heart, lungs, and digestive organs. The older and newer fibers combine on each side to form a right vagus nerve and left vagus nerve. They have slightly different functions on each side since we are not fully symmetrical. The nerves have afferent (sensory, to the brain) fibers and efferent (motor, from the brain) fibers, creating feedback loops that regulate the organs and keep them functioning in their ideal range.
Old: dorsal vagus
The dorsal (“back”) vagal nerve fibers start at the back of the brain stem. They are unmyelinated (“unsheathed”) and thus send relatively slow and imprecise signals. A strong signal (known as high tone) on these nerve fibers causes heart and breathing to slow dramatically. The body goes limp and freezes, or “plays dead.” This is different from a tense “must do!”/”can’t do!” freeze caused by conflicting impulses.
We share this ancient system with all vertebrates. For reptiles, slow heart and breathing allow them to dive underwater for longer periods and to survive a predator’s interest. For mammals, whose brains require more oxygen, a dorsal vagal freeze can be fatal if it goes on too long.
The nervous system resorts to the dorsal vagal freeze when it evaluates that it is our only hope for survival, or when it has given up on survival. It is completely involuntary and not a cause for shame or self-recrimination for “not fighting back.”
A less strong signal (low tone) on the dorsal vagus nerve fibers coordinates digestion, immune response, and cell repair. Our body goes into this important rest state when we feel safe and there are no demands on us.
If we experienced early developmental trauma, our nervous system might not have learned how to access this state. If we have PTSD from shock trauma, we might remain too vigilant to rest deeply. This leads to chronic issues with digestion, immune response, and fatigue. See Find Calm: Practice Rest and Regulation for body-centered tools to restore the nervous system.
New: ventral vagus
The ventral (“front”) vagal nerve fibers start at the front of the brain stem. They are myelinated (“sheathed”) and thus send fast, precise signals. All mammals have this system, which evolved to help mammals get the dependable high oxygen levels we need, as well as conserve energy when we are at rest.
Vagal brake on heart rate
The human heart has a pacemaker that beats around 100 times per minute. A strong signal on the ventral fibers of the vagus nerve slows the pacemaker, acting as a brake on the heart rate. Since ventral vagus signals are fast and precise, the vagal brake can be adjusted nearly instantaneously to slow or speed the heart as needed. When we stand up from sitting and chatting with someone, our heart beats faster without adrenal hormones.
The ventral vagus fibers help initiate each inhalation (in-breath). Exhalations (out-breaths) happen on their own as the diaphragm relaxes. During the inhalation signal, the vagal brake signal to the heart is interrupted, and the heart speeds up a little bit. This difference in heart rate between inhalation and exhalation is known as Respiratory Sinus Arrhythmia1, or RSA. It is used in medical experiments to measure the strength of the ventral vagal signal non-invasively, mostly in children where RSA is more pronounced.
The higher the ventral vagal tone, the more pronounced the RSA difference becomes. Higher ventral vagal tone is better, leading to a lower resting heart rate, healthier digestion, and more energy available for social engagement and attachment bonds.
The ventral vagus fibers originate in the same area of the brain stem as nerves controlling facial expressions, speech, singing, and swallowing. They coordinate our social engagement: using our avenue of expression to talk with and respond to other people. They sensitively detect friendly or threatening behavior, and adjust the vagal brake as necessary. When we feel safe, our heart rate slows, and when we feel threatened, it speeds up.
Release your ears
The ventral vagus nerve fibers interact with our hearing as well. They tighten the muscles of the middle ear, tuning it toward higher conversational pitches rather than the lower pitch of a predator’s roar. If we grew up in an abusive environment where humans were the source of danger, we might unconsciously tighten our middle ear all the time, straining to hear approaching voices that signal danger.
Try asking your middle ears on both sides to relax. Even if you do not know how to contact them with precision, you might notice a release in your jaw and maybe even your shoulders.
Breath supports calm
Stephen Porges points out that playing a wind instrument makes ideal contact with the ventral vagus: a slow controlled exhalation, attention to facial muscles to maintain embouchure, and engagement with others when playing in a group or for an audience. Singing does the same thing.
To calm your system, take a quick breath in, then blow out gently for as long as the breath lasts. Try it a few times and see if your heart rate slows down.
Our behavior makes sense
When we understand the underlying structure of our nervous system, we can make sense of our behavior more easily. No wonder it was hard to make small talk when we were already frightened. No wonder we find it hard to rest if we never got a chance to feel safe when our nervous system was developing. No wonder we got still and quiet when we thought there was no way out. Our nervous system is doing its best to help us survive. We can help it learn new patterns that serve us better in the present.
1. Respiratory (breathing) Sinus Arrhythmia (lack of rhythm) gets “Sinus” from the sinoatrial node of the heart, which is the heart’s pacemaker. “Atrial” comes from the atrium, one of the sections of the heart. “Sino” comes from the sinus venarum, a cavity (sinus means open space or cavity) that exists in the embryonic heart, becoming part of the atrial wall after birth. The sinoatrial node is in that part of the wall of the right atrium in the heart.↩
- Book: “The Polyvagal Theory” by Stephen W. Porges is a dense compilation of his research papers in technical medical language.
- Video: Stephen Porges on Polyvagal Theory and Healing Trauma by Elizabeth Bader. He explains his theory much more clearly in interviews. This is a summary blog post and 41 minute interview with Stephen Porges.
- Transcript: Transcript of Stephen Porges talking about the Polyvagal Theory for Shrink Rap Radio.
- More about vagal tone and RSA on Wikipedia.