The vagal system is a stress response system that helps us deal with crises. It pits reason and emotion against one another, in states of emergency or high stress. When we’re angry or upset or fearful, most of us find it difficult to think clearly. How often we come up with the perfect comeback long after the argument is over! The reverse is also true: If, when upset, you can get yourself to stop and think, you’ll probably calm down to some degree. Along with reason, the other capacity that shuts down with strong negative emotion is social engagement, the set of various feelings thoughts, and behaviors that enables us to interact with one another in positive ways. The vagal system mediates these inverse, “push-me/pull-you” functions: clear thinking and social engagement vs. states of stress and emergency.

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The relationship between states of mind and behavior operates at a neurobiological as well as a psychological level. Changes between stress and calm are mediated by the autonomic nervous system (ANS), the system that controls automatic bodily functions such as respiration, sleep, and digestion, and the physiological responses that generate emotion.  When we’re safe and in business-as-usual mode, our systems are dominated by the branch of the ANS called the parasympathetic nervous system (PNS), which enables clear thinking and social engagement as well as aforementioned ongoing bodily processes. But when the environment poses danger, such as a snake in your path or a nasty rival, the sympathetic division of the ANS (the SNS) takes over. Two of its most important subsystems associated with stress are the fight-or-flight response, which generates an immediate reaction to danger, and the hypothalamic-pituitary-adrenal axis, better known as the HPA axis response, which activates some time afterward (although this can be just minutes later) and which is necessary for a sustained reaction to stress.

Stephen Porges and his research team have formulated a convincing and widely accepted theory about how we switch from one system to the other. This is “the polyvagal perspective,” called a perspective rather than a system because it’s really a way of viewing how the ANS works. When you look at the ANS from the polyvagal perspective, you see that the vagal brake is the primary mechanism by which the body orchestrates the degree to which each system activates.

The ventral or “smart” vagus, which is in charge of these shifts between calm and stress, originates in the brainstem, the lowest area of the brain, continuous with the spine. In one direction, the ventral vagus connects to the heart’s pacemaker, a muscle called the sinoatrial node. Its crucial function is to enable us to match our reactions to what the environment requires by varying heart rate, which is a cause as well as an indicator of excitement. If heart rate is sufficiently accelerated we go into fight-or-flight mode, and if the threat is sustained, we segue into HPA axis mode; the SNS dominates.  If excitement is not required, heart rate slows down, and the PNS takes over. Porges calls this part of the ventral vagus (which controls heart rate), the “vagal brake,” because it increases and decreases heart rate just as the brake on a car regulates speed. When Elinor hears of Edward’s engagement, the vagal brake is lifted. When she recovers sufficiently to speak, the brake is applied, perhaps lightly in this case, since she’s still upset.

The automotive metaphor makes sense if you think of driving downhill rather than on a level surface. You need to press the brake to keep cruising at an acceptable speed. Without the restraint of the vagal brake, we would quickly go downhill biologically. The heart would beat much more quickly, initiating sympathetic stress responses, and we all know how unhealthy it is to live in a constant state of stress. Actually without any vagal restraint, the heart would beat too quickly to sustain life.

In the other direction (coming from the brainstem), the ventral vagus connects to important parts of the social engagement system, which is the set of brain areas involved in emotions, thoughts, and behaviors that activate when we interact socially with others. This part of the vagus is known as the tenth cranial, or head, nerve. It controls muscles of the head and face, most crucially around the eyes, which are so important for signaling emotion. It also connects to the staepedius muscle in the ear, which opens passageways to increase our capacity for hearing, vital to conversation. The ventral vagus therefore coordinates responses in two directions; with the vagal brake lifted, we are likely to have concentrated, stressed facial expressions (for instance, anger, fear, grief) as well as an accelerated heart rate, but when it’s on, our heart rates are normal, and our faces are likely to signal a willingness to engage with others.  Elinor is fairly good at controlling the outward expression of emotion, but even she must have registered shock—or felt her facial muscles attempting to do so—when Lucy shared her news.

In addition to inducing stress or calm, the smart vagus orchestrates all the states of mind and body that we experience between these extremes, and these are usually the norm (just as when driving we don’t stop altogether, unless we’ve arrived at our destination, nor do we bolt through city streets at eighty miles an hour). And so full sympathetic dominance rarely lasts for long. Elinor was dumbfounded at Lucy’s confession, but the peak of her upset subsided quickly, and parasympathetic regulation began to function sufficiently for her to answer Lucy. Yet she wasn’t calm and relaxed either. These in-between states show that the smart vagus mediates simultaneous input from the PNS and the SNS, largely determining how our bodies and brains respond to the environment at each and every moment.

The ventral vagus is truly “smart,” figuring out how we need to react according to the information it receives. If some people, like Elinor’s high-strung sister Marianne, send the message far too often that a stress response is needed (such people are said to have poor vagal tone), it is the fault of our imperfect tendencies as a species. Elinor has struck a balance between thought and feeling, sense and sensibility, cruising along at an acceptable speed, but many people tend to over- or under-regulate their responses. The smart vagus is only as emotionally intelligent as the person whose mind, brain, and body it helps to operate. You might think of the smart vagus as a biological form of Artificial Intelligence, responding in the best way to input, but still dependent on a programmer.

Abridged and adapted from Jane on the Brain: Exploring the Science of Social Intelligence with Jane Austen by Wendy Jones, published December 2017 by Pegasus Books. © Wendy Jones, 2017.

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