From Born to Be Good: The Science of a Meaningful Life by Dacher Keltner (pages 228-230):
[The vagus nerve] resides in the chest and, when activated, produces a feeling of spreading, liquid warmth in the chest and a lump in the throat. The vagus nerve … originates in the top of the spinal cord and then winds its way through the body…, connecting up to facial muscle tissue, muscles that are involved in vocalization, the heart, the lungs, the kidneys and liver, and the digestive organs. In a series of controversial papers, physiological psychologist Steve Porges has made the case that the vagus nerve is the nerve of compassion, the body’s caretaking organ.
…Porges notes that the vagus nerve innervates the muscle groups of communicative systems involved in caretaking – the facial musculature and vocal apparatus. In our research, for example, we have found that people systematically sigh – little quarter-second, breathy expressions of concern and understanding – when listening to another person describe an experience of suffering. The sigh is a primordial exhalation, calming the sigher’s flight/flight physiology, and a trigger of comfort and trust, our study found, in the speaker. When we sigh in soothing fashion, or reassure others in distress with our concerned gaze or oblique eyebrows, the vagus nerve is doing its work, stimulating the muscles of the throat, mouth, face, and tongue to emit soothing displays of concern and reassurance.
Second, the vagus nerve is the primary brake on our heart rate. Without activation of the vagus nerve, your heart would fire on average at about 115 beats per minute, instead of the more typical 72 beats per minute. The vagus nerve helps slow the heart rate down. When we are angry or fearful, our heart races, literally jumping five to ten beats per minute, distributing blood to various muscle groups, preparing the body for fight or flight. The vagus nerve does the opposite, reducing our heart rate to a more peaceful pace, enhancing the likelihood of gentle contact in close proximity with others.
Third, the vagus nerve is directly connected to rich networks of oxytocin receptors, those neuropeptides intimately involved in the experience of trust and love. As the vagus nerve fires, stimulating affiliative vocalizations and calmer cardiovascular physiology, presumably it triggers the release of oxytocin, sending signals of warmth, trust, and devotion throughout the brain and body, and ultimately, to other people.
Finally, the vagus nerve is unique to mammals. Reptilian autonomic nervous systems share the oldest portion of the vagus nerve with us, what is known as the dorsal vagal complex, responsible for immobilization behavior: for example, the shock response when physically traumatized; more speculatively, shame-related behavior when socially humiliated. Reptiles’ autonomic nervous systems also include the sympathetic region of the autonomic nervous system involved in flight/flight behavior. But as caretaking began to define a new class of species – mammals – a region of the nervous system, the vagus nerve, emerged evolutionarily to help support this new category of behavior.