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background as briefly and clearly as possible for the non-technical reader, I shall present a simplified version and the use of jargon and acronyms. If the reader wishes to resolve the resulting imprecision, hovering over a highlighted word or phrase will show the technical meaning I intend, and clicking will take you to the glossary for a definition or more information. Of course the system is much more complicated than this1.All vertebrate animals have a complex neural system that automatically and unconsciously monitors the environment for immediate danger or safety, and coordinates adaptive responses throughout the body. The Fight-or-Flight Response is activated by perceived danger, and regulates almost every organ of the body to prepare for vigorous physical effort to survive an immediate threat. In order to achieve peak performance in moments of crisis, other non-urgent functions are temporarily shut down, such as feeding, digestion, immune response, and reproduction. These somatic effects underlie visceral sensations associated with the emotion of fear, such as a racing or pounding heart, sweaty palms, shallow and rapid breathing, dry mouth, or a sinking feeling in the stomach. The Fight-or-Flight Response is energetically costly, and not sustainable.
The exact opposite occurs during the opposing “Rest-and-Digest Response,” which is most active when conditions of safety are detected. This neural pathway slows the breath and heart rate, and promotes regenerative and self-sustaining functions throughout the body. In a healthy person, these opposing systems dynamically engage and disengage in response to the demands of each situation. By way of illustration and summary, Figure 1 presents a sketch of what might be happening in these pathways in different situations.
It is extremely important to recognize that this mind-body interface is a two-way street. The brain centers that regulate Fight-or-Flight and Rest-and-Digest responses are themselves receiving sensory information from within the body, such as sensations of breathing, heart rate, hunger or nausea. Thus not only do these neural pathways cause changes throughout the entire body, but the reverse is also true: the internal states of the body can influence the activity of these neural pathways.
The signals underlying Rest-and-Digest functions travel through the vagus nerve, and therefore the level of activity in this system is sometimes referred to as “vagal tone.” This is an oversimplification, in that different parts of the vagus nerve are involved in different and sometimes even opposing functions. The Rest-and-Digest system can be roughly divided into two subdivisions. Both subdivisions can slow down the heart rate, but one of them uniquely synchronizes the heart with breathing, resulting in a distinct heart rhythm. In research, vagal tone is often measured by heart rate variability which is an indirect measure of this distinct heart rhythm. High vagal tone by this measure is generally associated with states of calmness. For now, I will refer to activity in this subdivision of the Rest-and-Digest system as the Calming Response; but other distinctive features of this subdivisio
