We will define Tone as the level of activation in a system when it is not doing anything. Muscle tone, for example, is the level of muscle tension in a relaxed body. Our focus today, is the Tone of the Autonomic Nervous System (ANS).
Autonomic Nervous System
The ANS is the part of the brain-body interface which controls our physiologic functions in relation to our mood states. It maintains a homeostasis between two major branches: Parasympathetic Nervous System (PSNS); and Sympathetic Nervous System (SNS).
PSNS is the rest-and-digest (or feed-and-breed) mode, which controls maintenance functions in the body. Items such as sleep, elimination, digestion, sexual response, sweating, heart rate, breathing, blood pressure and others are controlled by the PSNS. The parasympathetic mode of operation should ideally be the dominant mode most of the time. It is performed by numerous sources from the brain down to the gut. Parasympathetic function tends to shift among tasks as the body’s needs change.
Sympathetic is the fight-or-flight emergency response mode of the system. Whenever an event occurs which the brain’s screening system identifies as potentially dangerous or extremely important, the SNS takes over operation of the body. Ideally, this mode is used only occasionally. Unlike the PSNS, which uses multiple drivers to handle various tasks, the sympathetic mode has a single driver.
When we speak of tone in the ANS, we are talking about the homeostasis between sympathetic and parasympathetic modes. As sympathetic becomes more dominant, which can occur in an ongoing uncontrollable situation such as a war zone, the level of energy in the system increases, and its ability to perform maintenance functions is reduced.
Which functions are most affected can differ with the individual. Some people will have difficulty sleeping, others may experience digestive problems, others may have circulation issues or experience constipation/diarrhea.
We can think of the ANS in terms of its flexibility. When the system has high tone levels, it requires greater levels of energy to maintain homeostasis. As this occurs over longer periods of time, an individual can begin to experience what are called rebound effects. For example, when a brain becomes highly sympathetic in a stressed individual, at moments when it shifts from sympathetic to parasympathetic—that is, the stress level suddenly drops—the ANS reduces its energy below the levels to which it has become accustomed. The result can be that the system experiences being too relaxed and suddenly rebounds to sympathetic mode for no discernible reason. The individual experiences a sudden physiological response like racing heartbeat, difficulty breathing or high levels of anxiety and, by focusing on these, produces a so-called “panic attack”. This represents a sympathetic rebound. Parasympathetic rebounds, such as migraine headaches or irritable bowel syndrome, also occur, when the system makes an unaccustomed rapid return to maintenance mode.
As the level of tone in the ANS remains high over an extended period, the first indication is breakdown in maintenance functions. Following this, rebound effects may begin to occur.
Tone and Emotional State
The ANS controls maintenance activity in the body in relation to the emotional state in the brain. This effect can occur in both directions. High levels of stress or anxiety can change how our bodies operate, but changing physiological states can also change our emotional states. When an individual focuses on slowing breathing and relaxing the body, this can give the message to the ANS that the stress or anxiety state is no longer active. On the other hand, when anxiety becomes a standard state in the brain’s activation patterns, it can produce lasting maintenance problems in the body.
As we deal with tone issues in the coming sections, we’ll see the importance of 3 approaches. First will be to reduce the stress response; second will be to reduce levels of tone in the ANS; but our brain-training key will be to shift the stable activation patterns in the brain which help to maintain stress and ANS responses. This third approach is what will result in long-standing changes an ANS tone.
In the following sections we’ll discuss the brain structures that turn on and off the sympathetic response. We’ll also define the experiences of “stress” and “trauma” which drive the response.