Sympathetic Dominance

Getting stuck in the “fight or flight mode”

The autonomic nervous system (ANS) controls the “unconscious” functions of the body such as digestion, heart rate and blood pressure.  These functions vary according to demand without our direct thought about their activity.  This is in contrast to the peripheral nervous system (PNS) which controls voluntary function.  When we want to raise our arm, we activate the process through the peripheral nerves.  When we eat, in contrast, we do not have to think about activating digestion.

The autonomic nervous system senses internal circumstances and varies factors such as blood pressure, heart rate, digestive activity, inflammation, blood sugar and several more variations in function.  The ANS has two divisions, sympathetic (SNS) and parasympathetic (PNS).

The SNS is known for activating “fight or flight”.  It increases blood pressure, heart rate, inflammation, blood sugar, alertness in the brain and suppresses all aspects of digestion.

The goal of the SNS activation is the basic changes needed for survival during danger.  While this served man well for the first 5-6 million years of living in a dangerous environment, its over-activation is now a major driver of the most common chronic diseases.

It is acceptable and necessary for the SNS to activate all of the above changes for short periods of time in response to danger.  Unfortunately, modern humans do not differentiate “psychological stress” from danger, and the same changes in blood pressure, heart rate, blood sugar, inflammation, brain alertness and digestion occur.  The difference is that “stress” is very frequent and chronic in modern life, where true danger was in infrequent, short periods for our distant ancestors.  There is an excellent video of this where one of our best neuroendocrinologist’s, Robert Sapolsky of Stanford University, explains how humans react to stress. CLICK HERE TO VIEW VIDEO

The job of the PNS is to take over after the danger or stress has passed causing “rest and repair” restoring normal body function.  The following chart contrasts the functions of the SNS and the PNS:

Unfortunately, modern life has a lot more “stress” than “peaceful/mellow” so we spend a lot more time in sympathetic activation.  This exploits a trait of the nervous system which is that it learns.  This is called neuroplastic behavior.

Neuroscientists will often use the simple phrase “when neurons (nerve cells) fire together, they wire together.”  Repetition in the nervous system eventually teaches it to preferentially activate in this learned pattern.  Once the SNS has learned to be more active than the PNS, its activation features shown above become ongoing.  This learned dominance of the SNS over the PNS is called sympathetic dominance.

When digestion is suppressed on an ongoing basis, problems such as reflux and IBS occur.  Blood pressure stays up causing hypertension.  Blood sugar stays elevated and triggers pre-diabetes.  When brain activation becomes sustained, it causes anxiety and sleep difficulty.

The graphic above is of a heart rate variability test of a patient with hypertension.  The top portion shows a 4-minute period looking at heart rate beats per minute.  The constant increase and decrease in heart rate is from a phenomenon called respiratory sinus arrhythmia.  It is caused by constant switching from sympathetic dominance to parasympathetic dominance which occurs with inspiration and expiration.  It is a simple way to examine the balance in the amount of activity in the two divisions of the autonomic nervous system.

The low frequency signal in the data below the graph represents that signal from the SNS, while the high frequency signal represents that of the PNS.  The ratio of these shown on the last line should ideally be about 1:1.  The Low frequency: high frequency ratio is 9.6, or very sympathetic dominant.

The follow-up test results are shown in the graphic to the left.  The low frequency:high frequency or SNS:PNS has reduced to 4.4 in just 4 weeks of the training discussed below.  More importantly, the patient’s blood pressure normalized within the 4 weeks.

Recent advances in techniques to retrain the balance in the autonomic nervous system have brought a new dimension to treating all of the above disorders.  These techniques include heart rate variability biofeedback training, transcutaneous vagus (the main parasympathetic nerve) stimulation and neurofeedback, or EEG guided brain biofeedback.

These procedures work on the same concept that caused the ANS to learn a bad pattern of activation, “neurons (nerve cells) fire together, wire together.”  With time, the normal functioning pattern of ANS control are retrained.  Combinations of these procedures can be used resulting in restoration of balanced autonomic function and resolution of the related health problems such as digestive complaints, high blood pressure, non-specific inflammation, blood sugar elevations, anxiety/depression and sleep difficulty.

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