Vagal tone

Vagal tone refers to activity of the vagus nerve, an important component of the parasympathetic branch of the autonomic nervous system. This division of the nervous system is not under conscious control and is largely responsible for regulation of the body at rest. Vagal activity results in diverse pleiotropic effects, including: lowered heart rate, changes in vasodilation/constriction, and glandular activity in the heart, lungs, and digestive tract. Because the vagus nerve is importantly involved in heart rate regulation though its action on pacemakers in the heart, vagal tone is easily assessed by heart rate.

In this context, tone specifically refers to the continual nature of baseline parasympathetic action that the vagus nerve exerts. While vagal input is continual, the degree of stimulation it exerts is regulated by a balance of inputs from both divisions of the autonomic nervous system and reflects the general level of parasympathetic activity. Vagal tone is typically considered in the context of heart function, but also has utility in assessing emotional regulation and other processes that alter, or are altered by parasympathetic activity.

Heart rate is controlled largely by the heart's internal pacemaker activity. In normal hearts the main pacemaker is a collection of cells on the border of the atria and vena cava called the sinoatrial node. Heart cells exhibit automaticity, or the ability to generate electrical activity independent of external stimulation. As a result, these cells spontaneously generate electrical activity that is subsequently conducted throughout the heart, resulting in a regular heart rate. This is termed sinoatrial pacemaking.

Without external stimulation sinoartial pacing results in a heart rate of approximately 100 beats per minute. However, branches of the vagus nerve acts on this node and slow its conduction. This is mediated by the neurotransmitter norepinephrine and downstream changes to calcium handling by heart cells.

Vagal tone is not directly measured, instead it is inferred by measuring processes that the vagus nerve alters - specifically heart rate and heart rate variability. Increased vagal tone (and thus vagal action) is associated with a diminished and more variable heart rate. Vagal tone generally serves as a measure of parasympathetic nervous system activity that promotes "rest and digest".

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