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Vagal tone is activity of the vagus nerve (the 10th cranial nerve) and a fundamental component of the parasympathetic branch of the autonomic nervous system. This branch of the nervous system is not under conscious control and is largely responsible for the regulation of several body compartments at rest. Vagal activity results in various effects, including: heart rate reduction, Vasodilation of Blood vessel, glandular activity in the heart, , and digestive tract, liver, immune system regulation as well as control of gastrointestinal sensitivity, motility and inflammation.
In this context, tone specifically refers to the continual nature of baseline parasympathetic action that the vagus nerve exerts. While baseline vagal input is constant, the degree of stimulation it exerts is regulated by a balance of inputs from sympathetic and parasympathetic divisions of the autonomic nervous system, with parasympathetic activity generally being dominant. Vagal tone is frequently used to assess heart function, and is also useful in assessing emotional regulation and other processes that alter, or are altered by, changes in parasympathetic activity.
Measurements of vagal tone can be performed by means of either invasive or noninvasive procedures. Invasive procedures are in the minority and include vagus nerve stimulation by specific manual, breathing or electrical techniques. Noninvasive techniques mainly rely on the investigation of heart rate and heart rate variability.
In absence of external stimuli, sinoatrial pacing generally, while awake, maintains the heart rate in the range of 60–100 beats per minute (bpm). The two branches of the autonomic nervous system work together to increase or slow the heart rate. The vagus nerve acts on the sinoatrial node, slowing its conduction and modulating vagal tone, via the neurotransmitter acetylcholine and downstream changes to Ion channel and calcium of heart cells. Because of its effect on heart rate, and cardio health, vagal tone can be measured and understood by examining its correlation to heart rate modulation and heart rate variability.
During inhalation, the intra-thoracic pressure lowers due to the contraction and downward movement of the diaphragm and the expansion of the chest cavity. Atrial pressure is also lowered as a result, causing increased blood flow to the heart, which in turn decreases firing response which diminishes vagal tone. This causes an increase in heart rate.
During exhalation, the diaphragm relaxes, moving upward, and decreases the size of the chest cavity, causing an increase in intrathoracic pressure. This increase in pressure inhibits venous return to the heart resulting in both reduced atrial expansion and increased activation of baroreceptors. This relieves the suppression of vagal tone and leads to a decreased heart rate.
RSA has been found to increase in subjects in resting state and to decrease in states of stress or tension. It is increased in supine position and decreased in prone position, and is on average higher and more pronounced during the day as compared to the night. RSA has also been extensively used to quantify vagal tone withdrawal in graded orthostatic tilt.
Typically, expression of RSA decreases with age. However, adults in excellent cardiovascular health, such as endurance runners, swimmers, and cyclists, are likely to have a more pronounced RSA. Professional athletes on average maintain very high vagal tone and consequently higher RSA levels. RSA is less prominent in individuals with diabetes and cardiovascular disease.
The polyvagal theory by Stephen Porges is an influential model of how the vagal pathways respond to novelty and to stressful external stimuli. The theory proposes that there are two vagal systems, one that is shared with and Amphibian and a second, more recent, system that is unique to . The two pathways behave differently and can work against each other. This theory can account for several Psychophysiology phenomena and psychosomatic illnesses. However, recent studies indicate that the vagal "system" described by Porges as being unique to mammals existed long before the evolution of mammals.
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