Vagal tone

Vagal tone refers to 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/constriction of vessels, glandular activity in the heart, lungs, and digestive tract as well as control of gastrointestinal sensitivity, motility and inflammation. Vagal tone refers to 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/constriction of vessels, glandular activity in the heart, lungs, and digestive tract 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 manual or electrical techniques. Noninvasive techniques mainly rely on the investigation of heart rate and heart rate variability. In most cases, vagal tone is not measured directly. Instead the processes affected by the vagus nerve – specifically heart rate and heart rate variability – are measured and used as a surrogate for vagal tone. Increased vagal tone (and thus vagal action) is generally associated with a diminished and more variable heart rate. However, during graded orthostatic tilt, vagal tone withdrawal is an indirect indicator of cardiovascular fitness. Heart rate is largely controlled by the heart's internal pacemaker activity. In a healthy heart, 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, the ability to generate electrical activity independent of external stimulation. The electrical activity spontaneously generated by the sinoatrial node sets the pace for the rest of the heart. In absence of external stimuli, sinoatrial pacing generally 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 ionic currents and calcium of heart cells. Because of its effect on heart rate, vagal tone can be measured by investigating heart rate modulation and heart rate variability. Respiratory sinus arrhythmia (RSA) is typically a benign, normal variation in heart rate that occurs during each breathing cycle: the heart rate increases when breathing in and decreases when breathing out. RSA was first recognized by Carl Ludwig in 1847 but is still imperfectly understood. It has been observed in humans from the early stages of life through adulthood, and is found in several different species. 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 triggers baroreceptors which act to diminish 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 reduced activation of baroreceptors. This relieves the suppression of vagal tone and leads to a decreased heart rate.