The vagus nerve, also called the tenth cranial nerve, starts in the brain and runs down the trunk of the body, with branches innervating the major organs. It interfaces with the parasympathetic nervous system and helps to regulate the heart, lungs, and the digestive system. It's a bi-directional nerve, meaning it both sends signals from the brain to the organs and the organs send messages back to the brain.
The vagus nerve is an important sensor and regulator of basic functions including breathing, heart rate, the relaxation response, the gut-brain connection, and the formation of memories. The motor vagus nerve normally holds inhibitory influence over both systemic inflammation and the autonomic functions that become dysfunctional in POTS.
Because it is a bidirectional nerve, there are important functions for both the sensory (afferent) and motor (efferent) branches.
When the vague nerve detects pro-inflammatory cytokines such as tumor necrosis factor-alpha or interleukin 1-beta, chemoreceptors in the sensory vagus nerve send a signal into the brainstem that triggers both glial cell activation within the central nervous system as well as the general innate immune response sometimes called the sickness response. The motor vagus nerve is responsible for an anti-inflammatory pathway. Vagus is thus important for maintaining homeostasis and preventing an overreactive immune response that can cause major injury or death. For example, vagus nerve stimulation is being used experimentally as an adjunct treatment for sepsis, which can cause death via a massive cytokine storm (e.g. Wang et al 2016 Int Rev Immunol).
Vagal tone is a measure of the constitutive output of the motor branch of the vagus nerve. It is frequently measured with the "tilt-table test" because the vagus nerve is responsible for the autonomic changes that allow us to go from lying down to standing up without fainting.
With a loss of vagal tone, both the antiinflammatory pathway and parasympathetic inhibition over autonomic systems are diminished. According to the vagus nerve infection hypothesis, exaggerated sensory vagus nerve signaling leads to loss of vagal tone and therefore a loss of control over inflammation and in some cases POTS.
- Dr. Diana Driscoll: Vagus Nerve and POTS/Mast Cell Activation, The Low Histamine Chef, January 12 2016
- 9 Nervy Facts About the Vague Nerve, Mental Floss
- Vagus Nerve Anatomy - Medscape