What is the vagus nerve?

The vagus nerve is the longest and most complex nerve in the human body. Its name originates from the Latin word for “wandering”, as it travels from the brain though to the abdomen, connecting vital organs such as the heart and large intestine along its way.

As the primary structure of the parasympathetic nervous system, the vagus nerve plays a crucial role in overseeing a number of body functions including digestion, mood, hypothalamic -pituitary- adrenal axis function, respiratory function, blood glucose regulation and anti-inflammatory mechanisms. (Breit)

The vagus nerve is comprised of 20% efferent nerves, which carry information from the brain to the organs, and 80% afferent nerves, which carry information from the organs to the brain, making it an intricate bidirectional highway of information and a crucial component of the gut-brain axis. (Breit, Howland) The high percentage of afferent nerves highlights the role of the internal organs as sensing organs which influence the state of the parasympathetic nervous system.

Vagal tone

Vagal tone refers to the strength of the innervation of the vagus nerve and can determine the output of the parasympathetic nervous response. Poor vagal tone has been found in a number of conditions which are also impacted by stress including irritable bowel syndrome, inflammatory bowel disease (IBD) (Bonaz) depression (Tan) and poor emotional and attentional regulation in children. (Mclaughlin) A number of modern day lifestyle behaviours are causative factors for poor vagal tone including sleep deprivation (Westphal), stress (Bonaz) and sedentary lifestyle (Wichi), creating a bidirectional relationship between poor vagal tone and associated conditions.

The mind body connection

The vagus nerve plays a pivotal role in regulating both our physiology and our emotions. Of note, vagus nerve function is implicated in various mind-body conditions such as depression and stress. It is well established that the gut microbiome plays a role in the expression of depressive states, and research is suggesting that vagal signalling is fundamental to its function. Probiotic therapy, which may have anti-depressant effects, was ineffective in vagotomised mice, suggesting that vagal signalling is required for positive feedback in the gut-brain axis. (Breit) Dampening of vagal signalling may exacerbate mind-body conditions by impacting organ to brain signalling. Furthermore, stress and low moods are known to inhibit vagal tone, placing further stress on an already precarious situation. (Bonaz)

Vagus nerve stimulation

It is hypothesised that improving vagal tone may strengthen signalling between the brain and organs, and vagus nerve stimulation (VNS) has been shown to have a number of physical and emotional benefits. VNS refers to any activity or technique which stimulates the vagus nerve, thus increasing its tone. (Howland) Research shows that VNS affects the activity of serotonin, noradrenaline and other neurotransmitters involved in mood regulation. (Howland) VNS has also been found to have anti-inflammatory effects and may restore homeostasis of the gut-brain axis (Bonaz), therefore, improving vagal tone may be a promising adjunct treatment for a number of health conditions such as IBD, depression and other mind-body disorders. (Breit)

Clinical research conducted in VNS generally employs electric nerve stimulating devices which may be impractical in the natural medicine clinic. Nevertheless, a number of self-administered techniques are known to improve vagal tone include and may be prescribed and practiced without the need for complicated machinery or practitioner oversight. These include:

• Music - music stimulates the vagus nerve receptors in the ear, which may promote relaxation by decreasing heart rate, blood pressure, respiration rate, increase motility and relaxation of sphincters. (Helm)
• Cold immersion - applying a cold stimulus to the face (such as cold water) was shown to induce strong vagal activity and stimulate the parasympathetic nervous system. It improved various physiological markers of stress such as heart rate, cortisol and heart rate variability. (Richer)
• Exercise - moderate intensity interval training was shown to stimulate vagus nerve and autonomic nervous system activity. (Kai)
• Massage - 60 minutes of foot reflexology resulted in higher vagal modulation and lower blood pressure in patients with coronary artery disease. (Lu)
• Singing, humming and gargling - Ancient yogic texts document a number of exercises such as singing, humming and gargling to encourage parasympathetic nervous system dominance and stress reduction.

VNS is a promising cost-effective adjunct treatment for mind-body disorders.

References

• Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2018). Vagus Nerve as Modulator of the Brain-Gut Axis in Psychiatric and Inflammatory Disorders. Frontiers in psychiatry, 9, 44. https://doi.org/10.3389/fpsyt.2018.00044
• Howland R. H. (2014). Vagus Nerve Stimulation. Current behavioral neuroscience reports, 1(2), 64–73. https://doi.org/10.1007/s40473-014-0010-5
• Tan, C., Yan, Q., Ma, Y., Fang, J., & Yang, Y. (2022). Recognizing the role of the vagus nerve in depression from microbiota-gut brain axis. Frontiers in neurology, 13, 1015175. https://doi.org/10.3389/fneur.2022.1015175
• Bonaz, B., Bazin, T., & Pellissier, S. (2018). The Vagus Nerve at the Interface of the Microbiota-Gut-Brain Axis. Frontiers in neuroscience, 12, 49. https://doi.org/10.3389/fnins.2018.00049
• Kai, S., Nagino, K., Ito, T., Oi, R., Nishimura, K., Morita, S., & Yaoi, R. (2016). Effectiveness of Moderate Intensity Interval Training as an Index of Autonomic Nervous Activity. Rehabilitation research and practice, 2016, 6209671. https://doi.org/10.1155/2016/6209671
• Lu, W. A., Chen, G. Y., & Kuo, C. D. (2011). Foot reflexology can increase vagal modulation, decrease sympathetic modulation, and lower blood pressure in healthy subjects and patients with coronary artery disease. Alternative therapies in health and medicine, 17(4), 8–14.
• Richer, R., Zenkner, J., Küderle, A., Rohleder, N., & Eskofier, B. M. (2022). Vagus activation by Cold Face Test reduces acute psychosocial stress responses. Scientific reports, 12(1), 19270. https://doi.org/10.1038/s41598-022-23222-9
• Helm, A. S. H., & Ramezani, M. (2021). The effect of music and vagus nerve to improve various Diseases. Pakistan Journal of Medical and Health Sciences, 15(6), 1854–1857. https://doi.org/10.53350/pjmhs211561854
• McLaughlin, K. A., Rith-Najarian, L., Dirks, M. A., & Sheridan, M. A. (2015). Low vagal tone magnifies the association between psychosocial stress exposure and internalizing psychopathology in adolescents. Journal of clinical child and adolescent psychology : the official journal for the Society of Clinical Child and Adolescent Psychology, American Psychological Association, Division 53, 44(2), 314–328. https://doi.org/10.1080/15374416.2013.843464
• Westphal, W. P., Rault, C., Robert, R., Ragot, S., Neau, J. P., Fernagut, P. O., & Drouot, X. (2021). Sleep deprivation reduces vagal tone during an inspiratory endurance task in humans. Sleep, 44(10), zsab105. https://doi.org/10.1093/sleep/zsab105
• Wichi, R. B., De Angelis, K., Jones, L., & Irigoyen, M. C. (2009). A brief review of chronic exercise intervention to prevent autonomic nervous system changes during the aging process. Clinics (Sao Paulo, Brazil), 64(3), 253–258. https://doi.org/10.1590/s1807-59322009000300017