In the human body, the autonomic nervous system is related to important vital functions and it is controlled basically by the hypothalamus, a smaller part of our brain. The autonomic nervous system is divided into two parts, which are known as sympathetic and parasympathetic, and they control the diameter of pupils, heart rate, contraction of the heart, the diameter of vessels and airways, secretions and sweating, digestion, voiding, defecation, metabolism, reproduction, etc. If stable functional dysfunction occurs in the system; hypertension, obesity, depression, chronic pain, sleep disorders, chronic fatigue, gastrointestinal disorders, immune system-related disorders can be seen. The treatment of the functional impairment cures the diseases stated above in addition to epilepsy, addiction problems, and certain disorders.
Nowadays stress (physical and psychological loading of the body) is seen as the primary factor causing a functional disturbance in the autonomic nervous system and frequently activates the sympathetic nervous system in the body to make a ‘’fight or flight’’ response. In acute stress, this is a life-saving condition, but when the condition becomes chronic it results in psychosocial and physical disorders.
Transcutaneous vagus nerve stimulation activates the noninvasively auricular afferent part of the 10th cranial nerve, vagus, and the latter is the most important division of the parasympathetic system. The auricular vagus innervates the medial part of the tragus, concha, and external ear canal, so electrical stimulation of the ear vagus can regulate the overactivated sympathetic nervous system and by this way treat stress-related disorders. When we consider the time, place variability, and long duration of the stress sources, chronic stimulation of the vagus nerve can be expected to develop as a good method for protecting and improving personal and communal health.
General Information
In our bodies, there is a system called the autonomic nervous system which works involuntarily (autonomously), adapts the body to internal and external changes, and controls vital structures like the heart, lungs, vessels, bowels, reproductive organs, etc. It is composed of two parts: one is the sympathetic nervous system which gives rise to ‘’fight or flight’’ response especially in dangerous situations, attacking conditions in the external environment; and the other is the parasympathetic nervous system which calms the body, enables growth and development, mostly has the internal properties, and is known as ‘’rest and digest’’ shortly. The center of the autonomic nervous system is the hypothalamus in the brain, a part of the limbic system. The hypothalamus controls lots of vital functions together with the autonomic nervous system (Özden AV, 2015).
The nerves emanating from the hypothalamus travel certain paths in the brain and spinal cord, then stimulate the sympathetic and parasympathetic nerves which are dispersed to the entire body. The parasympathetic nervous system is composed of cranial nerves 3, 7, 9, and 10 with sacral nerves 2-4. Sympathetic nerves exit from the levels of the spinal cord between thoracic 1 and lumbar 2. The sympathetic nervous system uses adrenalin and noradrenalin molecules in end-organs; the parasympathetic nervous system uses acetylcholine instead. The functions of the two systems are summarized in the below table (Özden AV, 2015).
| Functions of the sympathetic and parasympathetic nervous system | ||
| Sympathetic system | Parasympathetic system | |
| Pupil size | Enlarges | Constricts |
| Heart rate | Elevates | Decreases |
| Heart contraction | Elevates | Decreases |
| Vessels | Enlarges or constricts | Enlarges |
| Airways | Enlarges | Constricts |
| Secretions-sweating | Elevates sweating | Elevates secretions |
| Bowel movements | Decreases | Elevates |
| Voiding | Blocks | Causes |
| Defecation | Blocks | Causes |
| Lipid tissue | Decreases | Elevates |
| Blood glucose and fatty acids | Elevates | Generally decreases |
| Genital tract (woman) | Orgasm | Clitoral erection- Elevates secretions |
| Genital tract (man) | Ejaculation-orgasm | Erection |
The Vagus nerve plays a great role in the regulation of homeostasis. By the neuro-endocrine-immune connections, it can control the cardiorespiratory system, digestive system, inflammation, psychological conditions, and pain (Yuan, 2015). Vagus nerve stimulation started in the 19th century but significant usage became widespread in the 1990s for the treatment of drug-resistant epilepsy and depression. In 2005, The US Food and Drug Administration (FDA) approved left implanted vagus nerve stimulation for use in the treatment of resistant epilepsy and chronic resistant depression. In 2015 again, FDA accepted vagus nerve stimulation as safe and reliable for obesity treatment. Vagus nerve stimulation is also suggested for heart insufficiency. Transcutaneous vagus nerve stimulation is approved in Europe for treatment of epilepsy and depression in 2010, and also for pain in 2012. Vagus nerve stimulation is being investigated as a possible remedy for diseases like bipolar disorder, Alzheimer’s disease, insulin resistance, asthma, fibromyalgia, traumatic brain injury, stroke, tremor, and kinetic disorders (Yuan, 2015).
In their study in 2014, Clancy and her friends stimulated tragus with TENS current and observed a decrement of sympathetic activity. Busch et al (2013) showed that pain perception decreases when concha and tragus are stimulated. Straube et al (2015) observed that left concha stimulation for 4 hours per day in a period of 3 months decreases chances of headache by more than 50%. Jacobs et al (2015) declared that transcutaneous vagus nerve stimulation improves memory in the elderly. When we look at the interrelation between depression, inflammation, metabolic syndrome, and heart diseases, it shows that vagus nerve states are in an important position (Howland RH, 2014).
As a result, autonomic nervous system dysfunction conditions such as hypertension, obesity, chronic pain, depression, sleep disorders, chronic fatigue, gastrointestinal disorders, and the immune system-related disorder can be seen. The treatment of the functional impairment is a remedy to the diseases stated above in addition to epilepsy, addiction problems like disorders. Stimulation of the afferent branch of the vagus nerve in the ear seems to play an important role in the treatment of autonomic nervous system dysfunction. In the near future, it is possible to see this method spreading all over the world and could constitute a huge portion of the treatment options.
References
Busch V, Zeman F, Heckel A, Menne F, Ellrich J, Eichhammer P. The effect of transcutaneous vagus nerve stimulation on pain perception–an experimental study. Brain Stimul. 2013 Mar;6(2):202-9.
Clancy JA, Mary DA, Witte KK, Greenwood JP, Deuchars SA, Deuchars J. Non-invasive Vagus Nerve Stimulation in Healthy Humans Reduces Sympathetic Nerve Activity. Brain Stimulation 7 (2014) 871-877.
Howland RH. Vagus Nerve Stimulation. Curr Behav Neurosci Rep. 2014 June; 1(2): 64–73.
Jacobs HIL, Riphagen JM, Razat CM, Wiese S, Sack AT. Transcutaneous vagus nerve stimulation boosts associative memory in older individuals. Neurobiology of Aging 36 (2015) 1860-1867.
Özden AV. Bazen Her Şey Sinirsel, Bedenimizi kontrol eden otonom sinir sistemine bir bakış. 2015, Agate Yayınları.
Straube A, Ellrich J, Eren O, Blum B, Ruscheweyh R. Treatment of chronic migraine with transcutaneous stimulation of the auricular branch of the vagal nerve (auricular t-VNS): a randomized, monocentric clinical trial. The Journal of Headache and Pain (2015) 16:63.
Yuan H, Silberstein SD. Vagus Nerve and Vagus Nerve Stimulation, a Comprehensive Review: Part II. (Headache 2015, DOI: 10.1111/head.12650)
Yuan H, Silberstein SD. Vagus Nerve and Vagus Nerve Stimulation, a Comprehensive Review: Part I. (Headache 2015, DOI: 10.1111/head.12647)
