Vagus Nerve

Original Editor - Kanza Imtiaz

Top Contributors - Kanza Imtiaz, Lucinda hampton and Tony Lowe

This article is currently under review and may not be up to date. Please come back soon to see the finished work! (31/01/2021)

Introduction

The vagus nerve is the tenth cranial nerve (CN X) and is the longest mixed cranial nerve. The literal translation of the vagus is 'wanderer,' which aptly represents its widespread interfacing of cortex, brainstem, hypothalamus, and the body. Its afferent and efferent pathways comprise about 80% and 20%, respectively.

This nerve is the sensory network that tells the brain what’s going on in our organs, most specially the digestive tract (stomach and intestines), lungs and heart, spleen, liver and kidneys, not to mention a range of other nerves that are involved in everything from talking to eye contact to facial expressions and even your ability to tune in to other people’s voices. It is made of thousands upon thousands of fibers, operating far below the level of our conscious mind. It plays a vital role in sustaining overall wellness. It is an essential part of the parasympathetic nervous system, which is responsible for calming organs after the stressed “fight-or-flight” adrenaline response to danger.

The vagus nerve runs from the brain through the face and thorax to the abdomen. It is a mixed nerve that contains parasympathetic fibres.

The vagus nerve has two sensory ganglia (masses of nerve tissue that transmit sensory impulses): the superior and the inferior ganglia.

1. The branches of the superior ganglion innervate the skin in the concha of the ear.
2. The inferior ganglion gives off two branches: the pharyngeal nerve and the superior laryngeal nerve.

The recurrent laryngeal nerve branches from the vagus in the lower neck and upper thorax to innervate the muscles of the larynx (voice box).

The vagus also gives off cardiac, esophageal, and pulmonary branches.

In the abdomen the vagus innervates the greater part of the digestive tract and other abdominal viscera.

The accessory nerve (CN XI) joins the vagus nerve just distal to the inferior ganglion.

Structure and Function

The vagus nerve contains somatic and visceral afferent fibers, as well as general and special visceral efferent fibers. See Table Below:

Components	Function	Central component	Cell bodies
Special Visceral Efferent	Swallowing and phonation	Nucleus ambiguus	Nucleus ambiguus
General Visceral Efferent	Involuntary muscle control (cardiac, pulmonary, esophageal) Innervation to glands throughout the gastrointestinal tract	Dorsal motor nucleus	Dorsal motor nucleus
Special Visceral Afferent	Sensations of taste from the tongue and epiglottis	Nucleus tractus solitarius	Inferior ganglion
General Visceral Afferent	Sensations from the pharynx, larynx, trachea, esophagus, and the abdominal and thoracic viscera	Nucleus tractus solitarius	Inferior ganglion
General Somatic Afferent	Innervation to the external ear and tympanic membrane	Nucleus of the spinal tract of trigeminal	Superior ganglion

Course

Upper portions of glossopharyngeal, vagus, and accessory nerves

The Vagus nerve

Exits the brain from the medulla oblongata of the brainstem and travels laterally exiting the skull through the jugular foramen.

It descends within the carotid sheath where it is located posterolateral to the internal and common carotid arteries, and medial to the internal jugular vein.

At the base of the neck, the nerve enters the thorax, where the right and left vagus nerve travels on a different path.

1. The right vagus enters the thorax by crossing the first part of the subclavian artery and posterior to the innominate artery; then travels behind the primary right bronchus and esophagus to form the esophageal plexus with the left vagus nerve.
2. The left vagus enters the thorax by passing between the left common carotid and left subclavian arteries, then travels behind the primary left bronchus and into the esophagus.

Parasympathetic Functions

In the thorax and abdomen, the vagus nerve is the main parasympathetic outflow to the heart and gastro-intestinal organs.

The Heart: Cardiac branches arise in the thorax, conveying parasympathetic innervation to the sino-atrial and atrio-ventricular nodes of the heart. These branches stimulate a reduction in the resting heart rate. They are constantly active, producing a rhythm of 60 – 80 beats per minute. If the vagus nerve was lesioned, the resting heart rate would be around 100 beats per minute.

Gastro-Intestinal System: The vagus nerve provides parasympathetic innervation to the majority of the abdominal organs. It sends branches to the oesophagus, stomach and most of the intestinal tract – up to the splenic flexure of the large colon. The function of the vagus nerve is to stimulate smooth muscle contraction and glandular secretions in these organs. For example, in the stomach, the vagus nerve increases the rate of gastric emptying, and stimulates acid production.

Anti‐inflammatory properties of the vagus nerve

A low vagal tone, as assessed by heart rate variability, a marker of the sympatho-vagal balance, is observed in functional digestive disorders and inflammatory bowel diseases. To restore a normal vagal tone appears as a goal in such diseases. Among the therapeutic tools, such as drugs targeting the cholinergic system and/or complementary medicine (hypnosis, meditation…), deep breathing, physical exercise, VN stimulation (VNS), either invasive or non-invasive, appears as innovative. There is new evidence supporting the role of VNS in the modulation of gastrointestinal functions.

Brain and viscera interplay within the autonomic nervous system where the vagus nerve (VN), containing approximately 80% afferent and 20% efferent fibres, plays multiple key roles in the homeostatic regulations of visceral functions. Recent data have suggested the anti‐inflammatory role of the VN. This vagal function is mediated through several pathways. The first one is the anti‐inflammatory hypothalamic–pituitary–adrenal axis which is stimulated by vagal afferent fibres and leads to the release of cortisol by the adrenal glands and efferent (activation of the CAP) fibres. Given its position as a key element of the autonomic nervous system in the brain–gut interactions the VN seems to be a good therapeutic target in inflammatory conditions of the digestive tract (e.g. IBD) and also other inflammatory conditions such as RA, and others.  

The use of neuromodulation by bioelectronics devices as a treatment is an emerging field in the domain of bioelectronic medicine. It could be an alternative non‐drug therapy to conventional treatment or could be combined with such treatments, but further investigation in a large longitudinal cohort of patients is required.

Vagus Nerve as Modulator of the Brain–Gut Axis in Psychiatric and Inflammatory Disorders

The vagus nerve represents the main component of the parasympathetic nervous system, which oversees a vast array of crucial bodily functions, including control of mood, immune response, digestion, and heart rate. It establishes one of the connections between the brain and the gastrointestinal tract and sends information about the state of the inner organs to the brain via afferent fibers.

• There is preliminary evidence that vagus nerve stimulation is a promising add-on treatment for treatment-refractory depression, posttraumatic stress disorder, and inflammatory bowel disease.
• Treatments that target the vagus nerve increase the vagal tone and inhibit cytokine production. Both are important mechanism of resiliency. The stimulation of vagal afferent fibers in the gut influences monoaminergic brain systems in the brain stem that play crucial roles in major psychiatric conditions, such as mood and anxiety disorders.
• There is preliminary evidence for gut bacteria to have beneficial effect on mood and anxiety, partly by affecting the activity of the vagus nerve. Since, the vagal tone is correlated with capacity to regulate stress responses and can be influenced by breathing, its increase through meditation and yoga likely contribute to resilience and the mitigation of mood and anxiety symptoms.

Resources

Vagus nerve stimulation

• https://www.mayoclinic.org/tests-procedures/vagus-nerve-stimulation/multimedia/vagus-nerve-stimulation-video/vid-20115582

References