Vagus Nerve Anatomy

The vagus nerve is the longest cranial nerve. It contains motor and sensory fibers and, because it passes through the neck and thorax to the abdomen, has the widest distribution in the body. It contains somatic and visceral afferent fibers, as well as general and special visceral efferent fibers. (See Table 1, below.)

Table 1. Summary of Central Connections, Components, Function, and Peripheral Distribution of the Vagus Nerve (Open Table in a new window)

Exit from the brain

The vagus nerve exits from the medulla oblongata in the groove between the olive and the inferior cerebellar peduncle. It leaves the skull through the middle compartment of the jugular foramen, where it has upper and lower ganglionic swellings, which are the sensory ganglia of the nerve. The superior ganglion (jugular) is less than 0.5 cm in diameter, while the inferior (nodose) ganglion is larger (2.5 cm) and lies 1 cm distal to the superior ganglion (see the image below). The vagus nerve is joined by the cranial root of the accessory nerve (cranial nerve XI), just below the inferior ganglion. (See the images below.) ^{[1, 2, 3, 4]}

Course of the vagus nerve

The vagus nerve descends vertically within the carotid sheath posterolateral to the internal and common carotid arteries and medial to the internal jugular vein (IJV) at the root of the neck.

The right vagus crosses in front of the first part of the subclavian artery and then travels into the fat behind the innominate vessels. It then reaches the thorax on the right side of the trachea, which separates it from the right pleura. It then inclines behind the hilum of the right lung and courses medially toward the esophagus to form the esophageal plexus with the left vagus nerve. (See the image below.)

The left vagus crosses in front of the left subclavian artery to enter the thorax between the left common carotid and subclavian arteries. It descends on the left side of the aortic arch, which separates it from the left pleura, and travels behind the phrenic nerve. It courses behind the root of the left lung and then deviates medially and downwards to reach the esophagus and form the esophageal plexus by joining the opposite (right) vagus nerve.

The anterior and posterior gastric nerves are then formed from the esophageal plexus. The anterior gastric is formed mainly from the left vagus, but it does contain fibers from the right vagus.

Similarly, the posterior gastric nerve is formed mainly from the right vagus but contains fibers from the left vagus nerve. The gastric nerves supply all abdominal organs and the gastrointestinal tract ending just before the left colonic (splenic) flexure (see the images below).

Vagus nerve branches in the jugular foramen

The meningeal branch arises at the superior ganglion and reenters the cranium through the jugular foramen to supply the posterior fossa dura.

The auricular branch supplies sensations to the posterior aspect of the external ear (pinna) and the posterior part of the external auditory canal. It arises also from the superior ganglion and enters the mastoid canaliculus in the lateral part of the jugular foramen. It exits again through the tympanomastoid suture of the temporal bone to reach the skin. It communicates with branches of the seventh (facial) and ninth (glossopharyngeal) cranial nerves.

Vagus nerve branches in the neck

The branches in the next consist of the following:

• Pharyngeal branches

• Superior laryngeal nerve

• Recurrent laryngeal nerve

• Superior cardiac nerve

Pharyngeal branches

The pharyngeal branches arise from the inferior ganglion and contain sensory and motor fibers. The motor fibers are contributed by cranial nerve XI. They reach the middle constrictor muscle after crossing between the external and internal carotid arteries. They reach the pharyngeal plexus formed by cranial nerve IX and the sympathetic chain. Branches of the pharyngeal plexus supply the pharyngeal muscles and mucous membrane and palate except for the tensor palatini muscle.

The intercarotid plexus, at the carotid bifurcation, is also formed by vagal fibers from the pharyngeal plexus, joined by glossopharyngeal and sympathetic fibers. These vagal fibers and visceral afferents mediate impulses set up by the chemoreceptors in the carotid body.

Superior laryngeal nerve

The superior laryngeal nerve passes between the external and internal carotid arteries at the level of crossing of cranial nerve XII. At the tip of the hyoid, the superior laryngeal nerve divides into the external and internal branches. The internal laryngeal nerve pierces the thyrohyoid membrane to enter the larynx. The external nerve passes inferiorly with the superior thyroid vessels to the inferior pharyngeal constrictor muscle. The cricothyroid muscle is supplied by the external branch of the superior laryngeal nerve. The internal branch of the superior laryngeal supplies most of the mucosa above the glottis. It is divided into the following 3 divisions:

• First division - Supplies mucosa of the laryngeal surface of the epiglottis

• Middle division - Supplies the mucosa of the true and false vocal folds, as well as the aryepiglottic fold

• Inferior division - Supplies the arytenoid mucosa, anterior wall of the hypopharynx, upper esophageal sphincter, and part of the subglottis (the major part of the subglottis is innervated by the ipsilateral recurrent nerve)

Recurrent laryngeal nerve

The recurrent laryngeal nerve is also known as the inferior laryngeal nerve. The right nerve branches from the vagus at the root of the neck around the right subclavian artery. It courses superiorly in the tracheoesophageal groove to enter the larynx between the cricopharyngeus and the esophagus.

The left recurrent laryngeal nerve has a similar course to the right recurrent, except that it loops around the aortic arch distal to the ligamentum arteriosus.

The main trunk of the recurrent lies in a triangle bound laterally by the common carotid artery, the IJV, and the vagus nerve and medially by the trachea and esophagus. The recurrent nerve passes under the posterior suspensory ligament of Berry (located on either side of the trachea, extending from the cricoid cartilage and the first 2 tracheal rings to the posteromedial aspect of the thyroid gland), before entering the larynx (see the image below). A few variations may occur in this area (see Natural Variants).

All the intrinsic laryngeal musculature is supplied by the ipsilateral recurrent nerve except the cricothyroid muscle, which is supplied by the superior laryngeal nerve. The interarytenoid muscle is the only one that receives a bilateral supply (ie, from the left and right recurrent laryngeal nerves).

The ramus communicans, or nerve of Galen, connects the superior and the recurrent laryngeal nerves. It provides the tracheal and esophageal mucosa and smooth muscle with visceral motor input.

Superior cardiac nerve

The superior cardiac nerve is made up of 2-3 branches. They communicate with the sympathetic fibers.

Vagus nerve branches in the thorax

The inferior cardiac branch is also called the ramus cardiaci inferiors. On the right side, it arises from the trunk of the vagus as it lies beside the trachea. On the left side, it originates from the recurrent laryngeal nerve only. These branches end in the deep part of the cardiac plexus.

The anterior and posterior bronchial branches are distributed as 2-3 branches on the anterior surface of the root of the lung. They form the anterior pulmonary plexus after joining branches from the sympathetic trunk. The posterior bronchial branches are larger than the anterior and lie on the posterior surface of the root of the lung to form the posterior pulmonary plexus (with contributory sympathetic fibers) as well.

The esophageal branches are anterior and posterior branches. Together they form the esophageal plexus. The posterior surface of the pericardium is supplied by filaments from this plexus.

Vagus nerve branches in the abdomen

The gastric branches (rami gastrici) supply the stomach. The right vagus forms the posterior gastric plexus and the left forms the anterior gastric plexus. The branches lie on the posteroinferior and the anterosuperior surfaces, respectively.

The celiac branches (rami celiaci) are derived mainly from the right vagus nerve. They join the celiac plexus and supply the pancreas, spleen, kidneys, adrenals, and intestine.

The hepatic branches originate from the left vagus. They join the hepatic plexus and through it are distributed to the liver.

The recurrent and external branches of the superior laryngeal nerves carry parasympathetic fibers from the dorsal motor nucleus to the subglottis and supraglottic regions, respectively. The superior cervical ganglion sends sympathetic innervation. ^{[4, 5, 6]} Histologic sections have revealed the presence of Meissner corpuscles, Meckel cells, and taste buds scattered in the larynx.

The mucosal surface sensory receptors are more numerous on the laryngeal surface of the epiglottis than on the true vocal folds. On the other hand, the chemoreceptors are limited to the supraglottic mucosa.

The position of the recurrent laryngeal nerves is very important for the thyroid surgeon. ^{[7, 3, 8, 9]} The following are some anatomic variations:

The recurrent nerve has been observed to branch into 2 nerves before entering the larynx. The anterior (or medial) branch supplies the adductor muscles, while the posterior (or lateral) supplies the abductors. Most of the time, the branching occurs 0.6-3.5 cm below the cricoid cartilage. In rare instances, the recurrent nerve may have 4-6 branches. These may be esophageal branches or they may supply the inferior pharyngeal constrictor. Any nerve in the surgical field should not be sacrificed unless it is invaded by malignancy.

In a small number of cases, the right subclavian artery is retroesophageal and arises from the aorta distal to the ligamentum arteriosus. In these cases, the right recurrent nerve enters the larynx without looping around the artery.

The recurrent laryngeal nerve passes under the ligament of Berry before entering the larynx. In 0.25% of cases, it passes over the ligament.

A branch of the inferior thyroid artery may pass deep to the ligament of Berry or along its inferior edge. Indiscriminate clamping of this artery could jeopardize the recurrent nerve.

A small portion of the thyroid gland may exist deep to the ligament and lateral to the recurrent nerve. This should be removed with great care, without injuring the nerve, during thyroidectomy, because it may contain carcinoma.

The types of fibers that constitute the vagus nerve perform different physiologic roles (see the image below, as well as Table 2), as follows: ^{[10, 11, 12, 13, 14]}

The parasympathetic efferent fibers (efferent general visceral or visceral motor) are distributed to the thoracic and abdominal viscera; to the bronchial tree; to the heart (inhibitory fibers); and to the esophagus, stomach, and small intestine (motor fibers). In addition, they are distributed as secretory fibers to the stomach and pancreas. They arise from the dorsal motor nucleus of the vagus.

The somatic motor fibers (efferent special visceral or branchial motor) arise from the cells of the nucleus ambiguus, which is the motor nucleus for striated muscles of the pharynx, larynx and palate in the middle of the upper part of the medulla. It is adjacent to the respiratory motor neurons in the brainstem. (See the image below.)

The sensory fibers (afferent general and special visceral) arise from the cells of the jugular ganglion and ganglion nodosum (superior and inferior ganglia of the vagus, respectively). When traced into the medulla, they end by arborizing around the cells of the inferior part of a nucleus, which lies beneath the ala cinerea in the lower part of the rhomboid fossa. These are the sympathetic afferent fibers. A few of the taste fibers of the vagus nerve descend in the fasciculus solitarius and end around its cells.

The somatic sensory fibers (afferent general somatic) are only a few in number. From the posterior aspect of the external auditory canal and the back of the external ear, they join the spinal tract of the trigeminal nerve as it descends in the medulla. They have connections with the thalamus, sensory cortex, and medullary and spinal nuclei.

Table 2. The Pathway According to the Type of Nerve Fibers of the Vagus Nerve (Open Table in a new window)

Cough is a very common symptom for physicians' visits all over the world. It serves to protect the respiratory tract from exogenous organic, thermal, and chemical irritants. According to its characteristics, cough can be split into 2 distinct types, aspiration cough, which is loud and involuntary, and urge-to-cough sensation, which describes an irritant, scratchy, and controlled cough of slowly increasing intensity.

Acute cough mostly develops because of infection of the respiratory system and ends after approximately 1 month. In contrast to this, bacterial infection with pathogens like Adenovirus, Bordetella pertussis (whooping cough), or Mycoplasma can last up to 2 months.

The cough reflex is initiated by different receptors. Dinh et al (2013) summarized the cough reflex and receptors as being divided into the following groups ^[15] :

• Two Aδ-fiber types – (1) rapid-adapting mechanoreceptor (RAR) and (2) slow-adapting mechanoreceptor (SAR)

• C-fiber receptor

The stimulus is detected by cough receptors that conduct the signal to the cerebral cough center via vagal-sensory neurons. The cough itself is mediated by efferent motoneurons.

The event of coughing can be divided into the first phase, consisting of fast inspiration with an opened glottis, followed by compression with a closed glottis with an increase in tracheal pressure, and, ultimately, maximum expiration with an opened glottis.

Neurologically, the cough reflex consists of 5 sequential parts: (1) the cough receptors; (2) the primary afferent fibers of the vagus, trigeminus, and glossopharyngeus cranial nerves; (3) the cough center in the medulla (nucleus tractus solitarius); (4) the afferent fibers of the phrenic, accessory, and the recurrent laryngeal nerves, and (5) the diaphragm, the abdominal wall, and intercostal and laryngeal musculature.

Beaumont et al ^[16] found that chronic vagus nerve stimulation (VNS) mitigates myocardial infarction–induced remodeling of the intrinsic cardiac nervous system along with the cardiac tissue it regulates in guinea pigs.

Lagae et al published a study on the efficacy of VNS therapy in a highly drug-resistant childhood epilepsy patient group. They confirmed the efficacy of VNS in children. A larger study is needed to determine the contribution of different factors (such as age at implantation, etiology, and epilepsy duration) related to this therapy. ^[17]  

Intraoperative neuro monitoring (IONM) during thyroid or parathyroid surgery is now recognized as an adjunct to visual nerve identification. It improves the outcomes among patients undergoing these types of surgery. ^{[18, 19]}  Laryngeal electromyography is effective in determining the origin of unilateral vocal-fold paralysis. It has also an important therapeutic and prognostic role if a lack of reinnervation potentials is noted, which is a possible indication for early medialization laryngeal surgery.