Vagus Nerve

Origin

The vagus nerve originates in the medulla oblongata of the brainstem. The specific brainstem nuclei giving rise to the motor fibers of the vagus nerve are the nucleus ambiguus and the dorsal vagal nucleus. Sensory vagus nerve fiber cell bodies are located in the inferior and superior vagal ganglia and send axons to the brainstem’s nucleus of the solitary tract and spinal nucleus of trigeminal nerve.

Course

The vagus nerve exits the medulla laterally, just posterior to the pontomedullary junction. From here, it runs laterally through the jugular foramen to exit the skull. After exiting the skull, the vagus nerve is located medial to the styloid process. It runs down to the root of the neck posterior to and between the internal jugular vein and the internal carotid artery/common carotid artery.

At the root of the neck, the course of the left and right vagus nerve begin to differ. The right vagus nerve runs down across the anterior surface of the right subclavian artery, then moves posteriorly to the level of the brachiocephalic artery to reach the right side of the trachea. It passes down posterior to the right primary bronchus and hilum of the right lung to the esophagus. Here it passes to the posterior surface of the esophagus and contributes to the posterior esophageal plexus. These fibers continue down the esophageal plexus to reach the abdomen by passing through the esophageal hiatus.

The left vagus nerve runs down across the anterior surface of the left subclavian artery and aortic arch. It passes anterior to the descending portion of the aortic arch, just lateral to the ligamentum arteriosum, and posterior to the pulmonary trunk, then posterior to the left primary bronchus and hilum of the left lung. Once below the hilum, the left vagus nerve moves onto the anterior surface of the esophagus. Here it spreads and contributes to the anterior esophageal plexus as it runs down to the esophageal hiatus and into the abdomen.

Branches

The first and second branches of the vagus nerve, the meningeal and auricular branches, emerge from the vagus nerve before it leaves the cranium. External to the cranium, a series of pharyngeal branches emerge and travel to the laryngopharynx. A carotid branch also emerges and contributes to the innervation of the carotid sinus, along with the glossopharyngeal nerve and cervical sympathetic trunk.

The superior laryngeal nerve emerges from the vagus nerve in the neck. The asymmetric right and left recurrent laryngeal nerves ascend from the brachiocephalic artery and aortic arch, respectively, to the larynx. The recurrent laryngeal nerves also give off branches that travel to the cardiac plexuses.

The superior and inferior cervical cardiac nerves emerge from the vagus nerve in the cervical region and travel inferiorly to the cardiac plexus. Below the level at which the recurrent laryngeal nerves branch off, the vagus nerve continues inferiorly, giving off thoracic cardiac branches to the cardiac plexuses. In addition to the cardiac branches, branches of the vagus nerve travel to the pulmonary plexuses to serve the lungs.

As the remainder of the vagus nerve reaches the esophagus, it spreads into the esophageal plexus, rotating from the left and right sides to the anterior and posterior of the esophagus, respectively. These are the anterior and posterior vagal trunks which run down the esophagus to the esophageal hiatus and into the abdomen.

In the abdomen, the vagus nerve forms gastric, hepatic, celiac, and mesenteric branches to innervate the abdominal viscera (Snell, 2010; Standring, 2016).

Supplied Structures & Function

The meningeal branch carries general somatic pain sensation from the posterior cranial dura mater. The auricular branch carries general somatic sensation, such as pain, from the external ear, external tympanic membrane, and external acoustic meatus.

The pharyngeal branches carry general visceral sensation from the laryngopharynx back to the medulla and send special visceral motor axons to pharyngeal constrictors. These fibers are required for constriction of the pharynx and the ability to swallow properly. A carotid branch carries special visceral sensory information from the carotid body and sinus and helps monitor blood pressure and gas concentrations (Andrew, 1954).

The superior laryngeal nerve carries general visceral sensation from the laryngopharynx and larynx back to the medulla and sends special visceral motor innervation to the cricothyroid muscle. The right and left recurrent laryngeal nerves innervate the larynx. These carry general visceral sensation from the larynx and supply special visceral motor innervation to all laryngeal muscles except the cricothyroid muscle. The recurrent laryngeal nerves also give off parasympathetic branches that travel to the cardiac plexuses to innervate the heart.

The superior and inferior cervical cardiac nerves innervate the cardiac plexus and the heart. These general visceral efferent, or parasympathetic fibers, are associated with slowing the heart rate. The thoracic cardiac branches to the cardiac plexuses are also parasympathetic and slow the heart rate down.

The pulmonary branches of the vagus nerve include both general visceral sensory and general visceral motor (parasympathetic) functions. Visceral sensory fibers from the lining of the lung travel via the pulmonary vagal branches. These are involved in cough reflexes. Pulmonary vagal general visceral motor fibers act on the smooth muscle of the lungs. In general, this parasympathetic innervation of the lungs acts to constrict airways.

The esophageal plexus and the anterior and posterior vagal trunks innervate the esophagus. General visceral afferents and general visceral efferent vagal fibers mediate sensation of the esophageal lining and motor innervation of the esophageal musculature, respectively. Motor innervation allows for peristaltic movement of food down the esophagus.

In the abdomen, the vagus nerve forms gastric, hepatic, celiac, and mesenteric branches to innervate all the abdominal viscera from the esophagus down to the splenic flexure of the large intestine, including the spleen and kidneys. The vagal innervation of the abdominal viscera is both general visceral sensory and general visceral motor, allowing for peristaltic movement of the gut contents. The hindgut, adrenal glands, and reproductive organs are not innervated by the vagus nerve (Snell, 2010; Standring, 2016).

List of Clinical Correlates

—Gag reflex

—Hoarseness

—Cough reflex

—Sleep apnea

—Bradycardia

—Hypotension

—Vagal nerve stimulation

References

Andrew, B. L. (1954) A laryngeal pathway for aortic baroceptor impulses. The Journal of physiology, 125(2), 352-360.

Snell, R. S. (2010) Clinical Neuroanatomy. Wolters Kluwer Health/Lippincott Williams & Wilkins.