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The Vagus Nerve

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Anatomy

The Vagus Nerve is the tenth of the Twelve Cranial Nerves. The fibers consist of efferent motoric and parasympathetic fibers and afferent sensory fibers. In this article the pathways will be discussed. The word afferent means toward the centre, as in from a peripheral area of a limb to the central nervous system. The word efferent is the opposite of afferent, meaning away from the centre and toward the periphery; when the stimulus is carried back to the brain from a peripheral area.

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Parasympathetic Efferent Pathway

The pathway for the efferent parasympathetic fibers of the vagus nerve is the following (From the Initiation in the brain to the termination in the periphery):

  • The dorsal vagal nucleus is situated behind the vagal nerve trigone and contributes the parasympathetic and viseroafferent fibers.
  • The parasympathetic fibers merge together with the motoric fibers within the vagus nerve and synapse upon the superior ganglion, otherwise known as the jugular ganglion of the vagus nerve.
  • As the postganglionic fibers leave the jugular ganglion and continue to synapse upon the inferior ganglion, also referred to as the nodosum ganglion of the vagus nerve, the nerve bundle exits the skull through the jugular foramen along with the glossopharyngeal nerve (CN IX) and the accessory nerve (CN XI).
  • From this point on (the nodosum ganglion of the vagus nerve) the efferent fibers diverge. The pathway described is that of the parasympathetic fibers. The motoric pathway can be seen below.
  • The postganglionic fibers continue within the main nerve bundle of the vagus nerve with specific branches that are explained below.
  • The superior cervical cardiac branch divides from the main vagus nerve at the level of the cervical vertebrae and descends separately and medially to the carotid artery before innervating the superior area of the heart.
  • A second branch that leaves the main nerve bundle further down (at the level of the bifurcation of the carotid artery into Internal and external branches) passes laterally to the carotid artery and terminates in the same area as the superior cervical cardiac branch. This nerve has the same name, but is the inferior version.
  • A small branch that leaves the bundle just above the level of the aortic arch, known as the cardiac thoracic branch, also terminates in the same place.
  • At the same level as the cardiac thoracic branch stems the recurrent laryngeal nerve which turns back under the aortic arch and follows the trachea to the larynx and innervates it.
  • Once the vagus nerve reaches the aortic arch it divides into the pulmonary, cardiac and oesophageal plexuses.
  • The oesophageal plexus continues towards the stomach into the anterior vagal trunk and then divulges the anterior gastric branches, the hepatic branch, the celiac branch, the celiac and superior mesenteric ganglia and the celiac plexus, the hepatic plexus, the pyloric branch and the intestinal branches which form a plexus of their own via various anastomoses.

Motoric Efferent Pathway

The pathway for the efferent motoric fibers of the vagus nerve is the following (from the initiation in the brain to the termination in the periphery):

  • The ambiguous nucleus is situated behind the taenia ventriculi quarti and contributes the motoric fibers that innervate the musculature of the larynx and the muscles of the pharynx.
  • For the steps between the nucleus and the inferior ganglion please see the previous pathway as the fibers run together!
  • The postganglionic fibers stemming from the Inferior ganglion diverge from the shared pathway with the parasympathetic fibers and will remain that way until the termination of the fibers in the anatomical structures. The pathway described is that of the motoric fibers.
  • The motoric fibers continue to run within the pharyngeal branch of the vagus nerve forming part of the pharyngeal plexus and giving innervating muscular branches to the superior and middle pharyngeal constrictor muscles, the levator palatini muscle, the salpingopharyngeus muscle, the palatoglossus muscle, the palatopharyngeus muscle and the stylopharyngeus muscle.
  • An additional branch known as the superior laryngeal nerve runs postganglionically and innervates the inferior pharyngeal constrictor muscles as well as the cricothyroid muscle motorically through and internal branch (sensory and parasympathetic fibers) and an external branch (motoric fibers for the cricothyroid muscle).

Afferent Pathway

The pathway for the afferent sensory fibers of the vagus nerve is the following (from the initiation in the periphery to the termination in the brain):

  • The anterior vagal trunk collects the lower gastrointestinal tract branches which include the anterior gastric branches, the hepatic branch, the celiac branch, the celiac and superior mesenteric ganglia and the celiac plexus, the hepatic plexus, the pyloric branch and the intestinal branches which form a plexus of their own via various anastomoses, before continuing upwards past the stomach and into the oesophageal plexus.
  • The pulmonary, cardiac and oesophageal plexuses merge into the main vagus nerve at the level of the aortic arch.
  • The recurrent laryngeal nerve stems from the vagus and turns back under the aortic arch and follows the trachea to the larynx and innervates it.
  • At the same level as the recurrent laryngeal nerve a small branch leaves the bundle just above the level of the aortic arch, known as the cardiac thoracic branch and terminates in the superior border of the heart between the pulmonary veins and just below the aortic arch.
  • A second branch that leaves the main nerve bundle further up (at the level of the bifurcation of the carotid artery into internal and external branches) passes laterally to the carotid artery and terminates in the same area as the cardiac thoracic branch. This nerve is known as the inferior cervical cardiac branch.
  • The superior cervical cardiac branch divides from the main vagus nerve at the level of the cervical vertebrae and descends separately and medially to the carotid artery before innervating the heart by synapsing in the same place as the cardiac thoracic branch.
  • The fibers continue upwards within the main nerve bundle of the vagus nerve and synapse upon the nodosum ganglion of the vagus nerve, also known as the inferior ganglion. Here both afferent and efferent fibers of all kinds diverge and follow the same path to or from the brainstem.
  • Some of the fibers that join the afferent nerve bundle also come from the upper gastrointestinal tract, namely the pharyngeal branch that gives sensory fibers to the lower part of the pharynx and also gives branches to the pharyngeal plexus and a communicating branch to the carotid sinus.
  • The other branch is that of the superior laryngeal nerve, which gives sensory fibers to the upper larynx.
  • As the postganglionic fibers leave the nodosum ganglion, they enter the skull through the jugular foramen and synapse on the jugular ganglion or superior ganglion.
  • From the jugular ganglion, vagal fibers are given to the auricular nerve and the meningeal nerve.
  • Fibers finally synapse on either the solitary tract (vasoafferent fibers) or the nucleus of the spinal tract (somatoafferent fibers) in the brainstem.
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Show references

Reference:

  • Frank H. Netter, Atlas der Anatomie, 5th Edition (Bilingual Edition: English and German), Saunders, Chapter 1, Plate 96, 114-115 and 125, Published 2010.

Author:

  • Dr. Alexandra Sierosławska

Illustrators:

  • Vagus nerve - Yousun Koh 
  • Vagus nerve - Yousun Koh 
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