Vagus nerve (CN X)
The vagus nerve (tenth cranial nerve, CN X, also called the vagus, latin: nervus vagus) is the cranial nerve with the widest distribution in the human body, because it innervates not only structures in the head, but also runs through the neck, thorax and abdomen, supplying most of the visceral organs there.
The vagus nerve is a mixed nerve, it contains somatic and visceral afferent fibers, as well as general and special visceral efferent fibers. The vagus nerve fibers originate from three nuclei:
- nucleus ambiguus,
- dorsal nucleus of vagus nerve,
- solitary tract nucleus.
The nucleus ambiguus consists of bodies of motor neurons that give rise to special visceral efferent fibers, which provide innervation for skeletal muscles of branchial arch origin, such as the cricothyroid muscle, muscles of the pharynx, and the intrinsic muscles of the larynx.
The dorsal nucleus of the vagus nerve contains general visceral efferent neurons, which provide parasympathetic innervation to the viscera.
The solitary tract nucleus (also called the nucleus of the solitary tract) contains neurons which receive information from special visceral afferent fibers carrying sensory information (taste sensation) from the epiglottis, and general visceral afferent fibers that transmit sensory information from the mucosa of the soft palate, pharynx, and larynx.
The vagus nerve can be divided in cranial, cervical, thoracic and abdominal parts.
Cranial part of vagus nerve
Rootlets that arise from the medulla oblongata form a smaller inferior and lager superior bundles that collectively form the vagus nerve, after it exits the medulla through the posterolateral sulcus along with the glossopharyngeal nerve (CN IX) and the cranial root of the accessory nerve (CN XI). The vagus nerve travels through the jugular foramen accompanied by the accessory nerve, both sharing an arachnoid and a dural sheath. Below the jugular foramen the vagus nerve has two enlargements, the superior (jugular) and the inferior (nodose) ganglion, where pseudounipolar neurons are located.
The superior ganglion of the vagus nerve carries cell bodies of afferent somatosensory neurons. Most of them give rise to axons that enter the auricular nerve, a branch of the vagus nerve providing sensory information from the external auditory meatus. Some of the fibers form the meningeal branch of the vagus nerve innervating the dura mater in the posterior cranial fossa, and several fibers transmit impulses from the taste receptors in the mucosa of the epiglottis and the epiglottic vallecula.
The inferior ganglion of the vagus nerve contains bodies of somatic, special and general visceral afferent neurons with axons synapsing centrally in the solitary nucleus and their dendrites receiving sensory information from the larynx, lungs, heart, and the alimentary tract segment from the pharynx down to the transverse colon.
Cervical part of vagus nerve
After leaving the skull through the jugular foramen, the vagus nerve descends in the neck covered by the carotid sheath first between the internal carotid artery and the internal jugular vein, then between the common carotid artery and the internal jugular vein. The cervical part of the vagus nerve gives off four sets of branches:
- pharyngeal branches,
- superior laryngeal nerve,
- recurrent laryngeal nerve,
- cardiac branches.
The pharyngeal branches of the vagus nerve arise from the superior part of the inferior vagal ganglion and also contain filaments form the accessory nerve (CN XI). In the neck, the pharyngeal branches of the vagus nerve travel between the external and internal carotid arteries to the superior border of the middle pharyngeal constrictor, where they distribute into several filaments, which enter branches of the sympathetic chain and glossopharyngeal nerve (CN IX) and participates in forming the pharyngeal nerve plexus. The pharyngeal nerve plexus together with the accessory nerve innervate all pharyngeal muscles (except the stylopharyngeus muscle, which is innervated by the CN IX), mucous membrane in the lower part of the pharynx, and the muscles of the soft palate (except the tensor veli palatini muscle, which is innervated by CN V3).
The superior laryngeal nerve is a branch of the vagus nerve, which carries both sensory and motor fibers. The superior laryngeal nerve arises from the middle part of the inferior ganglion of the vagus nerve and in its course receives a branch from the superior cervical sympathetic ganglion, then travels alongside the pharynx to the internal carotid artery, where it divides into internal and external branches of the laryngeal nerve. The internal branch of the superior laryngeal nerve supplies sensory innervation to the pharynx, but the external branch of the superior laryngeal nerve has motor fibers, which innervate the cricothyroid muscle.
The recurrent laryngeal nerve is a mixed branch of the vagus nerve containing motor and sensory fibers. The recurrent laryngeal nerve differs in origin and course on the left and right side. The term `recurrent` indicates that they travel in the opposite direction to the nerve they branch from. On the left side the left recurrent laryngeal nerve loops below the aortic arch, while the right nerve loops under the first part of the right subclavian artery, in this level both nerves give cardiac filaments to the deep cardiac plexus. After that the left and right recurrent laryngeal nerves ascends near a groove between the esophagus and trachea, closely to the medial surface of the thyroid gland and then penetrates the lower border of the inferior constrictor muscle, entering the larynx. The motor fibers of the recurrent laryngeal nerve innervates all laryngeal muscles, except the cricothyroid. It also communicates with the internal laryngeal nerve, supplying sensory (afferent) fibers to the laryngeal mucosa under the vocal folds, as well as transmitting afferent fibers from the laryngeal stretch receptors.
There are two sets of cardiac branches arising from the vagus nerve, the inferior and superior cervical cardiac branches.
The inferior cervical cardiac branches of the vagus nerve arise from the vagus nerve, on the right side, from the trunk of the vagus nerve and from the recurrent laryngeal nerve, while on the left side they arise from the recurrent nerve only. The inferior cervical cardiac branches descend behind the subclavian artery and along the front of the trachea, to join the deep part of the cardiac plexus.
The superior cervical cardiac branches of the vagus nerve arise as two branches from the superior cervical ganglion. They run down the neck behind the common carotid artery, and in front of the longus colli muscle and crosses in front of the inferior thyroid artery, and the recurrent laryngeal nerve. The right branch joins the deep part of the cardiac plexus, while the left branch runs in front of the left common carotid artery and across the left side of the arch of the aorta, to join the superficial part of the cardiac plexus.
Further the course of the vagus nerve differs on both sides. The right vagus nerve travels posterior to the jugular vein, crosses the first part of the subclavian artery and enters the thorax through the superior thoracic aperture. On the left side the vagus nerve enters the thorax between the left common artery and subclavian arteries, posterior to the brachiocephalic vein.
Thoracic part of vagus nerve
In the thorax, both the left and right vagus nerves run behind the pulmonal radix and around the middle part of the esophagus, and the fibers mix together to form the esophageal plexus. The inferior fibers of the esophageal plexus form the anterior and posterior vagal trunks. The anterior vagal trunk distributes fibers on the anterior surface of the esophagus. It consists primarily of fibers from the left vagus. The posterior vagal trunk consists primarily of fibers from the right vagus nerve and they are distributed on the posterior surface of the esophagus. Both trunks descend through the esophageal hiatus in the diaphragm into the abdominal cavity.
Abdominal part of vagus nerve
In the abdomen, the anterior vagal trunk runs along the lesser curvature of the stomach where it divides into several sets of branches: the anterior gastric branches, which form the anterior gastric plexus and supply the stomach; the hepatic branch that travels along the lesser omentum to the liver, and the celiac branch, which consists of small branches providing parasympathetic innervation to the celiac nerve plexus. The posterior vagal trunk runs along the greater curvature of the stomach to the posterior surface, where it splits into several sets of branches: the posterior gastric branches, which form the anterior gastric plexus, branches to the kidneys, liver and biliary tract, and the celiac branch, which joins the celiac nerve plexus and supplies almost all of the abdominal organs.