Vagus (X) Nerve

The dorsal vagal nucleus is a mixed nucleus that represents fused visceral afferent and efferent columns of neurons. It consists of a longitudinal column of cells lying beneath the vagal trigone in the fourth ventricle floor, lateral to the hypoglossal nucleus, extending nearly the length of the medulla oblongata. The special and general visceral afferent fibers ending in the nucleus are the central processes of pseudounipolar sensory cells in the inferior vagal ganglion (or nodose ganglion); peripheral processes of the sensory cells convey impulses from the heart, aorta, trachea, bronchi, lungs, most of the alimentary tract (from the lower pharynx almost to the left colic flexure), liver, pancreas, and possibly from the kidneys. Sensory fibers that carry taste sensation from the epiglottis, pharynx, and hard and soft palates are also located in the nodose ganglion. Preganglionic efferent fibers carrying impulses for the same structures originate in the dorsal vagal nuclei and are distributed through direct vagal branches to the viscera or through branches of the cardiac, celiac, and abdominal plexuses (anterior and posterior vagal trunks). Vagal preganglionic fibers synapse in ganglia located near or within the viscera they innervate. Because of this arrangement, vagal parasympathetic postganglionic fibers are relatively short and more limited in their distribution than their sympathetic counterparts. The general somatic afferent fibers are the pseudounipolar cells of the superior vagal ganglia (or jugular ganglia) involved with sensory impulses conducted through the auricular and meningeal vagal branches, although the fibers in the latter branches may be derived from interconnections between ganglia and upper cervical spinal nerves. Central processes of the superior vagal ganglion cells probably end in the spinal nuclei of the trigeminal nerves.

The solitary tract nucleus receives afferent special sensory taste fibers, traveling in the superior laryngeal vagal branches from the mucous membrane of the epiglottis and the epiglottic valleculae. In addition, general visceral sensations from the larynx, oropharynx, linings of the thorax, and abdominal viscera also project to the solitary tract nucleus. The nucleus ambiguus develops from special visceral efferent columns and forms a row of discrete, multipolar neurons located deeply in the reticular formation of the medulla oblongata. Its axons emerge in the glossopharyngeal and vagal nerves and in the cranial parts of the accessory nerves. The glossopharyngeal and vagal fibers distribute mainly to the intrinsic laryngeal and pharyngeal muscles (except tensor veli palatine [CN V] and stylopharyngeus [CN IX]), while the accessory fibers serve mainly the sternocleidomastoid and the trapezius muscles. The lower precentral gyrus controls vagal motor function.

VAGUS NERVE

The vagus nerve contains both afferent and efferent parasympathetic fibers that are widely distributed to visceral and vascular structures in the neck, thorax and abdomen, somatic sensory fibers in the auricular and meningeal branches, some special sensory fibers (taste) of the superior laryngeal branch, and special visceral efferent fibers that arise in the nucleus ambiguus and are distributed mainly to laryngeal and pharyngeal muscles.

Each vagus nerve emerges from the lateral medulla oblongata along the posterior sulcus as 8 to 10 rootlets above the rootlets of the glossopharyngeal nerve and cranial parts of the accessory nerve. The rootlets coalesce to form a nerve that exits the skull through the jugular foramen, together with the glossopharyngeal and accessory nerves, the sigmoid sinus, and several other blood vessels. Within or inferior to the jugular foramen, the vagus nerve expands into superior and inferior ganglia.

The superior vagal ganglion (jugular ganglion) communicates with the nearby superior cervical sympathetic trunk ganglion and the facial, glossopharyngeal and accessory nerves. It gives off a recurrent branch to the meninges of the posterior cranial fossa, an auricular branch that carries somatic sensory impulses from parts of the tympanic membrane and the external acoustic meatus, and a pharyngeal branch that, along with the glossopharyngeal nerve, forms the pharyngeal plexus and sends motor fibers to the muscles of the soft palate and pharynx.

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