The Vagus Nerve – Anatomy, Blood Supply, Functions

The vagus nerve, or CN X, the tenth cranial nerve, is a nerve that serves numerous important functions. While the majority of the fascicles function in parasympathetic activity, the vagus nerve also contains somatic sensory, visceral sensory, and branchial motor fibers. In Latin, vagus means, “wandering, straying.” The vagus nerve is thus named because it follows a complex course throughout the body to innervate several organs; fibers originate from the dorsal motor nucleus and nucleus ambiguus in the ventral medulla oblongata of the brainstem, with terminal branches reaching the splenic flexure of the colon. A critical division during the nerve’s course from rostral to caudal occurs as it enters the abdominal cavity: it splits into an anterior trunk and a posterior trunk. The structural and functional properties of the anterior trunk, along with its relevant surgical and clinical considerations, will be examined in this article.[rx][rx][rx]

The vagus nerve, historically cited as the pneumogastric nerve, is the tenth cranial nerve or CN X, and interfaces with the parasympathetic control of the heart, lungs, and digestive tract. The vagus nerves are normally referred to in the singular. It is the longest nerve of the autonomic nervous system in the human body and comprises sensory and motor fibers. The sensory fibers originate from neurons of the nodose ganglion, whereas the motor fibers come from neurons of the dorsal motor nucleus of the vagus and the nucleus ambiguous.^[rx]

Structure

Upon leaving the medulla oblongata between the olive and the inferior cerebellar peduncle, the vagus nerve extends through the jugular foramen, then passes into the carotid sheath between the internal carotid artery and the internal jugular vein down to the neck, chest, and abdomen, where it contributes to the innervation of the viscera, reaching all the way to the colon. Besides giving some output to various organs, the vagus nerve comprises between 80% and 90% of afferent nerves mostly conveying sensory information about the state of the body’s organs to the central nervous system. The right and left vagus nerves descend from the cranial vault through the jugular foramina, penetrating the carotid sheath between the internal and external carotid arteries, then passing posterolateral to the common carotid artery. The cell bodies of visceral afferent fibers of the vagus nerve are located bilaterally in the inferior ganglion of the vagus nerve (nodose ganglia).

The right vagus nerve gives rise to the right recurrent laryngeal nerve, which hooks around the right subclavian artery and ascends into the neck between the trachea and esophagus. The right vagus then crosses anterior to the right subclavian artery, runs posterior to the superior vena cava, descends posterior to the right main bronchus, and contributes to cardiac, pulmonary, and esophageal plexuses. It forms the posterior vagal trunk at the lower part of the esophagus and enters the diaphragm through the esophageal hiatus.

The left vagus nerve enters the thorax between left common carotid artery and left subclavian artery and descends on the aortic arch. It gives rise to the left recurrent laryngeal nerve, which hooks around the aortic arch to the left of the ligamentum arteriosum and ascends between the trachea and esophagus. The left vagus further gives off thoracic cardiac branches, breaks up into the pulmonary plexus, continues into the esophageal plexus, and enters the abdomen as the anterior vagal trunk in the esophageal hiatus of the diaphragm.

Branches

• Pharyngeal nerve
• Superior laryngeal nerve
• Superior cervical cardiac branches of vagus nerve
• Inferior cervical cardiac branch
• Recurrent laryngeal nerve
• Thoracic cardiac branches
• Branches to the pulmonary plexus
• Branches to the esophageal plexus
• Anterior vagal trunk
• Posterior vagal trunk
• Hering-Breuer reflex in alveoli^[rx]

The vagus runs parallel to the common carotid artery and internal jugular vein inside the carotid sheath.

Nuclei

The vagus nerve includes axons which emerge from or converge onto four nuclei of the medulla:

• The dorsal nucleus of vagus nerve – which sends parasympathetic output to the viscera, especially the intestines
• The nucleus ambiguus – which gives rise to the branchial efferent motor fibers of the vagus nerve and preganglionic parasympathetic neurons that innervate the heart
• The solitary nucleus – which receives afferent taste information and primary afferents from visceral organs
• The spinal trigeminal nucleus – which receives information about deep/crude touch, pain, and temperature of the outer ear, the dura of the posterior cranial fossa and the mucosa of the larynx

Function

The vagus nerve supplies motor parasympathetic fibers to all the organs (except the adrenal glands), from the neck down to the second segment of the transverse colon. The vagus also controls a few skeletal muscles, including:

• Cricothyroid muscle
• Levator veli palatini muscle
• Salpingopharyngeus muscle
• Palatoglossus muscle
• Palatopharyngeus muscle
• Superior, middle and inferior pharyngeal constrictors
• Muscles of the larynx (speech).

This means that the vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve). It also has some afferent fibers that innervate the inner (canal) portion of the outer ear (via the auricular branch, also known as Arnold’s or Alderman’s nerve) and part of the meninges.^[rx]

Efferent vagus nerve fibers innervating the pharynx and back of the throat are responsible for the gag reflex. In addition, 5-HT₃ receptor-mediated afferent vagus stimulation in the gut due to gastroenteritis is a cause of vomiting.^[5] Stimulation of the vagus nerve in the cervix uteri (as in some medical procedures) can lead to a vasovagal response.

The vagus nerve also plays a role in satiation following food consumption.^[rx] Knocking out vagal nerve receptors has been shown to cause hyperphagia (greatly increased food intake).^[rx]

References

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