The Vagus Nerve
Imagine that it’s a Sunday afternoon. You’ve just eaten a delicious three course meal and now you’re sitting down on the couch to unwind and relax. You feel completely at rest, so much so, that you begin to doze off in and out of sleep. While you may think your body is as relaxed as you are, in fact, one of the divisions of your nervous system is hard at work. The parasympathetic nervous system is busy slowing your heart rate, regulating your breathing and shouting out orders to the organs of your digestive system. The ‘rest and digest’ period is well underway. One nerve in particular is working diligently, this is the vagus nerve.
The vagus nerve, or the 10th cranial nerve (CN X), is primarily associated with the parasympathetic division of the autonomic nervous system, however, it also has some sympathetic influence through peripheral chemoreceptors. The vagus nerve is a mixed nerve, as it contains both afferent (sensory) and efferent (motor) fibers. This means it is responsible for not only carrying motor signals to the organs it innervates, but it also carries sensory information from these organs back to the central nervous system.
Specifically, the vagus nerve contains:
- general afferent (sensory) fibers
- special sensory
- visceral afferent (sensory) fibers
- branchial efferent (motor) fibers
- visceral efferent (motor) fibers.
General afferent fibers are responsible for perceiving touch, pain, temperature, pressure, vibration and proprioceptive sensation from the posterior ear, external auditory meatus, and posterior and the external surface of tympanic membrane. Visceral afferent fibers are responsible for perceiving sensory input (with the exception of pain) from the viscera, or internal organs, of the body’s main cavity. Branchial efferent fibers innervate the muscles that develop from the branchial arches, such as the muscles of mastication, the tensor tympani and the tensor veli palatini. Special sensory convey taste from palate and epiglottis. Finally, visceral efferent fibers innervate the viscera, including all smooth muscle and glands.
The cranial nerves are 12 pairs of nerves that emerge from the brain, with a majority of the nerves originating in the brainstem. The cranial nerves collectively transmit efferent and afferent signals to and from the body, but primarily the head and neck. Some of the cranial nerves only carry either sensory or motor signals, while others, like the vagus nerve, are mixed and carry both. The cranial nerves emerge in pairs, however, they are often referred to in the singular.
The vagus nerve, or the 10th cranial nerve (CN X), is the longest and most complex of the cranial nerves. The vagus nerve differs slightly as it primarily supplies the organs of the chest and abdomen, as opposed to the head and neck. It is called “vagus” as it is a vagrant or wandering nerve going down to the abdomen. It is appropriately called the tenth cranial nerve as it is the tenth pair of nerves to emerge from the brain (in order of exiting cranial nerves from the front of the brain to the back).
By week 6 of gestation, the central nervous system is said to be in a five vesicle stage. These vesicles include the:
The medulla oblongata is derived from the myelencephalon.The motor fibers of the vagus nerve are derived from the basal plate of the medulla oblongata. Meanwhile, sensory fibers of the vagus nerve are derived from the cranial neural crest which arises from the ectoderm cell layer.
Within the medulla oblongata of the brainstem, there are 4 vagal nuclei, onto which axons of the vagus nerve emerge from or converge onto. These include:
- the dorsal motor nucleus
- the nucleus ambiguus
- the solitary nucleus
- the spinal trigeminal nucleus
The dorsal motor nucleus supplies parasympathetic efferents primarily to the gastrointestinal tract and lungs. The efferent fibers that arise from the nucleus ambiguus supply the muscles of the soft palate, pharynx and larynx. It also gives rise to branchial efferent fibers and preganglionic parasympathetic neurons for the heart.
The solitary nucleus receives primary afferents from visceral organs, as well as taste information. Finally, the afferents that converge on the spinal trigeminal nucleus relay sensory information regarding pain, temperature and deep touch of the outer ear, the dura of the posterior cranial fossa and the mucosa of the larynx.
The vagus nerve exits the brain from the medulla oblongata of the brainstem. Specifically, the nerves emerge by a series of rootlets between the olive, or the olivary body, and the inferior cerebellar peduncle.
It then travels laterally exiting the skull through the jugular foramen. The sensory ganglia of the the vagus nerve consists of a superior and inferior ganglionic swelling. The vagus nerve is joined by the cranial root of the accessory nerve (CN XI), just after this inferior ganglion.
The vagus nerve trunk subsequently passes down the neck between the carotid artery and the internal jugular vein, within the carotid sheath. At the base of the neck, the nerve enters the thorax, however, the right and left vagus nerve take different paths after this point. The left vagus nerve travels anterior to the aortic arch, behind the primary left bronchus and into the esophagus. The right vagus nerve travels behind the esophagus and primary right bronchus.
Both left and right vagus nerves subsequently enter the abdomen through the esophageal hiatus of the diaphragm and follow their own individual path to their terminal branches.
Branches of the Vagus Nerve
Branches in the Jugular Fossa
The Meningeal branch
The meningeal branch arises at the at the superior ganglion and re-enters the skull at the jugular foramen. This branch contains general afferent fibers and supplies the dura of the posterior cranial fossa .
The Auricular branch
The auricular branch, also referred to as Arnold’s Nerve, arises from the superior ganglion and re-enters the lateral portion of the jugular foramen via the mastoid canaliculus. The branch exits again through the tympanomastoid suture of the temporal bone to reach and supply the skin. This branch contains general afferent fibers and it innervates and supplies sensation to the the external tympanic membrane and a small portion of the posterior aspect of the external ear.
Branches in the Neck
The Pharyngeal Nerve
The pharyngeal branches arise from the inferior ganglion of the vagus nerve and contain visceral afferent fibers and motor fibers. The motor efferent fibers are supplied by the accessory nerve (CN XI) which joins the pharyngeal nerve.
The pharyngeal branch of the vagus nerve passes across the internal carotid artery to the middle pharyngeal constrictor muscle. Here, filaments of the pharyngeal branches form a plexus along with branches of the glossopharyngeal (CN IX) nerve, branches of the external laryngeal nerve and sympathetic fibers from the superior cervical ganglion. This is called the pharyngeal plexus, which supplies the pharyngeal muscles (excl. the stylopharyngeus muscle), the mucous membrane of the pharynx (excl. the stylopharyngeus muscle) and the soft palate (excl. tensor palatini muscle).
Branches from the pharyngeal plexus also contribute to the internal carotid plexus (located on the lateral side of the internal carotid artery) along with sympathetic and glossopharyngeal fibers. The vagal visceral afferent fibers are responsible for transmitting impulses from the chemoreceptors in the carotid body.
As the vagus nerve descends down the carotid sheath, it inter communicates with the filaments or branches of the cervical sympathetic trunk, and so from the neck downward it is considered a mixed parasympathetic-sympathetic nerve.
Superior laryngeal nerve
It is a structure of IV branchial arch and thus innervates the pharyngeal and laryngeal derivatives of this arch. The afferent fibers of the superior laryngeal nerve arise from the inferior ganglion of the vagus nerve. This branch receives some sympathetic fibers from the superior cervical ganglion. At the level crossing of the hypoglossal nerve (CN XII), the superior laryngeal nerve passes between the external and internal carotid arteries. It then divides into external and internal branches at the tip of the hyoid bone, which lies under the mandible.
The internal laryngeal branch enters the larynx through the thyrohyoid membrane and it supplies most of the mucosa above the glottis.
The external laryngeal branch travels to the inferior pharyngeal constrictor muscle.This branch innervates an intrinsic laryngeal muscle called the cricothyroid muscle. All other intrinsic laryngeal muscles are innervated by the recurrent laryngeal nerve, which is another branch of the vagus nerve, discussed below.
Recurrent laryngeal nerve
Also known as the inferior laryngeal nerves, there are two recurrent laryngeal nerves, one on the right side of the body and one on the left. They were appropriately given the name recurrent laryngeal nerves as they follow a recurrent course and travel in the opposite direction to the nerve they branched from. The recurrent laryngeal nerve contains branchial efferent fibers. The main trunk of the nerve is bound medially by the trachea and esophagus and laterally by the common carotid artery, the internal jugular vein and the vagus nerve. The right nerve branches from the vagus nerve at the base of the neck, travels under the subclavian artery, and then courses upwards in the tracheoesophageal groove and enters the larynx. The left nerve has a similar pathway, however it loops around the aortic arch distal to the ligamentum arteriosus.
Both right and left recurrent laryngeal nerves are given off by the vagus nerves after they enter into the thorax, so they are sometimes included in the branches of thorax, especially the left recurrent as it arises at the level of the aortic arch. The recurrent nerves then ascend back to the larynx.
As mentioned, all the intrinsic laryngeal muscles, with the exception of the cricothyroid muscle, are innervated by the ipsilateral recurrent nerve. The one exception of this is the interarytenoid muscle which receives bilateral innervation. The superior and recurrent laryngeal nerves connect as the ramus communicans, which provides visceral efferent innervation of the esophageal mucosa and smooth muscle.
Superior cardiac branches
The superior cardiac nerve branches off the vagus nerve at the upper (superior) and lower (inferior) parts of the neck. Therefore, there are two branches of the superior cardiac nerve on each side. The left superior branch descends lateral to the trachea, in front of the esophagus and deep to the aortic arch and merges with the deep part of the cardiac plexus. The left inferior branch also descends lateral to the trachea and then passes over the aortic arch and merges with the superficial part of the cardiac plexus.
Both the right superior and inferior branches descend deep to the subclavian artery to diverge into the deep part of the cardiac plexus.
Branches in the Thorax
Inferior Cardiac nerve
On the left side, the inferior cardiac nerve arises from the recurrent laryngeal nerve. On the right side, it arises from the trunk of the vagus beside the trachea. Both left and right branches terminate in the deep part of the cardiac plexus.
The cardiac plexus, responsible for innervating the heart, receives fibers from the cardiac nerves of the vagus, recurrent laryngeal nerves and from the cervical ganglia of the sympathetic trunk.
Anterior Bronchial Branches
Two or three small anterior bronchial branches are located on the anterior surface of the root of the lung. Along with contributions from the sympathetic trunk, these branches form the anterior pulmonary plexus which innervates the bronchial tree and the visceral pleura.
Posterior Bronchial Branches
The posterior branches are generally larger and more abundant than the anterior branches and are located on the posterior root of the lung. These branches form the posterior pulmonary plexus along with contributions from the third and fourth thoracic ganglia of the sympathetic trunk. The posterior pulmonary plexus innervates the same structures as its anterior counterpart.
Esophageal branches of the vagus nerve, along with visceral branches of the sympathetic trunk, form the esophageal plexus. The esophageal branches extend from above and below the bronchial plexus. Filaments from the esophageal plexus project to the posterior surface of the pericardium. The esophageal branches are motor and sensory to the esophagus.
Branches in the Abdomen
The branches of the right vagus nerve forms the posterior gastric plexus on the postero-inferior surface of the stomach, while the branches of the left vagus nerve forms the anterior gastric plexus on the antero-superior surface of the stomach. Both of the divisions run between the layers of lesser omentum. The fibers from the anterior gastric extend as far as the pylorus and the upper part of the duodenum, while posterior vagal trunk ,in addition to posterior gastric branches, sends fibers to major abdominal autonomic plexus from which vagal fibers are distributed to the territories of celiac, renal and superior mesenteric arteries.
Celiac branches of the vagus nerve are primarily derived from the right vagus nerve. These branches join the celiac plexus, which innervates the pancreas, kidneys, spleen, suprarenal bodies and intestine.
Hepatic branches of the vagus nerve are primarily derived from the left vagus nerve. These branches join the hepatic plexus which innervates the liver.
Vagus Nerve Stimulation
Vagus nerve stimulation involves the implantation of a stimulator device under the skin in the chest, where a wire from the device is wrapped around the left vagus nerve in the neck. Regular, mild electrical impulses are delivered to the brain through the vagus nerve. Vagus nerve stimulation is used as a treatment for certain types of epilepsy and treatment-resistant depression. Medics are not exactly sure how vagal stimulation works but it is thought that the treatment alters mood and controls seizures by altering norepinephrine release, increasing levels of the inhibitory GABA neurotransmitter or by inhibiting cortical activity.
Research is currently being carried out to investigate the possible use of vagal nerve stimulation in the treatment of other conditions such as multiple sclerosis and Alzheimer’s disease.
A vagotomy is a surgical procedure in which a portion of the vagus nerve is severed to reduce acid secretion in the stomach. It is usually performed to help manage peptic ulcer disease. There are different types of vagotomies that are performed depending on the condition of the patient. The basic types of vagotomy include truncal vagotomy, selective vagotomy and highly selective vagotomy. A truncal vagotomy cuts the trunk of the vagus nerve before it enters the abdomen. Selective vagotomy involves severing between the anterior and posterior nerves of Laterjet and denervating the pylorus. Highly selective vagotomy involves denervation of the fundus and body of the stomach, while preserving the antrum and pylorus.
Lesions of the Vagus Nerve
The symptoms of a lesion along the vagus nerve are dependent on where the lesion is located. As the vagus nerve and its branches supply many different structures in the body, symptoms may vary from palatal and pharyngeal paralysis to abnormalities in the gastric acid secretion and heart rate.
Unilateral lesions of the recurrent laryngeal branch of the vagus nerve can result in vocal cord paralysis in the paramedian position. The result of this is a hoarse and breathy voice, and diplophonia may also occur. Bilateral recurrent laryngeal lesions can result in paralysis of both vocal cords, causing a whisper-type voice and possible death due to obstruction of the trachea by the cords. On the other hand, unilateral lesions of the superior laryngeal nerve generally don’t result in dysphonia, however, bilateral lesions may restrict vocal pitch control.