Vagus nerve

Hello everyone! This is Joao from Kenhub, and welcome to this tutorial, where I'm going to be talking about the vagus nerve. The very first thing we see here on the screen is an image of the brain as seen from an inferior view where we can see the cranial nerves emerging. This little pair of nerves highlighted in green here are what we're interested in today as these are the two vagus nerves emerging from the brain.

The vagus nerves are the tenth pair of cranial nerves and it's the longest of the twelve pairs of cranial nerves. The name comes from the Latin and means the "wandering nerve" which describes its wide distribution throughout the body as we can see now on this image. It performs many important roles within the parasympathetic nervous system. The system is one of the two antagonistic systems making up the autonomic nervous system and is craniosacral which means that it arises from the central nervous system in the cranium and from the sacral region.

The vagus nerve actually carries seventy five percent of the cranial parasympathetic function so it's pretty vital for a life. In this tutorial, we'll be describing the vagus nerve and its general course as a whole and then we'll take a closer look at the right and left vagus nerves and finally at its three functional pathways which include the parasympathetic efferent pathway, motor efferent pathway and the afferent pathways.

Before we continue, it's important to remember what we mean by the terms efferent and afferent. An efferent nerve is one that carries impulses from the central nervous system to the periphery while an afferent nerve brings signals from the periphery to the CNS or the central nervous system.

I also want to draw your attention to the picture here. We're viewing the body in through its left side showing the vagus nerve highlighted in green, the structures it innervates along with their major arteries, and a few other structures to help orientate us. This is the view we'll be exploring in this tutorial.

So let's start, and first we'll look at the general course of the vagus nerve. At the top of the picture here, we have the cranial nerves leaving the medulla oblongata, the vagus nerve on each side passes anteriorly then dives inferiorly to leave the skull through the jugular foramen. Through the foramen, it is accompanied by the glossopharyngeal nerve which is the ninth cranial nerve as well as the accessory nerve which is the eleventh cranial nerve. There are two sensory ganglia in each vagus nerve at the jugular foramen which are the superior vagal ganglion and the inferior vagal ganglion. When the three nerves pass through the jugular foramen, they go their separate ways and the vagus nerve continues down the neck within the carotid sheath.

The vagus nerve on each side runs posterolateral to the carotid arteries and medial to the internal jugular vein then passes in front of each subclavian artery. As the right and left vagus nerves approach the arch of the aorta, they branch to form the cardiac plexus, the pulmonary plexus which isn't shown on our image but is located approximately here, the right recurrent laryngeal nerve, the left recurrent laryngeal nerve, and the oesophageal plexus. The paths of the right and left vagus nerves differ as they prepare to form the oesophageal plexus so let's take a look at these differences right now.

We're going to take a look at the right vagus nerve first. If we change the angle of our image we are now looking from the front into the open chest cavity, we can see that after passing over the right subclavian artery, the right vagus nerve angles slightly medially to meet the esophagus at the level of the sternal angle. Here the right vagus nerve spirals posteriorly around the esophagus where it becomes the posterior vagal trunk and contributes fibers to the oesophageal plexus highlighted here and posterior gastric branch of the posterior vagal trunk.

The right vagus nerve provides most of the fibers of the celiac plexus. This forms the celiac ganglia sitting on the front of the aorta as you can see. And this gives off the pancreatic plexus then the splenic plexus, the renal plexus, the adrenal plexus which we can't see in this illustration, the intestinal branch of the vagus nerve, and the intermesenteric plexus. It's useful to note that these branches leave the front of the aorta then travel alongside of their organs' respective arteries. This is just one of many ways your body has evolved for efficiency.

Now, we'll move over to look at the left vagus nerve which is highlighted in green on this image, and from here, it continues inferiorly before curving around the arch of the aorta to run onto the front of the esophagus. As with the right, this happens at the level of the sternal angle. When it meets the esophagus, it forms the anterior vagal trunk and contributes the rest of the oesophageal plexus.

Now let's take a closer look at the anterior vagal trunk switching here the highlights to show you the anterior vagal trunk which is giving off the anterior gastric branch of the vagus nerve highlighted here. The anterior vagal trunk then also gives off the hepatic branch of the vagus nerve which in turn provides most of the nerve fibers to the hepatic plexus. An important structure to note at this point is the superior mesenteric ganglion shown here. Although near the path of the vagus nerve, it does not actually contain vagus nerve fibers. In fact, it provides sympathetic innervation to the small intestine rather than parasympathetic.

Having discussed the anatomical path of the vagus nerve, it is time to now look at each functional pathway. For each, we'll look at which nucleus in the brain each pathway originates or terminates and then describe the fibers' paths. We'll start with the parasympathetic efferent pathway whose fibers originate from the dorsal vagal nucleus approximately here in where you see this green which lies in the floor of the fourth ventricle. The parasympathetic efferent fibers merge with the motor efferent fibers to exit through the jugular foramen and, together, they continue down the neck within the carotid sheath giving off minor branches to provide parasympathetic innervation to the pharynx, larynx and other neck structures.

The first three major branches contribute to the cardiac plexus. The first branch is the superior cardiac branch of the vagus nerve shown here which leaves the vagus nerve on either side at the level of the first cervical vertebra. The second branch to split from the vagus nerve is the inferior cardiac branch here. A third branch is the thoracic cardiac nerve which we can't see on our image here but think of it leaving just above the aortic arch. These three branches form the cardiac plexus on the superior surface of your heart.

At or below the level of the aortic arch, the vagus nerve starts to split profusely although the actual level varies from person to person. The branches become the pulmonary plexus which as before can't be seen in this image but is located approximately here and the oesophageal plexus with its anterior and posterior vagal trunks. As we saw before, the oesophageal plexus then gives rise to the various plexuses that supply the abdominal organs.

So that's an overview of the parasympathetic efferent pathway. Next, we'll describe its neighbor – the motor efferent pathway. These motor efferent fibers are responsible for innervating the muscles of the larynx and pharynx. They can be controlled consciously thanks to fibers coming from the cortex and unconsciously through reflex arcs. The conscious cortical fibers together with the afferent fibers of the vagus meet and synapse centrally here in the nucleus ambiguus. Efferent fibers join those of the parasympathetic efferent pathway to leave the cranium through the jugular foramen and continue down the neck.

The motor efferent pathway has three main branches, the first of these is the pharyngeal branch which comes off first around the level of the pinna of the ear. It contributes motor innervation to the pharyngeal plexus supplying the muscles of the palate and pharynx. The superior laryngeal nerve is the second going its own way just after the pharyngeal branch has left. It contributes to the pharyngeal plexus and innervates muscles of the pharynx and larynx. The recurrent laryngeal nerves take the remaining fibers from the motor efferent pathway and diverge from the vagus nerve at the different level on each side.

Highlighted here is the left recurrent laryngeal nerve leaving the left vagus nerve by curving underneath the aorta to then come back up into the neck. The right recurrent laryngeal nerve splits from the main body of the right vagus nerve by hooking under the right subclavian artery to climb its way back to the larynx. The recurrent laryngeal nerves are the primary motor supply to the muscles of the vocal cords. Just to note here that the cough reflex is one well-known reflex that is performed through the motor efferent pathway.

That's us done with what the vagus nerve makes through its parasympathetic and motor pathways. Now let's look at what the vagus nerve senses for us by describing its afferent pathways. Remember that the term afferent refers to nerve fibers that brings signals from the periphery to the central nervous system. Mostly, the vagus nerve is autonomic so we'll focus on that but it does have a function in conscious sensation that we'll mention before we finish.

There are two components of the autonomic afferent pathway – the general branches provide visceral sensation from the heart, thoracic cavity, abdomen and the carotid and aortic bodies while the special branches bring sensation from the epiglottis and special sense of taste from the back of the mouth. These branches are also responsible for stimulating the cough reflex. Their paths are the same as those of the efferent fibers, the only difference being that the impulses travel along the fibers in the opposite direction. That is, signals come from the periphery to the central nervous system.

Talking about the general branches, the oesophageal plexus collects sensory nerve fibers of the abdominal plexuses. It meets with the pulmonary plexus and recurrent laryngeal nerves at the level of the arch of the aorta. The branches from the aortic body join here to the three main branches of the cardiac plexus meet the vagus nerve above the aorta and then as the vagus nerve, they will rise through the neck. The special branches are carried within the pharyngeal plexus and pass between the internal and external carotid arteries to join the vagus nerve at the inferior vagal ganglion.

The inferior ganglion is made of the cell bodies of the general and special autonomic afferent nerves. The postganglionic axons enter the skull through the jugular foramen and arch posteriorly to then synapse in the nucleus tractus solitarius. They synapse onto intermediate afferent fibers which connect to the efferent nuclei to effect parasympathetic reflexes.

The final pathway to talk about is the afferent somatic pathway. This is the conscious sensation component of the vagus nerve. It carries sensory information from the outer ear which includes the pinna, external auditory meatus and tympanic membrane. The auricular branch of the vagus nerve collects the cutaneous nerves from these areas and pierces through the tympanomastoid suture. From there, it travels medially and enters the cranium through the jugular foramen and its cell bodies make up the superior vagal ganglion. The postganglionic fibers continue and synapse within the spinal trigeminal nucleus connecting nerves then spread throughout the brain but most notably go to the cortex allowing conscious sensation.

That's just about everything we'll be covering on this tutorial. I know it's a lot to take in for the vagus nerve but before we finish, I want to mention a quick note on why it is important to understand the vagus nerve anatomy.

Here we can see the right and left recurrent laryngeal nerves highlighted in green. These travel down the neck to then return around and go back to innervate the muscles of the larynx. If either of those nerves are damaged, the muscles of the larynx can be weakened or paralyzed depending on the degree of injury. This is known as recurrent laryngeal nerve palsy. Weakened laryngeal musculature or one-sided paralysis causes hoarseness while bilateral paralysis may cause aphonia which is a complete loss of voice.

There are various causes of recurrent laryngeal nerve palsy. If a patient presents with hoarseness, then the diagnosis of bronchial or oesophageal cancer should be excluded. As these tumors grow, they can compress either recurrent laryngeal nerve and inhibit signal transmission. In a similar way, enlarged mediastinal lymph nodes can compress the left recurrent laryngeal nerve in the mediastinum of the chest.

Surgery and penetrating injury can cause direct trauma to the nerve with surgery on the thyroid and parathyroid glands being of note. Furthermore, some higher lesions such as lesions of the brainstem can elicit recurrent laryngeal nerve problems, however, these are usually accompanied by symptoms associated with other nerves in the region of the lesion.

Now before we finish this tutorial, let's do a brief recap. In this tutorial, we first discussed the general course of the vagus nerve which emerges here from the medulla oblongata and leaves the cranium via the jugular foramen. From there, it travels down the neck through the carotid sheath. As the right and left vagus nerves approach the arch of the aorta, they both branch to form the cardiac plexus, the pulmonary plexus, the right recurrent laryngeal nerve, and the left recurrent laryngeal nerve.

We then looked in more detail at the right vagus nerve which leads to several plexuses that supply the abdominal organs including the celiac plexus, the pancreatic plexus, the splenic plexus, the renal plexus, the adrenal plexus as well as the intestinal branch of the vagus nerve, and the intermesenteric plexus. The left vagus nerve forms the anterior vagal trunk which contributes to the rest of the oesophageal plexus. This in turn gives off the anterior gastric branch of the vagus nerve in addition to the hepatic branch of the vagus nerve which in turn provides most of the nerve fibers to the hepatic plexus.

After looking at the anatomy of the vagus nerve, we then looked at its motor, autonomic and sensory functions and the pathways involved in making those functions happen. First, we looked at the parasympathetic efferent pathway which originates in the dorsal vagus nucleus and gives off several branches to provide parasympathetic innervation to the pharynx, larynx and other neck structures as well as three major branches that contribute to the cardiac plexus.

Next, we looked at the motor efferent pathway which can be controlled both unconsciously and consciously. These fibers meet and synapse here in the nucleus ambiguus. They then leave the skull via the jugular foramen and travel down the neck to innervate the muscles of the larynx and pharynx. Next, we looked at the afferent pathways, the first of which being the autonomic afferent pathway whose branches follow the same routes as the efferent pathways but in the opposite direction back towards the central nervous system. Finally, we looked at the somatic afferent pathway. This is the conscious sensation component of the vagus nerve and carries sensory information from the outer ear.

And that concludes our tutorial. The vagus nerve is a tricky devil so if you don't get it at first, don't sweat. Just go back to the start of the pathway and try again. And thanks for watching this tutorial and all the best for your studying.