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Glossopharyngeal nerve

Course, branches and nuclei of the glossopharyngeal nerve.

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Transcript

Hello everyone! This is Joao from Kenhub, and welcome to another tutorial, where this time we're going to be talking about the glossopharyngeal nerve. The first thing I want to show you guys here on this screen is this view – this lateral view of the brain. When we turn it, we can see these yellow structures which are called cranial nerves. These beautiful structures are all seen from then the inferior view of the brain. The glossopharyngeal nerve belongs to this set of neural structures.

Now if we zoom in, we can see the glossopharyngeal nerve highlighted in green. You could also see here that it's a bilateral nerve which means that you can find one on each side like we do for the other cranial nerves. The glossopharyngeal nerve is ninth out of the twelve cranial nerves and is typically abbreviated as CN Roman numeral for nine. Before we talk about the different points related to the glossopharyngeal nerve, I want to show you guys the main image we will be exploring today here on this tutorial.

Now this lady was kind enough to let us use her profile picture so we can have this overlay of the main image that we're going to be exploring throughout this tutorial. So we will be exploring these structures that I am overlapping here just to give you an idea where they are located in your body. But note that this is a schematic view which shows structures placed slightly different from what they usually are in the body. The reason why we use this schematic view is that they allow us to see things a little bit better – how things actually connect to one another.

On the image, you see a few well-known structures here – the tongue which is cut, the pharynx, a few cranial nerves coming from the brainstem, and the focus of this video which is then the glossopharyngeal nerve seen here highlighted in green and see how the nerve is then connected, for lack of better word, to different structures that it's innervating, so you see an overview here.

Now the first thing I want us to do before we explore the course and branches of the glossopharyngeal nerve – so this image that we just looked at in a bit more detail – I want us to briefly explore its general anatomy. Every good nerve in your body is responsible for innervating one structure and the glossopharyngeal nerve is no exception so we're going to be seeing that there are a few structures innervated by this nerve – so all the sites where axons associated with the glossopharyngeal nerve are located, these axons contact area such as the parotid gland which you see here on this image, the pharyngotympanic tube, the tympanic cavity and membrane, the tongue, the palatine tonsil, the pharynx and one artery – this one here, the common carotid artery. These peripheral areas are connected to the brainstem specifically the lateral aspect of the medulla oblongata which you can see here on the image. This is also more clearly seen on this one, this image here with it being located just superiorly to the vagus nerve which you can see right here.

Now from here, the glossopharyngeal nerve passes through the jugular foramen which is this structure seen here highlighted in green on this image of the superior view of the base of the skull. Remember that the word foramen means opening, so this is where the nerve will be passing through on the skull. If you go back to the image of the glossopharyngeal nerve, you can see this circle showing where the nerve exits the jugular foramen. From there, the nerve branches and then supplies all the structures that I mentioned to you guys before.

At this point of the tutorial, we are ready to dive a bit deeper and explore the details related to the glossopharyngeal nerve. This particular cranial nerve can be considered one of the most complex as it deals with many different functions associated with the head and neck anatomy. And these functions include general sensation of the tongue and throat, also special sensation of taste, and visceral sensation from specific structures of the common carotid artery. There are also other components – motor from the parasympathetics and finally, somatic motor function. Throughout this tutorial, we will discuss the anatomy of the glossopharyngeal nerve associated with each of these functions so keep that in mind as we move on.

Let's begin by discussing the general sensory component of the glossopharyngeal nerve. As we're discussing sensation, let me add here that it can also be termed afferents because the action potentials are being conducted along axons towards the central nervous system. The types of sensation transmitted by general sensory axons include pain and touch which is shown here by this quite painful image I would say so please don't try this at home, and also temperature. The glossopharyngeal nerve, the sensations are from the tongue, tonsils, pharynx, tympanic cavity, pharyngotympanic tube and external ear which we will be discussing next.

So, let's first look at the sensation from the tongue. The glossopharyngeal nerve receives general sensory information from your tongue via the lingual branches which are the structures that you see now highlighted in green on this image. It's important to note that only the posterior third of the tongue is supplied by the glossopharyngeal nerve. Now, general sensation to the anterior two-thirds are then supplied by the mandibular division of the trigeminal nerve.

Next, we're going to look at sensations from the tonsils. The palatine tonsil will also send afferents to the glossopharyngeal nerve via the tonsillar branches which you see here in green. These branches form a part of a plexus of nerves with the contribution from the maxillary division of the trigeminal nerve and together these branches also provide sensory supply from areas of the soft palate and also the oropharyngeal isthmus.

Now as I mentioned on the previous slides, the next sensations that we're going to talk about are those that come from the pharynx carried out by pharyngeal branches which you can see now highlighted in green. The pharyngeal branches of the glossopharyngeal nerve supply the mucosa of the oropharynx and the laryngopharynx, however, much of the nasopharynx is supplied by the maxillary division of then the trigeminal nerve. The pharyngeal branches of the glossopharyngeal nerve will form the afferent limb of the gag reflex with the vagus nerve forming the efferent link causing the gagging movement of the pharyngeal muscles.

Next on our list, we're going to look at the sensations coming from the middle ear and the pharyngotympanic tube. Now parts of the middle are also supplied by the glossopharyngeal nerve which can seem strange to some people, however, as the glossopharyngeal nerve exits the your cranial cavity, there is a small branch which is called the tympanic branch which I am pointing at it right now with this arrow. This will then pass superiorly back into the petrous part of the temporal bone where the tympanic cavity is housed, which is this cavity which you now see highlighted from the section of the ear. This is the location for the conductive osseous apparatus of your middle ear.

Once in the middle ear, the tympanic branch ramifies – so it branches – on the tympanic membrane to form the tympanic plexus – this one that you see now highlighted on the image. This neural plexus sends general sensory information from your middle ear to the brainstem via the glossopharyngeal nerve. Also, there is a tubal branch – this structure that you see here highlighted in green. This tubal branch will be joining the tympanic plexus bringing with it sensory information from the pharyngotympanic tube which is this osseocartilaginous tube connecting the nasopharynx and the middle ear.

The final area of general sensation received by the glossopharyngeal nerve arises from the external ear. The innervation of the external ear is a bit complex as you probably know because it is the location where skin from developmental branchial skin meets the post branchial skin. In other words what I'm trying to say here is this is where developmentally different areas of skin come together or unite and bringing with them various nerves. However, the contribution of the glossopharyngeal nerve called the communicating branch of the vagus nerve seen here highlighted in green will piggyback on the auricular branch of the vagus nerve. Now this communication can explain why patients who experience tonsillar pain can also have refer to both external and middle ear from the tympanic branch.

All the branches that we have just mentioned come together and eventually pass to the brainstem through the superior and inferior glossopharyngeal ganglia which you can see here highlighted in green on this image. This is where cell bodies of the sensory neurons are then located. Once through the jugular foramen like you see here on the image, they enter the lateral aspect of the medulla oblongata. Pain and temperature fibers will synapse in the spinal nucleus of the trigeminal nerve seen on this green highlight while touch fibers will synapse in the main sensory nucleus of the trigeminal nerve which you also see here now highlighted in green.

Now if you remember from the list of functions we talked about before, the next function we will be talking about is special sensation. For special sensation, you only need to remember one – taste. The glossopharyngeal nerve is involved with sensation of taste. This sensation is picked up by taste buds specifically the posterior third of the tongue while the anterior two-thirds are supplied by the facial nerve.

The nerves involved in the transmission of taste through the glossopharyngeal nerve, we have already meet them – the lingual branches of the glossopharyngeal nerve – which you see now highlighted in green on this image. These axons pass through the superior and inferior ganglia which you see here on the image of the glossopharyngeal nerve then through the jugular foramen right here and into the medulla oblongata. And here, they synapse in the solitary nucleus – this structure that we have just highlighted here in green.

Now that we just finished talking about the taste sensation, let's move to the next function on our list – visceral sensation. The visceral sensory component of the glossopharyngeal nerve receives inputs from two structures that are associated with the bifurcation of the common carotid artery. Now, these specialized structures are the carotid body seen on the image highlighted in green and the carotid sinus. Now, these neural structures aid in the homeostatic mechanisms of the body.

Let's take a closer look at these two structures. the carotid sinus as you can see now is a small dilation of the lower end of the internal carotid artery and it is a specialized what we call baroreceptor which monitors blood pressure by sending signals to the central nervous system as the arterial wall stretched with varying blood pressures. While the carotid body is a small reddish structure located posteriorly at the bifurcation of the common carotid artery, its function is to transmit information on blood chemistry, therefore, is what we call a chemoreceptor. Axons from the carotid body and sinus will travel the carotid branch of the glossopharyngeal nerve and synapse in the solitary nucleus.

Now let's move on and next on our functions list, we have the parasympathetic function. The parasympathetic function of the glossopharyngeal nerve supplies secretomotor fibers to the parotid gland which is a very well-known gland. This is one of the major salivary glands located on each side of your face just anterior and inferior to the ears. The gland is then highlighted here in green on this image. The parasympathetic motor fibers originate from the inferior salivatory nucleus seen here in green from this dorsal view of the brainstem.

If we zoom in a bit on our image, we can see that these fibers travel out of the cranial cavity via the jugular foramen which is this circle right here. Now, they then pass through the superior and inferior glossopharyngeal nuclei then they hitch a ride with the tympanic branch of the tympanic plexus in the middle ear which is this structure that you see here highlighted in green.

Parasympathetic axons from the plexus are given off as the lesser petrosal nerve which passes from the middle ear and eventually passes down through the foramen ovale which you see here highlighted in green on this image of the superior view of the skull. And note that the lesser petrosal nerve passes through this foramen with the mandibular division of the trigeminal nerve which is this nerve right here. From here, the parasympathetics travel to a structure called the otic ganglion – this tiny structure highlighted in green – and then they synapse. Note that on the zoomed-in image of the glossopharyngeal nerve that this right here is the mandibular division of the trigeminal nerve. After that, they jumped onto the auriculotemporal nerve which is a branch from the mandibular division of the trigeminal nerve and this nerve passes close to the parotid gland, anatomically speaking, and we can see the parasympathetic fibers leaving to finally supply it.

Now, it's time to move on to the last function on our list which is somatic motor function. We'll start off with the easiest. The glossopharyngeal nerve only innervates one somatic muscle – the stylopharyngeus – via a muscular branch. This muscle is better seen from this posterior view of the pharyngeal muscles where you see the stylopharyngeus highlighted in green. These bilateral muscles descend from the styloid process of the temporal bone which you can see a little bit here to pass between the superior and middle pharyngeal constrictor muscles right here. The glossopharyngeal nerve stays close to this muscle as it passes towards the pharynx, tonsils and tongue.

I will quickly go back to this image of the cranial nerves' nuclei seen from the dorsal view of your brainstem where you can see that the axon supplying this muscle originate from the nucleus ambiguus in the medulla oblongata.

Now that we covered all the functions and relating anatomy of the glossopharyngeal nerve, I would like to briefly cover a couple of main clinical notes. A common test to check the integrity of the glossopharyngeal nerve is to touch the oropharynx or posterior aspect of the tongue with a tongue depressor. This will then initiate the gag reflex. The gag reflex has two components – an afferent limb and an efferent limb. The afferent limb or sensory limb is formed by the glossopharyngeal nerve which transmits the touch signals back to your brainstem where they synapse with axons on the vagus nerve to form then the efferent limb or motor limb which sends signals to the muscles of the pharynx which causes then a person to gag.

Before we finish, I would like to do a quick summary of what we have learned. We first went over the basic anatomy of the glossopharyngeal nerve then went over the details by splitting the tutorial into functions of the glossopharyngeal nerve and for each function, we looked at the anatomy of the glossopharyngeal nerve relating to that specific function.

We started with general sensation. We saw the lingual branches of the glossopharyngeal nerve which are responsible for the sensation of the tongue, the tonsillar branches of the glossopharyngeal nerve involved in the sensation of the tonsils, the pharyngeal branches of the glossopharyngeal nerve which are responsible for the afferent innervation of the pharynx, the tympanic branch seen here where the arrow is of the glossopharyngeal nerve which is involved in the innervation of the middle ear and pharyngotympanic tube. As part of that, we looked at the tympanic plexus and the tubal branch of the tympanic plexus.

Now as part of the sensation of the external ear, we saw the communicating branch of the vagus nerve. Then we looked at the structures involved in special sensation and saw the involvement and connection of the lingual branches of the glossopharyngeal nerve and their path to the solitary nucleus. We went on to talk about the visceral sensation where the glossopharyngeal nerve receives input from two structures – the carotid body and the carotid sinus.

Next, we looked at the parasympathetic functions where we saw that the nerve supplies secretomotor fibers to the parotid gland, and then going from the inferior salivatory nucleus to the tympanic plexus, the lesser petrosal nerve then through the foramen ovale, the otic ganglion and to the auriculotemporal nerve. Lastly, we looked at somatic motor function where the glossopharyngeal nerve innervates the stylopharyngeus muscle with axons coming from the nucleus ambiguus.

For the clinical notes relating to the glossopharyngeal nerve, we looked at the clinical testing of this nerve and essentially the two components of the gag reflex.

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