What's up? Time Talks Med here. Let's continue our cranial nerve series.
Cranial nerves are 12 pair of nerves that exit the brain and the brainstem. And in this segment, we will talk detailed about the 9th cranial nerve, the glossopharyngeal nerve. And we will do that by first making a quick scheme of the glossopharyngeal nerve pathway.
Then we will break down the functional components of this nuclei, along with its respective nuclei in the medulla. We will go through the course and distribution. by going through its intracranial course first, then the extracranial course and its branches.
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Alright, let's continue with the video. Let's start by making a little scheme, and then we'll start talking about it in detail. Now, The glossopharyngeal nerve plays a crucial role in transmitting sensory information from the back of the throat, taste sensation and saliva production, and it's involved in swallowing and speaking.
This nerve has altogether four nuclei that are located within the medulla oblongata. We got a nucleus ambiguus, which is a motor neuron, inferior salivatory nucleus, which is parasympathetic, Nuclei of the solitary tract, which is functionally divided into a viscero-sensory part and a gustatory part, for taste. And we got the spinal nucleus of the trigeminal nerve, which is somatosensory. Now what happens is, nerves are going to go in and out through the medulla, on the lateral aspect of the olive, called the retro-olivary groove.
So it has a motor component, parasympathetic component, and the sensory components. The motor part will go through the jugular foramen, through the superior and inferior ganglion of the glossopharyngeal nerve. Then it's going to go innervate the stylopharyngeal muscle. Then we got a parasympathetic component that takes a little bit of a complicated route.
It's going to go through the jugular foramen, then give off the tympanic nerve. The tympanic nerve is going to go through the tympanic catilicholus, and then enter the tympanic cavity. Within the tympanic cavity, it's going to form the neural tympanic plexus.
Now, this plexus is formed by two other nerves as well, one being the sympathetic carotid-tympanic nerve that extend from the internal carotid plexus, and a small branch from the facial nerve called the communicant branch of the tympanic plexus. But from the tympanic plexus, there is a parasympathetic branch that's going to go out called the lesser parasympathetic branch. petrosal nerve.
It goes through the canal of the lesser petrosal nerve and leaves the cranium as it passes through the sphenopetrosal fissure, or the foramen ovale, or the petrosal foramen. There are variations to this nerve on how it exits the skull, but primarily, I think most sources say the foramen ovale. But what you need to know is that this nerve will provide preganglionic parasympathetic innervation to the aortic ganglion, which then form postganglionic parasympathetic fibers that run together with the auricotemporal nerve of the trigeminal nerve to innervate the parotid gland and make it salivate.
Let's continue to the sensory ones. There are fibers that come from the carotid body and sinus, from the body receptors and chemoreceptors there. They're going to take these impulses towards the inferior ganglion of the glossopharyngeal nerve and then go to the nucleus of the solitary tract. Then there are some special sensory fibers for the sense of taste from the posterior 1 third of the tongue.
These fibers are going to take fibers towards the inferior ganglion, and then go further and synapse with the gustation part of the nucleus of the solitary tract. The last fibers, which I've highlighted here in green, are somatic sensory fibers, innervating the mucosal membranes along with different regions. There are some general sensory fibers coming from the posterior 1 third of the tongue, and taking these impulses towards the cell bodies of the superior ganglion of the glossopharyngeal nerve.
Then fibers are going to continue off towards the spinal nucleus of the trigeminal nerve. So we got two fibers now from the posterior 1 third of the tongue. One part for taste, and one part for general sensory, like touch and pain and temperature.
Then there's a branch coming in along the tympanic nerve from the mucous membranes of the middle ear, tympanic membranes, and part of the external ear. Then there's a large area in the pharyngeal region that is supplied by this nerve, through the pharyngeal plexus. This plexus is formed from a union of branches from the vagus nerve, laryngeopharyngeal nerves from the cervical sympathetic plexus, and the glossopharyngeal nerve. So, fibers from the glossopharyngeal nerve will also take fibers towards the superior ganglion and then to the spinal nucleus of the trigeminal nerve. And, And there are fibers coming from the mucous membranes of the palatine tonsils as well.
So this is the general scheme of this nerve. It's not really that complicated. But one thing I want you to understand before we talk about this one in detail, and you probably already see it from the color coding here.
But we generally divide this nerve into functionally different components, and it becomes easier to remember it like this. So the glossopharyngeal nerve is a mixed nerve. It has a motor component, sensory component, and a parasympathetic component. The motor component innervates the stylopharyngeal muscle, and the parasympathetic fibers go towards the parotid gland for saliva production.
The sensory is subdivided into visceral part and somatic part. The visceral part is further subdivided into two parts, general visceral sensory fibers from the carotid bodies, and special visceral for the sense of taste from the posterior 1 third of the tongue. The somatic sensory fibers are going to come in from all the different regions within the pharyngeal and middle ear area. So this is how it looks like.
Special visceral efferent fibers are motor fibers that go to the stylopharyngeal muscle, with its central nucleus being the nucleus ambiguous. General visceral efferent fibers being parasympathetic fibers, coming from the inferior salivatory nucleus, going out as the tympanic nerve, forming the tympanic plexus, then through the lesser petrosal nerve, providing preganglionic parasympathetic fibers to the otic ganglion, which then sends fibers further to the parotid gland. There are general visceral afferent fibers from the carotid sinus and body, taking information towards the inferior ganglion of the glossopharyngeal nerve, then to the nucleus of the solitary tract.
There are special visceral afferent fibers, for the sense of taste from the posterior 1 third of the tongue, going towards the inferior ganglion, then towards the nucleus of the solitary tract. And there are general somatic afferent fibers, that takes in general sensory information from the pharynx, tympanic membranes and the posterior 1 third of the tongue, towards the superior ganglion of the glossopharyngeal nerve, then towards the spinal nucleus of the trigeminal nerve. Alright, knowing all that, let us dive into the actual pathway in a visual setting, starting with the nuclei.
So here you see the spinal cord, medulla oblongata, cerebellum, pons, mesencephalon, and the diencephalon. If you now focus on the lateral aspect of the brainstem, you will see this. So again, we got the nucleus ambiguous, giving off the special visceral efferent fibers, so motor fibers.
There are the nuclei of the solitary tract, receiving both general visceral afferent fibers from the carotid sinus and the carotid body, as well as special visceral afferent fibers from the taste sensation of the tongue. There are the inferior salivatory nucleus, giving off general visceral afferent fibers, so parasympathetic innervation of the parotid gland. And there's the spinal nucleus of the trigeminal nerve.
getting general somatic afferent fibers. They're all going to go out from the medulla oblongata, from the posterior lateral part of the medullary olive. From here, we can divide its cores into intracranial cores and extracranial cores, and the dividing line between them would be the jugular foramen, from where this nerve exits the cranium.
There is really not much to talk about when it comes to the intracranial cores luckily, since it doesn't branch off at this point. All it does is emerge from the medulla, a series of rootlets between the olive and inferior cerebellar peduncle. It then goes along the cranial fossa and exits through the jugular foramen. Let us now look at this image from this perspective.
We'll see this. After it exits the jugular foramen, you'll immediately see two ganglia. If you zoom in, you'll see them better. These are called the superior ganglion of the glossopharyngeal nerve.
and the inferior ganglion of the glossopharyngeal nerve. Or you can also call it the petrous ganglion. And the reason why I'm being so specific here is because the vagus nerve also has a superior ganglion and an inferior ganglion. So I want to be very very specific in saying the superior and the inferior ganglion of the glossopharyngeal nerve. Alright, let us now go through some side branches.
And the first one that I want to talk about here is called the tympanic nerve. And again, this nerve contains the parasympathetic branch of the glossopharyngeal nerve, as well as some general somatic afferent fibers. It's going to ascend upward through the tympanic canaliculus and enter the tympanic cavity.
When it reaches the tympanic cavity, the tympanic nerve forms the tympanic plexus, along with two other nerves. The two other nerves that help form this plexus are the sympathetic carotid-tympanic nerve from the internal carotid plexus, and a branch from the facial nerve, called the communicant branch. The tympanic plexus then sends branches that will innervate the area of the middle ear, tympanic membrane, and some parts of the external ear, and send these general somatic afferent fibers towards the superior ganglion of the glossopharyngeal nerve, and then towards the spinal nucleus of the trigeminal nerve.
From the superior part of the plexus, the lesser petrosal nerve is going to come out. It then extends upward, passing through the canal of the lesser petrosal nerve, to reach the anterior side of the temporal pyramid. The nerve leaves the cranium as it passes through the foramen oval, or some variations might say the sphenopetrosal fissure or the petrosal foramen.
But it finally ends in the otic ganglion. This is important because the lesser petrosal nerve carries the secretory parasympathetic fibers that synapses with the cells in the otic ganglion. In that way, postganglionic fibers join the auricotemporal nerve from the trigeminal nerve to reach the parotid gland and supply it with the parasympathetic innervation that is necessary for proper salivation. So that is this nerve.
Next, we got the carotid sinus nerve. This nerve consists of general visceral afferent fibers that descend along the body of the internal carotid artery, aiming towards the bifurcation of the common carotid artery. And it ends by sending terminal branches to the carotid sinus and the carotid body.
So what it does is that it transmits sensory information from the chemoreceptors in the carotid body, helping to regulate the blood oxygen and carbon dioxide levels. and sensory information from the body receptors located within the carotid sinus, aiding in regulating the blood pressure. Those fibers will take signals towards the inferior ganglion of the glossopharyngeal nerve, and then send the fibers further towards the nuclei of the solitary tract. Then we got the lingual branches, which are terminal branches of the glossopharyngeal nerve. It's going to go towards the posterior 1 third of the tongue, and provide two types of fibers.
What were those two fibers? The lingual branches contain special visceral afferent fibers that provide a sense of taste from the valet papilla, taking the sensations toward the inferior ganglion and then towards the nucleus of the solitary tract. It also contains some general somatic afferent fibers, providing general sensory information from the posterior one-third of the tongue as well, Taking those impulses towards the superior ganglion and then towards the spinal nucleus of the trigeminal nerve So that is that one. Next we got the stylo pharyngeal nerve This is a motor branch or special visceral efferent fibers Providing motor innervation for the stylo pharyngeal muscle. It goes down and reaches the muscle from its lateral surface as you see here This muscle specifically helps with basically elevating the pharynx during swallowing and speech.
It's a small but a very important muscle for swallowing and speech. Alright, then we got the pharyngeal branches, which remember are general somatic afferent fibers that take in general somatic information from several regions in the pharynx towards the superior ganglion of the glossopharyngeal nerve. and then further towards the spinal nucleus of the trigeminal nerve. Now, what happens is, three nerves go towards the pharynx, and then go together to form a plexus, called the pharyngeal plexus.
Those nerves are branches from the vagus nerve, the laryngeo-pharyngeal nerve from the cervical sympathetic plexus, as well as the pharyngeal branches from the glossopharyngeal nerve. The sensory component belongs to the glossopharyngeal nerve, but But this plexus do provide some motor innervation for the pharyngeal muscles as well, through the vagus nerve. Alright, so that was everything I had for the glossopharyngeal nerve.
Here again you see the scheme for the glossopharyngeal nerve. It's just schematic, not exactly anatomically correct, but I hope it will help you gain a good understanding of this nerve. So, we now covered the glossopharyngeal nerve.
The next video is going to be about the 10th cranial nerve, the vagus nerve. Thank you so much for watching another one of my videos. If you enjoyed, learned something from it, please remember to like, comment your favorite moment, subscribe, turn on those notifications.
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