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Salivary glands

Salivary glands and surrounding structures.

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Transcript

Hey everyone! It's Nicole from Kenhub, and today we're going to be talking about the main salivary glands. And in this tutorial, we're going to be looking at this image of the right lateral view of the head with the skin cut away and over here, of course, we have the tongue highlighted in blue. Before we go on to talk about the different salivary glands, let's first define a salivary gland. And a salivary gland is the secretory gland that secretes tasteless, clear saliva. The saliva has many important functions such as keeping the mucous membrane in the mouth moist, lubrication of food, prevention of tooth decay and, of course, the salivary glands are also where the digestion of food starts.

So now that we know what the salivary glands are, let's talk a little bit about their divisions. And the salivary glands in our body can be divided into the major and the minor glands. But in this video, we're going to be talking about the major salivary glands which are the sublingual, submandibular and the parotid glands. And as we talked about these, we'll talk through the borders, the pathway of the ducts, the blood supply and the innervation of these glands.

So let's begin by talking about the borders of the major salivary glands. And there are several borders of the major salivary glands, the first being the floor of the mouth which is this collection of muscles just here in the blue triangle, the inferior surface of the tongue which is where the salivary glands will open into. So now that we've defined both of those, let's begin by looking at the floor of the mouth and what's located in this region.

So the floor of the mouth is mainly formed by the mylohyoid muscle, the geniohyoid muscle which is not illustrated in our image of the lateral base exposed but you can see it here on the skeleton and the mandible, and coming back to this image, you can see that we've dotted the geniohyoid out in blue. The floor of the mouth also has the digastric muscle and the stylohyoid muscle and it's in this region where you'll find the sublingual and the submandibular glands and their related structures. But, first, let's go through some of the muscles we just mentioned starting with the mylohyoid muscle which is highlighted in green in this image, and as you can see, the mylohyoid muscle makes up the majority of the floor of the mouth and supports the salivary glands.

The geniohyoid muscle, seen here in our image of the mandible and the hyoid bone, depresses the mandible and helps open the mouth and it also pulls the hyoid forward and up during swallowing – and you can see the hyoid bone highlighted in blue on our right. The long thin muscle that you can see below the mylohyoid muscle highlighted in green in our image is the digastric muscle. The digastric muscle is made up of two bellies – an anterior belly and a posterior belly. As you can see on the image, the two belies are interconnected by a tendon that is held at the level of the hyoid bone and this tendon is known as the connective tendon. Now the muscle that you can see surrounding the posterior belly of the digastric muscle is the stylohyoid muscle, and the stylohyoid muscle also makes up the floor of the mouth and inserts into the hyoid bone.

Let's now have a look at the inferior surface of the tongue where the salivary gland ducts will open into, and the inferior surface of the tongue rests on the floor of the floor of the mouth. As you can see on our image on the right, you can see the tongue which I'm pointing out with my blue arrow and the tongue is lifted so we can see the lower surface and the structures beneath it. The first one is the frenulum of the tongue which is highlighted in green and that's mucosal fold that extends to the gingiva or the gums that is seen at the midline on the lower surface of the tongue when you lift it up.

On the floor of the oral cavity, the mucosa contains a long narrow mucosal fold on either side known as the sublingual fold and this fold covers the sublingual gland which we'll talk a little bit more about a bit later. And near where the frenulum of the tongue starts on the floor of the oral cavity, there's a small bump known as the sublingual caruncle where the ducts of the large sublingual gland and the submandibular gland open together or near each other.

So now we've gone through the basic anatomy of the oral cavity and looked at its borders, we can start looking at the different salivary glands starting with the sublingual gland. And as you can see, the sublingual gland is an almond-shaped gland and it's also the smallest gland out of the major salivary glands. It has several ductal openings that run along the sublingual folds and also secretes the smallest portion of saliva every day. The sublingual glands also lie bilaterally on the floor of the mouth and as we mentioned before, the floor of the mouth is formed by the mylohyoid, the geniohyoid, the digastric and the stylohyoid muscles and let's just briefly go over them again. So, here we have the mylohyoid muscle, the geniohyoid muscle now dotted out in blue, the digastric muscle is just here and our stylohyoid is down here.

The sublingual gland is bordered by the mandible anteriorly which you can see highlighted in blue and by the genioglossus muscle posteroinferiorly – and you can see that highlighted in blue on the image on our right. And the tongue covers the gland superiorly and both the sublingual glands unite to form a horseshoe shape under the frenulum of the tongue which is seen in the midline under the tongue.

Let's talk a little bit now about the pathway of the sublingual duct, and the sublingual gland has many small ducts such as ducts of Rivinus and the duct of Bartholin that open along the margin of the sublingual folds. But what's important to know in this slide is that the duct of the major sublingual gland complex opens to the sublingual caruncle which you can see highlighted in blue which is the small elevated bump which we saw before and the numerous minor sublingual glands give short ducts that open along the sublingual fold and you can see these dotted out in blue as well.

As for the arterial blood supply of the sublingual glands, there are two separate arteries that supply it. And on the image you can see the external carotid artery which branches into the lingual artery and on this image, you can see the lingual artery giving off a branch called the sublingual artery which you can see here in the image traveling towards the sublingual gland. And you'll note that the artery is a little bit small in this image so we're just going to zoom in a little a bit and you can see the sublingual artery just here. And the second artery that supplies the sublingual gland is the submental artery which is a branch of the facial artery.

As for the venous drainage, the veins responsible for draining the sublingual gland are the veins with corresponding names of those with the arteries so the sublingual vein and the submental vein which drain into the lingual and facial veins. And in this image, we've highlighted the facial vein which as you can see drains into the common facial vein which itself drains into the internal jugular vein. And the arteries that align just anterior to the internal jugular vein is the carotid artery.

Moving onto the sublingual gland's innervation, the innervation of the sublingual gland is carried out by the chorda tympani nerve which contains fibers originating from the facial nerve which is cranial nerve seven and this nerve is important to know as it contains secretomotor fibers which are responsible for the nerve stimulus of saliva secretion. Also, the chorda tympani joins the lingual branch of the mandibular nerve and carries taste sensation from the anterior two-thirds of the tongue and parasympathetic ganglionic fibers to the submandibular ganglion for the innervation of the submandibular and the sublingual salivary glands. And the sympathetic fibers travel to it via the plexus along the lingual artery but don't worry if you didn't catch that, we'll go through that again.

Let's zoom in a little bit and as you can see, we have our chorda tympani and as we said before, the chorda tympani joins with the lingual nerve which is a branch of the mandibular nerve to carry taste fibers from the anterior two-thirds of the tongue and also the secretomotor fibers which you can see down here which carry innervation to the submandibular ganglion. And if you didn’t get that pathway just there, we're going to go over it again one more time a little bit later in the tutorial, so don't despair.

Let's move on now to the submandibular gland, and as you can see, the submandibular glands are located both superiorly and inferiorly to the inner aspect of the mandible in the submandibular triangle of the neck. And the submandibular triangle of the neck is sometimes called the digastric triangle. Let's talk through the borders of the submandibular triangle, and it is bordered superiorly by the inferior border of the mandible and the mastoid process of the skull which I've drawn for you as this upside down triangle just here on the right. It's bordered posteriorly by the posterior belly of the digastric muscle seen here on the image as well as the stylohyoid muscle which you can now see highlighted in blue. Anteriorly, it's bordered by the anterior belly of the digastric muscle which you can see just here and the floor of the submandibular triangle is made up of the mylohyoid muscle – this muscle that you can see on the image which is cut.

Let's talk now about the submandibular duct which is also known as Wharton's duct. And the submandibular duct is around about five centimeters long and arises between the mylohyoid and the hyoglossus muscle and so I've pointed out the mylohyoid again for you in blue and as the hyoglossus muscle is not in this image but I've dotted it out for you in blue. The duct then travels along the superior surface of the mylohyoid muscle seen here on the illustration and then passes forward medial to the sublingual gland to open on the sublingual caruncle, and the sublingual caruncle is situated on each side of the base of the lingual frenulum.

Let's now briefly talk about the arteries responsible for supplying the submandibular gland with arterial blood and the submandibular gland has the same arterial blood supply as the sublingual gland. So, if you remember from previous slides, the sublingual gland was supplied by the submental artery and the sublingual artery and so, of course, you can see them again in our image. And as we mentioned before, these two arteries are branches of the lingual artery which arises from the external carotid and the facial arteries. Similarly, the submandibular gland is mainly drained by the submental vein which anastomosis with the sublingual vein and the facial vein joins the submental and the sublingual veins which together drain into the internal jugular vein.

So as we mentioned before, the innervation of the submandibular and sublingual glands are the same, and if you remember from our previous slides, both are innervated by secretomotor fibers of the facial nerve which is cranial nerve seven. The parasympathetic innervation from the chorda tympani nerve from the facial nerve joins the lingual nerve in the same pathway and we're going to go through this pathway one more time just for your benefit. So, we have the chorda tympani and we have the lingual nerve, we have our secretomotor fibers coming off the lingual nerve just as before and, of course, we have the submandibular ganglion. But, in this image, I just wanted to draw in fibers from the submandibular ganglion to innervate the submandibular gland and here is some fibers innervating the sublingual gland.

The sympathetic innervation from the superior cervical ganglion accompanies the lingual artery to the submandibular tissue. I'm just going to explain this a little bit more because it's not entirely clear in this image. So, the superior cervical ganglion is located down below this image somewhere around C2 and C3 and it gives off fibers that run along with the lingual artery of the submandibular gland and the ganglion.

Now that we're finished talking about the submandibular and sublingual glands, let's move on now to the parotid gland. And the parotid gland, as you can see, is the largest of the major salivary glands. It's located in the front of the ear, the preauricular region, and, of course, the parotid gland has borders and these are, so the anterior border is the masseter muscle which is one of the muscles of mastication and you can see here highlighted in blue. It's also anteriorly bordered by the mandibular ramus which you can't see in this image but I've dotted out for you in blue. It's superiorly bordered by the zygomatic arch which again you can't see but I've dotted it out for you while posteriorly, the parotid is bordered by the mastoid process of the skull and the sternocleidomastoid muscle which is this big muscle you can see clearly here on the right neck, and you can also feel this muscle when you turn your head from side to side.

So the parotid gland has an irregular-shaped parotid bed or nest which can otherwise be described as the structure surrounding and touching the parotid gland, and these structures are mostly found behind the ramus of the mandible and in front of the styloid process and we're going to go through its borders as well. So, the parotid bed is bordered anteriorly by the masseter muscle, the ramus of the mandible and medial pterygoid muscle. Posteriorly, it's bordered by the sternocleidomastoid as well as the posterior belly of the digastric muscle. Medially, the parotid nest is bordered by the stylohyoid muscle which, of course, runs from the styloid process of the hyoid, the stylopharyngeus muscle which runs from the styloid process to the thyroid cartilage and the pharyngeal wall as well as the styloglossus muscle which runs from the styloid process of the hyoglossus muscle and the mandible and draws the tongue backwards. It's also bordered by the external carotid artery which divides into the superficial temporal artery that travels on the temporal muscle and the maxillary artery. And superiorly, the parotid bed is bordered by the zygomatic arch.

So we wanted to talk about the parotid nest because the parotid nest has quite a few important structures. And these structures include the parotideomasseteric fascia, the divisions of the facial nerve which you can see here in the image and exits the parotid nest, the auriculotemporal nerve from cranial nerve V3 found near the acoustic meatus. We also have the parotid lymph node, the retromandibular vein which has two divisions – a posterior division seen here and an anterior division seen here – as well as the external carotid artery also seen here on the image.

As we mentioned before, the facial nerve is one of the contents of the parotid nest and is in close contact with the parotid gland. The facial nerve runs through the parotid gland and comes out with five separate branches as seen on this picture, and this group of nerves is also called the parotid plexus from which the branches of the facial muscles arise. And we're going to go through these branches right now beginning with the temporal branches which innervate the muscles of facial expression above the eye and they're roughly in the area of the blue circle, the zygomatic branches which cross the upper cheek and supply the orbicularis oculi muscle which is the muscle that encircles the eye, the buccal branches that innervate the muscles responsible for facial expression below the orbit and above the chin, the marginal mandibular branches which are responsible for innervating the risorius muscle which is the muscle involved with smiling and the muscles of the lower lip and chin as well as the cervical branch which is the most inferior branch and it descends and innervates the platysma of the neck which you can see in the image on our right.

And to help you remember all the different branches of the facial nerve, we have this nifty little image to show you. So, a way to remember the order and the location of the nerves is to place an open hand on the side of the face and where your fingers fall the temporal branch is going to be where you fifth finger is, your fourth finger is going to be in the zygomatic area, the third finger is going to be in the buccal area, your second finger which will be on the chin area will be the marginal mandibular branch, and the first finger which is your thumb is where the cervical branch runs.

A couple more things I want to talk about in this area, the parotid gland is enclosed by a tough fascial capsule called the parotid fascia and it's part of the parotideomasseteric fascia surrounding the parotid gland and the masseter muscle and this sheath is part of the deep cervical fascia which splits into the superficial and deep layers to enclose the parotid gland and lies in front of the external acoustic meatus on the posterior part of the masseter muscle and covers the temporomandibular joint.

One more structure I want to look at in this region is the parotid duct and you can see it highlighted in green on our image on the right. Now, the parotid duct which is also called the parotid duct of Stensen is an important structure because it serves as the path that saliva takes from the parotid gland to the mouth. And as you can see, it passes horizontally across the masseter muscle and pierces the buccinator muscle – and you can see both of those highlighted in blue – to open up behind the second maxillary molar into the oral cavity. And the artery that you can see here above the parotid duct is the transverse facial artery which is a branch of the superficial temporal artery also seen here that runs across the face.

Of course, we want to talk about the supply of the parotid gland and the blood supply to the parotid gland comes from the branches of the external carotid artery which if you remember is this artery down here which are the superficial temporal, the maxillary and transverse facial arteries. The superficial temporal artery which you can see here on the image runs superiorly from the superior portion of the parotid gland to the tip, the maxillary artery exits at the medial portion of the parotid and supplies the infratemporal fossa and the pterygopalatine fossa, and the transverse facial artery branches off from the superficial temporal artery and runs up superior to the parotid duct as you can see on the image. And the transverse facial artery supplies the parotid glands, the parotid duct and the masseter muscle.

Let's, of course, have a look at the venous drainage of the parotid gland. And the parotid gland is drained mainly by the retromandibular vein which is formed by the maxillary vein and the superficial vein that come together and the retromandibular vein we've covered before but I'll go through one more time. So as you can see, the retromandibular vein bifurcates into anterior and posterior branches of the retromandibular vein. The parotid gland is a little bit different to the other two glands in that it's innervated by different sources. So let's talk through this.

The sensory innervation is provided by the auricotemporal nerve from the mandibular branch of the trigeminal nerve or cranial nerve five. The parasympathetic innervation which stimulates the saliva production is carried from the glossopharyngeal nerve which gives off the lesser petrosal nerve to synapse in the otic ganglion and you can see their pathway here highlighted. The parotid gland also receives sympathetic innervation from direct fibers of the external carotid plexus and you can see that scribbled out down here.

So now that we've gone through the major salivary glands, I want to go through some clinical notes, and the major syndrome I want to talk about is related to the parotid gland and that's know as Frey syndrome, and Frey syndrome is also known as the auriculotemporal syndrome. And Frey syndrome is a rare neurological disorder that results from damage to or near the parotid glands or from damage to the auriculotemporal nerve commonly from a surgery. So, if that happens, the patient will not be able to control sweating that occurs behind the cheek, the ears and the temporal region after eating certain foods that produce a strong salivary response. A lot rarer are tumors in the parotid gland that they can also develop although eighty percent of them are usually found to be benign. One more thing is a viral infection called mumps and this mostly affects children and is clinically presented with typical swelling of the parotid glands, fever and vomiting.

With the submandibular glands, there's a condition where a calcified mass forms and this is called sialolithiasis. Sialolithiasis is the most common disorder of the salivary glands. It usually presents as swelling and pain in the cheek when eating and treatment usually involves moist heat and NSAID use but it can also be aided by sucking on something sour like a lemon which can sometimes help pass the stone. But if the stone persists, surgery is usually required.

Of course now that we're finished the tutorial, I want to go over what we talked about today and, of course, we talked about the major salivary glands – the sublingual which is the smallest of the glands, the submandibular gland and the parotid gland which is the largest of the major salivary glands. We also talked about the borders of the oral cavity which included the floor of the mouth which holds the mylohyoid muscle, the geniohyoid muscle which you can see dotted in blue on our right here, the digastric muscle which has an anterior and a posterior belly and the stylohyoid muscle which surrounds the digastric muscle. The other border of the oral cavity and of the salivary glands is the inferior aspect of the tongue which includes the frenulum of the tongue which you can see when you lift up the tongue, the sublingual fold which is where ducts from the sublingual gland empty into as well as the sublingual caruncle which you can see highlighted in green.

And, of course, we went through the sublingual gland which in terms of its borders include the mandible, the genioglossus muscle and the tongue and in terms of its duct, the major pathway is through the sublingual caruncle where it's minor pathway includes the sublingual fold – again, you can see the little ducts opening out there. The sublingual gland is supplied by the sublingual artery and the submental artery, and its venous drainage is similarly drained by the sublingual vein and the submental vein. And we talked about how the sublingual gland is innervated by the chorda tympani.

For the submandibular gland, we talked about its location which is in the submandibular triangle and how the pathway of its duct which you can see highlighted in green runs towards the sublingual gland. And, of course, the arterial supply and the venous drainage and the innervation of the submandibular gland is the same as the sublingual gland.

For the parotid gland, we talked about the borders which include the masseter muscle, the mandibular ramus, the zygomatic arch, the mastoid process and the sternocleidomastoid muscle and in terms of its duct, we talked about how it traverse across the masseter muscle and the buccinator muscle to open out behind the second maxillary molar. It's arterial supply include the superficial temporal artery, the maxillary artery and the transverse facial artery which runs superior to the parotid gland, and the venous drainage includes the retromandibular vein which has an anterior and posterior part.

The parotid gland is innervated by the auriculotemporal nerve and the glossopharyngeal nerve as well as the direct fibers of the external carotid plexus. We also talked about the parotid nest whose borders include the masseter, the ramus of the mandible, the medial pterygoid muscle, the sternocleidomastoid muscle, the posterior belly of the digastric muscle, the stylohyoid, the stylopharyngeus, the styloglossus muscle, the external carotid artery and the zygomatic arch. We also talked about the contents of the parotid nest which include the parotideomasseteric fascia, the facial nerve, the auriculotemporal nerve, the parotid lymph node, the retromandibular vein and the external carotid artery.

In our clinical notes, we talked about the conditions of the parotid gland which included Frey syndrome which as you remember arises from damage to the auricotemporal nerve. We talked about tumors eighty percent of which are mostly benign and mumps which is a viral infection more common in children. We also talked about a condition of the submandibular gland which is probably the most common condition out of all of the conditions and that is sialolithiasis which is when a calcified mass forms within the submandibular duct causing pain and swelling in the cheek.

And that's all we have time for today. Thanks for watching!

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