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Origins, insertions, innervation and functions of the muscles of mastication.
Hello everyone! This is Joao from Kenhub, and welcome to another tutorial where, this time, we’re going to be covering the muscles of mastication.
Now, apart from your teeth and also your tongue, certain muscles known as the masticatory muscles or, like you saw on the title, the muscles of mastication, they take part in specific movements of this joint right about here, the temporomandibular joint, during the process of mastication which, then, allows the initial stages of your digestion to occur.
Now, what is this strange word, mastication? Now, mastication is the process of grinding and chewing food into smaller pieces in your oral cavity, turning it into the… what’s known as a food bolus. This mass can then be swallowed really easily and then further digested as it goes through your digestive tract.
Now, let’s move on and talk about, then, the list of muscles that we’re going to be covering here on this tutorial. And the muscles of mastication are exactly a group of four muscles. This includes the masseter, the temporalis, lateral pterygoid, and the medial pterygoid. What we’re going to be doing is covering the origin, insertion, and also the innervations and functions of these muscles.
Now, let’s start with the very first one here on our list, the masseter muscle, seen here, highlighted in green. Now, the masseter is the most superficial muscle of all muscles of mastication. It is a thick, rectangular muscle consisting of two parts specifically, and both parts originate from right here, as you can see, this portion of the skull known as the zygomatic arch, and this will serve as the origin point for the masseter muscle.
Now, as I mention, this muscle is divided into two parts. The first one is known as the superficial part. And keep in mind that, right now, we’re looking at the inferior view of the skull where you can see here the masseter muscle, more specifically, the superficial part of the masseter muscle. And on your right side, now, I’m showing you the deep part of this muscle.
Now, let’s look at the superficial part of the masseter muscle also highlighted here in green. And now, we’re looking at the lateral view of the skull. And the superficial part of the masseter, as mentioned in the previous slide, originates from the anterior two-thirds of the zygomatic arch.
And in the same way as the superficial part, the deep part that is seen now, highlighted in green, originates from the posterior one-third of the zygomatic arch that runs further dorsally to the outer surface of the mandibular ramus. At this point, the muscle can be palpated easily from the oral cavity along your cheek.
Now that we discussed the origin points for the superficial and deep parts of the masseter, it is time for us to move on, then, to the insertion point for this muscle. Now, both parts will have a common insertion point because the masseter promptly terminates at the mandibular angle and the inferior and lateral borders of the mandibular ramus in a bony elevation that is known as the masseteric tuberosity, and it’s just about here. So this is going to serve as the insertion point for this muscle specifically.
And now, let’s move on to the innervations of the masseter. Now, this muscle is going to be supplied or innervated by the masseteric branch that is a branch coming from the mandibular nerve.
The last part for the masseter is going to be the functions or actions that are related to this muscle—so the actions that this muscle is going to perform within the different joints and bones of the skull. Now, the first one is elevation, as you can see here, and the masseter functions to elevate the mandible, causing a powerful closure of the jaw. The other one is going to be protrusion of the mandible, which is the contraction of the superior part, which protrudes the mandible forward as you see here on this arrow. The masseter also acts to help stabilize tension of the auricular capsule of the temporomandibular joint, this joint right about here that I had been pointing out on this tutorial.
And if you remember well from the list that we discussed in the beginning, the next muscle is seen here, highlighted in green. This is the temporalis. Now, the temporalis is a flat, fan-shaped muscle of mastication that you find on the lateral side of your skull and covers a large part of the temporal bone—hence the name. Now, due to its size, it can be palpated without any difficulty, especially when you have or you open and close your mouth alternately.
Now, the first thing that I want to talk about the temporalis would be its origin point, and the origin point for this muscle will be the temporal fossa and also the temporal fascia which completely covers this muscle, but you cannot see clearly here on this image. But you can also consider the temporal fascia as another point of origin that… for this muscle specifically.
Now that the origin point for the temporalis is covered, it is time to logically move to the insertion point for this muscle, and from the temporal fossa that we talked about, the origin point, this muscle will descend through the gap between the zygomatic arch and the skull forming the thick tendon and inserts then at the coronoid process of the mandible. As you can see clearly on this image, you notice here that the muscle is going through the zygomatic arch, this gap between the zygomatic arch and the rest of the skull, and then it will insert right about here on the coronoid process of the mandibulum or the mandible.
Now, let’s move on and talk about the innervations of the temporalis. The temporalis is going to be innervated by branches of this nerve here, the mandibular nerve as well, known as the deep temporal nerves.
In terms of functions, we’re going to be talking about the functions of the temporalis. The temporalis is the most powerful muscle of the temporomandibular joint. This muscle has fibers which run in different directions and work together to elevate and move the mandible dorsocranially, facilitating strong closure of your jaw.
Now, the posterior of the temporalis muscle fibers run almost horizontally and pull the mandible backwards, which then allows you to retract the mandible. Now, this then, this portion of the temporalis muscle, will function to retract the mandible as you can see also here, represented on this arrow.
Now, the muscle also helps with lateral excursion of the mandible, lateral excursion is sideward movement of the mandible between the position of closure and the position in which cusps of opposing teeth are in vertical proximity. So basically, in other words, lateral excursion means when you move your jaw, your lower jaw, into the different sides: right and left.
Now, let’s move on to the third muscle on our list, this one seen highlighted in green. Now, if we were to remove right here, we just cut the zygomatic arch, and now, we’re exposing entirely this muscle that is known as the lateral pterygoid. The lateral pterygoid muscle lies superior to this muscle right about here that we’re going to talk about next that is known as the medial pterygoid. It is comprised also of two heads which lie almost horizontal or horizontally to each other. And these heads can be seen on the next slide.
The first one is known as the superior head, which is a bit smaller than the other one that is known as the inferior head. So now, we’re also looking at the inferior view of the mandible, where we can look at the superior and inferior heads of the lateral pterygoid.
Let’s start talking about the superior head of the lateral pterygoid about its origin point. Now, the origin for the superior head of the lateral pterygoid is going to be the infratemporal crest of the greater wing of the sphenoid. The inferior head of the lateral pterygoid muscle, as seen now, highlighted in green, is then going to originate from the lateral surface of the lateral plate of the pterygoid process. As you can clearly also see here on this image, this is the origin point for the inferior head of the lateral pterygoid.
Now, let’s move on to, then, the insertion point for the superior head of the lateral pterygoid, and this will be, then, the temporomandibular joint, which you can see clearly here that will serve as the insertion point for this portion of the muscle and also, or more specifically in terms of the points of the temporomandibular joint that will serve as insertion or attachment points for the superior head or going to be specifically the capsule and the disk of this joint.
Let’s move on and talk about the inferior head of the lateral pterygoid also, now, highlighted in green. The insertion point for this portion of the muscle is going to be, then, the condylar process of the mandible as you can also see clearly, here, on this image right about here. This is the insertion point for the lateral pterygoid, the inferior head of the lateral pterygoid.
We covered all the attachment points for the lateral pterygoid muscle. It is time for us to cover its innervations, and all you need to know is that this muscle is going to be innervated by what is known as the lateral pterygoid nerves which are also branches of the mandibular nerve.
Knowing that, we’re going to move on and talk about the functions and the actions that are related to the lateral pterygoid, and this includes depression of your chin as you can see here on the image. Another one will be protrusion of the mandible, and the other one is going to be lateral movement—so the lateral pterygoid is able to produce lateral movements on the lower jaw or movement side to side when you move your jaw side to side as I mentioned before.
Now that we talked about the lateral pterygoid, it would be… would make sense that one that is known as the medial pterygoid, one muscle known as the medial pterygoid existed and it does. So the medial pterygoid muscle, along with the masseter, forms what is known to be as a muscular sling, and I wanted to show you here on this image on the lower portion or the inferior portion of the jaw as I showed you before. You can clearly see here the medial pterygoid. You also see here the masseter, and it kind of forms like a sling as you can see, a muscular sling as you can see, a muscular sling that encompasses the mandible. And just a side note, you can see here the lateral pterygoid that we talked about before.
Now, moving on from this image here, I want to go back to the image that I showed you of the medial pterygoid. So I can add here that this muscle also has two heads that you need to remember, and these two heads, as you probably guessed, is going to be a deep head, as you can see here on the image, clearly highlighted in green, and then if you have a deep head, you’re going to have a superficial head that is also highlighted in green.
And this might sound confusing because here, on the left side, you’re looking at the deep head, and it looks more superficial, but keep in mind that you’re looking at the inferior view of the jaw, which lets us understand that the deep head should be found more inferiorly than the superior head. And this is clearly seen here on these two images. So the superficial head is clearly found superior to the deep head, and that’s why we call it the superficial head of the medial pterygoid.
Let’s start with the origin points for the medial pterygoid, starting off with the deep head. And the origin point for this portion of this muscle is going to be, then, the surface of the lateral plate of the pterygoid process, and also the pterygoid fossa is going to serve as an origin point for the deep head of the medial pterygoid.
Now, we’re going to have to look at the superficial head of the medial pterygoid, and the superficial head, in terms of origin points, is going to have one that you need to remember, and this is going to be the maxillary tuberosity that will serve as the origin point for the superficial head.
We just completed the origin points of the medial pterygoid muscle. It is time for us to move on, then, to the insertion point, and the pterygoid tuberosity found on the medial surface of the ramus of the mandible will serve as insertion point for this muscle specifically.
Now, it is time for us to continue on to the innervations of the medial pterygoid, and this muscle is going to be, then, innervated by the medial pterygoid nerve, which is also a nerve of the mandibular nerve.
Let’s move on and talk about the functions and actions associated to the medial pterygoid, and the medial pterygoid can be palpated medially to the ramus of the mandible, both intraorally and also extraorally. Now, this muscle is able to elevate the mandible, as you can see here on the arrow, also protrusion of the mandible, and finally, lateral excursion or movement side to side of your lower jaw.
We are done talking about all the four muscles of mastication, but before we go, I wanted to add just a quick note here on this structure that you see highlighted in green. And if you know this well, this is known as the parotid gland. The parotid gland lies on the lateral side of the masseter with its duct coursing underneath the zygomatic arch across the muscle. So I wanted to just highlight this important structure that has some sort of connection here with these muscles—at least topographically speaking, and I wanted to add here on this tutorial. But this is it for the muscles of mastication.