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Muscles of Mastication

Origins, insertions, innervation and functions of the muscles of mastication.

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Transcript

Chewing – something that we do every day without much thought. It’s easy and effortless. Surely, it can't require that much coordination? Wrong.

Movements of the jaw or, to use its proper name, the mandible, need to be dynamic enough to contribute towards speech, be strong enough to bite through a chunky carrot, and also be able to move with the endurance required to chew a piece of gum through that three-hour exam. So, what allows the mandible to meet all of these demands? The answer to this is found in this tutorial on the muscles of mastication.

During this tutorial, we'll briefly discuss the anatomy of the mandible focusing specifically on its bony landmarks relevant to the muscles of mastication. We’ll then learn all about the temporomandibular joint, which is formed between the mandible and the skull such as its joint morphology and range of movement. Once we know what is being moved and where, we'll get our teeth into the anatomy and function of the four muscles of mastication, which are the medial pterygoid muscle, the lateral pterygoid muscle, the masseter muscle, and finally, the temporalis muscle. Once we've chewed our way through the anatomy, we'll have a clinical note for dessert.

But before we tuck in, let's just get familiar with why these muscles exist. They exist because of mastication, which is the process of grinding and chewing food into smaller pieces in the oral cavity, transforming it into a lovely congealed mass or, if we're going to use the proper term, a food bolus. So why do we need this bolus?

Well, two reasons. Firstly, try swallowing a sprout. It’s just not gonna happen unless a bolus is made which can be swallowed with ease. Secondly, mastication is the first step of digestion and increases the surface area for digestive enzymes to act upon. Let’s now have a look at some of the key components of mastication.

So in case you're wondering why we're talking about a bone in a muscle video, the mandible serves as the insertion point for each of our four muscles of mastication, so it’s handy to get familiar with its basic anatomy. If we disarticulate the mandible from the skull – as we are doing right now – we can see that the mandible can be separated into three parts – the body, the two rami on either side, and the angle of the mandible which joins the rami to the body.

The rami have a very distinct shape mostly due to two processes which project superiorly. The most anterior is the coronoid process and the most posterior is called the condylar process. The condylar process has a slender stalk called the neck of the mandible which flares outward superiorly to create a long protuberance at the top called the head of the mandible. The head is covered with articular cartilage and this design is functionally important as the head of the mandible forms the lower articulating surface of the temporomandibular joint.

Here’s our mandibular head and just look at how perfectly it fits into the mandibular fossa of the temporal bone. That’s actually where the temporomandibular joint gets its name from because it's the articulation between the temporal and the mandibular bones.

So this joint is a little bit special as it doesn't really fit into a typical synovial joint cavity such as pivot or ball-and-socket joint. So what kind of joint is it then? Well, if we zoom out a little bit to this lateral view of the skull and take the longitudinal section of the joint, we can see that it's what we call a modified hinge-type joint. So a hinge joint usually only moves in one axis to flex and extend such as your elbow and knee joints. So, therefore, what is so special about a modified hinge joint?

Well, it has a joint capsule which is divided into two compartments by an articular disk and we can see that just here. The superior compartment is responsible for translational movement which basically means side-to-side. The inferior compartment allows for pivoting movement so when you want to open and close your mouth. These compartments allow the mandible to have a wide range of movement without being a ball and socket which would compromise stability. And just before we move on to the main course, if you want more information on the mandible and the temporomandibular joint, don't forget to check out our separate articles on them.

So as we mentioned earlier, the four bilateral muscles which make up the muscles of mastication are the medial pterygoid muscle, the lateral pterygoid muscle, the masseter muscle, and the temporalis muscle, and we'll start with the most superficial of these – the masseter.

The masseter is the strongest muscle of mastication and is roughly rectangular in shape. It is covered with a thin, but very strong masseteric fascia which acts to compartmentalize it from surrounding structures. One example of a surrounding structure is the parotid gland and duct which courses anterolaterally across its surface, and we can see the gland here in this illustration. The masseter muscle itself is separated into two layers known as the superficial and deep layers.

We’ll first look at the superficial layer highlighted here in green, and this layer specifically originates from the maxillary process of the zygomatic bone and the inferior border of the anterior two-thirds of the zygomatic arch. We can see that the muscle runs inferiorly down to its insertion on the lateral or external surface of the ramus and the angle of the mandible.

Staying within this same lateral view if we now highlight the deeper layer of the masseter muscle, we can see that this layer originates from the deep or the inferior surface of the posterior third zygomatic arch. Now, don't let the viewing angle fool you. The deep layer is actually much larger than the superficial and is also more muscular. Its fibers run inferiorly to insert along the lateral surface of the mandibular ramus as high up as the coronoid process. And you'll be pleased to hear that both layers of the masseter have the same innervation and blood supply. The masseter is innervated by the masseteric nerve which is derived from the anterior division of the mandibular nerve and blood comes from the masseteric artery which is a branch of the maxillary artery.

Oh and by the way, the maxillary artery and the mandibular nerve are going to come up a lot because they supply all of these branches to the muscles of mastication.

So both layers of the masseter contract together to elevate the mandible so you can close your mouth and occlude your teeth during mastication to crush up food, and this function contributes to prehension which is the act of grabbing food or taking a bite out of something. Contraction of just the superficial layer assists in protrusion of the mandible which means you can push out your chin.

So let's move on now to the temporalis, which is a flat fan-shaped muscle located on the lateral side of the skull, and it covers a large part of the temporal bone from which it gets its name. Because of its size, it can be easily palpated by opening and closing the mouth. Can you feel it?

The temporalis muscle arises from the temporal fossa up to the inferior temporal line and the temporal fascia which completely covers the muscle. From here, the temporalis descends through the gap between the zygomatic arch and the skull forming a thick tendon which inserts at the apex and medial surface of the coronoid process of the mandible.

The temporalis is innervated by both the anterior and posterior deep temporal branches of the mandibular nerve which we can see here, and this muscle gets its blood supply from the anterior and posterior deep temporal arteries which branch off from the maxillary artery.

The temporalis can be divided into two functional parts by the direction of its fibers. So we have the anterior portion which has fibers which run vertically and whose contraction results in the elevation of the mandible resulting in occlusion of the teeth for prehension, and it also has a posterior portion which has fibers which run almost horizontally and will assist in elevation of the mandible, but importantly also act to pull the mandible backwards in retraction. And it's quite special as it's the only muscle which is able to perform retraction.

Now to see the next muscles, we're going to need to cut the zygomatic arch here and also over here, and if we remove this little bit of bone and the temporalis, we can see deeper into the skull and reveal the pterygoid muscles.

So we're going to start with the lateral pterygoid muscle which is highlighted here in green and it lies superior to the medial pterygoid muscle and is comprised of two heads which lie almost horizontally to each other to create a triangular shape. There’s a smaller superior head and a much larger inferior head, so let's have a look at these in a little bit more detail.

So in this slide, just the superior head has been highlighted and it originates from the infratemporal crest of the greater wing of the sphenoid bone and its fibers travel posteriorly to insert into the joint capsule of the temporomandibular joint.

Let’s now have a look at the larger inferior head, and it originates from the lateral surface of the lateral pterygoid plate of the sphenoid bone which we can see here. I'm sorry there's a lot of laterals in there, but one plus of this is that if you can remember the name, you'll remember exactly where the muscle comes from.

So unlike the superior head, this inferior head has fibers which insert directly onto the mandible – at the pterygoid fovea to be precise – which is on the neck of the condyloid process. Both parts of the lateral pterygoid are innervated by the lateral pterygoid nerves which are again branches of the mandibular nerve and blood is delivered by the pterygoid branch of the maxillary artery.

So what does this muscle do? Don’t let its relatively small size deceive you, this muscle has a lot of functions. The horizontally-oriented fibers of the lateral pterygoid work to pull the head of the condylar process anteriorly and inferiorly. So with bilateral contraction, our lateral pterygoids protrude our mandible and our chin will move forwards and slightly downwards which we can describe as protrusion and depression of the mandible, and this helps to initiate the opening of the mouth. It also should be noted that once opened by the lateral pterygoids, the main depressors of the mandible are the digastric and geniohyoid muscles.

Remember we said that the superior head inserts directly into the articular capsule? Well this means that when the superior head contracts, it will pull the articular disc anteriorly. This ensures that the articular disc doesn't get left behind during protrusion and maintains contact with both the condylar head and the mandibular fossa. The superior head also helps to stabilize the condylar head of the mandible in the temporomandibular joint during retraction and elevation by the temporalis muscle, and this helps to ensure the integrity of the joint and to prevent dislocation.

Okay so I did warn you that this muscle has a lot of functions and we're not finished just yet. So the lateral pterygoids can also cause side-to-side movement of the mandible if contracting unilaterally. Are you confused? Well, let's get a posterior view of the skull up to help us out. When one of the lateral pterygoid muscles contracts in isolation, it causes the mandibular condyle on the same side to rotate medially causing medial movement of the mandible and this is really important in trituration or grinding of food.

The lateral pterygoid isn't the sole lateral mover and is assisted by our next muscle – the medial pterygoid muscle. So along with the masseter, this muscle contributes to the formation of a muscular sling which encompasses the mandible. In fact, it's quite comparable to the masseter because of its similar function and its quadrangular shape, and also just like the masseter, it has a superficial part and a deep part. So let's discuss the superficial one first.

So the superficial part of the medial pterygoid originates from the tuberosity of the maxilla which is located just beneath behind your wisdom teeth – or where they used to be if you had them removed – and it attaches to the pyramidal process of the palatine bone, and from here, it runs inferiorly to insert onto the medial surface of the ramus and the angle of the mandible.

The deep head of the medial pterygoid arises from the medial surface of the lateral plate of the pterygoid process of the sphenoid bone – I know that's a long one and all of these medials and laterals are definitely getting a bit confusing. Perhaps it's better if we take a moment before we discuss the insertion to have a closer look at the pterygoid process.

So here it is – the sphenoid bone. It’s pretty funky-looking, don't you think? So in this posterior view, we have highlighted the bilateral pterygoid processes and each process has a medial and a lateral plate and each plate has medial and lateral surfaces. So where do our pterygoid muscles fit in? Well, they both attach to this – to this lateral plate over here. So if we now put the bone back into the skull and add some musculature, we can see that the inferior head of the lateral pterygoid muscle originates from the lateral surface of the lateral plate and the deep head of the medial pterygoid originates from the medial surface. Does that clear things up? Awesome! So let's return from our little detour and talk insertions.

Luckily for us, both heads of the medial pterygoid insert into the same place- the medial surface of the angle of the mandible. Innervation for this muscle is from the medial pterygoid nerve which is – you guessed it – also a branch of the mandibular nerve. And for once to make things even easier, the blood supply is from the pterygoid branch of the maxillary artery just like we saw with the lateral pterygoid muscle.

Although not as strong as the masseter, the bilateral contraction of the medial pterygoids will assist in the elevation of the mandible so you can occlude your teeth and take a bite out of something. Remember that this is called prehension. The medial pterygoids will also assist the lateral pterygoids in protrusion of the mandible if contracting bilaterally or it can result in side-to-side movements if contracting unilaterally so you can grind your food up in trituration.

Whew! So that's the muscles covered. Now let's talk clinical.

Temporomandibular joint syndrome, also known as temporomandibular joint disorder or dysfunction, is a term used to describe pain and dysfunction in the muscles of mastication and the temporomandibular joints. The syndrome is known to be caused by multiple factors, but it's still pretty poorly understood how these factors relates to one another and contributes to the syndrome. It’s been suggested that individuals can be predisposed to the syndrome through predisposing factors such as genetics, hormonal, and anatomical factors.

The syndrome could arise following a physical trauma such as blunt force trauma or whiplash and this is called posttraumatic temporomandibular joint syndrome. It’s also possible for the syndrome to develop from an unknown cause which is termed idiopathic temporomandibular joint which is termed idiopathic temporomandibular joint syndrome. But regardless of what the etiology is, the syndrome can be approximately classified as being muscle- or joint-related.

Muscle-related means that the pain originates from the muscles of mastication as a result of abnormal function, inflammation, or hyperactivity and if the symptom is joint-related, then this means that the temporomandibular joint is displaying signs of degeneration.

Patients suffering with the syndrome will display three cardinal symptoms and these are pain and tenderness upon palpation or contraction of the muscles of mastication, limited range of mandibular movement, noises from the joint during mandibular movement such as clicking, popping, or grating and this is caused by abnormal articular disc displacement during movement of the mandible. And these classic symptoms are used for clinical diagnosis and may get worse during periods of emotional stress such as depression and anxiety.

Treatment is typically multidisciplinary and is specific to the identified etiology. For example, surgery can be used to amend any damage caused by blunt force trauma, and in contrast, cognitive behavioral therapy can be used in situations where the syndrome has developed in a patient who is highly stressed. In most patients, pain and anti-inflammatory medication is given to help manage symptoms. And if factors such as stress can be managed and any aggravating factor is corrected, the prognosis for patients with temporomandibular joint syndrome is generally pretty good with patients displaying no long-term damage or predisposition to arthritis in later life.

Now to help you digest all of this information, let's do a brief recap of everything we've learned from this tutorial.

Mastication is the process of grinding and chewing food into smaller pieces and it involves three components – the mandible, the temporomandibular joint, and the four muscles of mastication. The mandible is a U-shaped bone which makes up the lower third of your face and it consists of a body and angle, a ramus, a coronoid process, and a condylar process. The head of the condylar process articulates with the temporal bone of the skull at the temporomandibular joint.

So this is a special type of hinge joint which has a joint capsule split into two compartments by an articular disc. The superior compartment allows for side-to-side movement and the inferior compartment allows for pivoting movements. And these movements require muscles, and the first that we discussed was the masseter.

The masseter is the most powerful muscle of mastication and consists of two parts – the superficial part and the deep part. It is innervated by the masseteric nerve and both layers function to elevate the mandible so that you can occlude your teeth and take a big bite out of an apple. The superficial layer of the masseter specifically assists in protrusion.

The temporalis is the largest muscle of mastication and it consists of anteriorly-positioned vertical fibers and posteriorly-positioned horizontal fibers, which are all innervated by deep temporal branches of the mandibular nerve. Both the anterior and posterior fibers act to elevate the mandible whereas only the posterior fibers can perform retraction.

The lateral pterygoid has superior and inferior heads which are both innervated by the lateral pterygoid nerve. When contracting bilaterally, the inferior head is the main muscle for causing protrusion of the mandible. This protrusion unlocks the mandible so that it can then be depressed. The superior head acts to make sure that the articular discs stays on top of the condyle during protrusion and also has an important role in preventing the mandibular condyle from being pulled too far posteriorly by the temporalis during retraction and elevation. When contracting unilaterally, the lateral pterygoid produces medial movement away from the side which is contracting and this motion contributes towards trituration or the grinding up of food.

The medial pterygoid is sort of like a mini-masseter in its fiber direction and function and it also has a superficial and deep part – just like the masseter. Both heads are innervated by the medial pterygoid nerve and function to elevate and protract the mandible, and when contracting unilaterally, this muscle also assists the lateral pterygoid in causing medial movement for trituration.

And that brings us to the end of our video. I hope you've enjoyed it. Happy studying and see you next time!

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