MASTICATION & DYNAMICS OF OCCLUSION

The doctor said this is one of the most important lectures for the final exam and it will take a big portion for the final exam. It is divided into two lectures, today and the second part will be covered next week. You must remember every part of the lecture especially the dynamics of occlusion. Definition of mastication: Mastication is the dynamic process of chewing. It involves movement of mandible with the maxilla which is in a fixed position that prepares food for swallowing. (This means that only the mandible is moving but the maxilla is fixed). Everything that is related to dynamics starting from entry of food in mouth until food is ready for swallowing is called mastication.

Sequence of food Intake: Incision : the process start by incising the food (the front teeth is called incisors because they incised food). For example: when you have an apple you cut the part of the apple using your front teeth to bring it inside your mouth.

However, you dont incised all food. Sometimes when eat using spoon we dont incised. Examples of food that we can incise are fruits, a roll of sandwich, McDonalds burgers and etc.

Fragmentation of food by approximation is when you approximate which basically involve moving mandible against maxilla. It also involves rotation of the TMJs. Because the TMJs are also moving in the incision process.

Transport: After the food is cut/incised the food is transport between premolars and molars teeth. Mastication : After transport of food we begin the process of mastication. It is the fragmentation of food (food into pieces) by approximation of mandible against maxilla to start cutting the food into pieces.

Once the food is mixed with saliva it is transported to the left and right side of your teeth back and forth. Now the food is ready to be swallowed. So the transport is a preparatory to swallowing then the swallowing process occurs, (or deglutition occurs which is a scientific name for swallowing) SO THAT IS THE SEQUENCE FOR FOOD INTAKE _________________________________________________________________________

The Muscles of Mastication

When we are talking mastication, it involves the movement of mandible. There a number of muscles involved in it. These are group into elevators and depressors. We are going to talk about the functions of these muscles. Muscles that elevate, depress, rotate (move the mandible to one side. The depressors functions only when the hyoid bone is fixed with the infrahyoid muscles. SO For these muscles to depress the mandible, the hyoid bone must be fixed!! If the hyoid bone is not fixed, it will not be able to move the mandible downward. What will happen? The hyoid bone will move upward. So in order to move the mandible downward, you should fix the hyoid bone How can we fix the hyoid bone? By assisting muscle called infra hyoid muscle. Are they mastication muscle? Not necessary, they are assisting muscle. They assist the muscle of mastication. But they are not involved directly in process of mastication. Infra hyoid muscle cover the neck, we have , omohyoid, thyrohyoid, supra thyroid. These muscles are supplied by ansa cervicalis. Also the muscle of the lips and cheek are important in moving pieces of food within the mouth. to mix the food with the saliva. So you need to move the food one side to another. And move it anteriorly and posteriorly. So these muscles are important in moving the food and mix it with the saliva. All of these are called assisting muscle. And finally we have head and neck muscle. For example the sternocleidomastoid and the trapezius muscle. These are also assisting muscle. But these are only evidence in animal. Have you seen the video of animal like a lion catching a deer? When these lions in fight, they want to cut the flesh. So the only way to cut it, is to move the head to one side like this. (dr move is head to one side). This is done y one of big muscle scm, and trapezius. BUT it is not actually evidence in human. We usually do not do that. Unless, you are eating a very tough sandwich. You need to move your head to cut the

piece of sandwich. But normally when we eat we dont need to use this big muscle to cut the food. But they are much very important in carnivores. So greater forces are needed to cut the piece of food and capture animals. This is evidence in animal. SO THOSE ARE THE ASSISTING MUSCLES _________________________________________________________________________ But the primary and important muscles are called muscle of mastication. They function in elevating te mandible include the masseter, temporalis and medial pterygoid muscle. These muscles they elevate the mandible. The temporalis muscles have another function. The masseter muscle has also another function in addition to elevating the mandible. We will see that. I am not so concern about the origin and the insertion and also the anatomical details of these muscle because this is oral physiology . But I want you to see this masseter muscle. It has 3 layers. So this is the superficial layer. If you move this superficial layer you will see two deep layers. The two deep layers are running exactly vertical. But the superficial muscle is oblique. So what is the function of these deep layer muscles? They elevate the mandible vertically without any problem. but what is the function of these superficial layer. It is running obliquely. So that is why

The mandible will not be able to be brought forward. What will happen to the mandible? It will move to the other side. So, the function of this superficial layer of masseter is bring the mandible forward if they work bilaterally . If the muscle works unilaterally , it will deviate the mandible to other side.

The temporalis muscle is a big muscle and it is attached to the coronoid process of the mandible. We have some details in the insertion of this muscle but we are not concern about it. This muscle is a powerful elevator but only when this fibres work. What happen when this horizontal fibres work alone? It will be

brought backward.That is why the main function of this muscle is to bring mandible forward however if most horizonal fibre of this muscle contract, it will bring the mandible backward. This is the only muscle that bring the mandible backward. They only function bilaterally cannot function unilaterally. Please remember the muscle is a powerful elevator but also when the horizontal fibres, the most posterior fibres function alone, they break the mandible backward. It is just opposite to the masseter muscle because it elevates the mandible and but it can bring the mandible forward.

The medial pterygoid is not really a strong muscle. Its main functions to bring the mandible upward (elevator). It is NOT related to retrusion and protrusion and lateral movement.

Someone asked the doctor a question regarding the retrusion movement of the mandible. It is by the temporalis muscle but only on the horizontal fibres (it brings the mandible backward). If you remove temporalis muscle, you can see a muscle located deep. What is this muscle, it is lateral pterygoid. The lateral pteregoid DOES NOT elevate the mandible. The lower part of lateral pterygoid is attached to lateral part of lateral pterygoid plate. It run backwards to be attaced to the neck of mandible. When the muscle contracts, it will move forward. So, it is a major protrusor muscle only if it move bilaterally. If unilaterally, it will move to opposite side. It does not only move neck of mandible but also move the disc of the TMJ. As we have discussed last week, it is important that the head of the condyles is moving with the disc at the same time. Otherwise we will have problem clicking or inability to move the mandible or unable to open the mouth.

DEPRESSORS
Now we have the depressors, what are the muscles that depress the mandible? Of course we consider the infrahyoid as assisting muscles sternohyoid, thyrohyoid, omohyiod. We have the suprahyoid muslcles, the suprahyoid muscles are considered as muscles of mastication but the infrahyoid muscles are only assisting muscles. The digastric, it is one of the main depressors. This is the digastric muscle- it is attached to the digastrics fossa of the mandible and here it has an intermediate tendon, and then it goes upward to be attached to the medial surface of the mastoid process. When this muscle contracts, it opens the mandible. But this muscle is only involved by the way, in the first part of opening the mandible. If you remember last time we said the first part of mandibular opening, it only involves rotation of the condyles without displacement, without moving. So when you open your mouth for only 20mm, the first 20mm of opening the mouth, the condyles are not going to be moving against the cranium or against the mandibular boneONLY ROTATING. So this movement is caused by the suprahyoid muscles, particular to the digastric muscle. We have mylohyoid, stylohyoid and geniohyoid, all of these muscles are involved in the first part of opening the mandible. What continues opening the mandible? The lateral pterygoid muscle, because it is attached to the neck of the mandible and it causes the movement of the condyle forward.

If you remember we mentioned digastric before in Oral Histology, we said it has 2 pili. By the way this muscle is supplied here by trigeminal nerve and supplied here by facial nerve- it has dual innervations. As a result this muscle is embryologically developing from two brancial arches (first and second). This is not important in Oral Physiology.

DIRECTION OF FORCES BY MASTICATORY MUSCLES

As for the direction of forces by masticatory muscles, different parts of a muscle produce different movements as we discussed. If you want to understand this point, please remember the movement in the temporalis muscle and the masseter muscle and also the lateral pterygoid because it depends on the area of that muscle that is active.

For example take a temporalis muscle, if the area that is active in contraction is the most horizontal and posterior fibers of that muscle, the mandible would be retruded. But if the other areas are active, the mandible would be elevated. So this is an example of the temporalis muscle.

Mandibular elevation is a composite activity of a number of muscles. When you elevate the mandible, you cannot say that you only have one muscle- it is actually a composite ya3ni it is a complex of movement. So many muscles would be involved at the same time in mandibular movement. But remember we discussed, we said the masseter muscle is involved in powerful contraction of the mandible. The masseter muscle is involved in the type of movement that will continue over a long period of time. If you remembered we said the masseter muscle is like a marathon athlete, able to continue working for a long period of time without being tired but the temporalis muscle it is like a short distance athlete- they can run, they are very powerful but only for a short distance. Thats why in carnivores where they need very big forces, they need a very big temporalis, because the temporalis tends to be very huge in carnivores in mid-ages. But in herbivores they do not need force, they need a muscle that can withstand fatigue and can withstand long term action without being tired. This is the masseter, the masseter muscle in herbivores it is really very huge. Also the masseter muscle is involved in lateral movement, and we know herbivores move their mandible laterally to munch but in carnivores they do not move their mandible laterally so thats why they do not need a huge masseter. So the masseter muscle tends to be very small in carnivores but very big in herbivores. Temporalis muscle tends to be very huge in carnivores but very small in herbivores. Please remember that, this is importantat least two questions will be about this point.

Characterization of Masticatory Muscles

The masseter and the medial pterygoid these are related to power production. Remember this power production, asts for a long period of time. So these muscles they can withstand being induced for a long period of time without being tired. Temporalis for stabilizing of mandibular position and also for allowing masseter to take action. Temporalis is only needed for short term power. The lateral pterygoid: continuous workload at low forces with fatiguing so it can be fatigue easily. And also stabilizing the position of temperomandibular joint, why? Because it is attached to the neck of the mandible below TMJ and also attached to the disck and capsule of the TMJ. The digastric muscle is slow, fatigue, resistant. It cannot actually resist fatigue, it can easily get tired. The digastric is also important during the elevation of the mandible. How the digastric muscle is important? It is a depressor muscle. How do you say it is important in elevating the mandible? It is important in making brakes, to brake your mandible as teeth are closed. Imagine that you are going to close fast, if you dont have a brake, you will injure your teeth. Theoretically, imagine that we want to cut all the muscles that depress the mandible, what happens to your mandible?? All the time you want to close your mouth, it will close very fast. The same goes to when you want to close a door. When there is nothing stopping it, it will close very fast. So, the digastric muscle functions in braking the elevator muscle. Otherwise, you will injure your teeth. For this reason, you need the depressor muscles to be active when elevating your mandible. Depression is faster than elevation, to depress the mandible is faster than to elevate you mandible. Why? Because when you depress the mandible, you do not have any activity of the elevator, but when you elevate the mandible you have an activity of the depressor. When you depress the mandible, you dont have a brake, so that you can easily depress the mandible but not easily elevate the mandible because you have brake by the depressor muscles.

Sensory receptors in masticatory muscles

We know that the masticatory muscles are innervated by motor nerve, but we also have the sensory nerve. What is the function of sensory nerve? They control the muscle movement. These nerves, all the time they send information to the brain, telling the brain about the activity within that muscle. Without them, the muscle will work by itself and the brain will not be able to control the movement of the muscle. For example, when I extend my arm, these sensory nerves send the information to my brain that the biceps muscle has contracted because of this movement. So, this is important. Otherwise, the brain will ignore it. So, these sensory nerves they exist in palatal muscle, they are undistributed in the muscles of mastication. In muscle of mastication they are also present but they are not distributed in all part evenly. These are called muscle spindle and golgi tendon organ. Many muscle spindles occur in elevator and tongue muscle, few in depressor muscle. Golgi tendon organ not known as they exist in elevator muscle, they protect against the overdevelopment of muscle tension and performed by periodontal ligament receptor. In periodontal ligament there is a receptor called propioreceptors ( we discussed this before, it is important to send information to the brain telling the brain about the position of mandible in relation to cranium ). So, this golgi tendon organ have the same function as the proprioreceptors. The periodontal ligament receptor can act like golgi tendon which important in limiting the force applied in mastication. Otherwise, when you close/bite your teeth you will have too much pressure on teeth. Now when you bite your teeth how can you know that it is enough? It is with the propioreceptors. If you want to appreciate the importance of this point, ask an old man wearing a denture. They are not able to sense the amount of force because they dont have teeth.

Electromyography of masticatory muscles

Electromyography of masticatory muscle is a complex combination of activation in specific movement. If you put electrode inside the masticatory muscle, we will see complex combination of activation in a specific movement. These electrodes are connected to a computer and will tell you about the active area inside these muscles.

Masseter, temporalis and medial pterygoid are activated in a sequence during mandibular approximation. So when you approximate the mandible, when you elevate mandible, first muscle is masseter then temporalis then medial pterygoid. This is the sequence of the muscle. Masseter Temporalis Medial pterygoid

Digastric burst of activity during elevation to brake the rate of occlusion forces. We said the digastric muscle is important in providing braking to the elevation. So it will have burst of braking which is the function of digastric muscle.

Activity in the masseter muscle begins late during elevation and ceases before the stroke is completed. This activity of masseter muscle is in mastication, but here this is the activity of the masseter muscle in elevation. Elevation of the mandible - when you open to the maximum and then you elevate the mandible. This is in mastication. Thats why please remember many students get confused. Because here we said the first activity for the masseter muscle. It begins late during elevation. Here we talk about the activity during mastication, the chewing of food.

Sternocleidomastoid (SCM) is active in clenching. When you clench, you put your teeth against each other and exert more force. You will notice that SCM is active.

END OF LECTURE =) made by: Farah Aiman, Haniz Amirah, Ernida Wijaya, Haizun, Farhana, Nur Izyan