Overview

What is mastication?

Mastication can be described as the process of breaking up food and mixing it with saliva in the mouth, a medical term for chewing. Mastication is the first stage of digestion. Mastication is a complex technique; in which muscles are required to be powerful enough to break through parts of the food. Enough dexterity is required by these muscles, so not to injure the tongue.

Mastication is described as a coordinated movement. The masticatory cycle aids digestion by increasing the surface area of food for enzymes to work on. Forming a bolus of food, ready for deglutition.

 When ever you come across a new muscle in Anatomy, you must learn the following:  

• Name
• Origin/Insertion
• Movement/Action
• Innervations
• Blood supply

This will help, when it comes to studying the lectures and in the dissection room. It is in your best interest to learn this so you are well prepared for your spotter in the summer exam period. From knowing the origin/insertion you can identify most muscles in the head and neck when it comes to the spotter. The tables on each of the muscle pages, act as a quick revision aid, but it is also important to understand the development and problems associated with muscles of mastication for those of you who will go on to study Anatomy of Facial Development.                                                               

Photograph courtesy of the dissection room, University of California. Permission granted.

There

Image courtesy of https://www.flickr.com/photos/choho-digees/1484828155/ which has been labelled by the author under the creative commons license.

The main muscles and there actions

There are four main muscles of mastication, namely masseter, temporalis, external pterygoid and internal pterygoid. All supplied by a branch of the mandibular division of the trigeminal nerve. These are the four muscles of mastication that cross and act on the Temporo-Mandibular Joint (TMJ.) Muscles of mastication allow the movement of the mandible but not the condyle head. Muscles of mastication allow the movement of the mandible but not the maxilla which is an integral part of the skull.

The TMJ in situ lies bilaterally, anterior to the tragus of the ear. The TMJ is described as a compound synovial joint. The TMJ consists of a fibrocartilaginous surface and articular disc allowing tremendous range of movements in mastication, speaking and facial expression.

Temporo-mandibular fossa is located on the zygomatic process of the temporal bone. The mandible is in fact the largest non-articulating bone in the body.

At the Temporo-Mandibular Joint (TMJ), movements which occur include protraction, retraction, depression and elevation of the mandible. The 4 muscles of mastication help the TMJ to make the grinding movements what we call mastication.

The joint receives nerve innervation from the auriculotemporal and masseteric branches of V3. This does not just rotate in the articular fossa but also moves forwards onto the articular eminence of the temporal bone, taking the articular disc with it.

The tongue also has a role in mastication. The tongue consists of skeletal muscle. Cheek (Buccinator) muscles also plays a small part in mastication. The tongue consists of both intrinsic and extrinsic muscles. The tongue receives motor innervation from CNXII (hypoglossal.)

The suprahyoid muscles can help mastication and form the floor of the mouth.

The action of this group of muscles is to: 

• -open the mouth

• -deviate the mandible laterally.

Mastication cycle: 

Opening phase: mouth open, mandible depressed.

Closing phase: Mandible rose towards maxilla.

Occlusal phase: Mandible stationary

The mouth is closed by contraction of the masseter, temporalis, and medial pterygoid muscle. The lateral pterygoid protracts the mandible and, assisted by digastric and mylohyoid. Retraction of the mandible occurs from the posterior fibres of the temporalis.

The mandibular division of the trigeminal nerve (V3) contains sensory and  motor fibres and enters the infratemporal fossa through the foramen ovale in the sphenoid bone. The first branch ascends through the foramen spinosum and receives sensation from the meninges in the middle cranial fossa. The second branch is motor, supplying the medial pterygoid. The main trunk continues after the two branches and descends beneath the lateral pterygoid. The anterior division is mainly masseteric, deep temporal, lateral pterygoid and buccal branches. 

When the mandible is fully depressed the joint is relatively unstable and dislocation may occur, when the head of the mandible moves in front of the articular eminence and resulting in the person not being able to close the mouth.

Development 

Muscles of mastication are derived from the mesoderm of the mandibular arch. Most of the head muscles originate from somitomeres which are composed of seven incompletely segmented mesenchymal populations, which are masticatory 4,  6 qnd 7. The tongue muscles contribute to a subset of the head muscles, but tongue muscle progenitor cells are derived from hypaxial somites 2-5, not from the somitomeres, these migrate to the mandible where they form tongue muscle.

Muscles of mastication are derived from the first branchial arch. Hence, innervation is by the branches of the anterior branch of the mandibular division of CNV.