Functional Anatomy of the Muscles of Mastication

This interactive learning module aims to teach anatomy relating to the function of the four muscles of mastication; masseter, temporalis, lateral pterygoid and medial pterygoid.

Mastication

Mastication Process

Mastication is the process of chewing and grinding food between teeth so as to break it down into smaller fragments to prepare it for swallowing. It is the first step of digestion, and it increases the surface area of foods to allow more efficient break down by enzymes. During the mastication process, the food is positioned by the cheek and tongue between the teeth for grinding. Whilst many muscles are actually involved in mastication, the primary four muscle of mastication are masseter, temporalis, lateral pterygoid and medial pterygoid.

The movements of the mandible brought about by the muscles of mastication occur at the temporomandibular joints (TMJs). These movements involve elevation, depression, protrusion and retraction of the mandibular condyles.

Initially the incisors are used bite food off, which means the contraction of masseter, medial pterygoid and temporalis. Chewing, grinding or side-to-side movements are brought about by alternate contraction of the left and right pairs of pterygoid muscles. Masseter further contributes during chewing to maintain occlusion of the teeth. Temporalis is able to retract the mandible if food is sticky. Digastric, mylohyoid and geniohyoid may also help to separate the teeth if a food bolus is sticky. The tongue, orbicularis oris and both buccinators all contribute to keeping the bolus in the centre of the mouth. Once a suitable consistency has been achieved, the bolus is collected and swallowed.

What are the four muscles of mastication?

What other muscles are involved in the overall process of mastication?

Basic Anatomy

Temperomandibular Joint (TMJ)

  • Bilateral articulation between mandible and the skull
  • Complete intra-articular disc improves congruency and facilitates movements
    • Gliding and hinge movements possible
  • Synovial condyloid joint type between mandibular fossa of temporal bone and the mandibular head/condyle
  • Allows elevation, depression, protraction and retraction of the mandible
  • Lateral movements of the jaw are possible when only one condyle is protruded or retracted
    • The jaw rotates about a vertical axis through the contralateral condyle

Masseter

Masseter is a thick, quadrilateral muscle. Originating from the zygomatic arch and inserting onto the lateral surface of the mandibular ramus, its fibres run posteroinferiorly. It consists of superficial, middle and deep layers.

Origins:

Superficial – thick aponeurosis from the maxillary process of the zygomatic bone and the anterior two-thirds of the inferior border of the zygomatic arch.

Middle – medial surface of the anterior two-thirds and inferior border of the posterior third of the zygomatic arch.

Deep – medial surface of the zygomatic arch.

 

Insertions:

Superficial – Posteroinferior half of the lateral surface and angle of the mandibular ramus.

Middle – Central part of the lateral surface of the mandibular ramus.

Deep – Superior part of the mandibular ramus and the lateral surface of the mandibular coronoid process.

Vascular Supply:

Masseteric branch of the maxillary artery, facial artery and transverse facial branch of the superficial temporal artery.

 

Innervation:

The masseteric branch of the anterior trunk of the mandibular nerve (CNV3).

Temporalis

Temporalis originates from the whole of the temporal fossa. The fibres converge to form a tendon which passes in the gap formed between the zygomatic arch and the side of the skull to insert on to the coronoid process of the mandible. Temporalis’ anterior fibres are orientated vertically, the posterior fibres are almost horizontal, and the intervening fibres are at intermediate degrees of obliquity enabling it to function in a variety of ways.

Origin:

Temporal fossa up to the inferior temporal line, part of the zygomatic bone and the deep surface of the temporal fascia.

 

Insertion:

Medial, apex, anterior and posterior surfaces of coronoid process, anterior border of mandibular ramus.

Vascular supply:

Anterior, middle and posterior deep temporal branches from the second part of the maxillary artery.

 

Innervation:

Deep temporal branches on the anterior trunk of the mandibular nerve (CNV3).

Lateral Pterygoid

Lateral pterygoid is a short, thick muscle that is composed of two parts. Originating from two locations on the sphenoid bone, its fibres converge to pass posterolaterally to the neck of the mandible and joint capsule of the TMJ.

Origins:

Superior head – Infratemporal surface and infratemporal crest of sphenoid bone.

Inferior head – Lateral surface of lateral pterygoid plate of sphenoid bone.

 

Insertion:

Depression on front neck of the mandible (pterygoid fovea), capsule of the TMJ and anterior and medial borders of its disc.

Vascular supply:

Pterygoid branches from the maxillary artery and palatine branch of the facial artery.

 

Innervation:

One nerve for each head of lateral pterygoid arises from the anterior trunk of the mandibular nerve (CNV3).

Medial Pterygoid

Medial pterygoid, similar to masseter, is a thick, quadrilateral muscle. It is composed of two heads; a larger, deep head and a superficial head. These arise from the sphenoid and palatine bones and converge and pass posterolaterally to insert onto the medial surface of the mandible.

Origins:

Superficial head – Medial surface of lateral pterygoid plate of sphenoid bone.

Deep head – Maxillary tuberosity and pyramidal process of palatine bone.

 

Insertion:

Posteroinferior part of medial surface of the mandibular ramus and the mandibular angle.

Vascular supply:

Pterygoid branches of the maxillary artery.

 

Innervation:

Medial pterygoid branch of the mandibular nerve (CNV3).

Match the muscles

Drag each muscle name to its corresponding picture.
  • Masseter
  • Temporalis
  • Medial Pterygoid
  • Lateral pterygoid

Pick the muscle

Which muscles partly originates from the lateral surface of the lateral pterygoid plate of the sphenoid bone?

Pick the muscle

  • Medial pteryoid (deep head)
  • Medial pterygoid (superficial head)
  • Lateral pterygoid (superior head)
  • Lateral pterygoid (inferior head)
Which takes origin from the medial surface of the lateral pterygoid plate of the sphenoid bone?

Match the descriptions

Match each muscle to its corresponding  origin or insertion.
  • Deep layer of masseter
    Originates from the medial surface of the zygomatic arch
  • Medial pterygoid
    Inserts onto the medial surface of the mandibular ramus and the mandibular angle
  • Superior head of lateral pterygoid
    Originates from the infratemporal surface and crest of sphenoid
  • Temporalis
    Inserts onto the medial, apex, anterior and posterior surfaces of coronoid process and the anterior surface of the mandibular ramus

Muscle Function

Individual Muscles

Masseter

Masseter is the most powerful elevator of the jaw, occluding the teeth during mastication. Its superficial fibres also contribute to protraction of the mandible.

Temporalis

Temporalis elevates the mandible, occluding the teeth. Its anterior fibres pull the mandible superiorly while the posterior fibres pull the mandibular head posteriorly into the mandibular fossa, as it articulates with the articular fact (anterior to the fossa) when the mouth is open. Therefore, temporalis contributes to both elevation and retraction of the mandible.

Lateral Pterygoid

Lateral pterygoid protracts and slightly depresses the jaw (this assists in separating sticky teeth to open the mouth). The superior head  of lateral pterygoid is thought to counter the backward pull exerted by the posterior temporalis fibres, when the jaw is closed. 

Medial Pterygoid

Medial pterygoid assists in elevating the jaw. It also contributes toward protraction. 

Pick the muscle

  • Masseter
  • Medial pterygoid
  • Lateral pterygoid
  • Temporalis
Which muscles protract the jaw?

Pick the muscle

Which muscle of mastication can retract the mandible?

Pick the muscle

  • Posterior fibres of temporalis
  • Lateral pterygoid
  • Superficial fibres of masseter
  • Deep head of medial pterygoid
Which muscle can assist in depressing the mandible?

Summary of Movements

Muscles elevating the mandible:

Masseter

Medial pterygoid

Temporalis

Muscles retracting the mandible:

Temporalis

(digastric, geniohyoid)

Muscles protracting the mandible:

Lateral pterygoid

Medial pterygoid

Masseter

Muscles depressing the mandible:

Lateral pterygoid weakly assists in first part

(digastric, mylohyoid, geniohyoid, platysma)

 

Fill in the muscle

The  head of  is thought to counter the posterior fibres of , which pull the mandibular head posteriorly when the mouth is closed.

 is the most powerful jaw elevator.  and  also elevate the jaw.

 fibres of  weakly protract the jaw, acting alongside the stronger protractors  and .

Movements of Mastication

Muscle Function During Mastication

1) The incisors are first used to bite off food as the mouth is closed by the jaw elevators (masseter, medial pterygoid, temporalis).

2) Food is chewed until a suitable consistency is reached. This involves side-to-side movement, consisting of unliteral protraction of one mandibular condyle rotating the mandible about a vertical axis through the contralateral condyle, on one side after another. This breaks down the food between occluded teeth.

Alternate contraction of unilateral pterygoid muscles, one side after another, brings about the side-to-side movements (protraction on one side after another), while jaw elevators (masseter, temporalis) also contract to occlude the teeth, to grind and crush the food.

3) As masster and temporalis elevate, and the pterygoid muscles protract the mandible on one side, the contralateral posterior fibres of temporalis retract the mandible at the same time, creating a diagonal movement of the mandibular teeth across the maxillary teeth, further ensuring food is broken down.

Important Points

Medial pterygoid is a particularly strong rotator of the mandible (unilateral protraction of the mandibular head) because of its oblique angle.

All the muscles of mastication are used simultaneously, coordinated in rhythm with eathother.

Buccinator, the tongue and orbicularis oris must also be considered when studying mastication, as they play an important role in keeping food in between the teeth, where the force of the primary muscles of mastication can be exerted to break it down.

Chewing

  • During chewing of food, contraction of masseter and temporalis protract the mandibular condyle on one side, while lateral pterygoid retracts the contralateral mandibular condyle.
  • During chewing of food, contraction of the pterygoid muscles protract the mandibular condyle on one side, while temporalis retracts the contralateral mandibular condyle.
Which statement is true of chewing food?

Pick the movement

  • Retraction
  • Elevation
  • Protraction
Which movement does part of temporalis bring about on the opposite mandibular condyle to the one that is protracting?

Match the movements

Match each muscle to its respective movements
  • Protraction
    Pterygoid muscles
  • Elevation
    Masseter and temporalis
  • Retraction
    Temporalis
  • Holding food bolus between teeth
    Buccinator, the tongue, orbicularis oris

Summary

Initially incisors bite off food as the jaw is  by ,  and . The food is then broken down by side-to-side movements which involve  of one mandibular condyle by the pterygoid muscles and  of the mandibular condyle by . Simultaneously, occlusion of the teeth is maintained by  and .

References

Reference List

References

Palastanga, N., Field, D., Soames, R., Bogduk, N. (1994) Anatomy and human movement: Structure and function. Oxford: Butterworth-Heinemann.

 

Snell, R.S. (2007) Clinical Anatomy By Regions. 8th edn. Philadelphia: Lippincott Williams & Wilkins.

 

Standring, S., Borley, N.R., Healey, J.C., Collins, P., Johnson, D., Crossman, A.R., Mahadevan, V., Gatzoulis, M.A., Newell, R.L.M. & Wigley, C.B., eds. (2008) Gray's Anatomy. 38th edn. London: Churchill Livingstone.