muscles of mastication slides(1)

Muscles of MasticationMuscles of Mastication

Alex ForrestAssociate Professor, Forensic OdontologyForensic Science Research & Innovation Centre, Griffith UniversityConsultant Forensic Odontologist, Queensland Health Forensic and Scientific Services, 39 Kessels Rd, Coopers Plains, Queensland, Australia 4108

Oral Biology

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Learning ObjectivesLearning Objectives

You should be able to explain the embryological origin of the muscles of mastication, and to explain the resulting common

motor nerve supply.

You should be able to explain the various systems by which the muscles of mastication can be classified, and to

demonstrate their ability to differentiate between the major and accessory groups of these muscles.

You should be able to demonstrate knowledge of the origins, insertions and the functions of each of the major muscles

during normal masticatory function.


As we talk about the muscles of mastication, we will involve ourselves in a discussion about bones, muscles and the

structures that ensure their viability and continued function.

We will be thinking about the functions of these muscles in a dynamic way, and trying to gain an appreciation of their

role in the living, moving head and neck.


When thinking about anatomy, remember that the bones provide crucial clues to us about the soft tissues. Recall

that the soft tissue structures were there first, and that the bones formed around them.

Recall also that the bones are part of a dynamic system called the musculoskeletal system.

This system is responsive to change. Enlarge the muscles and the bones alter accordingly. Re-attach the muscles

surgically in a different place, and the forces on bones are different following the procedure.


DefinitionDefinition

The Muscles of Mastication are defined as the muscles immediately concerned with the movements of the

mandible in mastication and speech.

Some texts include the digastric muscle as a muscle of mastication, based on its function, and there are some

arguments in favour of this approach.

Other texts define the muscles based on their nerve supply, and include only the anterior belly of digastric as such a muscle. Many such texts include the mylohyoid

also as a muscle of mastication.


We will include only the following muscles which are directly responsible for movements of the mandible at the TMJ:

Masseter

Temporalis

Medial Pterygoid

Lateral Pterygoid


We will include the following muscles as accessory muscles of mastication:


Digastric

Mylohyoid

MasseterMasseter

The masseter muscle is

quadrilateral in shape, and

consists of three layers which

blend anteriorly.

From Grays Anatomy, 35th Ed, Longman, London 1973, p. 1121

MasseterMasseter

It is covered by a strong layer of fascia

called the parotid fascia. This is derived

from the deep cervical fascia, and is firmly attached to the

surface of the muscle.

MasseterMasseter

Clemente CD, Anatomy, A Regional Atlas of the Human Body, Munich, Urban & Shwarzenberg, 1975, Diagram 451.

The masseter originates from the zygomatic process

and zygomatic arch, and inserts onto the

ramus of the mandible in three layers which leave

distinct oblique marks on the bone.

MasseterMasseter


It arises from the zygomatic process of the maxilla and lower border of the body of the zygomatic bone,

and anterior two-thirds of the lower border of the zygomatic arch.

MasseterMasseter

From Grays Anatomy, 35th Ed, Longman, London 1973, p. 257.

Copyright A. Forrest

It passes downwards and backwards to insert into the angle and much of the superficial surface

of the ramus of the mandible.

MasseterMasseter


The Superficial Layer is the largest layer, and arises by a thick aponeurosis from

the zygomatic process of the maxilla and lower border of the body of the zygomatic bone, and anterior two-thirds of the lower

border of the zygomatic arch. Its fibres pass downwards and backwards to insert

into the angle and lower half of the superficial surface of the ramus of the

mandible.

Note that intramuscular tendinous septa in this layer are responsible for ridges on the

bony surface.

MasseterMasseter



The Middle Layer arises from the deep surface of the anterior two-thirds

of the zygomatic arch, and from the lower border of the posterior third.

It inserts on the middle of the ramus of the mandible.

The Deep Layer arises from the deep surface of the zygomatic arch. It

inserts into the upper part of the ramus of the mandible and into the coronoid

process.

MasseterMasseter



The insertions of the separate layers can be seen on the mandible, and are separated by vague oblique lines on

the external surface of the ascending ramus.

You should examine a variety of mandibles, holding them in such a way that light falling across them casts a

shadow from these lines to make them visible.

Do not use a plastic skull for this purpose. A good-quality real skull will be needed.

MasseterMasseter

The masseter is supplied by the masseteric nerve, a motor branch of the anterior trunk of the mandibular

division of V3.

MasseterMasseter

The masseteric nerve passes from the

infratemporal fossa through the posterior part of the mandibular notch

along with the masseteric artery which is a branch of the maxillary artery, and

both then run into the deep surface of the muscle.

MasseterMasseter


Masseter is active during closure, and most active during clenching and during the forceful phase of a

chewing cycle.

It is primarily an elevator of the mandible.

MasseterMasseter

TemporalisTemporalis

The temporalis muscle is covered superficially by the temporal fascia. This is firmly attached

to the superficial surface of the muscle, and indeed the muscle

arises partly from it.

If followed upwards, the fascia attaches along the superior temporal

line.



The temporalis muscle is fan-shaped, and originates from the whole of the temporal fossa (except the part formed by the zygomatic bone), and from the deep

surface of the temporalis fascia.


Its fibres converge and descend in a tendon which

passes through the gap between the zygomatic arch and the side of the skull, to

insert upon the medial (deep) surface, apex, anterior and

posterior borders of the coronoid process of the

mandible, and the anterior border of the ramus of the

mandible down nearly as far as the third molar.



It is supplied by deep temporal branches of the

anterior trunk of V3, passing through the mandibular notch.

The vessels and nerve to masseter pass behind the

tendon of the temporalis.



Temporalis is an elevator of the mandible. It is also a retrudor of the mandible.

During closure, the posterior, more horizontal, fibres are the first of the elevators to activate, followed by the

oblique middle group, and then by the anterior vertical group - a wave of contraction starting posteriorly and

ending anteriorly in the muscle.


It is not a particularly powerful elevator compared to others, but is nonetheless most important.

It is believed to be functional mainly in the anteroposterior positioning of the mandible, and in the

maintenance of its posture.


Lateral PterygoidLateral Pterygoid

This muscle is short and thick,

and arises by two distinct heads.



The upper head originates from the infratemporal

surface of the greater wing of sphenoid, between foramen ovale and the infratemporal crest, and

from the infratemporal crest of the sphenoid (greater

wing).


Modified from Grays Anatomy, 35th Ed,

Longman, London 1973, p. 268

The lower headoriginates of the lateral surface of

the lateral pterygoid plate of sphenoid.



At their origins, the two heads are

separated by a slight space through which

the buccal nerve passes, and also the

second part of the maxillary artery if it lies deep to lateral

pterygoid.



The fibres of the upper head are

horizontal in direction, pass beneath the

articular eminence, and are attached to

the front of the articular disk of the temporomandibular

joint.



The fibres of the lower head run

upwards, backwards, and slightly outwards,

to attach to a small fossa on the anterior surface of the neck of

the mandibular condyle.



It is generally a depressor muscle. Specifically it is a protrudor. Only a small component of its fibres are angled

enough away from horizontal to produce a depressive action.


http://www.drjimboyd.com/lateralpteygoidsworktogether.jpg

The lateral pterygoid muscles from both sides acting together protrude the mandible.

One muscle, acting alone on one side, helps pull the condyle forwards, downwards and medially, swinging the mandible to

the opposite side.


http://www.drjimboyd.com/lateralpteygoidsworktogether.jpg

The muscle is active during the power phase of a chewing cycle, as it exerts control over the anteroposterior position of

the mandible.


Medial PterygoidMedial Pterygoid

The medial pterygoid is also a thick, quadrilateral

muscle.

It also arises by two heads.



The larger arises from the medial surface of the lateral pterygoid plate of the sphenoid bone, and the smaller

from the lateral surface of the

pyramidal process of the palatine bone and the tuberosity of the

maxilla.



It inserts onto the lower and posterior parts of

the deep surface of the mandibular ramus, as

far upwards as the mandibular foramen,

and to the deep surface of angle of the

mandible.


Modified from: Clemente CD, Anatomy, A Regional Atlas of the Human Body, Munich, Urban & Shwarzenberg, 1975, Diagram 459.

The medial pterygoid is

supplied by a branch from

the mandibular nerve V3.


Modified from: http://www.drjimboyd.com/TENSaccessibility.html

Medial Pterygoid is an elevator. It becomes highly active towards the end of a closing movement, and even more so

during clenching of the teeth.

In a chewing stroke, it assists in directing the mandible towards the contralateral side.


The masseter and medial pterygoid are active together in protrusive movements, and in lateral mandibular

movements, particularly so in movements towards the opposite side.

In both of these movements they maintain elevation of the anterior part of the mandible, whilst the condyle is

depressed.


Accessory Muscles of Mastication

Accessory Muscles of Mastication

DigastricDigastric

The digastric muscle is so-called because it has two bellies. This is an anatomist’s idea of a joke. They may not get out

much.

DigastricDigastric

Modified from: http://www.drjimboyd.com/TENSaccessibility.html

The muscle stretches between the mastoid

process of the cranium to the mandible at the

chin, and part-way between, it becomes a tendon which passes through a tendinous

pulley attached to the hyoid bone.

DigastricDigastric


Because the hyoid is a mobile bone, not

attached to the skeleton directly at any point, the

action of the digastric can be modified by the

position of the bone, and therefore the position of

the sling, which determines where in space the tendon is.

DigastricDigastric


The posterior belly of the muscle attaches in a

deep notch just medial to the mastoid process on

the temporal bone called the digastric notch.

DigastricDigastric

Modified from Grays Anatomy, 35th Ed, Longman, London 1973, p. 268

The posterior belly runs forward below the

mandible, and often beneath the cover of the superficial belly of

the submandibular gland, it starts to

become tendinous again.

DigastricDigastric


The tendon passes through the pulley which

originates as a thick band of fascia from the

greater cornu of the hyoid bone, and then it starts to form a second

muscle belly.

DigastricDigastric


The anterior belly of the digastric muscle originates from the first branchial arch, and therefore gains its motor supply from the mandibular division of the trigeminal nerve (V3), while the posterior belly originates from the second branchial arch and

therefore is supplied by the Facial Nerve (VII).

DigastricDigastric

The anterior belly attaches to the

mandible on the internal aspect at the

digastric fossa, slightly to the side of the

midline near the base of the mandible,

inferior to the genial tubercles.

DigastricDigastric


As it passes down towards the fascial sling, the tendon of the posterior belly of the digastric is surrounded by the tendon of the stylohyoid muscle, which splits around it, before attaching

to the hyoid bone slightly forward of the attachment of the digastric sling.

DigastricDigastric

Modified from Grays Anatomy,

35th Ed, Longman,

London 1973, p. 507.

If the hyoid bone is held down by the infrahyoid strap muscles, then contraction of the digastric causes the mandible to be

pulled inferiorly, opening the mouth.

If the mandible is held in the closed position, then the digastric muscles elevate the hyoid and therefore the larynx, as in

swallowing.

It seems that the digastric muscles always work together on both sides, rather than separately, and this makes sense,

given their function.

DigastricDigastric

MylohyoidMylohyoid

The mylohyoid muscles are best thought of as the

muscles forming the floor of the mouth, sometimes better

referred to as the oral diaphragm.

MylohyoidMylohyoid

http://sprojects.mmi.mcgill.ca/larynx/notes/anat/naview072.htm

They form a muscular floor to the

entire oral cavity which suspends the tongue and helps

position it vertically.

MylohyoidMylohyoid

http://sprojects.mmi.mcgill.ca/larynx/notes/anat/naview072.htm

The muscles themselves are

triangular sheets attached along the mylohyoid ridges or

lines of the mandible, and to the anterior part of the body of the hyoid

bone.

MylohyoidMylohyoid


Because the hyoid bone lies posterior to

the mandible, the muscles meet in front

of the hyoid in the midline in a tendinous raphe which continues all the way forwards to

the mandible.

MylohyoidMylohyoid

Jamieson, EB. Illustrations of Regional Anatomy, Section II. Edinburgh, E & S Livingstone, 8th Ed. P.81.

The digastric muscles attach to the mandible in the digastric fossae

inferior to the mylohyoid, and the geniohyoid muscles attach to the inferior

genial tubercles superiorly to the

mylohyoid.

MylohyoidMylohyoid

Jamieson, EB. Illustrations of Regional Anatomy, Section II. Edinburgh, E & S Livingstone, 8th Ed. P.81.

If you follow the mylohyoid lines forwards to the

midline on the mandible, you will see that the muscle

attaches to the mandible quite highly posteriorly, and

becomes progressively more inferior as one works

forwards, until the mylohyoid lines meet in the

midline, below the genial tubercles and above the

digastric fossae.

MylohyoidMylohyoid


The mylohyoid muscles also form from the first branchial arch tissue, and therefore they are provided with motor innervation by the mandibular division of the trigeminal

nerve (V3).

MylohyoidMylohyoid

The mylohyoid again has its function determined partly by the position of both the mandible and the hyoid bone.

MylohyoidMylohyoid

Where the mandible is fixed in position, it elevates the hyoid bone on contraction, and also elevates the tongue,

as in the first stage of swallowing.

Elevation of the hyoid bone is also important in closing the laryngeal inlet in swallowing.

If the hyoid bone is held down by the infrahyoid strap muscles, then the mylohyoid causes the mandible to be

depressed, opening the mouth.

MylohyoidMylohyoid

We have briefly described the muscles that control the position of the mandible, separating them into Muscles of

Mastication and Accessory Muscles.

You should correlate their origins and insertions with their functions to try and get a dynamic view of the way in which the position of the mandible is controlled, and integrate this with your knowledge of the movements of which the TMJ is

capable.

ConclusionConclusion

Normal SwallowingNormal Swallowing

In the mouth, the lips, teeth and tongue help prepare the bolus (food mass) for further stages of swallowing.

http://www.mdausa.org/publications/Quest/q64dysphagia.html


Access between the nasal cavity and mouth closes as the bolus moves into the pharynx (throat).



The bolus is propelled toward and into the oesophagus as the oesophagus entrance opens and the epiglottis helps guard against access to the lungs.



The airway reopens and the oesophagus entrance closes as muscle contractions move the bolus toward the stomach.


Learning ObjectivesLearning Objectives

You should be able to explain the embryological origin of the muscles of mastication, and to explain the resulting common

motor nerve supply.

You should be able to explain the various systems by which the muscles of mastication can be classified, and to

demonstrate their ability to differentiate between the major and accessory groups of these muscles.

You should be able to demonstrate knowledge of the origins, insertions and the functions of each of the major muscles

during normal masticatory function.

The End

muscles of mastication slides(1)

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