Muscles of Mastication Condensed Grayscale Slides

7/27/2019 Muscles of Mastication Condensed Grayscale Slides

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Muscles of MasticationMuscles of Mastication

Alex ForrestAssoci ate Profess or, Forensic Odonto logyForensic 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 whichthe 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 thestructures 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.


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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 aredifferent 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.



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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 isquadrilateral in

shape, and

consists of three

layers which

blend anteriorly.

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

MasseterMasseter


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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 thesurface 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 threelayers which leave

distinct oblique marks

on the bone.

MasseterMasseter


It arises from the

zygomatic process of

the maxilla and lowerborder 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


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It passes downwards

and backwards to insert

into the angle and much

of the superficial surface

of the ramus of themandible.

MasseterMasseter


The Superficial Layeris 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 themandible.

Note that intramuscular tendinous septa in

this layer are responsible for ridges on the

bony surface.

MasseterMasseter

From Grays Anatomy, 35th Ed,

Longman, London 1973, p. 281


The Middle Layerarises 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 ofthe mandible.

The Deep Layerarises 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

From Grays Anatomy, 35th Ed,

Longman, London 1973, p. 281


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


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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 ofthe maxillary artery, and

both then run into the deep

surface of the muscle.

MasseterMasseter

Clemente CD, Anatomy, A Regional Atlas of the Human

Body, Munich, Urban & Shwarzenberg, 1975, Diagram 455.

Masseter is active during closure, and most active

during clenching and during the forceful phase of achewing cycle.

It is primarily an elevator of the mandible.

MasseterMasseter

TemporalisTemporalis


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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 thetemporalis.





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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 verticalgroup - 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 themaintenance of its posture.


Lateral PterygoidLateral Pterygoid This muscle isshort and thick,

and arises by two

distinct heads.

Lateral PterygoidLateral Pterygoid



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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 crestof the sphenoid (greater

wing).


Modified from Grays

Anatomy, 35th Ed,

Longman, London 1973,

p. 268

The lower head

originates 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.




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The fibres of the

lower head run

upwards, backwards,

and slightly outwards,

to attach to a small

fossa on the anteriorsurface 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.


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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.


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The muscle is active during the power phase of a chewing

cycle, as it exerts control over the anteroposterior position of

the mandible.



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Medial PterygoidMedial PterygoidThe medial

pterygoid is also a

thick, quadrilateral

muscle.

It also arises by two

heads.

Medial PterygoidMedial Pterygoid


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 themandibular 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



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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 mandibletowards the contralateral side.


The masseter and medial pterygoid are active together in

protrusive movements, and in lateral mandibularmovements, 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


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DigastricDigastric

The digastric muscle is so-called because it has two bellies.

This is an anatomists 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 thechin, 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, theaction 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




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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 thickband 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 thefirst 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


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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


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The mylohyoid

muscles are best

thought of as the

muscles forming the

floor of the mouth,

sometimes betterreferred 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 helpsposition it vertically.

MylohyoidMylohyoid

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

The muscles

themselves are

triangular sheetsattached 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, themuscles 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.


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The digastric muscles

attach to the mandible

in the digastric fossae

inferior to the

mylohyoid, and the

geniohyoid muscles

attach to the inferiorgenial 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


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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 thiswith 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 thenasal cavity and mouth

closes as the bolus

moves into the pharynx

(throat).



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The bolus is propelled

toward and into the

oesophagus as the

oesophagus entrance

opens and the epiglottishelps guard against

access to the lungs.



The airway reopens and

the oesophagus entrance

closes as muscle

contractions move the

bolus toward thestomach.


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 whichthe 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

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