Muscles of mastication 3

Seminar on
Muscles of
mastication
www.indiandentalacademy.com

Introduction:
Man has 639 muscles , composed of 6 billion
muscle fibers . each fiber has 1000 fibrils , which means
that there are 6000 billion fibrils at work at one time or
another.
Muscles of mastication are the group of
muscles that help in movement of the mandible as during
chewing and speech. we need to study these muscles as
they control the opening & closing the mouth & their role
In the equilibrium created within the mouth. They also
play a role in the configuration of face.

Development of muscles
# Zygote –2 cell stage –4 cell stage—
morulla---blastula—blastocyte
# Formation of precordial bulge & cranial
swellings
# Formation of pharyngeal arches
# Muscles develop from mesoderm of the
pharyngeal arches

A
Notochordal plate
c Endodermwww.indiandentalacademy.com

Classification:
Muscles of mastication are mainly of four
types
a) Temporalis
b) Lateral pterygoid
c) Medial pterygoid
d) Masseter
Accessory muscles of mastication
a) Buccinator
b) Orbicularis Oris
c) Mentalis etc .

masseter :
(most powerful muscle of mastication )
Origin and insertion: -
Quadrilateral muscle consisting of 3 layers
a) Superficial layer: largest of the 3 layers.
Arises -maxillary process of zygomatic bone
- anterior 2/3rds of the inferior border of
the zygomatic arch
Inserted - angle& lower posterior half of the
mandibular ramus.
Intramuscular tendinous septa in this layer are
responsible for the ridges on the bone

b) Middle layer: Arises - medial aspect of the
anterior 2/3rds & lower border of posterior 2/3rds
Inserts- central part of the mandibular ramus
c) Deep layer: Arises- deep surface of the zygomatic
arch Inserts - upper part of the mandibular ramus
& coronoid process
Nerve supply: : Masseteric nerve ( nerve to
masseter )
A branch of the anterior trunk of the mandibular nerve
Actions :
Elevates the mandible.
- Its electric activity is minimal in resting position
- Increases during orthodontic treatment & reduceswww.indiandentalacademy.com

Facial asymmetry in a 6-year-old girl with torticollis.
Excessive muscle contraction can restrict growth in a way analogous to
scarring after an injury.
. Note that the asymmetry affects the entire side of the face, not just the
mandible.

Temporalis:
Origin and insertion :
- Arises- whole of the temporal fossa & deep surface of the
temporal fascia .
Descends into a tendon which passes through the gap
between zygomatic arch & the side of the skull.
- Attaches- medial surface, apex , anterior & posterior
borders of the coronoid process & the anterior border of
the mandibular ramus almost to the last molar tooth .
- anterior fibers -vertically , posterior fibers-horizontally,
& intervening fibers- intermediate degrees of obliquity ,
in the manner of fan.

Nerve supply :-
Deep temporal nerves , (branch of
anterior trunk of mandibular nerve)
Actions :-
- Elevates the mandible & so closes the mouth &
approximates the teeth
- contributes to side to side grinding movements .
- retract the mandible (posterior fibers)
- EMG activity of temporalis and masseter was (1)
higher in classIII cases compared to class II &
class I

Medial pterygoid :
Origin and insertion :Thick , quadrilateral
muscle , Origin- medial surface of the lateral
pterygoid plate & the grooved surface of the
pyramid process of the palatine bone .
- superficial fiber slip- lateral surfaces of the
pyramidal process & the maxillary tuberosity .
Attached by a strong tendinous lamina to the
posteroinferior part of the medial surfaces of the
mandibular ramus & angle , as high as
mandibular foramen & almost as far forwards as
the mylohyoid groove .www.indiandentalacademy.com

Nerve supply:-
Nerve to medial pterygoid
Actions :
- Assists in elevating the mandible .
- Along with lateral pterygoid protrudes the
mandible . when the medial& lateral pterygoid
muscles of one side act together , the
corresponding side of the mandible is rotated
forwards & to the opposite side ,
- produces side to side movements which are used
to triturate food.

Lateral pterygoid : -
Origin and insertion:
short thick muscle with 2 heads
a) Upper head : Arises- infratemporal surface & crest of
the greater wing of the sphenoid bone
b) lower head : Arises - lateral surface of the lateral
pterygoid plate ;
Inserted - depression on the front of the neck of the
mandible ,& into the articular capsule & disc of the
TMJ.
Lateral pterygoid muscles excision caused retardation of
(2) growth in few rats – minimal role in growth
• www.indiandentalacademy.com

Nerve supply: Nerve to lateral pterygoid
(branch of mandibular nerve )
Actions: -
a) Only muscle of mastication which helps in opening the
jaw . Backward gliding of the articular disc & mandibular
condyle is controlled by slow elongation of lateral
pterygoid
b)Acting with ipsilateral medial pterygoid , lateral
pterygoid advances the condyle of that side about a vertical
axis.
c) When medial & lateral pterygoids of the two sides act
together they protrude the mandible.www.indiandentalacademy.com

Orbicularis oris
Origin and insertion : consists of 4 independent
quadrants (upper, lower, left and right) each of
which contains a larger pars peripheralis and a
pars marginalis
Pars peripheralis is found in many mammals,
pars marginalis is absent in non-primate mammals
and uniquely developed with speech in the human
Marginal and peripheral parts are apposed along
lines that correspond externally to the lines of
junction between the red-lip and the skin.

Pars peripheralis
Pars peripheralis has, in each quadrant, a lateral stem
attached to the labial side of the modiolus over its full
thickness.
- stem fibers are thought to originate within the modiolus
-reinforced directly by fibers from the following muscles:
In the upper lip: from buccinator (Upper fibers and
decussating lower central fibers) and depressor anguli
oris
In the lower lip: from buccinator (lower fibers and
decussating upper central fibers), levator anguli oris and
superficial part of zygomaticus major.

The fibers of orbicularis oris enter their respective
superior and inferior labial areas and diverge to form
triangular muscular sheets
a) Thickest at the junctions between skin and red-lip
b) Thinner as they reach the limits of the labial region
The greater part of each sheet enters the free lip, where its
fibers aggregate into cylindrical bundles orientated
parallel to the red-lip margin. Fibers of the direct labial
tractors pass to their submucosal attachments between
these cylindrical bundles and between pars peripheralis
and pars marginalis
In the upper lip, highest fibers run near nasolabial
sulcus; a few fibers attach to the sulcus, and a few to the
nasal ala and septum.
In the lower lip, the lowest fibers reach and
attach to the mentolabial sulcus.

Pars marginalis
Developed to a unique extent in human lips and is associated
with speech and the production of some kinds of musical
tone.
At their medial end fibers meet and interlace with their
contra lateral fellows and attach to the red-lip dermis
At their lateral ends; the fibers converge & attach to the
deepest part of the modiolar base along a horizontal strip
level with a buccal angle.
In full thickness section at right angles
upper lip --“S” shape
lower lip -- continues curve
Nerve supply: Lower branch & mandibular marginal
branches of the facial nerve.www.indiandentalacademy.com

Scarring of the corner of the mouth is related to a burn at an
early age. From equilibrium theory, one would expect a
distortion in the form of the dental arch in the region of the
contracting scar, and exactly this occurs.

Buccinator
Origin and insertion ;
quadrilateral muscle- occupies the interval between the
maxilla and the mandible in the cheek. Attached to the
outer surfaces of the alveolar processes of the maxilla
and mandible opposite the molar teeth, and, behind, to
the anterior border of the pterygomandibular raphe
which separates the muscle from the superior constrictor
of the pharynx..
The posterior part of buccinator is deeply placed, internal
to the mandibular ramus and its attachments and in the
plane of the medial pterygoid plate; its anterior part
curves out behind the third molar tooth to lie in the
submucosa of the cheek and lips.

Relations:
Posteriorly, buccinator lies in the same plane as the
superior pharyngeal constrictor and is covered by
buccopharyngeal fascia. Superficially, a large mass of fat
separates its posterior part from the ramus of the
mandible, masseter and part of temporalis
Anteriorly, the superficial surface of buccinator is related
zygomaticus major, risorius, levator and depressor anguli
oris,
The parotid duct, pierces it opposite the 3rd upper molar
tooth; crossed by the facial artery, facial vein and branch
of the facial and buccal nerves. deep surface is related to
the buccal glands and mucous membrane of the mouth.

Nerve supply.
Buccinator is supplied by lower buccal branch of
the facial nerve.
Actions.
- Compress the cheeks against the teeth and gums;
- During mastication they assist the tongue in
directing food between the grinding molar teeth.
- When the cheeks have been distended with air, the
buccinators expel it between the lips.
(Latin buccinator = trumpeter)

Mentalis :
Conical fasciculus lying at the side of the frenulum of
the lower lip . the fibers arise from the incisive fossa
of the mandible & descend to attach to the skin of the
chin .
Actions :
• Raises the lower lip , mental tissue & mentolabial
sulcus
• wrinkles the skin of the chin
• Helps in protruding & everting (drinking )
Expresses doubt and disdain.
Nerve supply: lower branch & mandibular
marginal branches of the facial nerve.

modiolus
On each side of the face a number of muscles converge
towards a focus just lateral to the buccal angle, where
they interlace to form a dense, compact, mobile,
fibromuscular mass: the modiolus
• palpated by thumb and index finger
• resembles a cart wheel
• blunt cone
The base of the cone (basis moduli) is adjacent
and adherent to the mucosa;
The blunt apex of the cone (apex moduli) is about 4 mm
across, and is centered about 12 mm lateral to the
buccal angle

Cruciate modiolar muscles:
- zygomaticus major
- levator anguli oris
- depressor anguli oris
- platysma
resemble ‘X’ when vieved from apex
transverse modiolar muscles
- buccinator
- risorius
- various parts of orbicularis oris
- incisive inferioris
- incisive superioris

Direct labial tractors:
pass directly into the tissues of lower lip & not via
the moduli.
the force exerted by tractors is directed vertically at
approx. right angles to the fissure
Upper lip :-- labial part of levator labii superioris
alaque nasi, levator labii superioris & zygomaticus
major
Lower lip :--depressor labii superioris & pltysma
pars labialis
they are divided into
superficial group
intermediate group
deep group www.indiandentalacademy.com

superficial group :- Comprises a succession of fine
fiber bundles that curve anteriorly a short distance
before attaching in a series of horizontal rows to
the dermis between hair follicles, sebaceous &
sweat glands
deep group :- It is closely applied to the anterior
surface of pars peripheralis O.Oris, between the
parallel bundles of which it sends fine tractor
fibers posteriorly to attachments on the submucosa
& periglandular connective

intermediate group:- attaches to the dermis of
modified skin of the red lip , which they reach by
2 routes
• More superficial bundles continue past the
skin, red lip junction,then curve posteriorly over
pars marginalis O. Oris to punctate attachments on
the ventral half of the red lip margins
• Deeper bundles 1st
pass posteriorly between
parsperipheralis &pars marginalis then curve
anteriorly punctate attachment on the dorsal half
of the red lip dermis.

Movements of the lip :
act together or individually for ex partial contraction
of superior labial tractors can result in localized
elevation of segment of upper lip called the
“ canine snarl”
Lip protrusion -passive in its initial state. It may be
suppressed by powerful contrction of whole of
orbicularis oris or enhanced by selective activation
of the parts of the direct labial tractors
Activity of tractors is modified by the superimposed
activity of orbicularis oris & modiolar muscles

ccinator mechanism
Bone is hardest tissue --most responsive to change when
there is an alteration in the environmental balance
The teeth & supporting structure are constantly under the
influence of the contagious musculature.
WINDERS-- During mastication & deglutition, the tongue
may exert 2 or 3 times as much force on the dentition as
the cheeks at any one time . but the net effect is one
balance as tonal contraction ,peripheral fibers recruitment
of the buccal and labial muscles & atmospheric pressure
team of to offset the momentarily greater functional force
of the tongue

The buccinator mechanism runs lateral & posterior around
the corner of the mouth, joining other fibers of the
buccinator muscle which insert into the
pterygomandibular raphe just behind the dentition. At this
point it intermingles with fibers of the superior constrictor
muscle & continues posterior& medially to anchor to the
origin of superior constrictor muscle, the pharyngeal
tubercle of the occipital bone.
Opposing the buccinator mechanism is a very powerfull
muscle--the tongue . The tongue begins its manifold
activities even before birth, when it functions in the
swallowing of amniotic fluid
Best developed stucture in the body at birth
• www.indiandentalacademy.com

Buccinator mechanism
orbicularis orisSuperior
constrictor
pterygomandibular raphe
buccinator

Lip, tongue and cheek balancing forces on the
teeth and supporting bony structures.

Lip, tongue and cheek balancing forces on the
teeth and supporting bony structures

# MOSS states that
coronoid process----temporalis muscles
gonial angle--masseter and medial pterygoid
muscles
# SICHER supra orbital rim as a receptor of the
forces transmitted to it by the canine & zygomatic
pillars.
# LISCHER, included the act of muscle of
mastication as a factor in the restoration of normal
occlusion by orthodontic therapy.
•

Principles of muscle physiology
Einthoven in 1918
muscle in contraction --idiomuscular current --action
current
ALL OR NONE PHENOMENON
The intensity of contraction of any fiber is
independent of the strength of the exciting stimulus
provided that the stimulus is adequate.
Muscle tonus:
slight constant tension characteristic of all healthy muscle,.
Tonus is basis of reflex posture
Ex. Constant minimal contraction of
antigravity muscle to maintain a standing position

Equilibrium Theory
• dentition is in equilibrium
• If a tooth is subjected to a continuous force from an
orthodontic appliance, it does move. The force applied by
the orthodontist has altered the previous equilibrium,
resulting in tooth movement.
• . The bony processes are influenced by the muscles
attached.
• The density of the facial bones, like the skeleton as a
whole, increases when heavy work is done and decreases
in its absence.

Equilibrium Effects on the Dentition:
• Heavy intermittent pressure has no impact on the
long-term position of a tooth
• second possible contributor to the equilibrium is
pressure from the lips, cheeks, and tongue. These
pressures are much lighter than those from
masticatory function, but are also much greater in
duration

In this unfortunate patient, a large portion of the the cheek has
been lost because of a tropical infection. The out-ward splaying of the
teeth when the restraining force of the check is lost illustrates the effect
of a change in equilibrium

• Since the light pressures from lips, cheeks, and
tongue at rest are maintained most of the time, one
would expect these pressures to affect tooth
position.
• If an injury to the soft tissue of the lip results in
scarring and contracture, the incisors in this
vicinity will be moved lingually as the lip tightens
against them.
• On the other hand, if restraining pressure by the
lip or cheek is removed, the teeth move outward in
response to unopposed pressure from the tongue
resulting in labial
displacement of the teeth even though the lips andwww.indiandentalacademy.com

• intermittent shortduration pressures created
when the tongue and lips contact the teeth during
swallowing or speaking would have any
significant impact on tooth position
• masticatory forces, the pressure magnitudes
would be great enough to move a tooth, but the
duration is inadequate.
• same reasoning can be applied thumb sucking
• Whether a behavior pattern is essential or
nonessential, innate or learned, its effect on the
position of the teeth is determined not by the force
that it applies to the teeth, but by how long that
force is sustained.www.indiandentalacademy.com

Equilibrium Effects on Jaw Size
and Shape

• Normal muscle activity associated with
normal jaw relationship

Activity in class II malocclusion mandibular
retrusion & excessive apical base difference

Activity in classII malocclusion with deep
bite,the functional retrusion tendency

references
• Grays anatomy—38th
edition
• Principles and practice of Orthodontics by
T.M.Graber3rd
edition
• Contemporary Orthodontics by proffit
• Embryology –I.b.singh
• Grants atlas of anatomy
• Gyton physiology

journals
• Patterns of electromyographic activity in subjects with
different skeletal facial types Rodolfo Miralles,
Roberto Hevia. AJO DO1991
• Control of condylar growth - Whetten and Johnston –
AJODO 1985 sep
• Changes in masseter muscle activity during orthodontic
treatment evaluated by a 24-hour EMG system Keisuke
Miyamoto AJO DO 1996 NO.3
• The "three M's": Muscles, Malformation and
Malocclusion GRABER 1963 AJO DO

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Muscles of mastication 3

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Muscles of mastication 3