Muscles of mastication and its

physiology

By : Dr. Komal Sharma

Pg 1st year

Contents

Introduction

Development

Muscles of mastication-Anatomy

Physiology

Periodontal consideration

Conclusion

References

INTRODUCTION

Mastication:

Rhythmic opposition & separation

of jaws with the involvement of

teeth, lips, cheeks, tongue for

chewing of food in order to prepare

it for swallowing and digestion.

Main purpose of mastication is to

reduce the size of food particles to a

size that is convenient for

swallowing (Bolus formation) with

the help of saliva.

Development of muscles of mastication:

Muscles of mastication are derived

from first or mandibular arch (first

arch).

The muscular system develops

from intra embryonic mesoderm

from embryonic cells called

myoblast.

Lateral view of a four week

embryo showing muscles derived

from branchial arches

Muscles of mastication

Primary muscles of mastication:

Masseter

Temporalis

Lateral pterygoid

Medial pterygoid

Secondary muscles of mastication:

The suprahyoid group of muscles being used as secondary or supplementary

muscles. They are:

Digastric

Mylohyoid

Geniohyoid

MASSETER:

Quadrilateral, covers lateral surface

of ramus of mandible and consist of

three layers.

Three layers of masseter

Superficial layer (largest):

Origin:

From anterior 2/3 of lower border of

zygomatic arch and adjoining zygomatic

process of maxilla. Fibers pass

downward and backwards at an angle of

45 degree.

Insertion:

Into lower part of lateral surface of

ramus of mandible

Middle layer:

Origin:

From anterior 2/3 of deep surface and posterior 1/3 of lower border of

zygomatic arch.

Middle and deep fibres pass vertically downwards.

Insertion:

Into middle part of ramus.

Deep layer:

Origin:

From deep surface of zygomatic arch.

Insertion:

Into upper part of ramus and coronoid process of mandible.

Three layers are separated posteroinferiorly by an artery and a nerve.

Nerve supply:

Masseteric nerve, a branch of anterior division of mandibular nerve

Blood supply:

Maxillary artery, which is a branch of external carotid artery.

RELATIONS OF MASSETER SUPERFICIAL LAYER:

Platysma

Risorius

Zygomaticus Major

Parotid gland

Parotid duct

Branches of facial nerve

DEEP SURFACE Overlies the:

Insertion of temporalis and ramus of the mandible

In front buccinator and the buccal nerve

Massetric nerve and artery.

ANTERIOR MARGIN:

Projects over the buccinator and is crossed below by the facial vein.

POSTERIOR MARGIN:

Overlapped by the parotid gland.

Actions of masseter muscle

The whole muscle elevates the mandible

to close the mouth to bite.

The superficial fibers protract the

mandible.

TEMPORALIS MUSCLE:

Fan shaped, fits the temporal fossa.

Temporal Fascia:

Thick aponeurotic sheet - roofs over the temporal fossa & covers temporalis

muscle.

Superiorly, fascia- single layered & attached to superior temporal line.

Inferiorly, it splits into 2 layers which are attached to inner & outer lips of upper

border of the zygomatic arch.

Small gap b/w 2 layers contains - fat, a branch from the superficial temporal

artery & the zygomatico-temporal nerve.

Origin:

Temporal fossa excluding zygomatic bone.

Temporal fascia

Fibres Converge and pass through gap deep to zygomatic arch.

Insertion:

Margins and deep surface of coronoid process.

Anterior border of ramus of mandible.

Nerve supply:

Two deep temporal branches from anterior division of mandibular nerve.

Blood supply:

Deep temporal part of maxillary artery.

RELATIONS OF TEMPORALIS SUPERFICIAL:

Skin

Auricularis anterior

Temporal fascia

Superficial temporal vessels

Auriculotemporal nerve

Temporal branch of facial nerve

Zygomatic arch

Masseter

DEEP SURFACE:

Temporal fossa

Lateral pterygoid

Superficial head of medial pterygoid

Small part of buccinator

Maxillary artery

Deep temporal nerves

Buccal vessels and nerves

ANTERIOR BORDER:

Separated from zygomatic bone by a mass of fat

ACTIONS OF TEMPORALIS:

Elevates mandible

Posterior fibres retract the

protracted mandible

Helps in side to side grinding

movement

LATERAL PTERYGOID MUSCLE: SHORT, CONICAL, HAS UPPER

AND LOWER HEADS.

ORIGIN:

UPPER HEAD(small):

From infratemporal surface and crest of greater wing of sphenoid bone.

LOWER HEAD(larger):

From lateral surface of lateral pterygoid plate.

Fibers run backwards and laterally and converge

INSERTION:

PTERYGOID FOVEA

On the anterior surface of neck of mandible

Anterior margin of articular disc and capsule of temperomandibular

joint.Insertion is posterolateral and at a slightly higher level than origin.

Nerve supply:

A branch of anterior division of mandibular nerve

Blood supply:

Pterygoid branch of 2nd part of maxillary artery.

Relations of lateral pterygoid muscle

ACTIONS OF LATERAL PTERYGOID MUSCLE:

Depress mandible to open mouth, with suprahyoid muscle

Protract mandible

Left lateral pterygoid and right medial pterygoid turn the chin to left side as part

of grinding movements.

MEDIAL PTERYGOID MUSCLE: Quadrilateral, has a smallsuperficial and

a large deep head.

ORIGIN:

SUPERFICIAL HEAD (SMALL SLIP):

From tuberosity of maxilla and adjoining bone.

DEEP HEAD (QUITE LARGE):

From medial surface of lateral pterygoid plate and adjoining process of palatine

bone.

Fibres run downwards,backwards and laterally.

INSERTION:

Roughened area on the medial surface of angle and adjoining ramus of

mandible, below and behind the mandibular foramen and mylohyoid groove.

NERVE SUPPLY:

Nerve to medial pterygoid, branch of main trunk of mandibular nerve.

BLOOD SUPPLY:

Pterygoid branch of 2nd part of maxillary artery.

Relations of Medial Pterygoid:

Superficial Relations

The upper part of the muscle is separated from the lateral pterygoid

muscle by:

The lateral pterygoid plate;

The lingual nerve;

The inferior alveolar nerve

Deep Relations

The relations are:

Tensor veli palatini;

Superior constrictor of pharynx;

Styloglossus

Stylopharyngeus attached to the styloid process

ACTIONS OF MEDIAL

PTERYGOID:

Elevates mandible

Helps protraction of mandible

Right medial pterygoid with left

lateral pterygoid turn the chin to

left side

SECONDARY MUSCLES TAKING PART IN THE MASTICATION

4 primary muscles of mastication are in turn supported or supplemented by few

secondary muscles known as SUPRAHYOID GROUP of muscles they are:

Digastric

Mylohyoid

Geniohyoid

DIAGASTRIC MUSCLE: Two

bellies united by tendon

Origin –

Anterior belly from diagastric

fossa of mandible. Posterior belly

from mastoid notch of temporal

bone.

Insertion –Both meet at the

intermediate tendon and held by

the fibrous pulley.

ACTIONS:

Muscle - secondary role in mastication as depressor muscle + to action of

lateral pterygoid when mouth to be opened against resistance.

Elevation of hyoid bone

MYLOHYOID MUSLE: Flat

triangular

Origin –Mylohyoid line of

mandible.

Insertion – Middle & Anterior

fibers into median raphae. Posterior

fibers body of hyoid bone.

ACTION:

The secondary role of this muscle is evident as a depressor seen in action when

mouth is to be opened against resistance.

It elevates the floor of mouth to help in deglutition

GENIOHYOID: Short and narrow

muscle lies above mylohyoid

Origin –Inferior mental spine

Insertion – Anterior surface of

body of hyoid bone

ACTIONS:

Geniohyoid elevates the hyoid bone and draws it forward, thus acting as a

partial antagonist to stylohyoid.

When the hyoid bone is fixed, it depresses the mandible

Physiology: Neuromuscular

transmission The function of masticatory

system is complex.

A highly refined neurologic

control system regulates and co-

ordinates the activities of the

entire masticatory system.

It consists primary of nerves and

muscles, hence the term neuro-

muscular system.

Basic component - neuro-muscular system is motor unit consists of muscle

fibres - innervated by 1 motor neuron each neuron joins muscle fibres at motor

end plats- thick sarcolemma.

Cell membrane of nerve terminal or “PRE-JUNCTIONAL MEMBRANE” and

muscle fibre (end) plated - “POST JUCTIONAL MEMBRANE”.

Space b/w pre and post junctional membrane is called as “NEURO-

MUSCULAR CLEFT”.

Cleft contains choline esterase enzyme (which destroys acetyl choline).

Motor nerve terminal contains abundant mitochondria and vesicles containing

Ach (Acetylcholine).

Receptors of Ach are present in post junctional membrane (motor and plate).

SEQUENCE OF EVENTS IN NMT

Nerve impulse (action potential)reaches presynaptic nerve ending .

As it reaches presynaptic membrane it causes release, diffuses within few

hundred microsec. Across the very short distance to the post synaptic

membrane i.e. motor end plate.

A-ch attaches to nicotinic A-ch receptors on motor end plate surface & ses

permeability of motor end plate to Na+ (mainly) & other positive ions .

sed permeability of Na+ -- depolarization of post synaptic membrane--

generation of local potential ,called end plate potential

Resting membrane potential in skeletal muscle membrane is -90 mV. when end

plate potential reaches a threshold of 30-40 mV, it depolarizes the surface

membrane of muscle & results in generation of action potential (magnitude 120-

130 mV)

Spike potential thus sets up a propagated muscle action potential which can

travel in both directions along the muscle membrane.

Once it reaches the muscle cell then the muscle gives mechanical response by

contraction.

Clinical importance of NM junction:

Blocking of NM junction produces muscle relaxation, therefore:

Helps in surgical operations by providing open fields.

Reduces movements during electroconvulsive treatment of psychotic patients

Blockage of NM junction can be achieved by two ways:

By inhibiting release of A-ch from presynaptic membrane i.e. motor nerve

endings eg botulinum toxin

Drugs whih antagonize the action of A-ch on the post –synaptic membrane i.e.

motor end plate.

MUSCLE FUNCTION

Motor unit can carry only one action i.e. contraction or shortening, entire

muscle, has 3 potential function.

ISOTONIC CONTRACTION

When muscle shortens & moves a load, the contraction is isotonic. Hence

load remains constant & equal to muscle tension throughout the most of the

period of contraction

Occurs in the masseter, when the mandibular elevated forcing the teeth

through a bolus of food.

ISOMETRIC CONTRACTION

When a muscle does not shortern and length remains same (iso- same, metry-

length), but develops tension, the contraction is isometric.

Such type of contraction occurs when muscle attempts to move a load that is

greater than the tension developed in muscles, this occurs in masseter when an

object is held between the teeth. eg. Pipe or pencil

CONTRACTION RELAXATION

Stimulation of motor unit discontinued ----the fibres of motor unit relax &

return to their normal length.

Seen in masseter when mouth opens to accept new bolus of food during

mastication.

Periodontal consideration

BRUXISM:

Jaw clenching, with or without forcible excursive movements, where the

intensity of the clenching dictates the severity (or lack of) grinding.

Clenching- It can occur as a brief rhythmic strong contractions of the jaw

muscles during eccentric lateral jaw movements, or in maximum

intercuspation,

Bruxism is defined as diurnal or nocturnal parafunctional activity

including clenching, bracing, gnashing, & grinding of teeth.

Bruxism has potential to cause :

Tooth wear

Fracture

Periodontal & muscle pain

Major cause of mobility.

Causes:

Associated with stressful events

Non stress related or hereditary

Bruxism may lead to -tooth wear .

Treatment -coronoplasty -maxillary stabilization appliance

Clinical consideration

Masseter muscle hypertrophy is commonly seen in bruxism.

Bruxism is also associated with an increased rate of implant failure.

Many consider bruxism to be a contraindication to implant treatment.

Protective measures for implant patient with bruxism :

Creating narrow occlusal table with flat cusp angles

protected occlusion

regular use of occlusal guards.

MYOFASCIAL PAIN DYSFUNCTION SYNDROME(MPDS)

When muscle spasm develops , dysfunction as well as pain occurs and the condition

usually is designated as MPDS.

It is initiated as spasm of one or more masticatory muscle.

Most commonly involved muscles: lateral pterygoid and medial pterygoid muscle

Etiology:

Abnormal occlusion

Prosthetic problems

Malocclusion

Emotional problems

Hypermobility

Clinical features:

Age and sex distribution: Seen in middle age group with more predilections

for women.

Onset: Occurs in episodes of several times a day, at times, with extended

symptom free intervals. Usually during increased emotional tension.

Symptoms:

Masticatory pain- due to myalgia or arthralgia

Pain localized to preauricular area but radiate to temporal, frontal,& occipital

region.

Difficulty in chewing and restriction of mandible excursion.

Patient complains of noise on rubbing, grinding, clicking,and popping snapping

sounds on mandibular movement.

Laskin’s cardinal criteria of MPDS

Fascial space infection

Fascial spaces or compartments are generally "potential spaces" that become

opened or expanded by invading infection that intervenes between the structures

surrounding the space.

Such spaces are of particular significance in the head and neck as they may

serve as pathways for the spread of infection from one region to another.

Buccal space

Boundaries:

Superiorly: zygomatic arch.

Inferior: inferior border of mandible.

Laterally: skin & subcutaneous tissue.

Medially: buccinator muscle

,buccopharyngeal fascia.

Posteriorly: anterior edge of masseter

muscle.

Anteriorly: posterior border of

zygomaticus major & depressor anguli

oris.

Contents: Buccal fat pad, Stenson’s duct.

Facial artery.

Etiology:

Infected mandibular &

maxillary premolars & molars.

Clinical Features:

Obliteration of nasolabial fold.

Angle of mouth shifted to

opposite side.

Swelling in cheek extending to

corner of mouth.

Submental space

Submental space:

Boundaries:

Roof: mylohyoid muscle.

Inferior: deep cervical fascia,

platysma, superficial fascia & skin.

Laterally: anterior belly of

digastric.

Posteriorly: submandibular space.

Contents: Lymph nodes, anterior

jugular vein.

Etiology:

Infected mandibular incisors.

Anterior extension of

submandibular space.

Clinical Features

Chin appears glossy & swollen.

Pain & discomfort on swallowing

Submandibular Space

Boundaries:

Superiorly: mylohyoid muscle,

inferior border of mandible.

Inferior: anterior & posterior belly of

digastric.

Laterally: deep cervical fascia,

platysma, superficial fascia & skin.

Medially: hyoglossus, styloglossus,

mylohyoid muscle.

Posteriorly: to hyoid bone.

Anteriorly: submental space.

Contents:

Submandibular salivary gland

Proximal portion of Wharton’s duct

Lingual & hypoglossal nerves

Branches of facial artery- palatine, tonsillar, glandular, submental.

Etiology:

Infected mandibular 2nd & 3rd

molars.

From submental, sublingual

spaces.

Clinical Features:

Indurated swelling in

submandibular region.

Usually bulges over lower border

of mandible.

Pterygomandibular Space

Boundaries:

Superiorly: lower head of lateral

pterygoid muscle.

Laterally: medial surface of ramus.

Medially: medial pterygoid muscle.

Posteriorly: deep part of parotid.

Anteriorly: pterygomandibular raphe.

Contents: Inferior alveolar

neurovascular bundle, Lingual &

auriculotemporal nerves, Mylohyoid

nerve & vessels.

Etiology:

Infected mandibular 3rd molars.

Pericoronitis.

Infected needles or contaminated LA

solution.

Clinical Features:

Absence of extra-oral swelling

Severe trismus.

Difficulty in swallowing.

Anterior bulging of half of soft palate

& tonsillar pillars with deviation of

uvula to unaffected side.

Ludwig’s angina

It is a rapidly swelling cellulitis of

the sublingual and submaxillary

spaces, often arising from infection

of the tooth roots that extent below

the mylohyoid line of the mandible.

Involves submandibular, submental

and sublingual spaces bilaterally.

Most commonly encountered neck

space infection.

Fatal complications:

Respiratory obstruction

Generalized septicemia

Erosion of carotid artery

Cavernous sinus thrombosis

Clinical features:

Swelling is firm, painful & diffused

Woody tongue

Bull neck

Local anesthesia

Inferior alveolar nerve block: Also known as Mandibular block

Nerves anesthetised:

Inferior alveolar nerve

Incisive

Mental

Lingual

Indication:

Procedures on multiple mandibular teeth in one quadrant.

Area anesthetized:

Mandibular teeth,

Body & ramus of mandible

Buccal mucoperiosteum

Anterior two third of tongue (lingual nerve).

Procedure:

Height of injection determined by imaginary line extend posteriorly from

coronoid notch to pterygomandibular raphe as it turns upwards towards maxilla.

Anteroposterior site on injection: about three fourths the distance from anterior

border of ramus.

Penetration depth: till bone is touched usually 20-25 mm.

Complication:

Trismus

Transient facial palsy.

Hematoma.

Trismus

Trismus is a spasm of the muscles of mastication where opening of the mouth

becomes restricted.

Most commonly seen following an IA nerve block. Since an IA nerve block

will almost always cause soreness because the needle is inserted through

through muscle, causing an injury to the tissue by the needle.

Since an IA nerve block injures the muscles of mastication specifically the

medial pterygoid muscle, the patient constantly irritates the tissue when talking

and eating.

Trismus is more common when dental professionals have difficulty getting an

IA nerve block and require multiple cartridges of local anesthesia to succeed.

The tissues have been injured multiple times and larger volumes of fluid have

been deposited each time, stretching the tissue.

Patient does not notice it while in the

dental chair due to numbness but when

the patient wakes up the following

morning after 6-8 hours of not using

the mastication muscles, the muscles

go into spasm.

Gradually over the next day or so, the

patient becomes able to open his or

mouth a little more and eventually

maybe within 3 or 4 days, trismus is

gone.

Management includes using a warm,

moist compress on the area, but it just

takes time to go away.

PALPATION OF MASTICATORY

MUSCLES

Digital palpation: Regional muscles are examined for the tenderness and trigger

points using the digital palpation.

Temporalis muscle: Temporalis muscle can be seen and readily palpated

throughout entire length and breadth when the patient’s teeth are firmly

clenched.

Masseter muscles: They are most effectively examined by simultaneously

pressing them from inside and outside the mouth in the process of bimanual

palpation.

Lateral pterygoid muscle: The lateral pterygoid muscles are evaluated by

inserting a finger each behind the maxillary tuberosities.

Medial pterygoid is checked by running a finger in an anteroposterior direction

along the medial aspect of the mandible in the floor of mouth.

Masseter muscle

Temporalis muscle

Lateral pterygoid

Medial pterygoid

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