Introduction 

Mastication is the process to break down food for digestion by means of the masticatory forces generated. It is characterized by cyclic jaw movements in three dimensions (vertical, lateral, and anteroposterior). These various movements are produced by the coordinated contraction of the (jaw) masticatory, facial and tongue muscles. 

Muscles of mastication are divided into primary and accessory muscles. 

Embryology 

The primary muscles of mastication are derived from the first pharyngeal arch from the 7th week of intrauterine life. It is supplied by the mandibular division of the trigeminal nerve. The blood supply is by the second part of the maxillary artery, the terminal branch of the external carotid artery.

Primary Muscles of Mastication

The primary muscles of mastication include temporalis, masseter, medial pterygoid and lateral pterygoid. 

Temporalis:

It is a fan shaped muscle present in the temporal fossa. 

Origin

It originates from the temporal fossa excluding the zygomatic arch and from the temporal fascia.

Anterior fibres run vertically, middle fibres obliquely and posterior fibres run horizontally. All converge and pass through the gap deep to the zygomatic arch.

Insertion 

It inserts into the margins and deep surface of coronoid process and anterior border of the ramus of the mandible.

Nerve supply 

It is innervated by deep temporal nerves(2) of mandibular nerve.

Blood supply 

It is supplied by the deep temporal arteries(2), a branch of the second part of the maxillary artery.

Function

• Elevates the mandible.

• Helps in side to side grinding movements

• Posterior fibres retract the protruded mandible.

Masseter:

Quadrilateral muscle covering the lateral surface of ramus.

Origin

Superficial layer from anterior 2/3rd of lower border of zygomatic arch and adjoining zygomatic process of maxilla. It passes downwards and backwards at 45 degrees.

Middle layer from the lower border of the posterior 1/3rd of arch. It passes vertically downwards.

Deep layer from deep surface of zygomatic arch. It passes vertically downwards.

Insertion 

Superficial layer into lower part of lateral surface of ramus of mandible.

Middle layer into central part of ramus of mandible.

Deep layer into rest of ramus.

Nerve supply 

Massestric nerve of mandibular nerve.

Blood supply 

Masssetric branches of the second part of the maxillary artery.

Function

• Elevates mandible to close the mouth to bite.

• Superficial fibres causes protrusion.

Lateral pterygoid:

Short, conical muscle, has upper and lower heads. It is also known as an external pterygoid.

Origin

Upper head from the infratemporal surface and crest of greater wing of sphenoid bone.

Lower head front he lateral surface of lateral pterygoid plate. 

Fibres run backwards and laterally and converge for insertion.

Origin is medial to insertion.

Insertion 

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

Pterygoid fovea on the anterior surface of neck of mandible

Anterior margin of articular disc and capsule of TMJ.

Nerve supply 

Nerve to lateral pterygoid, a branch from anterior division of mandibular nerve.

Blood supply 

Pterygoid branches of the second part of the maxillary artery.

Function

• Depresses mandible to open mouth, with suprahyoid muscles.

• Protrudes the mandible.

• The right lateral pterygoid turns the chin to the left side and vice versa.

Medial pterygoid:

It is a quadrilateral muscle, also known as internal pterygoid.

Origin

Superficial head from the tuberosity of maxilla and adjoining bone.

Deep head from medial surface of lateral pterygoid plate and adjoining part of palatine bone.

The fibres run downwards, backwards and laterally. The 2 heads embrace part of lower head of lateral pterygoid.

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, a branch of the main trunk of mandibular nerve.

Blood supply 

Pterygoid branches of the second part of the maxillary artery.

Function

• Elevates the mandible.

• Helps in protrusion of mandible.

• The right medial pterygoid along with the right lateral pterygoid turn the chin to the left side as a part of grinding movements.

Accessory Muscles

The accessory muscles of mastication include buccinator, suprahyoid muscles (digastric muscle, mylohyoid muscle, and geniohyoid muscle), and infrahyoid muscles (the sternohyoid, sternothyroid, thyrohyoid and omohyoid muscle). Sometimes platysma is also used to depress the mandible.

Clinical Importance 

Any disorders, derangements in the muscles of mastication lead to various disorders like

• Masticatory myofascial pain

• Masticatory myalgia

• Masticatory myospasm

• Bruxism

• TMJ derangement, etc.