Muscles of Mastication

Original Editor - Olajumoke Ogunleye

Top Contributors - Olajumoke Ogunleye, Wendy Walker, Joao Costa, Kim Jackson, Vidya Acharya, Areeba Raja and Ahmed M Diab

Introduction and Overview

• The muscles of mastication are a group of muscles responsible for the chewing movement of the mandible at the temporomandibular (TMJ) joint, they enhance the process of eating, they assist in grinding food, and also function to approximate the teeth.

• The four main muscles of mastication originate from the surface of the skull and they attach onto the rami of the mandible at the TMJ.

• The movement performed by these muscles is elevation, depression, protrusion, retraction, and side to side movement.

• Unlike the muscles of facial expression that are innervated by the facial nerve (CN VII), the muscles of mastication are innervated by motor branches of the mandibular division of the trigeminal nerve (CNV3), while the main arterial supply is derived from branches of the maxillary artery.

Description

The muscles of mastication can be divided into the primary muscles and secondary or accessory muscles. The primary muscles include:

• Masseter
• Temporalis
• Lateral pterygoid
• Medial pterygoid

The secondary or accessory muscles are:

• Buccinator
• Suprahyoid muscles (digastric muscle, mylohyoid muscle, and geniohyoid muscle)
• Infrahyoid muscles (the sternohyoid, sternothyroid, thyrohyoid, and omohyoid muscle)

We will examine the muscles separately.

Masseter

It is a rectangular muscle that covers most of the lateral aspect of the ramus. It consists of three layers which blend anteriorly. They are the superficial layer, intermediate layer, and deep layer.

Origin and Insertion

The fibres of the muscle originate from the inferior zygomatic arch and the anterior two-thirds of the zygomatic arch with a connection to the posterior aspect of the zygomatic bone. The firers converge inferiorly forming a tendon that inserts at the outer surface of the mandibular ramus and the coronoid process of the mandible.

Nerve and blood supply

It is innervated by the masseter muscle from the masseteric nerve which is a branch of the mandibular nerve. Its blood supply is derived from the masseteric artery, which emerges from the maxillary artery.

Function

• The major function of the masseter muscle is to elevate the mandible, approximate the teeth.

• The intermediate and deep muscle fibres of the masseter function to retract the mandible and the superficial fibres function to protrude the mandible.

Temporalis

It is a fan-shaped muscle that fills the temporal fossa, with anterior fibres that have a vertical orientation, mid fibres have an oblique orientation, and posterior fibres have more of a horizontal orientation.

Origin and Insertion

It originates from the temporal fossa to the inferior temporal line of the lateral skull. The temporalis muscle fibres converge inferiorly forming a tendon that exists the temporal fossa passing underneath the zygomatic arch and inserting on the coronoid process of the mandible.

Nerve and blood supply

• It is supplied by the deep temporal nerve which is a branch from the anterior division of man ibular nerve.

• Blood supply of the temporalis muscle is from the deep temporal part of the maxillary artery and the middle temporal branches of the superficial temporal artery.

Function

• The function of the anterior and mid fibres of the temporalis muscle is to elevate the mandible.
• The posterior fibres of the temporalis muscle function to retract the mandible. It also contributes to side to side grinding movement.

Medial Pterygoid

The medial pterygoid muscle is a thick rectangular muscle with a superficial head and a deep head. The deep head of the medial pterygoid is larger than the superficial head.

Origin and Insertion

• The medial pterygoid muscle originates on the pterygoid process, which is a downward pointing process that extends from the sphenoid bone. The superficial head of the medial pterygoid has its origin from the maxillary tuberosity of the inferior maxilla and the deep head originates from the medial surface of the lateral pterygoid plate of the sphenoid bone.

• The medial pterygoid muscle fibres converge inferiorly, forming a tendon that inserts on the medial ramus of the mandible posterior and inferior to the mylohyoid groove of the mandible. The insertion of the medial pterygoid forms a tendinous band with the insertion of the masseter called the pterygoid-masseteric sling

Nerve and blood supply

• It is innervated by the branch of the main trunk of the mandibular nerve.

• Its blood is supplied by a pterygoid branch of 2nd part of the maxillary artery.

Function

The medial pterygoid muscle functions to assist with elevation and protrusion of the mandible. It also assists the lateral pterygoid muscle with side to side mandibular motion to help with the grinding of food.

Lateral Pterygoid

The lateral pterygoid is a short thick muscle with two head. The upper and lower head

Origin and Insertion

• The upper head arises from the infratemporal surface and infratemporal crest of the greater wing of the sphenoid bone. The lower head arises from the lateral surface of a lateral pterygoid plate of the splenoid bone.

• The lateral pterygoid muscle fibres converge inferiorly, forming a tendon that inserts into a depression; pterygoid fovea of the neck of the condylar process of the mandible along with the articular capsule and disc of the temporomandibular articulation.

Nerve and blood supply

• The lateral pterygoid is supplied by a branch of the anterior division of the mandibular nerve.

• Its blood supply is from the pterygoid branch of 2nd part of the maxillary artery.

Function

• The lateral pterygoid muscle functions as the sole muscle of mastication to causes depression of the mandible. This being the case, depression of the mandible is largely the result of gravity.

• It also assists with protrusion and side to side movement of the mandible.

Resources

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