Muscles of mastication

Temporalis muscle

The temporalis muscle is a large, flat muscle that lies within the temporal fossa of the skull. This fan-shaped muscle arises from the entirety of the temporal fossa below the temporal line, as well as the deep surface of the temporal fascia. Its muscle fibers converge anteriorly to form a tendon which runs deep to the zygomatic arch. The tendon then inserts on the apex and medial surface of the coronoid process, and the anterior border of the ramus of mandible.

The muscle is innervated by the deep temporal branches of the mandibular nerve, and vascularized by the deep temporal branches of the maxillary artery and middle temporal branches of the superficial temporal artery.

The temporalis muscle functions mainly as an elevator of the mandible. This function is largely produced by its anterior vertical fibres which are continually in action, opposing gravity when the mouth is closed. The contraction of the posterior, more horizontal fibers of the muscle produces a retraction of the mandible, pulling the jaw backwards. In addition, the temporalis muscle contributes to grinding movements by moving the mandible from side to side.

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Masseter muscle

The masseter muscle is a strong, quadrangular muscle that covers the lateral aspect of the ramus of the mandible. It is composed of two layers that slightly differ in their attachments:

• Its larger, superficial layer arises from the maxillary process of the zygomatic bone and the anterior two-thirds of the zygomatic arch. From this origin, these muscle fibers run inferiorly and posteriorly to attach to the lateral surface of the angle and lower half of the ramus of the mandible.
• The deep layer of the masseter muscle arises from the medial surface and inferior margin of the zygomatic arch. These fibers run vertically downwards to insert onto the upper part of the ramus of the mandible and the coronoid process.

The innervation of the masseter muscle stems from the masseteric nerve, a branch of the mandibular nerve. Its blood supply is derived from the masseteric artery, which emerges from the maxillary artery. The major function of the masseter muscle is to elevate the mandible, with a minor contribution to protrusion of the mandible.

Medial pterygoid muscle

The medial pterygoid muscle is a quadrangular muscle situated in the infratemporal fossa. It is composed of two heads that have two sets of origins.

• The larger deep head arises from the medial surface of the lateral pterygoid plate of the sphenoid bone and the adjacent pyramidal process of palatine bone.
• The smaller superficial head originates from the tuberosity of the maxilla.

Both heads encircle the lower fibers of the lateral pterygoid muscle. From their points of origin, the muscle heads converge and run posterolaterally in an oblique fashion to insert on the medial surface of the mandibular ramus, close to the angle of the mandible.

The medial pterygoid muscle is innervated by the medial pterygoid branch of the mandibular nerve. Its principal blood supply stems from the pterygoid branches of the maxillary artery.

The major functions of this muscle are elevation of the mandible and side-to-side movements when grinding and chewing. The medial pterygoid is also involved in protrusion of the mandible.

Lateral pterygoid muscle

The lateral pterygoid muscle is a triangular muscle that lies in the infratemporal fossa. Like the medial pterygoid muscle, the lateral pterygoid has two heads with two distinct origins.

• The smaller, superior head arises from the inferior surface of the greater wing and infratemporal crest of the sphenoid bone, which form the roof of the infratemporal fossa.
• The larger, inferior head originates from the lateral surface of the lateral pterygoid plate of the sphenoid bone.

The fibers from both heads merge and run posterolaterally to insert onto a shallow depression on the anterior aspect of the neck of the mandible called the pterygoid fovea. Additionally, some fibers insert on the joint capsule and articular disc of the temporomandibular joint (TMJ).

The lateral pterygoid muscle is innervated by the lateral pterygoid branch of the mandibular nerve and vascularized by the pterygoid branches of the maxillary artery.

The function of the lateral pterygoid depends on the degree of its contraction. Bilateral contraction of the lateral pterygoid muscles protrudes and depresses the mandible. A unilateral contraction on a particular side, in conjunction with the ipsilateral medial pterygoid muscle, moves the mandible to the opposite side. This allows for alternating side-to-side movements during chewing.

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