What is the anatomy of the masseter muscle, including its origin, insertion, nerve supply, and function?

Masseter Muscle Anatomy

Origin

The masseter muscle originates from two distinct layers with separate bony attachments on the zygomatic arch and temporal bone. 1

• Superficial layer: Arises from the anterior two-thirds of the lower border of the zygomatic arch 1
• Deep layer: Originates from the posterior third and medial surface of the zygomatic arch 1
• Coronoid part (third layer): A newly described distinct layer runs from the medial surface of the zygomatic process of the temporal bone—this represents the deepest portion of the muscle 1

The traditional teaching of a two-layered masseter has been updated by recent anatomical studies demonstrating this consistent third layer, which was historically mentioned but never properly characterized until 2022. 1

Insertion

The masseter inserts along the lateral surface of the mandibular ramus and angle, with the newly described coronoid part inserting specifically on the coronoid process. 1

• Superficial layer: Inserts on the angle and lower half of the lateral surface of the mandibular ramus 1
• Deep layer: Inserts on the upper half of the mandibular ramus and lateral surface of the coronoid process 1
• Coronoid part: Inserts specifically on the root and posterior margin of the coronoid process 1

Some masseter fibers also attach to the anterior surface of the temporomandibular joint disc in certain individuals, contributing to disc stabilization during mastication. 2

Nerve Supply

The masseter muscle receives motor innervation exclusively from the mandibular division of the trigeminal nerve (V3, cranial nerve V). 3, 4

• The mandibular nerve provides branchial motor innervation to all muscles of mastication, including the masseter 3
• When evaluating masticatory muscle weakness, the American College of Radiology recommends imaging the entire course of the mandibular nerve from brainstem through the masticator space, as pathology anywhere along this pathway can affect masseter function 4
• The innervation pattern follows a complex branching system where nerve branches reach various muscle bundles of the outer masticatory muscle group, which includes the masseter 5

Clinical Pitfall

Masseter muscle spasm following anesthetic administration may indicate malignant hyperthermia susceptibility and requires immediate recognition and trigger-free anesthesia. 4

Function

The masseter functions primarily as a powerful elevator of the mandible during mastication, with the newly described coronoid part specifically stabilizing the mandible through elevation and retraction of the coronoid process. 1, 6

Primary Functions:

• Mandibular elevation: The masseter is one of the strongest muscles of mastication, generating significant bite force 7
• Mandibular stabilization: The coronoid part elevates and retracts the coronoid process to stabilize the mandible during complex jaw movements 1
• Continuous loading: The masseter loads the mandible almost continuously throughout the day, either within cyclic activity bouts or with thousands of isolated muscle bursts 6

Integrated Functions:

• Mastication: Works with other masticatory muscles in chewing movements 7
• Verbalization: Contributes to speech through mandibular positioning 7
• Swallowing: Assists in the oral phase of swallowing 7
• Disc stabilization: In some individuals, anterior fibers attach to the TMJ disc and pull it anteriorly to steady it against masticatory movement 2

Biomechanical Characteristics:

• The masseter muscle activity is strongly linked to mandibular bone-strain amplitude—larger muscle activity produces larger bone-strain events 6
• Muscle-to-bone force transmission occurs at frequencies up to 15 Hz during habitual loading 6
• The muscle rarely allows mandibular strain events without simultaneous masseter activity, demonstrating its non-stop role in mandibular loading 6