What is the anatomy of the masseter muscle?

Anatomy of the Masseter Muscle

Structural Organization and Layering

The masseter muscle is a three-layered structure consisting of a superficial part, a deep part, and a newly described coronoid part (pars coronoidea), which runs from the medial surface of the zygomatic process of the temporal bone to the root and posterior margin of the coronoid process. 1

Three Distinct Layers

• Superficial part: Forms the outermost layer of the muscle 1
• Deep part: Lies beneath the superficial layer 1
• Coronoid part (pars coronoidea): The most recently described third layer, positioned deepest, originating from the medial surface of the zygomatic process of the temporal bone and inserting onto the root and posterior margin of the coronoid process 1

The coronoid part's fiber arrangement suggests it functions to stabilize the mandible by elevating and retracting the coronoid process 1. This three-layered architecture was consistently demonstrated across multiple anatomical studies using dissection, CT imaging, MRI, and histological analysis 1.

Internal Architecture and Compartmentalization

The masseter contains complex internal tendon architecture that subdivides it into multiple partitions, which can be further subdivided into neuromuscular compartments representing small motor unit territories. 2

• Individual masseter compartments possess unique biomechanical properties that, when activated individually or in groups, generate a wide range of sagittal and off-sagittal torques about the temporomandibular joint 2
• This compartmentalization allows for precise mandibular positioning and force generation during diverse activities including incising and power strokes 2

Innervation Pattern

The masseter muscle receives motor innervation exclusively from branches of the mandibular nerve (V3), the third division of the trigeminal nerve (CN V). 3, 4, 5

• The trigeminal nerve provides branchial motor innervation to all muscles of mastication, including the masseter 3
• The masticatory muscles can be classified into an inner group (lateral pterygoid) and an outer group (masseter, temporalis, medial pterygoid, and transitional bundles) based on nerve branching patterns 5
• The masseter belongs to the outer muscle group with specific positional relationships between muscle bundles and their innervating branches 5

Functional Characteristics

The masseter participates in mastication, swallowing, speech, mandibular positioning, and force generation, with functional demands requiring diverse force vectors determined by its complex internal architecture. 2, 6

• The muscle functions in moving and posturing the mandible during verbalizing, eating, and swallowing 6
• Myosin heavy chain (MyHC) fiber-type distribution in the adult masseter is sexually dimorphic and influenced by testosterone, causing a phenotype switch from slower to faster fiber-types in males 2

Developmental Considerations

• Masseter muscle form is pre-programmed prior to birth, while muscle fiber contractile characteristics are refined postnatally in response to functional requirements 2
• The development of complex organization, MyHC fiber-type expression, and endplate formation appear to be pre-programmed and not under control of the muscle nerve 2
• Secondary myotube generation and endplate maturation are nerve-dependent 2
• The masseter develops later compared with facial, tongue, and jaw-opening muscles, likely related to delayed functional requirements for chewing 2

Clinical Relevance

When evaluating masticatory muscle weakness or dysfunction, imaging should assess the entire course of the mandibular nerve from brainstem through the masticator space, as pathology anywhere along this pathway can affect masseter function. 3

• Masseter muscle integrity can be compromised in temporomandibular dysfunction (TMD) or myogenic disorders, resulting in pain, malfunction, inflammation, or swelling 6
• Bilateral masseter muscle hypertrophy can occur after puberty, causing aesthetic concerns (square-face appearance) and functional problems including limited mouth opening 7
• Masseter muscle spasm following anesthetic administration may indicate malignant hyperthermia susceptibility and requires trigger-free anesthesia 8
• Trismus can develop from radiation therapy affecting masticatory muscles, requiring range of motion exercises and stretching interventions 3