1.2 Muscles of Mastication & Facial Expression

Muscles of Mastication & Facial Expression

Quick Answer: The four muscles of mastication (Masseter, Temporalis, Medial Pterygoid, Lateral Pterygoid) are innervated by the mandibular nerve (CN V3). The lateral pterygoid is the only one of the four primary muscles of mastication that assists opening (depression) and protrusion; the suprahyoid muscles and gravity also help open the jaw, while the other three primary muscles elevate it. In contrast, all muscles of facial expression (including the Buccinator, Orbicularis Oris, and Mentalis) are innervated by the Facial Nerve (CN VII).

Dental assistants must understand the muscular systems of the head and neck to assist effectively during operative, prosthodontic, and oral surgery procedures, position dental dam clamps, maintain moisture control with retractors, recognize signs of temporomandibular dysfunction (TMD), and manage post-injection complications such as trismus.

---

1. Embryological Origin & Dual Innervation Rule

A central organizing principle tested on the DANB NELDA is the strict division between the muscles of mastication and the muscles of facial expression:

   EMBRYOLOGICAL & CRANIAL NERVE DIVISION RULE
   ========================================================================
   1st Branchial Arch  -->  Muscles of Mastication    -->  Trigeminal (CN V3)
   2nd Branchial Arch  -->  Muscles of Facial Expr.   -->  Facial (CN VII)
   ========================================================================

---

2. The Four Primary Muscles of Mastication

The four paired muscles of mastication attach directly to the mandible and execute all chewing movements (elevation, depression, protrusion, retrusion, and lateral excursion).

1. Masseter Muscle

• Description: A thick, quadrilateral, powerful muscle covering the lateral surface of the ramus.
• Origin: Zygomatic arch (superficial head originates from lower border of anterior 2/3; deep head originates from medial surface of posterior 1/3).
• Insertion: Lateral surface of the angle and ramus of the mandible.
• Action: Elevates the mandible (closes jaw) with immense vertical crushing force.
• Innervation: Masseteric nerve (branch of CN V3).
• Clinical Note: Readily palpated extraorally when the patient clenches their teeth; highly hypertrophied in patients with severe bruxism or clenching habits.

2. Temporalis Muscle

• Description: A broad, fan-shaped muscle filling the temporal fossa on the lateral aspect of the skull.
• Origin: Entire temporal fossa bounded by the inferior temporal line.
• Insertion: Coronoid process and anterior border of the mandibular ramus.
• Action:
  • Anterior/Vertical fibers: Elevates the mandible (closes jaw).
  • Posterior/Horizontal fibers: Retracts (retrudes) the mandible posteriorly.
• Innervation: Deep temporal nerves (branches of CN V3).

3. Medial Pterygoid Muscle (Internal Pterygoid)

• Description: A quadrilateral muscle situated on the medial aspect of the ramus, mirroring the masseter externally.
• Origin: Medial surface of the lateral pterygoid plate (sphenoid), pyramidal process of palatine bone, and maxillary tuberosity.
• Insertion: Medial surface of the angle and ramus of the mandible (pterygoid tuberosity).
• Action: Elevates the mandible (closes jaw); assists in lateral excursion.
• The Masticatory Sling: Together, the Masseter and Medial Pterygoid form an anatomical sling that cradles the mandibular angle, stabilizing the jaw and providing massive upward power.
• Innervation: Medial pterygoid nerve (branch of CN V3).

4. Lateral Pterygoid Muscle (External Pterygoid)

• Description: A short, conical, two-headed muscle lying deep in the infratemporal fossa.
• Origins:
  • Superior (Upper) Head: Infratemporal surface and crest of the greater wing of the sphenoid.
  • Inferior (Lower) Head: Lateral surface of the lateral pterygoid plate of the sphenoid.
• Insertions:
  • Superior head inserts directly into the articular capsule and anterior margin of the articular disc (meniscus) of the TMJ.
  • Inferior head inserts into the pterygoid fovea on the anterior neck of the mandibular condyle.
• Action:
  • Bilateral contraction: Depresses the mandible (opens the mouth) and protrudes the mandible forward.
  • Unilateral contraction: Produces lateral excursion (side-to-side chewing movement) by shifting the mandible to the contralateral (opposite) side.
• Innervation: Lateral pterygoid nerve (branch of CN V3).

---

3. Comparative Summary: Muscles of Mastication

---

4. The Temporomandibular Joint (TMJ) & Functional Mechanics

The TMJ is classified as a ginglymoarthrodial joint:

• Ginglymus (Hinge joint): Permits rotational movement around a transverse horizontal axis.
• Arthrodia (Gliding joint): Permits translational sliding movement along the articular eminence.

                      TMJ ARTICULAR ANATOMY
     Temporal Bone (Glenoid Fossa)       Articular Eminence
             \_____________________       /--------\
                                   \_____/          \
      [ Upper Synovial Cavity ] --> TRANSLATION / GLIDING
     ====================================================
          Articular Disc (Dense Fibrous Meniscus)
     ====================================================
      [ Lower Synovial Cavity ] --> ROTATION / HINGE
             /-----------------\
            /  Mandibular       \
           (      Condyle        )
            \                   /
             \                 /
              \--- Neck ------/

Components of the TMJ

1. Articular Disc (Meniscus): A dense, biconcave fibrocartilaginous pad positioned between the temporal bone and mandibular condyle. Its central portion is avascular and aneural to withstand extreme compressive chewing forces.
2. Retrodiscal Tissue (Bilaminar Zone): Highly vascularized and richly innervated connective tissue behind the disc. Compression or inflammation of this tissue is a major source of TMJ pain.
3. Synovial Cavities:
   • Upper Joint Cavity: Located between the disc and the glenoid fossa/articular eminence. Performs gliding/translation during wide mouth opening (protrusion and downward glide).
   • Lower Joint Cavity: Located between the condyle and the disc. Performs hinge/rotation during initial opening (first 20–25 mm of interincisal separation).
4. TMJ Ligaments:
   • Temporomandibular (Lateral) Ligament: Strengthens the lateral capsule and prevents excessive posterior/inferior condylar displacement.
   • Sphenomandibular Ligament: Extends from sphenoid spine to mandibular lingula; acts as a passive suspensory ligament.
   • Stylomandibular Ligament: Extends from styloid process to mandibular angle; limits extreme forward protrusion.

---

5. Major Muscles of Facial Expression (Perioral Group)

All muscles of facial expression insert into the dermis of the skin or adjacent muscles and are innervated by the Facial Nerve (CN VII).

                  KEY PERIORAL FACIAL MUSCLES

                      [ Zygomaticus Major ]  --> Elevates oral angle (Smile)
                                 \
   [ Buccinator ] ------------> [ ORBICULARIS ORIS ] <---------- [ Risorius ]
   (Compesses cheek against     (Encircle lips/closes mouth)     (Pulls angle
    molars; food bolus control)  /                           laterally)
                                /
                      [ Mentalis ]  --> Elevates/wrinkles chin; protrudes lip

Key Facial Muscles Relevant to Dental Assisting

• Orbicularis Oris: Circular muscle surrounding the oral aperture. Closes, compresses, and puckers the lips (the "kissing muscle"). Must be relaxed during intraoral impression taking and photography.
• Buccinator ("Trumpet Player's Muscle"):
  • Forms the major muscular wall of the cheek.
  • Origin: Alveolar processes of maxilla and mandible, and the pterygomandibular raphe.
  • Clinical Function: Compresses the cheek against the molar teeth during mastication, preventing food from collecting in the buccal vestibule and keeping the bolus on the occlusal table.
  • Landmark: Pierced by Stensen's duct (parotid duct) opposite the maxillary second molar.
• Mentalis Muscle:
  • Originates on the anterior mandibular incisive fossa and inserts into the skin of the chin.
  • Elevates and wrinkles the skin of the chin and protrudes the lower lip.
  • Clinical Trap: Hyperactivity or excessive tension in the mentalis muscle can dislodge a mandibular impression tray, complete lower denture, or dental dam frame.
• Zygomaticus Major & Minor: Elevate the angle of the mouth upward and laterally during smiling and laughing.
• Platysma: Broad, thin muscular sheet extending from the clavicle across the neck to the mandible. Depresses the lower lip and assists in jaw depression.
• Levator Labii Superioris & Depressor Labii Inferioris: Elevate upper lip and depress lower lip, respectively.

---

6. Hyoid Muscles: Suprahyoid & Floor of the Mouth

The hyoid muscles coordinate swallowing, tongue movement, and speech:

• Suprahyoid Group (Above Hyoid): Digastric, Mylohyoid, Geniohyoid, Stylohyoid.
  • Mylohyoid Muscle: Originates along the mylohyoid line on the medial mandible and forms the muscular floor of the mouth (intermandibular diaphragm). Elevates the tongue and floor of the mouth during swallowing.
  • Digastric Muscle: Has anterior belly (innervated by CN V3 via mylohyoid nerve) and posterior belly (innervated by CN VII); assists lateral pterygoid in opening the mandible.
• Infrahyoid Group (Below Hyoid): Omohyoid, Sternohyoid, Sternothyroid, Thyrohyoid. Act to depress the hyoid bone and larynx during phonation and terminal swallowing.

---

7. Clinical Practice Traps & Chairside Pearls for DANB NELDA

[!WARNING] DANB Exam Trap #1: Jaw Opening vs. Jaw Closing Questions frequently test which muscle opens the jaw. Three muscles close/elevate the jaw (Masseter, Temporalis, Medial Pterygoid). Only ONE primary muscle of mastication opens/depresses the jaw: the Lateral Pterygoid (assisted by the suprahyoid muscles).

[!CAUTION] DANB Exam Trap #2: Trismus Following Local Anesthetic Delivery Trismus (inability or severe restriction in opening the mouth) following an inferior alveolar nerve block (IANB) most commonly results from the needle inadvertently puncturing and traumatizing the Medial Pterygoid muscle in the pterygomandibular space, resulting in muscle spasm and localized hematoma formation.

[!NOTE] DANB Exam Trap #3: Innervation of the Buccinator Because the buccinator muscle functions intimately during chewing by positioning the food bolus, students often mistakenly classify it as a muscle of mastication innervated by CN V3. The buccinator is a muscle of facial expression innervated by the Facial Nerve (CN VII)! Note: The "long buccal nerve" (from CN V3) provides sensory innervation to the buccal gingiva and cheek mucosa, but does not provide motor innervation to the buccinator muscle.