Muscles of Mastication and Temporomandibular Joint

Muscles of Mastication

• Divided into two groups: principal muscles and accessory muscles
• Principal muscles:
  • Origin: from skull
  • Insertion: into ramus of mandible
  • Nerve supply: mandibular nerve (from trigeminal nerve, 5th CN)
  • Action: elevation and protraction of the mandible, except lateral pterygoid muscle which depresses the mandible
• Masseter:
  • Origin: from inner surface and lower border of zygomatic arch
  • Insertion: into outer surface of the ramus of mandible
  • Nerve supply: branch from anterior division of mandibular nerve
  • Action: elevation and protraction of the mandible
• Temporalis:
  • Fan-shaped muscle
  • Origin:
    • Floor of the temporal fossa
    • Temporal lines
  • Insertion: into the coronoid process of the mandible
  • Nerve supply: from the anterior division of the mandibular nerve
  • Action: elevation and retraction of the mandible
• Lateral Pterygoid Muscle:
  • Two heads (lower and upper)
  • Origin:
    • Lower head: from the lateral surface of the lateral pterygoid plate
    • Upper head: from the greater wing of the sphenoid
  • Insertion: into the pterygoid fovea
  • Nerve supply: branch from the anterior division of the mandibular nerve
  • Action:
    • Depression of the mandible
    • Protraction of the mandible
    • Side-to-side movements of the mandible (chewing movement) with medial pterygoid muscle
• Medial Pterygoid Muscle:
  • Two heads
  • Origin:
    • Superficial head: from the maxillary tuberosity
    • Deep head: from the medial surface of the lateral pterygoid plate
  • Insertion: into the inner surface of the ramus of mandible near its angle
  • Nerve supply: branch from the main trunk of the mandibular nerve
  • Action:
    • Elevation and protraction of the mandible
    • Side-to-side movement of the mandible with lateral pterygoid muscle

Tempromandibular Joint (TMJ)

• Type: synovial joint of condylar variety
• Articular surfaces:
  • Mandibular fossa and articular tubercle of temporal bone (above)
  • Head of mandible (below)
  • Intraarticular disc (fibrocartilage disc inside the joint cavity)
• Capsule: attached to margins of articular surfaces and circumference of the intraarticular disc
• Ligaments:
  • Lateral tempromandibular ligament (triangular in shape)
  • Stylomandibular ligament (extends from the tip of the styloid process to the posterior border of ramus of mandible)
  • Sphenomandibular ligament (extends from the spine of sphenoid to the lingula of mandible)
  • Pterygomandibular ligament or raphe (extends from the pterygoid hamulus to the posterior end of mylohyoid line of mandible)

Movements of the Mandible

• Depression (Opening of Mouth): produced mainly by lateral pterygoid muscle, helped by gravity and assisted by digastric, geniohyoid, and mylohyoid muscles
• Elevation (Closing of Mouth): caused by medial pterygoid, masseter, and temporalis muscles
• Protraction: done by lateral and medial pterygoids and masseter
• Retraction: done by posterior fibers of temporalis
• Side-to-side (Chewing) Movements: performed by alternate contraction of medial and lateral pterygoids on each side

Atlanto-Occipital Joint

• Type: synovial joint of ellipsoid variety
• Articular surfaces:
  • Occipital condyles (above)
  • Superior articular facet of the atlas (below)
• Capsule: attached to the margins of the articular surfaces, lined by synovial membrane
• Ligaments:
  • Anterior atlanto-occipital membrane (attached to the upper border of anterior arch of atlas and the anterior margin of foramen magnum)
  • Posterior atlanto-occipital membrane (attached to the upper border of posterior arch of atlas and the posterior margin of foramen magnum)
• Movements: flexion and extension of the head (nodding), lateral flexion of the head

Atlanto-Axial Joints

• Three synovial joints:
  • One median atlantoaxial joint
  • Two lateral atlantoaxial joints
• Lateral Atlantoaxial Joints: plane synovial joints between inferior articular facet of atlas and superior articular facet of axis
• Median Atlantoaxial Joint:
  • Type: synovial joint of pivot variety
  • Articular surfaces:
    • Odontoid process (dens) of axis
    • Articular facet on the posterior surface of the anterior arch of atlas
  • Ligaments:
    • Fibrous capsule
    • Transverse ligament
    • Ligaments connecting the axis with the occipital bone