Muscles of Mastication and Temporomandibular Joint

Study Notes

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