Head and Neck Anatomy

Head Anatomy and Function

Overview of the Chapter

  • Focus on the head; the neck is addressed in Chapter 9 (Neck and Trunk).

  • The atlanto-occipital joint is also discussed in Chapter 9.

Structure of the Skull

  • The head comprises the skull, divided into:
        - Cranial Bones: Protects the brain.
        - Facial Bones: Provides structure to the face.

  • Cranium:
        - Contains multiple bones that form a protective case for the brain.

  • Facial Bones:
        - The mandible articulates with the temporal bone at the temporomandibular joint (TMJ).

Temporomandibular Joint (TMJ)

  • A synovial joint responsible for jaw movement.

  • Notable articulations include:
        - Elevation/Depression
        - Protraction/Retraction
        - Lateral Deviation

  • Movement dynamics in TMJ:
        - Elevation and depression occur synchronously in both TMJs.
        - Protraction and retraction follow a similar rule of simultaneous action in both joints.
        - During lateral deviation, the left TMJ moves laterally while the right moves medially.

Movements at the TMJ

  1. Mandibular Depression: Lowering the jaw, which opens the mouth.

  2. Mandibular Elevation: Raising the jaw, which closes the mouth.

  3. Mandibular Protraction: Moving the jaw forward.

  4. Mandibular Retraction: Moving the jaw backward.

  5. Lateral Deviation: Side-to-side movement of the jaw.

  • Normal range of mandibular depression is approximately 2 inches between upper and lower teeth.

Movements in Joint Spaces

  • The TMJ consists of:
        - Upper Joint Space: Involves the retrodiscal tissue and gliding motions.
        - Lower Joint Space: Involves rolling of the condyle on the disc.

  • Depressing the Mandible:
        - 1st Movement: Concave/Convex Rule applied; as the mandibular condyle rolls on the disc, it glides anteriorly.
        - 2nd Movement: The disc glides anteriorly and inferiorly under the articular tubercle.

  • Elevation Sequence is the reverse of depression.

Bones and Landmarks

Cranial Bones (8 Total)

  • Frontal (1)

  • Temporal (2)

  • Sphenoid (2)

  • Parietal (2)

  • Occipital (1)

  • Joints between cranial bones are synarthroses (fused in adults).

Major Facial Bones (5 Total)

  • Maxilla (2)

  • Zygomatic (2)

  • Mandible (1)

  • The mandible forms the TMJ with the temporal bone.

  • The maxillae contribute to the overall protection of the CNS.

Landmarks of Specific Bones

Occipital Bone

  • Forms the base of the cranium.

  • Occipital Condyles: Articulate with the atlas.

  • External Occipital Protuberance: Midline prominence for muscle attachment.

  • Foramen Magnum: Large hole for spinal cord passage.

Frontal Bone

  • Forehead region; forms part of temporal fossa.

  • Orbital opening: Contributes to the eye socket.

Parietal Bone

  • Sits between the frontal and occipital bones, contributing to the lateral cranium.

Temporal Bone

  • Positioned laterally; key features include:
        - Articular Tubercle: Supports TMJ condensation.
        - Mandibular Fossa: Articulates with the mandibular condyle.
        - External Auditory Meatus: Ear canal.

Landmarks of the Sphenoid Bone

  • Forms part of the base of the skull and articulates with various bones.

  • Major features include the Greater Wing and the Lateral Pterygoid Plate.

Mandible

  • Anterior-inferior horseshoe-shaped bone articulated with the TMJ.

  • Important components include:
        - Body: Contains the lower teeth.
        - Ramus: Connection with the condyle.
        - Coronoid Process: Muscle attachment for masseter.
        - Mental Spine: Provides attachment for geniohyoid muscle.

Unique Structures of the Head

Hyoid Bone

  • A small horseshoe-shaped bone that supports the tongue and provides attachment for neck muscles.

  • No bony articulations. Suspended from styloid processes.

Thyroid Cartilage

  • Largest cartilage of the larynx, prominent in males, provides attachment for infrahyoid muscles.

TMJ Articular Disc

  • Fibrocartilage disc: Divides joint space.

  • Attached circumferentially to the capsule of TMJ.

Joints in the Head

Cranial Joints

  • All fibrous joints, termed sutures. Provide rigidity and protect the CNS.

Facial Joints

  • Formed by irregular bone fusions, such as the maxillae and zygomatic bones.

Review of the TMJ

  • A modified hinge joint (synovial), facilitating complex jaw movements via a disc structure.

Ligaments

  • Numerous ligaments support the TMJ, such as:
        - Lateral Ligament (Temporomandibular Ligament): Limits movements of the mandible.
        - Sphenomandibular Ligament: Suspends the mandible.
        - Stylomandibular Ligament: Limits anterior movements.

Muscles Involved with TMJ

  • Elevating Muscles: Temporalis, masseter, medial pterygoid

  • Depression: Lateral pterygoid

  • Protrusion: Lateral and medial pterygoid

  • Retrusion: Temporalis (posterior fibers)

  • Lateral Deviation: Medial pterygoid, lateral pterygoid across the sides.

Temporalis Muscle

  • Origin: Temporal fossa

  • Insertion: Coronoid process and ramus of the mandible

  • Action: Elevates/retracts mandible; lateral deviation unilaterally.

Masseter Muscle

  • Prime mover for jaw elevation.

  • Consists of superficial and deep parts.

  • Origin: Zygomatic arch

Medial and Lateral Pterygoid Muscles

  • Both contribute to various mandible movements, enabling complex actions during chewing.

Suprahyoid Muscles

  1. Mylohyoid: Supports tongue and floor of the mouth.

  2. Geniohyoid: Elevates hyoid and tongue.

  3. Stylohyoid: Elevates hyoid and tongue.

  4. Digastric: Elevates hyoid; has anterior and posterior bellies.

Infrahyoid Muscles

  1. Sternohyoid: Depresses hyoid/larynx.

  2. Omohyoid: Depresses/retracts hyoid/larynx.

  3. Sternothyroid: Depresses thyroid cartilage and hyoid.

  4. Thyrohyoid: Elevates thyroid cartilage, depresses hyoid.

Facial Muscles

  • Involved in facial expressions, each muscle has specific actions ranging from elevating eyebrows to compressing the cheeks.

  • Some examples:
        - Zygomaticus Major: Pulls mouth corners upward (smiling).
        - Buccinator: Compresses cheek, directs food between molars.
        - Frontalis: Lifts eyebrows (surprise).

Ocular Muscles

  • Control eye movements, including:
        - Rectus Superior/Inferior: Upward/downward rotation.
        - Orbicularis Oculi: Closes eyelids.

Tongue Muscles (Glossus)

  • Includes:
        - Genioglossus: Protrudes tongue.
        - Hyoglossus: Depresses tongue.

Clinical Applications

Application 8-1: TMJ Mechanics

  • Mechanical Advantage: TMJ acts as a third-class lever. Resistance arm longer than the force arm, needing greater force for a bite.

Application 8-2: Postural Influence

  • Poor posture impacts TMJ alignment, which can lead to dysfunction in biting, chewing, and swallowing.

Summary of Innervations

  • TMJ muscles are primarily innervated by the trigeminal nerve (CN V) with additional contributions from the facial nerve (CN VII) and hypoglossal nerve (CN XII) for suprahyoid and infrahyoid muscles.

Common Pathologies Related to TMJ

  • TMJ dysfunction: Associated with pain, reduced range of motion, or clicking sounds in the joint.

  • Bruxism: Teeth grinding can worsen TMJ disorders and lead to further complications.

  • Examining cervical posture is important for treatment.