Head, Neck, and Intrinsic Back Muscles

Objectives

• Muscles of the head, neck, and intrinsic back are part of the Axial muscles.
• The muscles of the head are further divided into muscles used for facial expression, and mastication.
• Muscles of the neck are divided into Suprahyoid and Infrahyoid muscles.
• Intrinsic back muscles are divided into superficial and intermediate muscles.
• Deep muscles are a third group, but are not covered.

Muscles of Head

• The muscles of the head are divided into two groups: muscles of facial expression and muscles of mastication.
• Facial Expression muscles are supplied by Cranial Nerve VII (Facial Nerve).
• Mastication (chewing) muscles are supplied by Cranial Nerve V3 (Trigeminal Nerve: Mandibular Division).

Cranial Nerve (VII)

• All muscles of facial expression are innervated by Cranial Nerve VII (Facial Nerve).
• These muscles insert into either the skin of the face or blend with another facial muscle.

Muscles of Facial Expression

• Occipitofrontalis (Frontal Belly)
• Originates: Epicranial aponeurosis
• Action: Moves scalp, eyebrows, wrinkles skin of forehead
• Innervation: CN VII (Facial N.)
• Occipitofrontalis (Occipital Belly)
• Originates: Epicranial aponeurosis
• Action: Retracts Scalp
• Innervation: CN VII (Facial N.)
• Orbicularis Oculi
• Originates: Skin surrounding eyelids
• Action: Closes eye (winking, blinking, squinting)
• Innervation: CN VII (Facial N.)
• Orbicularis Oris
• Originates: Encircles mouth; skin and muscles at angle to mouth
• Action: Compresses and purses lips (kiss muscle)
• Innervation: CN VII (Facial N.)
• Zygomaticus
• Originates: Skin at superolateral edge of mouth
• Action: Elevates corner of mouth (smile muscle)
• Innervation: CN VII (Facial N.)
• Buccinator
• Originates: Orbicularis oris
• Action: Compresses cheek; holds food between teeth when chewing
• Innervation: CN VII (Facial N.)
• Platysma
• Originates: Skin of cheek and mandible
• Action: Pulls lower lip inferiorly, tenses skin of neck
• Innervation: CN VII (Facial N.)

Cranial Nerve (V3)

• All muscles of mastication are innervated by the Mandibular Division of the Trigeminal Nerve (CN V3).

Superficial Muscles of Mastication

• Temporalis
• Insertion: Coronoid process of Mandible
• Action: Elevates and retracts Mandible
• Innervation: Mandibular Division of Trigeminal Nerve (CN V3)
• Masseter
• Insertion: Coronoid process, lateral surface, and angle of mandible
• Action: Elevates and protracts Mandible; prime mover of jaw closure
• Innervation: Mandibular Division of Trigeminal Nerve (CN V3)

Deep Muscles of Mastication

• Medial Pterygoid
• Insertion: Medial surface of mandibular ramus
• Action: Elevates and protracts mandible; produces side-to-side movement of mandible (grinding)
• Innervation: Mandibular Division of Trigeminal Nerve (CN V3)
• Lateral Pterygoid
• Insertion: Condylar process of mandible
• Action: Protracts mandible; produces side-to-side movement of mandible (grinding); depresses mandible
• Innervation: Mandibular Division of Trigeminal Nerve (CN V3)

Important Anatomical Landmarks of the Neck

• Hyoid Bone
• Unique in that it doesn't articulate with any other bone.
• Keeps the airway open superiorly.
• Located at the C3 vertebra level.
• Thyroid & Cricoid Cartilage
• Protects the Larynx (voicebox).
• Provides a site for muscle attachment crucial in vocalization and swallowing.

Sternocleidomastoid (SCM)

• Broad, straplike muscle deep to platysma
• Origin: sternum & clavicle
• Insertion: mastoid process (temporal bone)
• Unilateral action: lateral flexion, rotation of head to opposite side
• Bilateral action: flexes neck
• Innervated by CN XI (accessory nerve)
• Key LANDMARK for triangles of the neck which is the anterior triangle

Suprahyoid Muscles

• Digastric
• Insertion: Hyoid Bone
• Action: Depresses mandible; elevates hyoid bone
• Mylohyoid
• Insertion: Hyoid Bone
• Action: Elevates hyoid bone; elevates floor of mouth

Infrahyoid Muscles

• Sternothyroid
• Origin: Sternum
• Insertion: Thyroid cartilage
• Action: Depresses thyroid cartilage
• Thyrohyoid
• Origin: Thyroid cartilage
• Insertion: Hyoid bone
• Action: Depresses hyoid bone; elevates thyroid cartilage
• Sternohyoid
• Origin: Sternum
• Insertion: Hyoid bone
• Action: Depresses hyoid bone
• Omohyoid
• Origin: Scapula
• Insertion: Hyoid bone
• Action: Depresses hyoid bone; fixes hyoid during opening of mouth

Intrinsic Back Muscles

• "Back muscles proper" or "Deep back muscles”
• Innervation comes from posterior rami segmentally (from spinal nerve at specific level of muscle).
• Maintain posture and control movements of the vertebral column, including the cranium.
• Groups: Superficial, Intermediate, and Deep (not covered).

Intrinsic/Superficial Group: Splenius Muscle

• Splenius Capitis
• Origin: Spinous process of C7 to T4
• Insertion: Mastoid Process
• Nerve Supply: Posterior Rami
  • If acting unilaterally, laterally flexes and rotates to ipsilateral side
  • If acting bilaterally, extends head & neck
• Splenius Cervicis
• Origin: Spinous process of C7 to T4
• Insertion: Transverse Process of C1-C3 or 4
• Nerve Supply: Posterior Rami
  • If acting unilaterally, laterally flexes and rotates to ipsilateral side
  • If acting bilaterally, extends head & neck

Intrinsic/Intermediate Group: Erector Spinae

• Consists of three groups of muscle fibers:
• Iliocostalis (most lateral)
• Longissimus
• Spinalis (most medial - next to spinous processes of vertebrae)
• All three muscles have these features
• Origin:
  • Broad Tendon from iliac crest
  • Posterior Sacrum
  • Sacral and Lumbar spinous processes
• Insertion: Extensive Insertion: Ribs, Vertebrae, and Skull
• Nerve Supply: Posterior Rami
• Action:
  • Unilaterally: Laterally Flex vertebral column
  • Bilaterally: Extend vertebral column
• Easy way to remember order from lateral to medial is the mnemonic "I Love Spaghetti" for Iliocostalis - Longissimus - Spinalis.