Axial Muscles: Head, Neck & Back

• Axial muscles are categorized in Unit 3 Part A

Objectives List

• The muscles covered include those of the head, neck, and intrinsic back
• Head muscles are further divided into facial expression and mastication muscles
• Neck muscles are divided into suprahyoid and infrahyoid muscles
• Intrinsic back muscles include superficial and intermediate muscles

Muscles of Head

• Head muscles fall into two groups: facial expression and mastication (chewing)
• Facial expression muscles are supplied by Cranial Nerve VII aka the Facial Nerve
• Mastication muscles are supplied by Cranial Nerve V3 aka Trigeminal Nerve: Mandibular Division

Cranial Nerve (VII) - Facial Expression

• All facial expression muscles are innervated by CN VII (Facial Nerve)
• These muscles insert into the skin of the face or blend with other facial muscles

Muscles of Facial Expression Details

• Occipitofrontalis (Frontal Belly)
  • Inserts into the epicranial aponeurosis
  • Moves the scalp and eyebrows, and wrinkles the skin of the forehead
  • Innervated by CN VII (Facial N.)
• Occipitofrontalis (Occipital Belly)
  • Inserts into the epicranial aponeurosis
  • Retracts the scalp
  • Innervated by CN VII (Facial N.)
• Orbicularis Oculi
  • Inserts into the skin surrounding the eyelids
  • Closes the eye via winking, blinking, and squinting
  • Innervated by CN VII (Facial N.)
• Orbicularis Oris
  • Inserts into the skin and muscles at the angle to the mouth
  • Compresses and purses the lips, like when kissing
  • Innervated by CN VII (Facial N.)
• Zygomaticus
  • Inserts into the skin at the superolateral edge of the mouth
  • Elevates the corner of the mouth, as in smiling
  • Innervated by CN VII (Facial N.)
• Buccinator
  • Inserts into the orbicularis oris
  • Compresses the cheek and holds food between the teeth when chewing
  • Innervated by CN VII (Facial N.)
• Platysma
  • Inserts into the skin of the cheek and mandible
  • Pulls the lower lip inferiorly, and tenses the skin of the neck
  • Innervated by CN VII (Facial N.)

Cranial Nerve (V3) - Muscles of Mastication

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

Superficial Muscles of Mastication

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

Deep Muscles of Mastication:

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

Key Anatomical Landmarks of the Neck

• Hyoid Bone:
  • It is the only bone that does not articulate with another bone.
  • Keeps the airway open superiorly.
  • Located at the level of the C3 vertebra.
• Thyroid & Cricoid Cartilage:
  • Protect the larynx (voicebox).
  • Provide a site of muscle attachment critical in vocalization and swallowing.

Sternocleidomastoid (SCM):

• Broad, straplike muscle deep to the platysma.
• Origin: Sternum and clavicle.
• Insertion: Mastoid process (temporal bone).
• Unilateral action: Lateral flexion, rotation of the head to the opposite side.
• Bilateral action: Flexes the neck.
• Innervated by CN XI (accessory nerve).
• Key landmark for triangles of the neck, particularly the anterior triangle.

Suprahyoid Muscles:

• Digastric:
  • Insertion: Hyoid bone.
  • Action: Depresses the mandible and elevates the hyoid bone.
• Mylohyoid:
  • Insertion: Hyoid bone.
  • Action: Elevates the hyoid bone and elevates the floor of the mouth.

Infrahyoid Muscles:

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

Intrinsic Back Muscles

• Defined as "Back muscles proper" or "Deep back muscles”
• Innervation: Posterior Rami segmentally from the spinal nerve at the specific level of the muscle.
• Function: Maintain posture and control movements of the vertebral column, including the cranium.
• Groups: Superficial, Intermediate, and Deep.

Intrinsic/Superficial Group: Splenius Muscle:

• Origin: Spinous process of C7 to T4.
• Insertion:
  • Splenius Capitis: Mastoid Process.
  • Splenius Cervicis: Transverse Process of C1-C3 or 4.
• Nerve Supply: Posterior Rami.
• Main Action:
  • Unilaterally: Laterally flex and rotate to the ipsilateral side.
  • Bilaterally: Extend the head and 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)
• Origin: Broad Tendon from the iliac crest, posterior sacrum, and sacral and lumbar spinous processes.
• Insertion: Extensive insertion into the ribs, vertebrae, and skull.
• Nerve Supply: Posterior Rami.
• Main Action:
  • Unilaterally: Laterally flex the vertebral column.
  • Bilaterally: Extend the vertebral column.
• Easy way of memorizing: "I Love Spaghetti”. (Iliocostalis - I, Longissimus - Love, Spinalis - Spaghetti).