Anterior Thorax & Neck LO 9

9.) Identify and detail the muscles of the anterior neck: sternocleidomastoid m., infrahyoid mm.

Sternocleidomastoid m.

  • Superior attachments:
    • Mastoid process of temporal bone
    • Superior nuchal line of occipital bone
  • Inferior attachments:
    • Manubrium of sternum
    • Medial clavicle
  • Actions:
    • Bilateral contractions: flexes cervical vertebrae
    • Unilateral contraction: lateral flexes the neck & rotates the face in the opposite direction
  • Innervation:
    • Efferent: Accessory n. (CN XI)
    • Afferent: C2 & C3 fibers
    • It is uncommon that separate nerves provide afferent and efferent innervation to a muscle.
  • Anatomical relationships:
    • Forms the lateral boundary of the anterior cervical triangle and anterior boundary of posterior cervical triangle
      • Prominently visible and palpable landmark in the neck
    • External jugular v. typically sits anterior to this muscle

Infrahyoid mm.

The infrahyoid muscles are a set of 4 muscles (sternohyoid, omohyoid, sternothyroid, and thyrohyoid mm.) located inferior to the hyoid bone. This set of muscles is often referred to as strap muscles due to their ribbon/strap-like shape.

  • The names of the muscles indicate the attachment points:
    • Omo-: means shoulder (in this case, attached on the scapula)
    • Thyro(id): indicates the thyroid cartilage, not thyroid gland
  • Actions
    • Stabilization or depression of hyoid
    • Dependent on muscles, will pull larynx either superiorly, or inferiorly
  • Innervation
    • Ansa cervicalis (C1,2,3) for 3 of the 4 muscles
    • Thyrohyoid m. is innervated by fibers of C1 traveling with hypoglossal n. (CN XII)
  • Anatomical relationships:
    • Arranged in two layers
      • Superficial: sternohyoid & omohyoid
      • Deep: sternothyroid & thyrohyoid
    • To access the thyroid gland, these muscles have to be reflected or moved