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Sternocleidomastoid Muscle Anatomy

The sternocleidomastoid muscle is a large, strap-like muscle that forms a key landmark in the neck, dividing each side into anterior and lateral cervical regions (also known as anterior and posterior triangles).

Sternocleidomastoid Muscle Anatomy

The SCM has two distinct heads:

  1. Sternal head: A rounded tendon attaching to the manubrium of the sternum.
  2. Clavicular head: A thick, fleshy attachment to the superior surface of the medial third of the clavicle.

These two heads are separated inferiorly by a small triangular depression known as the lesser supraclavicular fossa. As they ascend, they merge into a single muscle belly that inserts superiorly at two points:

  • The mastoid process of the temporal bone
  • The lateral portion of the superior nuchal line of the occipital bone
See Also: Temporomandibular Joint Anatomy
Sternocleidomastoid Muscle

Embryological Development

The sternocleidomastoid muscle derives from paraxial (pre-optic) mesoderm and occipital (post-optic) somites, with partial contribution from neural crest cells. It begins to form around the 14th day of gestation. Recent research suggests that the progenitor cells of neck muscles, including the SCM, share space with heart progenitor cells within the cardiopharyngeal mesoderm during early development.

Innervation and Blood Supply

Innervation

The primary motor innervation of the sternocleidomastoid muscle comes from the spinal accessory nerve (CN XI). However, it also receives proprioceptive innervation from the ventral rami of C2 and C3 spinal nerves.

Blood Supply

The arterial supply is derived primarily from branches of the external carotid artery, specifically:

  • Occipital artery
  • Superior thyroid artery

Venous drainage is via the external jugular vein, which passes inferiorly and posteriorly to the sternocleidomastoid muscle.

Function and Biomechanics

The sternocleidomastoid muscle plays a crucial role in various neck movements:

  1. Unilateral contraction:
    • Rotates the head to the opposite side
    • Laterally flexes the neck to the same side
    • Slightly extends the neck
  2. Bilateral contraction:
    • Flexes the neck if the cervical spine is not fixed
    • Extends the head at the atlanto-occipital joint if the cervical spine is fixed
  3. Accessory respiratory function:
    • Elevates the sternum and clavicles during forced inspiration

The sternocleidomastoid muscle also contributes significantly to posture maintenance and has connections with the vestibular system, highlighting its role in balance and proprioception.

OriginSternal head: superior part of anterior surface of manubrium sterni
Clavicular head: superior surface of medial third of the clavicle
InsertionLateral surface of mastoid process of the temporal bone, Lateral half of superior nuchal line of the occipital bone
InnervationAccessory nerve (CN XI), branches of cervical plexus (C2-C3)
Blood SupplyOccipital artery
Superior thyroid artery
ActionUnilateral contraction: cervical spine: neck ipsilateral flexion, neck contralateral rotation
Bilateral contraction: atlantooccipital joint/ superior cervical spine: head/neck extension; Inferior cervical vertebrae: neck flexion; sternoclavicular joint: elevation of clavicle and manubrium of sternum

Clinical Significance

  1. Torticollis: SCM dysfunction can lead to various forms of torticollis, including:
    • Congenital muscular torticollis
    • Spasmodic torticollis
    • Ocular torticollis
  2. Surgical landmark: The SCM serves as an important landmark for various neck surgeries and procedures.
  3. Graft source: The SCM can be used as an autograft for reconstructive surgeries in the head and neck region.
  4. Accessory nerve injury: Trauma or surgical procedures involving the SCM can potentially damage the spinal accessory nerve.
  5. Temporomandibular joint (TMJ) function: The SCM plays a role in TMJ mechanics, with evidence suggesting its involvement in optimal occlusion.

To test the SCM, the head is turned to the opposite side against resistance (hand against chin). If it is acting normally, the SCM can be seen and palpated.

The sternocleidomastoid muscle, with its complex anatomy and multifaceted functions, remains a crucial structure for medical professionals to understand. Its role in neck biomechanics, respiration, and various pathologies underscores the importance of a thorough anatomical knowledge in clinical practice.

References & More

  1. Gray’s Anatomy: The Anatomical Basis of Clinical Practice
  2. Clinically Oriented Anatomy – 8th Edition
  3. Bordoni B, Jozsa F, Varacallo M. Anatomy, Head and Neck, Sternocleidomastoid Muscle. [Updated 2023 Apr 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: Pubmed
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