Sternocleidomastoid Detail

musculus sternocleidomastoideus detail

The SCM is the dominant lateral neck muscle visible as a cord from mastoid to sternal notch during contralateral head rotation. The posterior triangle between SCM and trapezius contains the brachial plexus and accessory nerve.

Nerve: Accessory nerve (CN XI) — motor; C2, C3… Blood Supply: Occipital and posterior auricular arteries Region: Neck

Anatomical Data

Origin, Insertion & Supply

Origin	Sternal head: anterior manubrium surface; Clavicular head: medial third of clavicle
Insertion	Mastoid process and lateral half of the superior nuchal line
Nerve Supply	Accessory nerve (CN XI) — motor; C2, C3 — proprioceptive
Blood Supply	Occipital and posterior auricular arteries

Biomechanics

Function & Actions

Actions	Ipsilateral lateral flexion; Contralateral head rotation; Bilateral: neck flexion and head extension

Clinical Relevance

Clinical Notes

SCM is the landmark for most neck surgery — the carotid sheath lies deep to it. CN XI injury from posterior triangle dissection produces SCM and trapezius palsy. SCM is used as a flap for pharyngeal fistula closure.

Palpation

Visible and palpable as the dominant anterolateral neck cord during resisted contralateral rotation.

Pathology

Common Injuries & Conditions

CN XI Palsy from SCM Division

Accessory nerve injury during neck dissection producing SCM and trapezius weakness with shoulder droop and inability to shrug.