Muscle
Sternocleidomastoid
Surface landmark and head-turning muscle
High-yield
Motor supply is primarily CN XI. Bilateral contraction can flex the lower cervical spine while extending the head at the upper cervical joints, so its action is more complex than simple neck flexion.
Overview
Sternocleidomastoid runs obliquely from the sternum and medial clavicle to the mastoid region. It separates the anterior and posterior neck triangles. One side tilts the head toward itself while turning the face away; paired activity depends on head and neck position.
Reference facts
- Origin
- Sternal head: manubrium. Clavicular head: medial clavicle, usually described as its medial third
- Insertion
- Mastoid process and lateral part of the superior nuchal line
- Action
- Ipsilateral side-bending and contralateral rotation; bilateral lower-neck flexion and upper-neck extension vary with posture
- Innervation
- Spinal accessory nerve (CN XI), with cervical sensory/proprioceptive contributions, chiefly C2–C3
- Blood supply
- Branches of the occipital and superior thyroid arteries, with additional regional contributions
Clinical correlation
Congenital muscular torticollis can involve a shortened, tight or fibrotic sternocleidomastoid. The usual posture combines tilt toward the affected side with rotation of the chin away from it.
Frequently asked questions
What does the sternocleidomastoid do?
Ipsilateral side-bending and contralateral rotation; bilateral lower-neck flexion and upper-neck extension vary with posture.
What nerve innervates the sternocleidomastoid?
The sternocleidomastoid is innervated by the spinal accessory nerve (CN XI), with cervical sensory/proprioceptive contributions, chiefly C2–C3.
Where does the sternocleidomastoid attach?
Origin: Sternal head: manubrium. Clavicular head: medial clavicle, usually described as its medial third. Insertion: Mastoid process and lateral part of the superior nuchal line.
What is the blood supply of the sternocleidomastoid?
Branches of the occipital and superior thyroid arteries, with additional regional contributions.