Sternocleidomastoid
The sternocleidomastoid (SCM) is a prominent muscle in the anterior neck with two heads. It flexes and rotates the head and is an important landmark for structures in the neck.
Origin, Insertion, Action, Innervation
OOrigin
Sternal head: Manubrium of sternum. Clavicular head: Medial third of clavicle.
IInsertion
Mastoid process of temporal bone and lateral superior nuchal line
AAction
- • Unilateral contraction: Lateral flexion to same side, rotation to opposite side
- • Bilateral contraction: Flexion of the neck, extension of the head at atlanto-occipital joint
- • Accessory muscle of inspiration (when head is fixed)
NInnervation
Accessory nerve (CN XI) for motor; C2, C3 for proprioception
Blood Supply
Occipital artery and superior thyroid artery
Clinical Relevance
Torticollis (wry neck) can result from SCM spasm or fibrosis. The SCM divides the neck into anterior and posterior triangles, important anatomical landmarks. Accessory nerve damage affects both SCM and trapezius.
Palpation
Easily palpated as the prominent muscle on the anterolateral neck. Turn the head against resistance to make the muscle stand out.
Study Tips
- ✓Name tells origin and insertion: Sterno-cleido-mastoid
- ✓Unilateral action = rotate to OPPOSITE side (chin goes away from contracted muscle)
- ✓With trapezius, only muscle innervated by accessory nerve (CN XI)
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Common questions about the sternocleidomastoid
The name describes the muscle's attachments: 'sterno' (sternum), 'cleido' (clavicle for origin), and 'mastoid' (mastoid process for insertion). It literally tells you where the muscle attaches.
The sternocleidomastoid is innervated by the accessory nerve (CN XI) for motor function, with proprioceptive fibers from C2 and C3. This is the same nerve that innervates the trapezius.
Torticollis (wry neck) is a condition where the head is tilted to one side and rotated to the opposite side due to SCM spasm or contracture. Congenital torticollis in infants may be due to birth trauma or intrauterine positioning.
The SCM divides the neck into anterior and posterior triangles, which contain important structures. It also serves as a landmark for locating the carotid artery, internal jugular vein, and various nerves.