Sternocleidomastoid
Neck · Head & Neck
The sternocleidomastoid is the large, strap-like muscle that stands out obliquely on either side of the neck, dividing it into anterior and posterior triangles. It has two heads of origin, a rounded sternal head from the manubrium and a flatter clavicular head from the medial third of the clavicle, which fuse and ascend to insert on the mastoid process of the temporal bone and the superior nuchal line of the occipital bone. Acting on one side, it laterally flexes the head to the same side and rotates the face to the opposite side; acting together, the two muscles flex the neck and assist forced inspiration. It is supplied by the accessory nerve (CN XI), with proprioceptive fibres from C2 and C3. Its prominence makes it a key surgical and clinical landmark.
Origin, Insertion, Action & Nerve
Origin
Sternal head: manubrium of sternum; Clavicular head: medial third of clavicle
Insertion
Mastoid process of temporal bone and superior nuchal line of occipital bone
Action
Unilateral: lateral flexion to same side, rotation to opposite side; Bilateral: flexes neck, assists forced inspiration
Nerve
Accessory nerve (CN XI); proprioception via C2–C3
Attachments explained
The Sternocleidomastoid is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Sternal head: manubrium of sternum; Clavicular head: medial third of clavicle
Insertion
Mastoid process of temporal bone and superior nuchal line of occipital bone
Action & function
When the Sternocleidomastoid contracts, it produces the following movements: Unilateral: lateral flexion to same side, rotation to opposite side; Bilateral: flexes neck, assists forced inspiration.
As part of the Neck group of the Head & Neck, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Sternocleidomastoid receives its nerve supply from the Accessory nerve (CN XI); proprioception via C2–C3. This nerve carries fibres from spinal segment(s) C2, C3.
Because a muscle can only contract when its nerve is intact, injury to the Accessory nerve or to its spinal roots (C2, C3) can weaken or paralyse the Sternocleidomastoid, impairing the movements it normally produces (lateral flexion to same side, rotation to opposite side). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Sternocleidomastoid (the OIANS method)
Master the sternocleidomastoid by working out its paradoxical rotation. Because it runs from the front of the chest up to the mastoid process behind the ear, contracting one side tilts the head toward that shoulder while turning the face away; acting on the same side rotates the face to the opposite side. Turning your own head against resistance and watching the opposite muscle stand out makes this crossed action stick.
Anchor the nerve as an exception worth remembering: unlike most neck muscles supplied by cervical rami, the sternocleidomastoid is driven by the accessory nerve (CN XI). This is why testing head turn against resistance is a standard screen for accessory nerve function, tying the anatomy, action, and clinical exam together.
Frequently asked questions
Where is the Sternocleidomastoid located?
The Sternocleidomastoid is a muscle of the Neck group, located in the Head & Neck.
What is the origin of the Sternocleidomastoid?
Sternal head: manubrium of sternum; Clavicular head: medial third of clavicle
What is the insertion of the Sternocleidomastoid?
Mastoid process of temporal bone and superior nuchal line of occipital bone
What movements does the Sternocleidomastoid produce?
Unilateral: lateral flexion to same side, rotation to opposite side; Bilateral: flexes neck, assists forced inspiration
What nerve supplies the Sternocleidomastoid?
Accessory nerve (CN XI); proprioception via C2–C3
Is the Sternocleidomastoid free to study in OIANS?
The Sternocleidomastoid is always free to browse. Its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory. Quiz and Flashcard practice for the Head & Neck is part of the one-time Lifetime upgrade, though; only the Upper Limb decks are free to test yourself on.
Related muscles
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