Sternocleidomastoid muscle

At a glance

English

Sternocleidomastoid muscle

Latin (TA2)

Musculus sternocleidomastoideus

System

Muscle

Region

Neck

Side

Paired, left and right

Arises from

The clavicle

About the sternocleidomastoid muscle

The sternocleidomastoid is a large, superficial strap of muscle on either side of the neck, running obliquely from the top of the breastbone and the clavicle up to the skull behind the ear. Its name lists its attachments: sternum, collarbone and mastoid process. It is easy to see and feel when the head is turned to the opposite side.

Below, it has two heads. The sternal head is a rounded bundle, tendinous in front, from the upper front surface of the manubrium. The clavicular head is flatter and wider and rises almost vertically from the top of the clavicle's inner third. A small triangular gap, the lesser supraclavicular fossa, separates them before they merge about halfway down the neck. The combined belly, narrow and thick in the middle and thinner at its ends, attaches by a strong tendon to the outer face of the mastoid process and by a thin aponeurosis to the outer half of the occipital bone's superior nuchal line.

Acting alone, one muscle tilts the head towards its own shoulder and rotates the face towards the opposite side. Acting together, the pair flex the neck and push the head forward, while extending the head at the top joints of the spine. When the head is fixed they can lift the sternum and help draw air in, working with the scalenes as accessory muscles of breathing.

The muscle divides each side of the neck into a front and a back triangle and lies over the carotid sheath, so it is a guide to the carotid artery, internal jugular vein and the cervical lymph nodes. The gap between its two heads is used to locate the internal jugular vein for central venous lines. It is tested in the cranial nerve examination because its motor supply is the accessory nerve, which it shares with trapezius, while sensory fibres come from C2 and C3. Shortening or spasm of the muscle tilts the head, which is known as torticollis or wry neck; in infants this can follow a fibrous swelling within the muscle.

Attachments, action and supply

Origin

Sternal head from the front of the manubrium; clavicular head from the top of the clavicle's inner third.

Insertion

Outer face of the temporal bone's mastoid process and the outer half of the occipital bone's superior nuchal line.

Action

One side tilts the head to the same side and turns the face to the opposite side; both together flex the neck, protrude the head and act as accessory muscles of inspiration.

Nerve supply

Accessory nerve (cranial nerve XI) for motor supply; branches of C2 and C3 for sensation and proprioception.

Blood supply

Sternocleidomastoid branches of the occipital and superior thyroid arteries, with smaller contributions from the posterior auricular and suprascapular arteries.