Sternocleidomastoid Muscle (Left)

Origin

The sternocleidomastoid muscle originates from two separate heads. Its sternal (or medial) head attaches to the superior portion of the anterior surface of the manubrium of the sternum. Its clavicular (or lateral) head attaches to the medial one third of the clavicle along its superior surface. A small triangular space, the lesser supraclavicular fossa, separates these two attachments. The clavicular head travels deep to the sternal head where they appear as a singular fleshy muscle.

Insertion

The sternocleidomastoid muscle inserts into the lateral aspect of the mastoid process of the temporal bone and the lateral one third of the superior nuchal line of the occipital bone. The fibers from clavicular portion primarily attach to the mastoid process while fibers from the sternal portion typically continue to the attachment on the occipital bone.

Key Features & Anatomical Relations

The sternocleidomastoid muscle forms an important landmark, where it delineates the boundary between the anterior and posterior triangles of the neck.

The sternocleidomastoid is covered by the superficial investing cervical fascia. Superficial to this fascia, the external jugular vein and platysma cover the sternocleidomastoid muscle. Additionally, cutaneous nerves from the anterior rami of the cervical plexus sit over the sternocleidomastoid muscle. These are the greater auricular and transverse cervical nerves. Near its superior attachment, the posterior border of the parotid gland overlaps the sternocleidomastoid muscle.

The sternocleidomastoid conceals much of the carotid sheath and its contents. The accessory nerve (CN XII) also passes deep to the sternocleidomastoid muscle before turning posteriorly as it courses towards the trapezius muscle.

Actions & Testing

Overall, the sternocleidomastoid muscle is involved in multiple actions:

- during unilateral contraction, it laterally flexes the neck to the same side at the cervical vertebral joints;

- during unilateral contraction, it rotates the head to the opposite side at the atlantoaxial joints;

- during bilateral contraction, it flexes the neck at the cervical vertebral joints;

- during bilateral contraction, it elevates the sternum and clavicles and can act as an accessory muscle of inspiration (Standring, 2016).

The sternocleidomastoid muscle can be tested by rotating the head to the opposite side against resistance, during which the muscle can be seen and palpated (Sinnatamby, 2011).

References

Sinnatamby, C. S. (2011) Last's Anatomy: Regional and Applied. ClinicalKey 2012: Churchill Livingstone/Elsevier.

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41st edn.: Elsevier Limited.