sternocleidomastoids

The sternocleidomastoids, also known as the sternocleidomastoid muscles, are paired muscles of the anterior and lateral neck. Each muscle has two heads: a sternal head arising from the manubrium and a clavicular head arising from the medial third of the clavicle. The fibers converge to insert on the mastoid process of the temporal bone and the adjacent superior nuchal line. They are prominent landmarks in neck anatomy and can act as accessory muscles of respiration in severe breathing effort.

Innervation and blood supply: The sternocleidomastoids are innervated primarily by the spinal accessory nerve (cranial nerve XI), with proprioceptive input from the cervical plexus (C2–C3). Arterial supply comes mainly from branches of the superior thyroid and occipital arteries, with additional contributions from the posterior auricular artery.

Actions: Unilateral contraction of a sternocleidomastoid rotates the head to the opposite side and laterally flexes the neck. Bilateral contraction flexes the neck and extends the head at the atlanto-occipital joints. The muscles also participate as accessory muscles of respiration during increased work of breathing.

Clinical relevance and variation: Variants include dual heads or accessory slips. Clinically, the sternocleidomastoids can be involved in congenital muscular torticollis in infancy, causing head tilt and facial asymmetry. In adults, injury to the spinal accessory nerve or chronic muscle spasm can produce torticollis or weakness. As superficial neck muscles, they commonly serve as key landmarks in physical examination and neck procedures.