sternokleidomastoideus

Sternokleidomastoideus, or sternocleidomastoid, is a paired muscle of the lateral neck. It originates from the manubrium of the sternum and the medial third of the clavicle and inserts on the mastoid process of the temporal bone and the superior nuchal line. The muscle runs obliquely across the side of the neck and forms the boundary between the anterior and posterior triangles.

Innervation and blood supply: The primary motor innervation is the spinal accessory nerve (cranial nerve XI), with proprioceptive fibers from the C2–C3 spinal nerves. Vascular supply comes from branches including the superior thyroid artery and the occipital artery, among others.

Function: Bilateral contraction flexes the neck. Unilateral contraction rotates the head to the opposite side and tilts it toward the same side. The muscle can also assist in elevating the sternum during deep inspiration when the neck is fixed.

Clinical relevance: The SCM is commonly involved in congenital muscular torticollis, wherein fibrosis or shortening of the muscle leads to a tilted head from birth. In adults, it may be affected by strain or spasms after trauma or whiplash. Clinically, it is a superficial landmark in the neck and its function is often assessed in neurological and musculoskeletal evaluations.