The sternocleidomastoid (SCM) is a paired muscle located on either side of the front of the neck. This muscle is easily visible beneath the skin, giving the neck its characteristic rope-like definition when contracted. The SCM plays a role in posture and a wide variety of head and neck movements. Understanding this muscle is important for comprehending the mechanics of the upper body and stabilizing the neck during complex movements.
Defining the Sternocleidomastoid
The sternocleidomastoid muscle belongs to the superficial layer of the anterior neck muscles. It extends diagonally across the side of the neck, where it divides the area into anterior and posterior triangles. Healthcare professionals use the SCM as a landmark to locate underlying structures like blood vessels and nerves. The name of the muscle itself, “sternocleidomastoid,” is a map of its bony attachments.
The name is derived from three roots. “Sterno-” refers to the sternum (breastbone), while “cleido-” refers to the clavicle (collarbone). The final part, “-mastoid,” points to the mastoid process, a bony prominence of the skull located just behind the ear.
Anatomical Attachments: Origin and Insertion
The sternocleidomastoid has two distinct heads that form its origin, the attachment point that remains fixed during contraction. The Sternal head is round and tendinous, arising from the upper part of the manubrium of the sternum. The Clavicular head is broader and more fleshy, originating from the superior and anterior surface of the medial third of the clavicle. These two heads are initially separated by a triangular space, but they blend together as the muscle travels upward.
As the muscle fibers ascend diagonally across the neck, they converge to a single insertion point on the skull. The insertion is the attachment site that moves as the muscle contracts. The SCM inserts onto the lateral surface of the mastoid process of the temporal bone. A portion of the muscle also attaches to the lateral half of the superior nuchal line, a ridge on the occipital bone.
Primary Actions and Movements
The functional role of the SCM varies depending on whether one muscle contracts independently (unilateral action) or if both sides contract together (bilateral action). Unilateral action causes two primary movements: lateral flexion of the neck and rotation of the head. The head is tilted toward the shoulder on the same side, while the chin is turned to face the opposite direction.
Bilateral contraction results in the flexion of the neck, bringing the chin toward the chest. The SCM can also assist in extending the head when the neck is already bent backward. Because the muscle attaches to the sternum and clavicle, it acts as an accessory muscle of respiration, helping to lift the upper chest during forced inhalation.
Clinical Significance and Associated Conditions
The location and function of the sternocleidomastoid make it important as an anatomical landmark. One common issue is the development of trigger points within the muscle, which are hyperirritable spots that cause referred pain. This pain often manifests as headaches, particularly in the temple or behind the eye, or as pain radiating down the neck.
A specific condition associated with the SCM is torticollis, or “wry neck,” which involves an abnormal, twisted neck posture. Congenital muscular torticollis is present at birth due to the shortening or tightness of one SCM muscle, causing the infant’s head to tilt and rotate. The SCM serves as a palpable landmark for clinicians, defining the location of important structures like the carotid artery and internal jugular vein.