Anatomy and supply
The sternocleidomastoid muscle is a two-headed neck muscle. It is innervated by the accessory nerve (cranial nerve XI) and direct branches of the cervical plexus (C1-C2). The sternal head originates from the manubrium sterni, the clavicular head from the middle part of the clavicle. The insertion is the mastoid process of the temporal bone and the superior nuchal line of the occipital bone.
The muscle lies very superficially so that it is both easily visible and palpable. The carotid pulse may be felt in the middle third of the front edge. Under the sternocleidomastoid region runs a neurovascular bundle containing the common carotid artery (medial), the internal jugular vein (lateral), the vagus nerve (dorsal) and the cervical ansa. When putting a central venous catheter (CVC) the medial edge of the sternocleidomastoid muscle serves as a lead structure. Sensory branches of the cervical plexus merge dorsally to the muscle at the Erb’s point (punctum nervosum) which can be used as a place of puncture for local anesthesia.
A unilateral contraction of the sternocleidomastoid muscle flexes the cervical vertebral column to the same side (lateral flexion) and rotates the head to the opposite side. A bilateral contraction elevates the head by dorsally extending the upper cervical joints. At the same time, it flexes the lower cervical column causing an overall bending of the neck towards the chest. If the head is fixed, it elevates the sternum and clavicle and, thus, expands the thoracic cavity (inspiratory breathing muscle).
During pathological changes of the sternocleidomastoid the clinical picture of the wryneck occurs (bending of the head to the affected muscle and rotation to the healthy side). A classic example for this condition is the muscular torticollis, a tonic spasm of the sternocleidomastoid. People with this disorder show difficulties swallowing, an extreme immobility of their throat, facial asymmetries and scoliosis. Approximately 0.5% of all newborn suffer under muscular torticollis however the etiology remains unclear.