The sternocleidomastoid muscle is a two-headed neck muscle, which true to its name bears attachments to the manubrium of sternum (sterno-), the clavicle (-cleido-), and the mastoid process of the temporal bone (-mastoid).
It is a long, bilateral muscle of the neck, which functions to flex the neck both laterally and anteriorly, as well as rotate the head contralaterally to the side of contraction. The muscle is closely related to certain neurovascular structures that pass through the neck on their way either to the head or to the periphery of the body.
Sternal head: superior part of anterior surface of manubrium sterni
Clavicular head: superior surface of medial third of the clavicle
|Insertions||Lateral surface of mastoid process of the temporal bone, Lateral half of superior nuchal line of the occipital bone|
|Innervation||Accessory nerve (CN XI), branches of cervical plexus (C2-C3)|
Unilateral contraction: cervical spine: neck ipsilateral flexion, neck contralateral rotation
Bilateral contraction: atlantooccipital joint/ superior cervical spine: head/neck extension; Inferior cervical vertebrae: neck flexion; sternoclavicular joint: elevation of clavicle and manubrium of sternum
|Clinical relations||Wryneck, torticollis|
This article will discuss the anatomy, function and clincal correlates of the sternocleidomastoid muscle.
Origins & Insertions
The sternal head originates from the manubrium sterni, while the clavicular head from the medial third of the clavicle. The insertion is the lateral surface of the mastoid process of the temporal bone and the lateral half of the superior nuchal line of the occipital bone.
It is innervated by the accessory nerve (cranial nerve XI) and direct branches of the cervical plexus (C1-C2).
Landmarks & Relations
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)
- 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 from muscular torticollis, however the etiology remains unclear.