Sternocleidomastoid muscle

The sternocleidomastoid muscle is a paired, superficial muscle in the lateral neck that originates from the manubrium of the sternum and the medial portion of the clavicle, and inserts on the mastoid process of the temporal bone. It functions primarily to rotate the head to the opposite side and flex the neck.It divides the neck into anterior and posterior triangles.

Anatomical details

1. Origin

• Sternal head: Anterior surface of the manubrium sterni.

• Clavicular head: Medial one-third of the superior surface of the clavicle

2. Insertion

Fibres ascend laterally, with the two heads gradually merging, and insert into the mastoid process of the temporal bone and the lateral part of the superior nuchal line of the occipital bone.

Nerve Supply

• Motor innervation: Spinal part of the accessory nerve (cranial nerve XI).

• Sensory (proprioceptive) fibres: Ventral rami of cervical spinal nerves C2 and C3 (and sometimes C4).

Actions

• Unilateral contraction: Tilts the head toward the same side and rotates it so that the face turns to the opposite side.

• Bilateral contraction:

  • Flexes the neck (bringing the chin to the chest).

  • When the neck is fixed, it assists in elevating the sternum during forced inspiration.

Relations and Landmarks

• Superficial: Covered by investing layer of deep cervical fascia; visible during head rotation.

• Deep: Carotid sheath (enclosing the common carotid artery, internal jugular vein, and vagus nerve) lies deep to it.

• Landmarks: Divides the neck into anterior and posterior triangles. The external jugular vein runs superficial to it.

Clinical correlation

Accessory nerve palsy can cause weakness of sternocleidomastoid and trapezius muscles and patient cannot rotate head or shrug shoulder effectively.