sternocleidomastoid muscle

Muscles of

the neck. Lateral view

A

cross-section diagram of the human neck at the level of C6 showing the fascia compartments, muscles, organs, bone, and major arteries, veins, and nerves.

Wikipedia • Public domain

Radiopaedia • CC-by-nc-sa 3.0 • de

The sternocleidomastoid muscle (also known as sternomastoid) is found in the neck. It has two heads that meld to form one insertion. Sternocleidomastoid muscle, along with the trapezius muscle, is invested by the superficial layer of the deep cervical fascia, which splits around it. Sternocleidomastoid muscle divides the neck into anatomical anterior and posterior triangles.

Summary

origin: manubrium, medial clavicle
insertion: mastoid process, superior nuchal line
innervation: accessory nerve (CN XI)
action: lateral flexion, flexion and rotation of the head on the neck

Gross anatomy

Origin

sternal head (round and tendinous): upper part of anterior surface of manubrium of sternum
clavicular head (thin and broader fleshy): upper surface of medial third of clavicle

Course

sternal head passes over the sternoclavicular joint and joins the clavicular head halfway up the neck
common muscle belly then courses posterolaterally to its insertion

Insertion

mastoid process of temporal bone
lateral half of the superior nuchal line of the occipital bone

Relations

upper half
- posteriorly: cervical plexus, which emerges from behind the lateral border of sternocleidomastoid muscle with some branches (lesser occipital, greater auricular and transverse cervical nerves) coursing anteriorly
- anteriorly: platysma muscle, external jugular vein
lower half
- posteriorly: anterior scalene muscle, common carotid artery, internal jugular vein, carotid sheath
- anteriorly: platysma

Arterial supply

superior thyroid and occipital arteries (both branches of the external carotid artery)

Innervation

spinal part (C2, C3) of the accessory nerve (cranial nerve XI)
- the nerve descends and enters the muscle from its deep surface

Action

when acting unilaterally: lateral flexion of neck to same side, rotates head to opposite side
when acting bilaterally: flexes neck, draws head ventrally and elevates chin, draws sternum superiorly in deep inspiration