CorpusHuman Anatomy
MuscularMuscle

Sternocleidomastoid

Musculus sternocleidomastoideus

Also known as: SCM, sternomastoid

A prominent two-headed strap muscle running obliquely from the sternum and clavicle to the mastoid process. It divides the neck into anterior and posterior triangles.

Show on body map Muscles — Posterior View

Overview

Sternocleidomastoid (SCM) has a rounded, tendinous sternal head from the front of the manubrium and a broader, fleshy clavicular head from the medial third of the clavicle. The small triangular gap between the two heads (the lesser supraclavicular fossa) overlies the internal jugular vein, a landmark for central venous access. The heads unite and ascend to the mastoid process and the lateral half of the superior nuchal line. SCM is the key landmark of the neck. In front of it lies the anterior triangle and behind it the posterior triangle. It covers the carotid sheath, the cervical plexus and the deep cervical lymph nodes.

Actions: acting alone, it tilts the head toward the same shoulder and rotates the face to the opposite side, turning the face upward. Acting together, the two muscles flex the neck (bringing the head forward), extend the head at the atlanto-occipital joints when the neck is flexed, and serve as accessory muscles of inspiration by lifting the sternum when the head is fixed. Its motor supply is the spinal accessory nerve (CN XI), which enters its deep surface and then crosses the posterior triangle to trapezius.

Location

Anterolateral neck, from the sternoclavicular region to the mastoid process.

Parts & subdivisions

  1. Sternal head
    Rounded tendon from the anterior surface of the manubrium.
  2. Clavicular head
    Flat fleshy head from the superior surface of the medial third of the clavicle.

Anatomy

Origin

Sternal head: anterior surface of the manubrium of the sternum. Clavicular head: superior surface of the medial third of the clavicle.

Insertion

Lateral surface of the mastoid process and the lateral half of the superior nuchal line.

Action

Unilaterally: lateral flexion of the neck to the same side and rotation of the face to the opposite side. Bilaterally: flexion of the neck, extension of the head at the atlanto-occipital joints, and elevation of the sternum in forced inspiration.

Innervation

Spinal accessory nerve (CN XI) for motor supply; branches of C2–C3 (cervical plexus), mainly proprioceptive.

Nerve roots

CN XI (spinal accessory nucleus, C1–C5 cord segments); C2–C3 (proprioception)

Arterial supply

Sternocleidomastoid branches of the occipital and superior thyroid arteries; suprascapular artery (inferiorly).

Relations

Superficial: platysma, external jugular vein, great auricular and transverse cervical nerves. Deep: carotid sheath (common/internal carotid artery, internal jugular vein, vagus nerve), cervical plexus, scalenes, deep cervical lymph nodes.

Function

  • Rotates the head to the opposite side and tilts it to the same side
  • Flexes the neck (bilaterally)
  • Accessory muscle of inspiration

Clinical significance

Congenital muscular torticollis

Fibrosis or a fibrous "tumour" of the SCM in an infant, often after a difficult breech delivery, tilts the head toward the affected side and turns the face to the opposite side. Most cases resolve with stretching physiotherapy.

Testing CN XI

Ask the patient to turn the head against resistance. Turning to the left tests the right SCM. Shoulder shrugging tests trapezius.

Accessory muscle use

Visible SCM contraction during breathing indicates respiratory distress, for example in COPD or acute asthma.

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.