Sternocleidomastoid

Anatomy · Systems & applied anatomy

Sternocleidomastoid (SCM) is a two headed muscle in the neck. Its primary function is rotation and flexion of the cervical spine. It is used anatomically to divide the neck into anterior and posterior triangles.

Sternocleidomastoid	Notes
Innervation	Motor: Accessory (spinal accessory) nerve (CN XI) Sensory: C2 and C3 ventral rami
Origin	Sternal head: Manubrium Clavicular head: Medial third clavicle
Insertion	mastoid process of the temporal bone of the skull superior nuchal line
Action	Unilateral contraction: contralateral cervical rotation ipsilateral cervical flexion Bilateral contraction: cervical flexion/extension (dorsal extension upper cervical joints) elevation of sternum assists in forced inhalation
Clinical testing	Ask patient to turn head (rotate) to one side against resistance provided by the clinician

Clinical note:The accessory nerve, which supplies SCM, runs obliquely behind the upper 1/3 of the SCM and is vulnerable to injury in trauma and surgery to the neck e.g. ACDF, cervical lymph node biopsy.

Video tutorial: anatomy and function of sternocleidomastoid