Muscles of Neck: Anterior View Anatomy
1. Sternocleidomastoid muscle
Origin (inferior attachment): This muscle has 2 heads of origin. The sternal head arises from the anterior surface of the manubrium of the sternum. The clavicular head arises from the superior surface of the medial third of the clavicle.
Insertion (superior attachment): Attaches to the lateral surface of the mastoid process of the temporal bone and the lateral half of the superior nuchal line.
Action: Tilts the head to 1 side, flexes the neck, and rotates the neck so the face points superiorly to the opposite side. When the muscles of both sides act together, they flex the neck.
Innervation: Accessory nerve (CN XI and C2 and C3).
Comment: When the head is fixed, the 2 muscles acting together can help elevate the thorax during forced inspiration. The sternocleidomastoid (SCM) is 1 of 2 muscles innervated by the spinal accessory nerve. Although the accessory nerve is classified as a cranial nerve, it does not possess any fibers originating from the brainstem. Its nerve fibers originate in the upper cervical spinal cord, so its classification as a “true” cranial nerve is problematic.
Clinical: The SCM is innervated by the accessory nerve (CN XI), and this nerve is susceptible to injury where it crosses the posterior cervical triangle between the SCM muscle and the trapezius muscle. CN XI innervates both of these muscles.
Torticollis is a contraction of the cervical muscles that presents as a twisting of the neck such that the head is tilted toward the lesioned side (ipsilateral) and the face away from the lesioned side (contralateral). Commonly, the SCM is affected unilaterally by this congenital fibrous tissue tumor.