The sternocleidomastoid muscle is one of the largest cervical muscles.
It is the most superficial muscle in the neck.
Its primary actions are rotation of the head to the opposite side and flexion of the neck.
When both sides of the muscle act together, it flexes the neck and extends the head.
When one side acts alone, it causes the head to rotate to the opposite side and flexes laterally to the same side.
The sternocleidomastoid is innervated by the accessory nerve.
The sternocleidomastoid muscle can be clearly observed when rotating the head.
Actions:
Unilaterally: contralateral cervical rotation, ipsilateral cervical flexion
Bilaterally: cervical flexion, elevation of sternum and assists in forced inhalation.
It originates at the manubrium of the sternum and the clavicle, and has an insertion at the mastoid process of the temporal bone of the skull.
It inserts at the mastoid process of the temporal bone of the skull by a thin aponeurosis.
It is thick and narrow at its center, and broader and thinner at either end.
The sternal head is tendinous in front, fleshy behind, arising from the upper part of the front of the manubrium sterni.
The sternocleidomastoid travels superiorly, laterally, and posteriorly from the sterna head.
The clavicular head of the sternocleidomastoid muscle is composed of fleshy and aponeurotic fibers, that arise from the upper, frontal surface of the medial third of the clavicle.
The clavicular head of the sternocleidomastoid muscle is directed almost vertically upward.
The two heads are separated from one another at their origins by the supraclavicular fossa.
They gradually blend, below the middle of the neck, into a thick, rounded muscle inserted by a tendon, into the lateral surface of the mastoid process, and at its superior border, and by a thin aponeurosis into the lateral half of the superior nuchal line of the occipital bone.
The sternocleidomastoid is innervated by the accessory nerve of the same side.
The accessory nerve supplies motor fibers to the sternocleidomastoid muscles, and the cervical plexus supplies sensation, including proprioception, from the ventral primary rami of C2 and C3.
The clavicular origin of the sternocleidomastoid varies greatly in breadth.
The supraclavicularis muscle arises from the manubrium behind the sternocleidomastoid and passes behind the sternocleidomastoid to the upper surface of the clavicle.
It also acts as an accessory muscle of respiration, along with the scalene muscles of the neck.
Cranial Nerve XI, the accessory nerve
signals the contraction or relaxation of the the sternocleidomastoid.
The Cranial Nerve XI, the accessory nerve’s nucleus is in the anterior horn of the spinal cord around C1-C3, where lower motor neuron fibers mark its origin.
The sternocleidomastoid muscle divides the neck into anterior and posterior cervical triangles which helps define the location of structures, such as the lymph nodes for the head and neck.
Structures relating to the sternocleidomastoid, include the common carotid artery, accessory nerve, and brachial plexus.
The triangle formed by the clavicle and the sternal and clavicular heads of the sternocleidomastoid muscle is used as a landmark in identifying the correct location for central venous catheter placement.
Contraction of the muscle gives rise to torticollis.