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Sternocleidomastoid – Functional Anatomy

Sternocleidomastoid Anatomy.

The sternocleidomastoid is a two-headed muscle that extends obliquely along the lateral neck.

Origin: The sternal head attaches to the superior aspect of the manubrium. The clavicular head attaches to the medial third of the clavicle.

Insertion: Both heads merge and attach to the mastoid process of the temporal bone and lateral third of the superior nuchal line of the occiput.

Function:

Because the SCM crosses so many joints, its function can be quite diverse and complex.

Bilaterally, when the atlantooccipital joint is stable, it flexes the neck. If the neck is stable, it extends the head on the atlantooccipital joint.

If the atlantooccipital joint extends and the neck flexes, SCM produces the “barking and biting” motion that juts the chin forward.

Bilateral contraction, when the neck and head are stable, lifts the rib case and assists on inhalation.

Unilateral flexion creates rotation of the head and lateral flexion of the neck.

This becomes very prominent when, in supine position, the head is turned to one side and lifted from its resting place.

The structure of the muscle, especially the clavicular head, has frequent variations. The clavicular head can be thin and narrow, attaching to only a small section of the medial clavicle. Also, it can be thick and extend over half of the clavicle. It occasionally has multiple heads attaching to the clavicle.

SCM has complex nerve interactions.

It is innervated by the spinal accessory nerve, which comes of the lower cervical spinal cord, ascends through the thecal sac, and then descends through the jugular foramen. This structure means that the upper cervical vertebrae and cranial base all impact its function.

The vagus nerve descends along the deep surface of the SCM as it extends from the cranial base into the anterior thorax. This structure impacts many regulatory functions, from blood pressure receptors to polyvagal regulation. See Therapist Notes.

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