Sternocleidomastoid Muscle; Origin, Nerve Supply, Function

Sternocleidomastoid Muscle is one of the largest and most superficial cervical muscles. The primary actions of the muscle are a rotation of the head to the opposite side and flexion of the neck. The sternocleidomastoid is innervated by the accessory nerve. It is given the name sternocleidomastoid because it originates at the manubrium of the sternum (Sterno-) and the clavicle (cleido-), and has an insertion at the mastoid process of the temporal bone of the skull.

The sternocleidomastoid muscle originates from the sternum and clavicle and extends in a posterior diagonal fashion to insert onto the ipsilateral mastoid process and lateral portion of the occipital ridge. The muscles work together to flex the neck and work individually to turn the neck and elevate the chin. With the head rotated away from the observer, the sternocleidomastoid muscle becomes a prominent surface landmark that divides the neck into the anterior and posterior triangles. The muscle and the mastoid process are important landmarks used to identify the spinal accessory nerve at its most exposed location in the posterior triangle

Anatomy of Sternocleidomastoid Muscle

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The sternocleidomastoid muscle originates from two locations: the manubrium of the sternum and the clavicle. It travels obliquely across the side of the neck and inserts at the mastoid process of the temporal bone of the skull. The sternocleidomastoid is thick and narrow at its center, and broader and thinner at either end.

The sternocleidomastoid muscle (SCM) divides the neck area into an anterior triangle and a posterior triangle. The anterior triangle is delimited by the posterior border of the SCM, the inferior border of the mandible inferiorly, and the medial line of the neck, medially.[rx] In the anterior triangle, we find the suprahyoid and infrahyoid muscles. The posterior triangle is delimited by the SCM anteriorly, by the clavicle inferiorly, and by the trapezius muscle posteriorly. Scalene muscles reside in the posterior triangle. The SCM is a large and easily recognizable and palpable muscle.[rx]

SCM can be divided into four portions

• Sterno-mastoid
• Sterno-occipital
• Cleido-mastoid
• Cleido-occipital

The muscle originates from the upper edge of the sternal manubrium, from the medial quarter of the upper face of the clavicle; the two muscle heads merge into a single muscle belly that is directed upwards and laterally. Insertions arrive at the mastoid process of the temporal bone and at the anterior portion of the superior nuchal line.[rx] SCM has fibers arranged in parallel; it is not a pennate muscle.[rx] SCM expresses greater strength and thickness in men than women; the sternomastoid portion is the muscle area that develops a greater percentage of contractile strength than the other portions. The cleido-occipital portion is the muscular area where less force develops.[rx]

Nerve supply of Sternocleidomastoid Muscle

The sternocleidomastoid is innervated by the accessory nerve of the same side. It supplies only motor fibers. The cervical plexus supplies sensation, including proprioception, from the ventral primary rami of C2 and C7.

Blood supply

The arterial blood to the sternocleidomastoid is mostly supplied by the branches of the external carotid artery, such as the sternocleidomastoid branch of the superior thyroid artery and the sternocleidomastoid branch of the occipital artery. The venous drainage from the sternocleidomastoid is provided by the sternocleidomastoid vein, which flows into the internal jugular or the superior thyroid vein.

Functions of Sternocleidomastoid Muscle

• The function of this muscle is to rotate the head to the opposite side or obliquely rotate the head. It also flexes 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 (ipsilaterally). It also acts as an accessory muscle of respiration, along with the scalene muscles of the neck.
• If the cervical spine is not fixed, this bilateral contraction determines a hyperlordosis of the cervical spine with an extension of the head and a bending of the cervical spine on the dorsal one.
• If the cervical spine is rigid and rectilinear due to the contraction of the paravertebral muscles, the simultaneous contraction of the SCM determines the flexion of the cervical spine on the dorsal spine and a flexion of the head forward.

References

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