A paired muscle in the superficial layer of the neck
that functions as landmark for the accessory nerve and as a boundary for multiple anatomical triangles, it recieves its name from its bifid muscular origins.
You can remember this muscles action easily, because torticolis demonstrates the same position of the head as isolated muscle contraction)! Essentially, the 3 movements it produces are all just bringing the mastoid as close as possible to the Sternoclavicular joint, which is the "wry neck position" of torticulis. They are:
When both muscles contract together, they PROTRUDE the head (which is a combination of both cervical spine flexion and atlanto-occipital extension.
This muscle is also the primary rotator of the lateral atlanto-axial joint.
At the atlanto-occipital joint, the majority of the fibres pass posteriorly, and so are extensors. However the most anterior run just anterior to the atlanto-occipital joint, and so this muscle can also theoretically flex the atlanto-occipital joint.
A round head originates from the sternum.
A thick fleshy head originates from the clavicle.
Courses superior medially with a very slight convexity due to the
investing fascia attaching at the hyoid.
Inserted into a curved line from the mastoid to the superior nuchal line of the occiput.
There are 5 discrete bundles of fibres that are recognised in the sternocleidomastoid muscle:
Sternomastoid - superficial and deep.
The spinal accessory nerve is said to enter the muscle between the cleidomastoid (CM) fibres and the others (the menmonic for that is in its name: s-CM)
By the spinal accessory Nerve (C1-6). The specific branch comes off just before the nerve enters the muscle. It's crossed by the sternocleidomastoid branch of the occipital artery.
Enclosed in an Investing Layer of Fascia
Internally traversed by The Accessory Nerve - enters SCM between CM + other fibres.
Deep to its upper half lies the cervical plexus.
Deep to its lower half lies the carotid sheath with omohyoid crossing it.
At its lateral border, approximately halfway along its course: the cervical nerve point.
Between its two heads: internal jugular vein.
At its inferio-lateral border: Subclavian artery pulsations at the lateral border.
Superficially is the External jugular vein coursing down.
The arterial supply is different for the
upper, middle and lower thirds.
Upper third: reliably by
Two SCM branches from Occipital Artery.
(upper branch guide to the accessory nerve).
Middle third, variable either: scm branch of superior thyroid (typically) or directly from external carotid.
Most commonly from branch of suprascapula.
The lower pedicle does not have the ability to ensure the full vascularization of the SCM muscle. A composite flap might be safely raised only if the integrity of both inferior and middle pedicles is respected.
occipital vein, posterior auricular vein, external jugular vein, and branches from the internal jugular vein
Migrates form the trunk
And gains attachment to the
Muscle arises dorsally and then migrates anteriorly, for this reason branchial pouch cysts are always infront of it (anterior to it).
supernumerary cleido-occipital muscle
Fusion with trapezius - both from posterior 6th branchial arch.
"The Investing layer that surrounds the
SCM attaches to the hyoid
--> explains the gentle convexity of the
The anterior horn cells of the muscle
are on the ipsilateral, not contralateral side!
Right cortex controls left limb muscles.
Richt cortex controls Right SCM to turn head Left.
So it means that head turning left and left arm muscles can be coordinated (makes sense).