By Elizabeth A. Murray, Mount St. Joseph University
The spine moves in many ways. Flexion—forward bending; extension—the erect posture that is a key function of the true back muscles; lateral bending—also aided by the true back muscles when only one side is used; and rotation—a function that the transversospinalis can assist abdominal muscles in performing. But what about the increased rotation that occurs in the neck because of the complex articulations of the first two cervical vertebrae?
Postural Muscles
The spine is a series of stacked bones, linked by the many joints and ligaments that unite the vertebrae into the spinal column. This makes the spine stable and gives it the mobility needed to be flexible—but the movements and much of the support comes from muscles.
The true back muscles move aspects of the spine and skull—and all are posterior to the longitudinal axis of the vertebral column. But along with the joints and ligaments, these muscles help stabilize the spine. They are our postural muscles active in all our body positions other than lying flat.
Subgroups of these postural muscles take turns all day long, rotating the job of keeping the erect posture needed by the spine when sitting, standing, walking, or running. By taking turns like this, our back doesn’t get tired. Instead, when we’re sitting or standing, our butts or legs get tired before our back does.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Suboccipital Muscles
Then there is a group of four muscles, known as the suboccipital muscles, that act—in part—as the steering mechanism at the base of the neck.
The occipital condyles of the base of the skull are convex; these articulate with the concave superior surfaces of the atlas—the C1 vertebra. The atlas has very flat inferior surfaces that form a joint with the flat superior surfaces of the axis—the C2 vertebra. But just as importantly, the axis has that superior projection called the dens or odontoid process that links with the anterior aspect of the atlas.
This atlantoaxial joint is the real source of the great degree of rotation we have between the head and neck—not actually between the skull and the vertebrae but between the atlas and the axis.
Suboccipital Triangle
The suboccipitals is a group of four bilateral muscles that form what is called the suboccipital triangle. The rectus capitis posterior major originates on the spinous process of the axis and inserts on the skull. The rectus capitis posterior minor starts on the posterior aspect of the atlas and inserts on the skull.
The superior oblique capitis originates on the skull and inserts on the transverse process of the atlas. And finally, the inferior oblique capitis originates on the spinous process of the axis and inserts on the transverse process of the atlas.
Together, the superior oblique capitis, inferior oblique capitis, and rectus capitis posterior major all act to rotate the skull and C1 on the axis—the action is ipsilateral. The muscles on the right rotate the head and the atlas to the right on C2. The superior oblique, rectus capitis posterior major, and rectus capitis posterior minor all insert on the head and so also act to extend the head.
Suboccipital Nerve
All four of these muscles are supplied by what is called the suboccipital nerve, which is the name given to the dorsal ramus of the first cervical nerve. The first cervical nerve comes out between the skull and C1, so the name for this dorsal ramus—the suboccipital nerve—is literal: the nerve comes out below the occipital bone of the skull. And the only job of this nerve is to supply those four small, but important muscles—the C1 nerve never reaches the skin, so one won’t find it on a dermatome map, and so is an exception to the rule of spinal nerves being mixed.
Another interesting thing is that the suboccipital nerve emerges from the triangle formed by the juxtaposition of the rectus capitis posterior major, superior oblique capitis, and the inferior oblique capitis. And deep within that triangle, if the muscles are parted slightly, one can see the vertebral artery—the same vertebral artery that snakes up through the transverse foramina of cervical vertebrae as it makes a horizontal journey from the transverse foramen of C1 and heads to the foramen magnum to enter the skull and help supply the brain.
So, as one can see, the spine—as the main axis of the body—has a wonderfully complex set of muscles to achieve the support, posture and movements we ask of it all day, every day. It’s incredibly important that we keep these muscles strong and flexible.
Common Questions about the Spine
The spine is a series of stacked bones, linked by the many joints and ligaments that unite the vertebrae into the spinal column. This makes the spine stable and gives it the mobility needed to be flexible—but the movements and much of the support comes from muscles.
The suboccipitals is a group of four bilateral muscles that form what is called the suboccipital triangle. The rectus capitis posterior major originates on the spinous process of the axis and inserts on the skull. The rectus capitis posterior minor starts on the posterior aspect of the atlas and inserts on the skull. The superior oblique capitis originates on the skull and inserts on the transverse process of the atlas. And finally, the inferior oblique capitis originates on the spinous process of the axis and inserts on the transverse process of the atlas.
The suboccipital nerve is the name given to the dorsal ramus of the first cervical nerve. It emerges from the triangle formed by the juxtaposition of the rectus capitis posterior major, superior oblique capitis, and the inferior oblique capitis.
