The cervical spine is broken up into 7 segments. C1 and C2 are atypical, whereas C3 to C7 are typical.
There are eight cervical nerve roots. C1 to C7 exit above their corresponding vertebrae. C8 exits between C7 and T1. Nerve roots C3 to C5 innervate the diaphragm. Nerve roots C5 to T1 make up the brachial plexus, a network of nerves that innervate the upper extremity.
The transverse foramen is an opening in the vertebrae between the vertebral body and the transverse process. The vertebral artery, after arising from the first part of the subclavian artery, passes through the transverse foramina of C6 to C1 and into the foramen magnum. Accompanying the vertebral artery through the transverse foramen is the vertebral vein. The vertebral vein mainly serves the cervical spinal cord and adjacent vertebrae. The cervical vertebrae are the only vertebrae with transverse foramen.
C1, the Atlas, is named after the Greek Titan who holds the world on his shoulders. Similarly, the occiput is held up by C1. C1 also articulates with the dens of C2, held secure by the cruciate ligament.
The transverse ligament, also known as the cruciate ligament, attaches to C1 and C2. It acts much like a seat belt originating from C1, sneaking over the dens of C2 and inserting into C1.
C2 is often referred to as the axis because half of the rotation of the cervical spine occurs here.
The dens, also known as the odontoid process, projects superiorly to articulate with the anterior arch of C1. The dens is named after the Latin word for “tooth”.
The alar ligaments are two strong fibrous cords a half-centimeter in width that attach the dens to the occiput. Along with the cruciate ligament, the alar ligaments help stabilize the upper cervical spine. These ligaments can be lax in certain diseases, such as Down Syndrome and Rheumatoid Arthritis.
The Joints of Luschka are present from C3 to C7. There are superolateral projections coming from the vertebral bodies in the cervical spine; these articulate with the vertebrae immediately superior to them. The articulations that these projections make are referred to as the Joints of Luschka. These joints exist to prevent herniated discs from impinging on cervical nerve roots.
C2 to C6 have bifid spinous processes; this means that their bony posterior projections fork into two.
The scalenes originate from the transverse processes of the cervical vertebrae. The anterior and middle scalenes then attach to the first rib. The posterior scalene attaches to the second rib. When both left and right scalenes contract, they flex the neck. When only one side contracts, they sidebend the neck ipsilaterally.
The sternocleidomastoid, also known as the SCM, is a muscle of the neck that runs from the nuchal line and mastoid process of the skull and attaches to the sternum and clavicle. When contracted unilaterally, the SCM will rotate the head contralaterally and sidebend the head ipsilaterally. The action of one SCM may be remembered as “STRAw”: Sidebend Towards, Rotate Away”. Contracted bilaterally, the SCM flexes the neck.
The Atlantal-Axial joint is where C1 and C2 meet. Half of the total rotation of the neck happens here. Almost no flexion or extension happens at this joint. For this reason, the AA joint is sometimes called the “No” joint, as it facilitates shaking the head “no”.
The Occipital-Atlantal joint is the occiput on top of C1. Half of the total flexion and extension of the neck happen at this joint. The OA joint has what is referred to as “type-I like mechanics” because rotation and sidebending occur to opposite sides. Unlike true type I mechanics, the joint may also be flexed or extended while rotated and sidebent to opposite sides. . Because of the large motions in flexion and extension, the OA joint is sometimes called the “yes” joint, as it facilitates nodding the head “yes”.
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