The spine, or vertebral column, is a segmental set of 33 bones and associated soft tissues that comprise the subcranial portion of the axial skeleton. It subdivides into five regions based on curvature and morphology: the cervical, thoracic, and lumbar spine, the sacrum, and the coccyx.  There are seven, twelve, and five articulating vertebrae in the cervical, thoracic, and lumbar spine, respectively.  These three regions, while somewhat similar in terms of bone morphology, variably balance spinal rigidity with flexibility and movement, and articulate in a particular fashion that contributes to the overall S-shaped curvature of the spine.  Meanwhile, the sacrum and the coccyx are two sets of fused vertebrae at the caudal aspect of the spine that convey no motion.  Five fused vertebrae typically form the sacrum, with four forming the coccyx.

The cervical spine, comprised of seven cervical vertebrae referred to as C1 to C7, is divided into two major segments: the craniocervical junction (CCJ) and the subaxial spine. The CCJ includes the occiput and the two most cranial cervical vertebrae known as the atlas (C1) and the axis (C2). The subaxial spine includes the five most caudal cervical vertebrae (C3-C5). As a whole, the cervical spine is responsible for supporting the weight of the cranium and allowing motion of the head and neck. 

The typical vertebrae have hallmark anatomic structures that are conserved across the cervical, thoracic, and lumbar regions.  Generally, each vertebra is comprised of a ventral vertebral body comprised of mostly trabecular cancellous and a denser, mostly cortical dorsal vertebral arch.  The vertebral body is the main site of intervertebral articulation and load-bearing.  Each vertebral body is linked to its cranial and caudal counterparts by an intervertebral disk.  The dorsal arch typically consists of a pair of pedicles arising from the dorsal vertebral body which unite dorsally by a pair of flat laminae.  The two laminae join at the midline producing a dorsal projection called the spinous process. The pedicles, laminae, and dorsum of the vertebral body form the vertebral foramen, a complete osseous ring that encloses the spinal cord. Its main role is to protect the spinal cord and its associated vascular structures.  Additionally, the transverse processes and the superior and inferior articular processes are present near the junction of the pedicles and the laminae. In the cervical spine, the costal process becomes the anterior part of the transverse process that encloses the vertebral artery foramen.[1]

Unique Features of the Cervical Spine

Despite displaying most of the typical vertebral hallmarks, a significant amount of anatomical variation exists within the cervical spine.  The main role of the cervical spine is to support and promote the movement of the head and neck.  Large vertebral bodies are not necessary considering the relatively small weight-bearing load at this level.  Thus, an increased range of motion takes priority over vertebral size and rigidity.  However, the additional motion and flexibility may carry an increased risk for injury of the spinal cord and its associated neurovascular structures. All seven cervical vertebrae have a transverse foramen within their transverse processes, where a pair of vertebral arteries travel cranially through the vertebrae starting at C6 before coursing medially over the arch of C1 towards the foramen magnum.  The spinous processes of the C2 to C6 vertebrae are bifid, C1 has a posterior tubercle instead of a spinous process, and C7 has a much larger and singular spinous process, known as the vertebra prominens, which is similar to those in the thoracic vertebrae.[2]

The Upper Cervical Spine – The Axis (C1) and the Atlas (C2)

The upper cervical region of the cervical spine includes C1 and C2, which are more commonly referred to as the atlas and the axis, respectively.  The main function of the atlas is to support and cradle the base of the occiput at the atlanto-occipital articulation.  As such, there are many features unique to the atlas not shared with the rest of the spine.  Most notably, the atlas lacks a vertebral body and instead forms a large ring-shaped fusion of anterior and posterior arches that allows C1 to accommodate the spinal cord as it exits the foramen magnum.  The atlas has pronounced concave and medially facing articular facets which accommodate the convex occipital condyles.  This morphology helps the joint contribute roughly fifty percent of flexion-extension of the neck but limit lateral displacement of the occiput.  These joints receive further stabilization from strong soft tissue ligaments that promote tight adherence to the occiput.[1][3] 

The axis, or C2, also possesses unique anatomical features.  Whereas the atlas is responsible for accommodating the occiput, the axis is the primary weight-bearing bone of the upper cervical region.  The hallmark feature is its odontoid process, or dens, which is a bony projection that extends cranially from the vertebral body.  It evolved from the body of the atlas and serves as the principal attachment point for the soft tissues that stabilize the atlantoaxial junction.  In contrast to the atlanto-occipital joint, the atlantoaxial junction is responsible for about fifty percent of the rotational motion of the cervical spine. The junction has three articulations: a midline atlanto-odontoid (or atlanto-dental) joint and a pair of atlantoaxial facet joints. The atlanto-odontoid joint permits the anterior arch of the atlas to pivot on the odontoid process. The lateral facet joints involve the articulation of the atlas’ inferior facets and the superior facets of the axis; these joints are fairly shallow to allow for significant motion.[1][2][3][4]

The Subaxial Cervical Spine – C3 to C7

All five vertebrae in this region share nearly identical morphological and functional features, and compared to the upper cervical spine, share most characteristics with the typical vertebral anatomy. Unique to these, all five vertebrae possess uncinate processes, which are bony prominences on the lateral edges of the vertebral body that articulate at Luschka joints to confer additional stability and prevent vertebral listhesis.[2]

There also exist some minor features of C7 that differentiate it from the rest of the subaxial region.  The transverse foramen of C7 is smaller in diameter in comparison to the rest of the region and typically does not house the vertebral arteries.  Instead, the vertebral arteries cross anteriorly to the transverse processes of C7 before proceeding cranially through the transverse foramina of C6.  Additionally, C7 is considered a transitional vertebra, and as such, has a spinous process and inferior facets that resemble thoracic vertebrae.  The vertebra also has a somewhat larger vertebral body.[5]

The vertebral column derives from somites, part of the paraxial mesoderm.  More than forty pairs of somites develop in a craniocaudal fashion alongside the notochord during the process of body axis elongation.  Under the direction of local factors secreted by the notochord, neural tube, ectoderm, and visceral mesoderm, the somites rapidly undergo endothelial to mesenchymal transition (EMT) to form the sclerotome, dermatome, and myotome.  The vertebral column is ultimately derived from just the mesenchymal sclerotome, whereas the dermomyotome form muscle cells and overlying dermis.[6][7]

During the fourth week of embryogenesis, sclerotome cells quickly develop around the notochord and the neural tube, signifying the initiation of the true vertebral column and skull base development.  Under the direction of multiple factors produced by the notochord, segmentation of the column progresses.  Cells that will ultimately form the annulus fibrosus and nucleus pulposus of the intervertebral discs coalesce between collections of sclerotome cells, and aggregations of these sclerotome cells will later fuse to give rise to independent vertebrae by the sixth week of embryonic development. Subsequently, ossification begins at three main ossification centers in the individual vertebra and will progress to five ossification centers through the first year of life. The two secondary centers of ossification that develop later will contribute to the formation of the vertebral growth plate.[4][6][7]

The ossification centers of the odontoid process are particularly important since they are often misidentified as fractures in the pediatric population. The junction between the dens and the axis vertebral body does not fuse until around six years of age, whereas the secondary ossification centers appear around the age of three years old, and fuses to the cranial aspect of the dens by age twelve.[4]

Distal to their bifurcation from the subclavian arteries, the vertebral arteries course cranially to the base of the skull.  They progress past the C7 vertebrae anteriorly before entering the transverse foramen of C6.  After exiting the C2 transverse foramen, the arteries bend laterally to enter the more widely placed transverse foramen of C1 and then revert medially over the superior aspect of the arch of the atlas before entering the base of the skull.  Main tributaries of the vertebral arteries, the cervical radicular arteries, supply blood directly to the cervical vertebral bodies.[8]

Also located in the cervical spine region are the carotid arteries.  The right common carotid artery is a branch of the brachiocephalic artery, whereas the left common carotid arises directly from the aortic arch.  The common carotids then bifurcate into the internal and external carotids at the C3 vertebral level.  Despite their association with the neck, they do not perfuse any structures in the cervical spine.[8]

The cervical spine functions as bony protection of the spinal cord as it exits the cranium.    Despite the presence of seven cervical vertebrae, there are eight pairs of cervical nerves, termed C1 to C8.  C1 through C7 exit the spine cranially to its associated vertebrae, while C8 exits caudally to C7. 

Direct innervation of the spinal column is highly complex, yet three discrete innervation sources have been identified in the ventral compartment of the cervical spine.  These include branches of the sympathetic trunk, the sympathetic rami communicantes, and the perineural vascular plexus of the vertebral arteries.  Meanwhile, the posterior aspect of the cervical spine receives innervation by the medial branch of the posterior primary rami.  Coursing through the intervertebral foramen with the vertebral arteries, the vertebral nerves are thought to supply additional sympathetic innervation.[9]