The human spine is a complicated anatomic unit, consisting of osseous, ligamentous, muscular, intervertebral, vascular, and neural elements. The interaction of these individual elements allows for motion, protection of the spinal cord, and the distribution of forces throughout the body.

The spinal cord is derived from the neural crest and neural plate early in embryologic development. Beginning at week 3, the embryo becomes planar before the development of the neural crest. A temporary structure known as the primitive groove appears around this time. This primitive groove will deepen and begin to fold on itself within the ectodermal layer of the embryo. When the cleft has completely closed, it becomes the neural tube. During the closing of the neural tube, the neural crest will form dorsally as the notochord remains ventral (Figure 1).

The neural crest will eventually form the peripheral nervous system, whereas the neural tube is the primitive form of the central nervous system/spinal cord. The notochord will form the structural elements of the spine, including the anterior vertebral bodies and intervertebral disks.

The failure of complete neural tube closure results in a variety of clinical pathology. Failure of cranial closure can result in anencephaly. Failure of caudal closure can result in spina bifida occulta, meningocele, myelomeningocele, or myeloschisis. Conversely, diastematomyelia is thought to be due to a remnant neuroenteric canal during the third and fourth weeks of gestation.

The development of individual vertebra occurs from the somites. These surround the notochord and neural tube. There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal somites. Of these, 31 somites persist. Each vertebra has three primary ossification centers—the centrum and one in each half of the vertebral arch. The centrum gives rise to the vertebral body anteriorly. The neural arch gives rise to the posterior elements, as well as the pedicles and a small portion of the anterior vertebral body. The costal ossification center
gives rise to the anterior portion of the lateral mass, transverse process, or rib depending on the region of the spine. In the intervertebral disk, the notochord gives rise to the nucleus pulposus, whereas the sclerotome gives rise to the anulus fibrosus. The development and segmentation of these structures happens simultaneously in the embryo, with failure of formation or segmentation leading to a variety of clinical pathologies (Figure 2).

The spinal cord is derived from the neural tube. The dorsally located cells become primarily afferent sensory pathways, whereas ventrally they become primarily efferent motor control pathways.¹,²

The spinal cord is the central neurologic element that provides a connection between the brain and the body. It is composed of highly organized pathways divided between efferent (outgoing) and afferent (incoming) signals.

The spinal cord changes position throughout development. At birth, the conus medullaris lies at the L3 level and migrates to the L1-L2 level by adulthood. As a result, the neurologic level of the spinal cord does not always correlate to the vertebral level, as described in
a 2019 study.³ There are 31 pairs of spinal nerves that correspond to the level at which they exit the spinal canal/neuroforamen. In the cervical spine, the roots exit above the same numbered pedicle, transitioning after C7 to exiting below the same numbered pedicle (ie, C4 root exits superior to the C4 pedicle, whereas the T2 root exits below the T2 pedicle).

As in the brain, the spinal cord is covered in three layers of meninges. The innermost layer is the pia mater, which is adherent to the underlying neural tissue. The next layer is the arachnoid mater, followed by the dura mater. Within the subarachnoid space there is a protective buffer of cerebrospinal fluid that surrounds the spinal cord. This fluid communicates with the cerebrospinal fluid produced in the cerebral ventricles (Figure 3).

On a microscopic level, the spinal cord is composed of two distinct types of tissues—gray and white matter. The gray matter corresponds to the neuronal cell bodies, whereas the white matter represents the myelinated axons. As the cord progresses from cranial to caudal, the proportion of gray matter decreases relative to white matter.

Dorsal cells are primarily sensory in nature whereas ventral cells are primarily motor (Figure 4). Vibration, deep pressure, and proprioception are transmitted via the dorsal columns. The lateral spinothalamic tract is responsible for pain and temperature sensations, whereas anterolaterally, the ventral spinothalamic tract is responsible for light touch sensation. Efferent motor function runs along the lateral corticospinal tract. The arrangement of myelinated fibers within these tracts is such that the upper extremities are deeper, with more distal targets such as the trunk and legs becoming progressively superficial.⁴

At each spinal level there are anterior and posterior rootlets. These consolidate to form anterior and posterior roots, ultimately combining to form a common
segmental nerve root with both motor and sensory components. The dorsal root ganglion lies on the posterior root and is the location of peripheral sensory nerve synapse with afferent signals.

The constellation of symptoms that occur in the pathologic state can often be traced to the disruption of the blood supply to grouped white matter tracts.

The human spine exhibits four distinct regions (cervical, thoracic, lumbar, and sacral), each associated with a different sagittal curvature (Figure 5). When summed, this curvature maintains the calvarium centered over the pelvis. In general, each vertebra consists of the same basic structure. Anteriorly there is the vertebral body, which consists of dense superior and inferior end plates filled with cancellous bone. Connecting the vertebral body to the posterior elements are the pedicles. The pedicle is a dense cortical strut that is filled with cancellous bone. The pedicle is also the superior border of the neuroforamen and connects to the superior articular process (SAP). The SAP is the posterior border of the neuroforamen and articulates with the inferior articular process (IAP) of the cranial level (ie, L4 IAP articulates with the L5 SAP). The SAP is confluent with the lamina. This is the dorsal shell that provides bony protection to the spinal cord and nerves. The IAP is an extension of the caudal aspect of the lamina. There is a dense region of laminar bone between the SAP and IAP that is responsible for weight transfer known as the pars interarticularis. Finally, there are the spinous process and transverse processes that serve as attachment points for various interspinous ligaments and muscular units.

The cervical region consists of seven vertebrae. Special consideration is given to the first two and last of these vertebrae. Because the cervical vertebrae require the least amount of weight bearing, their bodies are relatively small and thin with respect to the size of their posterior elements. Furthermore, they have greater medial-lateral dimension than anterior-posterior dimension. The first two cervical vertebrae are unique in their development and structure. This special articulation accounts for approximately 50% of the rotational, flexion, and extension capabilities of the cervical spine. The first cervical vertebra is known as the atlas, whereas the second cervical vertebra is known as the axis. Because of the articulation of the atlas with the skull, the superior articular facet joints of this vertebral body are unique in that they have very little slope, thus enabling articulation with the caudally directed occipital condyle. The atlas is also unique in that it does not have a true vertebral body or spinous process. The anterior arch of the atlas serves as an articulation point to the odontoid process that stems from the axis (dens). This articulation allows for stable rotation as well as resistance to horizontal translation and displacement. The transverse ligament runs across the anterior arch of C1, lying posteriorly to the tip of the dens. This maintains the pivotal relationship between the atlas and dens. This is supplemented by the apical ligament and paired alar
ligaments that confer stability to the occipitocervical junction (Figure 5).

In the cervical spine, the articular unit of SAP and IAP is referred to as the lateral mass. Additionally, there is a foramen that exists from C2 to C6 that houses the vertebral artery and its related venous plexus.

The lower cervical vertebra, C7, is notable for its elongated spinous process and trapezoidal shape. Because it is a transitional segment between cervical and thoracic regions, its superior end plate is smaller in the anterior-posterior dimension when compared with the inferior end plate.⁵

There are 12 thoracic vertebrae that are roughly sized between that of the cervical and lumbar vertebrae. The general shape of the thoracic vertebra is more heart-like when compared with the cervical or lumbar vertebra and tends to be elongated in anterior-posterior dimension. The unique and obvious characteristic of the thoracic vertebra is the addition of the costovertebral joint and support of the rib corresponding to the same vertebral level. This costovertebral articulation is located ventral to the SAP of the corresponding level and adjacent to the transverse process. Their physical relationship is such that multiple connecting ligamentous structures and a synovial cavity exist between the neck of the rib and the transverse process. There is no costovertebral articulation at T11 and 12 as these are transitional levels.

The lowest five vertebrae in the presacral spine make up the lumbar region. These vertebrae tend to be larger in all dimensions as they are responsible for supporting the most weight. They are larger in the medial-lateral
dimension than anterior-posterior and are recognizable from their cervical and thoracic counterparts because of the lack of a transverse foramen or costovertebral articulation. The lumbar vertebrae also have a pronounced mammillary process, which serves as the origin and insertion point of deep paraspinal musculature (Figure 6).

The sacrum is composed of five fused vertebrae forming a single triangular unit. This functions as the lumbopelvic connection point and keystone of the trunk and lower extremities. The orientation of the sacrum is in significant flexion such that there is a steep angle created between the lowest lumbar vertebra and the highest sacral element. This angle is very variable. The cranialmost aspect of the sacrum consists of a region known as the sacral ala. These are laterally based wings that connect to the upper region of the sacroiliac joint and provide a surgical utility as an area of dense bone that can be used in posteriorly based fusion procedures. Although fused, the first three sacral levels consist of all the basic elements of a normal vertebral body including neuroforamen and rudimentary disks; however, because of the sacral ala and sacroiliac joint, there is both a dorsal and ventral foramen that allows for the egress of a dorsal and ventral sacral nerve root.

Ligaments of the spine act as tensioners that achieve force transmission through the spine. The ligaments that compose the spinopelvic complex are the strongest in the human body. The ligaments that exist within each vertebral level add to the partial stability conferred by the osseous and muscular spinal elements.

The anterior longitudinal ligament is a broad-based, strong ligament that runs on the anterior surface of the entire vertebral column. It is composed of three distinct layers. The most superficial layer extends three to four vertebral levels. The middle layer spans two to three levels, whereas the deep layer extends only one vertebral level. The attachment point is at the anulus fibrosus with looser attachments to the vertebral body where it blends into the periosteum.

The posterior longitudinal ligament runs the length of the vertebral column and is continuous with the tectorial membrane. Similar to the anterior longitudinal ligament, the attachment point is the anulus fibrosus.

The supraspinous ligament is a well-developed and thick attachment between the tips of the spinous processes. It runs the length of the spine starting at the ligamentum nuchae and terminates on the sacrum (Figure 7).

The interspinous ligament is prominent in the lumbar spine and poorly developed elsewhere. It runs obliquely between spinous processes in the interval between the supraspinous ligament in the ligamentum flavum.

The ligamentum flavum is a subarticular ligament that runs from the medial aspect of the lamina and extends laterally to blend with the facet joint capsule. Superiorly it originates from the anterior surface of the cephalad lamina at approximately the middle region of the lamina. The inferior border creates a “shingles on a roof” structure with termination on the posterior border of the caudal vertebra. The ligamentum flavum has a high elastin content, which gives it a yellow color with elastic properties. It is sometimes referred to as the yellow ligament.