There are four curvatures in the adult spine—cervical, thoracic, lumbar, and sacral—with alternating convexity (Fig. 1.3). The cervical and lumbar curvatures are oriented with posterior concavity, known as lordosis, while the thoracic and sacral curvatures are oriented with anterior concavity, known as kyphosis.³ The curvatures of the vertebral column change through development, from the developing fetus to adulthood. The fetal spine is anteriorly concave through its entire length (Fig. 1.4). In adulthood, the sacrum and the thoracic curvatures retain this anterior orientation, while the cervical and lumbar curvatures reverse, making them primary and secondary curvatures, respectively.

The intervertebral discs are poorly vascularized, relying on the diffusion of nutrients from adjacent vertebrae.⁴ The innervation of the intervertebral discs is from the ventral rami and grey ramus communicans of the spinal nerves at the level above, so the L4-L5 disc is innervated by L4 (Fig. 1.5).⁵ Attached to this bony framework are ligaments, tendons, deep, intermediate, and superficial muscles, which will be expanded upon later in this chapter. When combined with these connective structures, the spine can move in flexion and extension, right and left side bending, and can rotate around the central axis. The segments of the vertebrae articulate with each other at Z-joints. From top to bottom, cervical to lumbar, the superior articular surface of the facet joints of the vertebrae shift from being primarily upward and posterior facing in the cervical vertebrae to lateral and posterior facing in the thoracic vertebrae and finally medial and posterior in the lumbar. This is a gradual transition from level to level, and the shift in orientation allows for different freedom of movement for each region of the spine. The lumbar and cervical spine are both able to side bend and rotate to a similar degree. The cervical spine has the largest range of motion in flexion, whereas the lumbar spine is most free in extension.⁶ The thoracic spine, locked in place due to the attachment of the thoracic cage, is the freest in axial rotation.⁷ The orientation of the facet joints can be best illustrated in Figs. 1.6 and 1.7.

The (Z-Joints) are true joints made up of articular surfaces, a synovial membrane, and a joint capsule. They are innervated by the dorsal rami of spinal nerves. Two pairs of medial branch nerves from the dorsal rami, one from the level above and one below, join to provide innervation to the joint. By naming convention this means at the cervical level these two medial branches come from the same corresponding nerve level, so C5-C6 Z-joints are innervated by C5 and C6 medial branch nerves. Alternatively in the thoracic and lumbar regions due to the existence of the C8 to T1 transition the level of innervation is shifted, meaning that the T11-T12 facet joint is innervated by T10 and T11 medial branches.³ The course of the posterior ramus of the spinal nerves can be seen in Fig. 1.8.

The nervous system of the spine is split between the central nervous system in the form of the spinal cord and the peripheral nervous system. As mentioned previously, the spine functions not only as rigid support and attachment for the body but also as a protective conduit for the spinal cord. The spinal cord is cylindrical in shape extends out from the skull via the foramen magnum and extends to roughly two-thirds the length of the spine. Spinal cord is contained inside the dural sack with denticulate ligaments anchoring the cord to the dura between nerve roots. Inside the dural sack anterior and posterior to the denticulate ligaments are the anterior and posterior nerve rootlets, which meet to form the spinal nerves. The dura, the arachnoid, and the pia mater make up the meninges. The pia mater is the tissue layer sitting directly on top the spinal cord, whereas the arachnoid is between the pia mater and the dural sac.³ The cord is bathed in cerebrospinal fluid, which is contained within the dural sac and below the arachnoid mater within the subarachnoid space (Fig. 1.9).

The spinal cord itself can be divided into white matter and grey matter, with the grey matter made up of cell bodies and the white matter made up of nerve tracts. The grey matter runs in a butterfly-shaped core through the center of the cord with white matter surrounding it. The spinal cord has anterior and posterior horns. The sensory function of the body arises in the posterior horn, whereas the motor function of the body arises from the anterior, making up the somatic nervous system.

The spinal nerves exit the spinal cord through neural foramina (visualized well in Fig. 1.8). From there, the nerves go on to innervate structures throughout the body. The sensory distribution of the spinal nerve levels is clearly mapped out in the dermatomes¹ (Fig. 1.10).