Between 15% and 40% of chronic low back pain is related to the Z joints.  The Z joint capsule receives a significant sensory innervation, [19, 20] much of which is probably related to nociception, that is, signaling potential or real tissue damage. The medial branches of the posterior primary divisions innervating a Z joint terminate as 1 of 3 types of sensory receptors:
free nerve endings (nociceptive),
complex unencapsulated nerve endings, and
encapsulated nerve endings.
The free nerve endings are associated with nociception. The ultrastructure of these receptors has been described. [20, 21]
The Z joint capsules throughout the vertebral column are thought to do little to limit motion.  However, the capsules probably help to stabilize the Z joints during motions.  The gross and microscopic anatomy of the Z joint capsules has been described in detail.
See the sections on Biomechanics and the
Somatic Nervous System
for further details of current research related to the Z joints and spinal adjusting.
Ligaments of the spine
Nerve tracing techniques indicate that stretching of spinal ligaments results in “a barrage of sensory feedback from several spinal cord levels on both sides of the spinal cord.”  The sensory information has been found to ascend to many higher (cortical) centers. Such findings provide provocative evidence that the spinal ligaments, along with the Z joint capsules, and the small muscles of the spine (interspinales, intertransversarii, and transversospinalis muscles) play an important role in mechanisms related to spinal proprioception (joint position sense) and may play a role in the neural activity related to spinal adjusting. 
Recent work has also been done assessing the structure of spinal ligaments. The attachment sites and dimensions of the anterior and posterior longitudinal ligaments [27, 28] and the innervation and gross and light microscopic structure of the ligamenta flava [29, 30] and iliolumbar ligaments  have been studied in detail.
The long posterior sacroiliac ligament may be important in transmitting loads from the lower extremity to the spine.  The strongest fibers course from the posterior superior iliac spine to the sacrotuberous ligament, and many important structures attach to this band, including the aponeurotic attachments of the common origin of the erector spinae muscle. The ligament is tensed during counternutation of the sacrum and slackened during nutation.  These findings are considered to be important by those involved with the study of the “kinetic chain concept” of load transmission from the lower extremity to the spine.
The intervertebral disk and intervertebral disk degeneration
Many relevant studies on the biology of the intervertebral disks (IVDs) have been completed in recent years. Disk degeneration is characterized by loss of fluid pressure, disruption or breakdown of collagen and proteoglycans, and sclerosis of the cartilaginous end plate and the adjacent subchondral bone. All of these hallmark signs of IVD degeneration can also occur as part of the normal aging process of the IVD. For these reasons, disk degeneration and normal aging of the disk are frequently discussed interchangeably, [33, 34] although the biochemical processes may be distinct. The IVD seems to age differently from other tissues, probably because of its lack of a blood supply, and the degenerative process may begin as early as 20 years of age (earlier in some cases).  In fact, certain teenagers may experience back pain because of IVD degeneration.  There is an extremely wide variation in aging and degeneration of the IVD. Some individuals in their 70s have disks of equivalent health to some in their 30s. The aging and degenerative stages of the IVD from prenatal development through the ninth decade of life have been worked out in considerable detail at the gross and light microscopic levels.
[34–40] Calcification of the IVD during the aging process is much more common than was once thought, being found in 58.3% of subjects at autopsy. Such calcification is “significantly underestimated” by conventional radiography. 
Several conditions promote or even possibly initiate disk degeneration. These include traumatic Schmorl's node formation, advanced aortic atherosclerosis,  and possibly, nicotine consumption.  The biochemistry of IVD degeneration is also being elucidated. In this regard, extruded nucleus pulposus has been found to spontaneously produce increased amounts of many chemokines that not only initiate a series of events that decrease the size of the IVD bulge but also result in IVD degeneration. [26, 35, 44–47]
Intervertebral disk protrusion
The normal mechanics of the IVD continue to be investigated. [33, 48–51]
In addition, the mechanisms involved in IVD protrusion and failure have been studied in detail [35, 39, 52–62], as well as the effects of changing intradiscal pressure. [63, 64, 65] In addition, a set of terms to be used when describing bulging of the IVD was established by the International Society for the Study of the Lumbar Spine.  This terminology included disk bulge, protrusion (tearing of some inner layers of the anulus fibrosus with the nucleus extending into the radial tear), extrusion (tearing of all layers of the anulus fibrosus allowing nuclear material to enter the vertebral canal), and sequestration (a piece of extruded nucleus breaks off of the host IVD). Much recent research related to IVD protrusion in the cervical and lumbar regions (protrusion in the thoracic region has not been studied as extensively) has found that IVD protrusion is a very dynamic process and that after approximately 1 to 3 weeks, IVD protrusion will usually begin a 2-month to 1-year process of resolution, resulting in significant resorption and, from a patient's standpoint (ie, pain), often complete remission of signs and symptoms. [33, 67–69] In fact, histologic evidence of resorption of sequestered nuclei pulposi has been found, [70, 71], and shrinkage of protruded nuclei pulposi has been seen on both computed tomography and MRI.  This provides hope to patients with protruded IVDs and for those using conservative methods to treat this condition. Adenovirus-mediated transfer of genes and the resultant production of therapeutic growth factors are being investigated as a means to further study the biology of the IVD and the potential for treatment of disk degeneration ; however, the low vascularity of the adult IVD may preclude the effective use of gene therapy in IVD disease.  Two published studies have shown that the inhibition of tumor necrosis factor–a (TNF-a) (extruded nucleus pulposus contains high levels of TNF-a) by a monoclonal antibody (Remicade [infliximab], Centocor, Inc., Horsham, Pa) is successful in alleviating sciatica. [74, 75] Finally, the mechanisms of radicular pain continue to be studied. [41, 76–78]
Innervation of IVDs
The significant innervation of the IVDs continues to be investigated in detail. [79–82] Degenerated disks have been found to receive increased innervation by sensory fibers conducting nociception.  The added innervation seems to be stimulated by Schwann cells of the nerves innervating the outer aspect of the anulus fibrosus.  Consequently, injured or degenerated disks are likely to be more sensitive to pain than normal disks.
Unique characteristics of the cervical IVDs
The cervical IVDs have been found to differ significantly from the lumbar disks. Rather than being made up of many lamellae, the anulus fibrosus of each cervical disk is composed of a single, crescent-shaped piece of fibrocartilage that is thick anteriorly and becomes very narrow laterally and posteriorly. 
Range of motion studies
Noteworthy studies measuring both the ranges of motion in various regions of the spine (eg, cervical region) and the motions between individual vertebrae continue. The latter activity has led to studies attempting to better understand the concept of coupled motions in the spine. Finally, significant findings related specifically to motions in the sacroiliac joints have also been published in recent years. These findings are summarized next.
Although ROMs (eg, cervical ROMs) can be measured reliably,  measurements made on different days of the same individual can vary considerably.  Coupled motion (eg, rotation of vertebrae during lateral flexion) of spinal segments continues to be actively studied.
Current investigators are finding that:
(1) these motion patterns are very complex;
(2) all spinal motions are coupled motions; and
(3) coupling differs from 1 motion segment to the next.
Furthermore, consensus has not been reached on many of these motion patterns.
The full ROM of the sacroiliac joint is not expressed until the extremes of hip motion are reached, moving an average of 7.5° (range, 3°–17°) in the sagittal plane during full flexion and extension of the hips.  Motions as high as 22° to 36° have been reported in preteen and early teenage children.  Contraction of the left and right transversus abdominis muscles increases stiffness of the sacroiliac joint, thus potentially reducing sprains of the ligaments that protect it. 
Morphometry means “measurement of an organism or its parts.” The past decade has seen many morphometric studies of various spinal structures. These studies allow for more accurate biomechanical and computer modeling (finite element analysis) studies to be performed and also allow for more accurate patient treatment protocols (surgical and manipulative) to be designed. Table 1 shows many of the morphometric studies performed since 1995, the region of the spine investigated, and the specific anatomical structure analyzed.
Certain anatomical findings can best be discussed with each spinal region. The following sections describe anatomical findings of particular significance in the cervical, thoracic, lumbar, and sacroiliac regions. Each of the topics discussed is related to an active area of research.