The neurological dysfunction associated with the vertebral subluxation may take many forms. A previous column discussed nerve compression physiology. This month, I will address the production of aberrant afferent input to the CNS as a result of vertebral subluxation.

The intervertebral motion segment is richly endowed by nociceptive and mechanoreceptive structures. As a consequence, biomechanical dysfunction may result in an alteration in normal nociception and/or mechanoreception. Aberrated afferent input to the CNS may lead to dysponesis. To use the contemporary jargon of the computer industry, "garbage in -- garbage out."

Appreciation of these processes begins with an understanding of the neuroanatomy of the tissues of the intervertebral motion segment.

Several papers have described the innervation of human cervical and lumbar intervertebral discs.

Bogduk et al observed that the lumbar intervertebral discs are supplied by a variety of nerves.

The sinuvertebral nerve supplies the posterior aspect of the disc and the posterior longitudinal ligament. The posterolateral aspects are innervated by adjacent ventral primary rami and from the grey rami communicantes. The lateral aspects of the disc are innervated by the rami communicantes. The anterior longitudinal ligament is innervated by recurrent branches of rami communicantes. [1]

Clinically, Bogduk stated that intervertebral discs can be a source of pain without rupture or herniation. Torsional stress may result in circumferential tears in the innervated outer third of the annulus. Compression injuries may lead to internal disruption of the disc, resulting in mechanical or chemical stimulation of the nerve endings in the annulus. [2]

Bogduk et al also examined the nerve supply to the cervical intervertebral discs. The sinuvertebral nerves were found to supply the disc at their level of entry as well as the disc above. Nerve fibers were found as deeply as the outer third of the annulus. [3]

Mendel et al stated that nerves were seen throughout the annulus. In addition, receptors resembling Pacinian corpuscles and Golgi tendon organs were seen in the posterolateral region of the disc. The authors conclude that human cervical intervertebral discs are supplied with both nerve fibers and mechanoreceptors. [4]

Human cervical facet joints are also equipped with mechanoreceptors. McLain found Type I, Type II, and Type III mechanoreceptors, as well as unencapsulated nerve endings in the cervical discs of normal subjects.

The author stated, "The presence of mechanoreceptive and nociceptive nerve endings in cervical facet capsules proves that these tissues are monitored by the central nervous system and implies that neural input from the facets is important to proprioception and pain sensation in the cervical spine. Previous studies have suggested that protection muscular reflexes modulated by these types of mechanoreceptors are important in preventing joint instability and degeneration." [5]

Wyke has described articular mechanoreceptors, and explored the clinical implications of dysafferentation in pain perception. [6,7]

Besides the discs and articular capsules, mechanoreceptors and other neural tissues have been described in the ligaments attached to the spine.

Jiang et al noted that Pacinian corpuscles were scattered randomly, close to blood vessels, whereas Ruffini corpuscles were seen in the periphery of human supraspinal and interspinal ligaments. [8]

Rhalmi et al found nerve fibers in the ligamentum flavum, the supraspinal ligament, and the lumbodorsal fascia. [9]

The authors of the remarkable book "Segmental Neuropathy," published by Canadian Memorial Chiropractic College, proposed the concept of a "neural image," dependent upon the integrity of neural receptors and afferent pathways. If afferent input is compromised, efferent response may be qualitatively and quantitatively compromised. [10]

Alterations in mechanoreceptor function may affect postural tone.

Murphy summarized the neurological pathways associated with the maintenance of background postural tone: "Weight bearing disc and mechanoreceptor functional integrity regulates and drives background postural neurologic information and function (muscular) through the unconscious mechanoreception anterior and posterior spinocerebellar tract, cerebellum, vestibular nuclei, descending medial longitudinal fasciculus (medial and lateral vestibulospinal tracts), regulatory anterior horn cell pathway." [11]

The anterior horn cells provide motor output which travels via motor nerves to muscle fibers. Although stimulation of articular mechanoreceptors may exert an analgesic effect, use of manipulation for the episodic, symptomatic treatment of pain is not chiropractic.

Correcting the specific vertebral subluxation cause is paramount to restoring normal afferent input to the CNS, and allowing the body to correctly perceive itself and its environment.

References

1. Bogduk N, Tynan W, Wilson AS: "The nerve supply to the human lumbar intervertebral discs." J Anat (1981 Jan) 132(Pt 1):39.

2. Bogduk N: "Pathology of lumbar disc pain." Manual Medicine (1990) 5(2):72.

3. Bogduk N, Winsor M, Inglis A: "The innervation of the cervical intervertebral discs." Spine (1988 Jan) 13(1):2.

4. Mendel T, Wink CS, Zimny ML: "Neural elements in human cervical intervertebral discs." Spine (1992 Feb) 17(2):132.

5. McLain RF: "Mechanoreceptor endings in human cervical facet joints." Spine (1994 Mar 1) 19(5):495.

6. Wyke B: "The neurology of joints. Ann R Coll Surg (Br) (1967):25.

7. Wyke B: "Neurology of the cervical spinal joints." Physiother (1979) 65:72.

8. Jiang H, Russell G, Raso VJ et al: "The nature and distribution of the innervation of human supraspinal and interspinal ligaments." Spine (1995 Apr 15) 20(8):869.

9. Rhalmi S, Yahia LH, Newman N, Isler M: "Immunohistochemical study of nerves in lumbar spine ligaments." Spine (1993 Feb) 18(2):264.

10. "Segmental Neuropathy." Canadian Memorial Chiropractic College. Toronto, Ontario. No date.

11. Murphy DJ: "Neurogenic posture." Am J of Clinical Chiropractic (1995) 5(1):16.