An immunohistochemical study of mechanoreceptors in lumbar spine intervertebral discs

Journal of Clinical Neuroscience Volume 17, Issue 6, June 2010, Pages 742-745 A. Dimitroulias, C. Tsonidis, K. Natsis, I. Venizelos, S.N. Djau, P. Tsitsopoulos andP. Tsitsopoulos FROM ABSTRACT The aim of our study was to determine the types of mechanoreceptors in the twolower intervertebral discs in normal adult cadaveric donors and to review theliterature.Twenty-five lumbar (L4 – 5 and L5 – S1) intervertebral discs were retrieved from 15fresh cadavers.We utilized immunoreactivity against the S-100 protein to localize specialized nerveendings. Immunoreactivity showed receptors in 92% of discs.The most frequent type had morphology resembling the Ruffini type receptor(88%), followed by the Golgi type.Free nerve fibers were frequently present.All neural structures were found in the superficial layers of the annulus fibrosus, inlongitudinal ligaments, or between these two.The anterior part of the L5 – S1 disc had a greater frequency of encapsulatedreceptors than the other parts, which may be correlated with the high shear forcesto which the lumbosacral junction is subjected. KEY POINTS FROM THIS STUDY: 1) The presence of nerve structures in intervertebral discs is well documentedsince 1932. 2) These receptors have a key role in the perception of joint position andadjustment of the muscle tone of the vertebral column.[Important: these nerves are responsive to changes in joint position andalignment, and they control vertebral column muscle tone]. 3) An important component of low back pain is an intense muscle spasm of thevertebral musculature, elicited through reflex arches mediated by specialized nerveendings. 4) There are four categories of joint receptors: A)) type I: encapsulated mechanoreceptors similar to Ruffini endings B)) type II: encapsulated mechanoreceptors similar to Pacinian corpuscles C)) type III: encapsulated mechanoreceptors similar to Golgi endings D)) type IV: unmyelinated free nerve endings and unencapsulated plexuses thathave nociceptive function. 5) The 25 discs used in this study were harvested from 15 human cadavers withthe mean age of 45.4 years (range, 15 – 66 years). None had any history of chroniclow back pain. 6) Nerves were not found in 2 out of 25 discs (8%). 7) The receptors most frequently encountered showed Ruffini receptormorphology [mechanoreceptors]. Ruffini receptors help maintain muscle tone (lowthreshold, slow adaptation). 8) The second most common receptors found had Golgi tendon organmorphologically [mechanoreceptors]. Golgi receprtors are activated at extremes ofjoint motion (high threshold, slow adaptation). 9) Free nerve fibers [pain afferents] were a frequent finding. In 16 of the 25discs [64%], free nerve fibers were found. It is assumed that those close to bloodvessels have a vasomotor role[sympathetic] while those away from vessels mayhave a nociceptive (small caliber) or a proprioceptive role (large caliber). 10) The neural elements were found in the longitudinal ligaments, the spacebetween the longitudinal ligaments and the annulus fibrosus, and in the outer thirdof the annulus fibrosus. 11) The greatest amount of mechanoreceptors were found at the anterior part ofthe L5 – S1 disc which may be correlated with the high shear forces to which thelumbosacral junction is subjected. 12) All neural structures detected were located either in the longitudinalligaments, or the superficial lamellae of the annulus fibrosus, which are the areassustaining the greatest pressure or tension during extremes of movement. 13) During axial loading of a motion segment, compressive stresses in thenucleus will generate tensile stresses in the peripheral annulus, which is rich inneural receptors. 14) In conclusion, this study confirms the existence of an abundant network ofencapsulated and non-encapsulated receptors in the intervertebral discs of thelower lumbar spine in normal human subjects. The principal role of encapsulatedstructures is assumed to be the continuous monitoring of position, velocity andacceleration (kinesthesia). Free nerve fibers are likely to be involved in nociceptionor regulation of vessel tone (autonomic fibers). COMMENTS FROM DAN MURPHY It is chiropractically important to understand that the intervertebral disc isinnervated with nerves that communicate with the central nervous system. Thesenerves tell the CNS about the mechanical status of spinal function and alignment ofthe spine. Undoubtedly, chiropractic adjustments influence these nerves both during anadjustment and afterwards as a consequence of improved biomechanical functionand posture.