Stereoscopic Structure of Sensory Nerve Fibers in the Lumbar Spine and Related Tissues

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Abstract
Neurotracer was applied to various sites in the rat lumbar spine and related tissues. The segmental distribution of labeled neurons in dorsal root ganglia (DRG) was investigated. To clarify the stereoscopic structure of afferent fibers innervating the lumbar spine and related tissues. Afferent fibers in the rat L5–L6 lumbar intervertebral disc are reported to originate from neurons in L1 and L2 DRG. However, anatomic studies determined that each dorsal ramus of the spinal nerve sends nerves to dorsal elements of the corresponding lumbar vertebra. Fluorescent neurotracer DiI (1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate) was applied to various sites of the lumbar spine and related tissues in the L2, L5, and L6 levels in rats. DRG were sectioned 3 weeks after DiI application. Rostrocaudal distribution of DiI-labeled neurons was investigated. At L5, DiI-labeled neurons were prominent in DRG L3 for the lamina, L2 for the spinous process, L2 for the back muscle fascia, and L1 for the skin. Dorsal elements are therefore innervated by neurons in more rostral DRG. In the transverse plane, the more distant from the DRG a site was, the more rostral the DRG innervating the site. This structure suggested a concentric innervation pattern in the transverse plane. Stereoscopically, the peripheral innervation territory of a lumbar DRG is conical, with the apex at the ganglion and the base circumference located on the dermatome. The lumbar spine itself is involved in the conical innervation territories of DRG.