Dermatomes of rats from C1 to T1 (forelimb) and from T12 to S2 (hindlimb) were determined by electrical stimulation of spinal nerves following the intravenous administration of Evans blue. After stimulation of the ventral ramus of a spinal nerve, a blue spotted area presenting the maximal innervation area of the spinal nerve appeared in the skin. Maximal innervation areas generally overlapped the adjacent areas. Each digit was innervated by two to three spinal nerves. Composite dermatomes were determined where boundary lines were defined as the midline of overlapping areas. Based on the composite dermatomes, boundary lines in the rat fore- and hindlimbs were revealed to loop around the antero-posterior axis showing a V-shaped pattern in the anterior and posterior aspects and converge to the ventral and dorsal midlines of the limb. The present dermatome chart may be applied to research concerning the segmental distribution of sensory C-fibers.
The dorsal portion of the L5-L6 disc of rats was shown to be multisegmentally innervated by the T13 to L6 dorsal root ganglia. The sensory fibers from T13, L1, and L2 dorsal root ganglia were shown to innervate the dorsal portion of the L5-L6 disc through the paravertebral sympathetic trunks. In contrast, those from the L3-L6 dorsal root ganglia may innervate the dorsal portion of the L5-L6 disc through the sinuvertebral nerves.
The results showed that the posterior portion of lumbar intervertebral discs was innervated by the sympathetic nerves multisegmentally and bilaterally.
Dermatomes and the associated central projection fields were studied with the application of fluorescent neurotracer, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), to 21 reference points on rat trunk and hindlimb skin. Segmental distribution and rostrocaudal central level of dorsal root ganglion (DRG) neurons innervating reference points were examined and DiI-induced fluorescent areas were mapped in the horizontal plane through lamina II of the dorsal horn. Segmental levels of DRG neurons innervating reference points were generally identical to the level determined using dye-extravasation methods. However, innervation of the first digit was situated in the L4 dermatome, not the L3 reported previously using those methods. Generally, afferents from a reference point projected to a single field in the ipsilateral dorsal horn. Reference points on ventral and dorsal median lines of the trunk were represented bilaterally. Afferents from reference points located on the ventral median line of the hindlimb projected to two separate fields: one on the medial margin of spinal cord segments L2-L5 and the other on the medial half of spinal cord segment L5. From the distribution of central projection fields of reference points, central projection fields of dermatomes were revealed as even in shape and located within corresponding spinal cord segments. The arrangement of peripheral and central fields of dermatomes and body surface regions suggests that peripheral and central projection fields of cutaneous afferent fibers are reshaped from the common prototypical pattern that exhibits an orderly and evenly sequenced arrangement.
We found that most small neurons innervating the disc were CGRP-ir. Furthermore, disc inflammation caused an increase in CGRP-ir neurons but not IB4-binding neurons, suggesting that CGRP-ir, nerve growth factor-dependent neurons are more responsible for discogenic pain.
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