Morphology and somatotopic organization of the central terminals of hindlimb hair follicle afferents in the rat lumbar spinal cord

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Abstract
The morphology of the central collateral arborizations of 24 A‐beta hair follicle afferents (HFAs) innervating different regions of the skin of the hindlimb were studied by the intra‐axonal injection of horseradish peroxidase (HRP) in adult rats. A total of 236 collaterals were recovered. These fell into three classes—complex, simple, and blind‐ending—based on numbers of boutons and terminal branch patterns. The morphology of the HFA central arbors innervating the lateral and medial leg and dorsum of the foot was flameshaped. Afferents with receptive fields on the glabrous‐hairy skin border consistently had extra terminal branches running ventromedially into laminae IV/V. Differences in the width of terminal arbors were found. HFA terminals innervating the lateral leg formed narrower sheets than those innervating the dorsum of the foot and toes. The somatotopic organization of the collaterals and terminal arborizations of individual afferents were analyzed both by considering all the collaterals along an axon's rostrocaudal extent and by only examining arbors with boutons (the complex and simple arbors). Thirty‐seven percent of blind‐ending and 18% of simple collaterals were found to overlap in the rostrocaudal direction with the complex arborizations of afferents whose receptive fields were in a different cutaneous nerve territory. There was no overlap between complex arborizations of afferents from different nerve territories. However, the complex arbors of afferents with receptive fields within a particular nerve territory showed considerable terminal overlap even if they had nonadjacent peripheral receptive fields. The topographical organization of the central terminals of HFAs, forms a coarse somatotopic map of overlapping terminals whereby a particular region of dorsal horn has a maximal, but not exclusive, input from a particular area of skin.

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