Projections from Pacinian corpuscles and rapidly adapting mechanoreceptors of glabrous skin to the cat's spinal cord.

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Abstract
Single axons innervating Pacinian corpuscles and rapidly adapting mechanoreceptors of the foot and toe pads were injected with horseradish peroxidase near their entrance of the lumbosacral spinal cord in cats anesthetized with chloralose and paralyzed with gallamine triethiodide. Subsequent histochemistry revealed the morphology of the intra-spinal parts of the axons. All Pacinian corpuscle axons. All Pacinian corpuscle axons that traced into the dorsal root bifurcated upon entering the cord into ascending and descending branches. All Pacinian corpuscle axons gave rise to collaterals entering the dorsal horn. The collaterals of Pacinian corpuscle afferent fibers had a distinctive morphology. They provided 2 regions of termination, a larger dorsal region in laminae III and IV and a smaller ventral region in laminae V and VI. Within the dorsal region the terminal axons ran mainly in the longitudinal axis of the cord and carried many boutons en passant. Within the ventral region the axons ran dorso-ventrally in the transverse plane of the cord. Although carrying some boutons en passant, they gave rise to clusters of boutons. The collaterals of rapidly adapting afferent fibers had a distinctive morphology different from the Pacinian corpuscle afferent fiber collaterals. The termination region of rapidly adapting afferents was limited almost exclusively to lamina III, with only slight extension into lamina IV. Boutons were mainly of the en passant type and terminal axons were generally orientated within the longitudinal axis of the cord. The morphology of the afferent fiber collaterals in relation to the physiology of the dorsal horn is discussed.