The morphology of hair follicle afferent fibre collaterals in the spinal cord of the cat

Abstract

The enzyme horseradish peroxidase (HRP) was injected into single axons that innervated hair follicle receptors to study the morphology of their collaterals in the dorsal horn of the cord. The axons were impaled near the dorsal root entrance zone in the lumbosacral spinal cord of anesthetized cats and HRP injected by passing current through the intra-axonal microelectrode. The morphology was revealed by subsequent histochemistry. Hair-follicle afferent fibers (13) were stained including 6 that innervated tylotrichs (type T hair follicle afferent units) and 1 that innervated guard hairs (type G unit). The remaining 6 axons were not classified according to hair type, but, on the basis of their axonal conduction velocities, would be either type G or T. Eleven axons could be traced back into the dorsal roots. Eight of these, upon entering the cord, turned and ran towards the brain. They did not divide into rostral and caudal branches. Three of the 11 did divide an gave rise to both rostral and caudal branches. Sixty-three collaterals were given off the 13 stained axons. All well-filled collaterals had a strikingly similar morphology. They descended through laminae I-III of the dorsal horn into the deeper parts of lamina IV or into lamina V, before turning and ascending back into superficial lamina IV and lamina III where they branched profusely to give rise to their terminal arborizations. Terminal boutons, most commonly of the en passant type, were numerous in lamina III, but were also seen in the dorsal part of lamina IV and in ventral lamina II. None were observed in dorsal lamina II or near the junction of the grey and white matter (lamina I) or in lamina V. The terminal arborizations of collaterals from a single hair follicle afferent fiber were in line with 1 another in the longitudinal axis of the cord. In the better-stained preparations the terminal arborizations of adjacent collaterals from a single axon formed a continuous longitudinal column through the dorsal horn. There was a gradual shift of the column of arborizations from lateral to medial as the more rostral collaterals were given off. The hair follicle afferent fiber collaterals are now identified as the collaterales grosses et profondes de la substance de Rolando'' of Ramon y Cajal which give rise to the ''flame-shaped arbors'' of Scheibel and Scheibel. The importance of the longitudinal organization of the terminal arborizations for an understanding of the topographical properties of dorsal horn neurons was discussed.