Central terminations of cutaneous mechanoreceptive afferents in the rat lumbar spinal cord

The morphology in the dorsal horn of the lumbar spinal cord of the collateral branches and terminal arborizations of three different types of low-threshold mechanoreceptor innervating the skin of the rat hindlimb has been studied by the intracellular injection of horseradish peroxidase into physiologically characterized afferent fibres. The central terminals of five rapidly adapting glabrous skin mechanoreceptors (RA), six hair follicle afferents (HFA), and four slowly adapting type I afferent fibres (SA I; two from glabrous and two from hairy skin) were recovered for detailed analysis. The number of collaterals per axon varied from eight to 12, and the length of axon stained from 3.5 to 4.7 mm. In each afferent, while the majority (52-67%) of the terminal arborizations displayed extensive branching with large numbers of en passant and terminal synaptic boutons, the arborizations at the caudal and rostral extremes of the terminal field tended to be much simpler and less profuse, with few and in some cases no boutons. This did not appear to be the consequence of inadequate filling. The general pattern of the terminal arborizations was one of mediolaterally compressed, rostrocaudally oriented sheets. In the case of the RA afferents there was no overlap between the adjacent terminal arborizations; a few of the arborizations of the SA I afferents overlapped; most, but not all, of the HFA terminal arborizations overlapped. The terminal arborizations of the HFAs had a distinctive morphology identical to the flame-shaped arbors described in earlier Golgi studies and included synaptic boutons extending from inner lamina II to lamina IV. The morphologies of the RA and SA terminal arborizations were similar to each other, but the former tended to be concentrated in lamina IV with branches in III and V, whereas the latter tended to V with few branches more superficial than lamina IV. There were also differences in the intercollateral spacing between these two types of afferent fibre. While there are similarities between the morphology of the central terminals of cutaneous low-threshold mechanoreceptors in the rat and those previously described in the cat (for example, the longitudinally continuous arrangement of the mediolaterally restricted flame-shaped HFA arborizations and the discontinuous RA arborizations arising from a dorsally located axon), there are also some major differences: the large number of HFA arbors extending to lamina IIi and to lamina IV rather than being restricted to lamina III, the deeper location of the RA arbors (in laminae IV and V rather than lamina III),(ABSTRACT TRUNCATED AT 400 WORDS)