Several morphological types of terminal arborizations of primary afferents in laminae I-II of the rat spinal cord, as shown after HRP labeling and Golgi impregnation.

The morphology of the terminal arborizations in laminae I-II of primary afferent fibers was studied in sections stained by the heavy metal (nickel and cobalt) intensification of diaminobenzidine (DAB) after crushing one dorsal root with horseradish peroxidase (HRP) crystals, and with the mixed Golgi method which duplicated the staining provided by the first method. Besides the flame-shaped arbors located in deep lamina IIi as an extension of the arbors of lamina III, which were derived from 1.7-micron thick stem fibers (probably A alpha beta fibers), six types of terminal arbors, all rostrocaudally oriented, arising from fine stem fibers and having preferential locations, were disclosed. The lateral third of laminae I-II contained a longitudinal plexus of parallel 0.8-micron thick stem fibers (C fibers) with longitudinal side branches generating many boutons en passant. Laminae I and IIo, in their middle third, contained dichotomizing longitudinal fibers with elongated boutons, arising from 1-micron thick stem fibers (C or A delta), and, in the medial third, a dense plexus with terminal networks carrying large boutons, which arose from 1.3-micron thick stem fibers (A delta). Fibers ending in terminal bouquets and issuing from 1-micron thick stem fibers (C or A delta) occupied the dorsal part of middle and medial lamina IIi, while the intermediate part contained clusters (swarms) of ultrafine boutons arising from extremely fine fibers. The whole medial lamina IIi also contained fine undulating fibers arising from 0.3 micron-thick stem fibers (C fibers) with large boutons near their ends. The functional meaning of this multiplicity of morphological types and locations is still unclear. It may be clarified when single unit analysis of HRP-injected fine fibers is made possible, or immunocytochemical stainings disclose the neurotransmitters utilized by each fiber type.