Spinal termination patterns of canine identified A‐δ and C spermatic polymodal receptors traced by intracellular labeling with Phaseolus vulgaris ‐leucoagglutinin

The spinal projection patterns of spermatic polymodal receptors were studied by intracellular labeling of functionally identified canine dorsal root ganglion (DRG) neurons with Phaseolus vulgaris ‐leucoagglutinin (PHA‐L). The processes of 2 C‐fiber and 1 A‐δ‐fiber spermatic polymodal receptor neurons were labeled well enough to trace their central terminations. The labeled C‐fiber DRG neurons were of medium size (mean diameter 36.8 and 40.7 μm). On entering the spinal cord, axons of the C‐polymodal receptors divided into rostral and caudal main branches that extended over 3 spinal segments (20 and 25 mm, respectively), and issued a total of 16 and 15 collaterals, respectively. The majority of collaterals ran in or along the lateral surface, but both neurons had 1 or 2 collaterals or terminal branches running through the middle, or along the medial, surface of the dorsal horn. Terminal swellings and en passant enlargements were observed mainly in laminae I, V, and VII. Some C‐fiber terminations appeared in lamina II and the adjacent lateral column. The A‐δ polymodal receptor had a termination pattern similar to that of the C‐fiber units with the exception of a shorter distance over which its 13 identified collaterals were issued (10 mm), and continuation of the rostral main branch into Lissauer's tract (traced for 3.6 mm) after all branches appeared. Two terminal branches were found running just above the central canal in another A‐δ neuron in which termination could be only partially traced. The density of terminals of spermatic thin afferents was lower and their rostrocaudal spread was more extensive than those reported in cutaneous C‐polymodal nociceptors. These results suggest that spermatic afferents are organized to exert a diffuse and wide‐spreading influence on the spinal neurons. © 1993 Wiley‐Liss, Inc.