The terminal arborization pattern of primary afferent fibers in the substantia gelatinosa of the spinal cord in the cat.

The spinal cords of adult cats were impregnated using the Golgi-Kopsch technique and cut in the sagittal plane. Richly arborizing axon-trees were found in the substantia gelatinosa (lamina II of Rexed). Axon arborizations, characterized by a long rostro-caudal course, a single-stem fiber, multiple-arborizing side branches and large terminal and en passant boutons, were subjected to a semi-quantitative and later to a computer-aided analysis. These arborizations, presumably of primary afferent origin (Réthelyi, 1977), are arranged in narrow sheets. The sheets of the individual arborizations are tilted with respect to the sagittal plane (the dorsal edge of the sheet having a more lateral position than the ventral edge), and tilted with respect to each other's plane, but they are rigorously oriented in the rostro-caudal direction. The average thickness of the sheets encompassing the terminal and en passant bulbs of these arborizations is 16.65 micrometer (S.D. = 2.86; n = 10). Occasionally, the side branches and terminal bulbs of a single stem fiber arborize into two separate sheets. These anatomical results suggest that the activity of the peripheral fibers spreads in long and narrow neuropil fields in the substantia gelatinosa. This pattern suggests a fine scale somatotopic arrangement.