Preterminal and terminal axon arborizations in the substantia gelatinosa of cat's spinal cord.

Extensive terminal branchings of fine fibers in the substantia gelatinosa of Golgi-Kopsch preparations of the adult cat spinal cord were subjected to a semi-quantitative analysis. transverse sections suggest that these fibers are probably unmyelinated primary afferent elements of dorsal root origin. In transverse sections these elements pass medially and ventrally and shortly disappear due to a change in orientation. Similar thin fibers in sagittal sections can be followed for several hundred microns as they give rise to side branches that also run mainly in a longitudinal direction. The side branches divide in turn to produce preterminal axon arborizations. The arborizations were distributed in 150 mum wide zones in the dorsal horn region corresponding to Rexed's lamina II. The end terminals are large bulbs, usually preceded by two to three equally large en passant enlargements. Seven to eight terminals stem from each side branch. The terminals and enlargements are arranged in narrow (16-26 mum thick) sagittal sheets. The terminals of several side branches often converge upon a common region so that clusters of terminals occur within the sagittal sheet. It is proposed that these observations are consistent with the substantia gelatinosa (lamina II) as the termination of unmyelinated (C) primary afferent fibers and that the latter are the only type of primary fibers ending in this portion of the spinal cord.