Projections of digit afferents to the cuneate nucleus in the raccoon before and after partial deafferentation

Within the cuneate nucleus of the raccoon, the representations of individual forepaw digits are anatomically separated by densely myelinated laminae. This unique arrangement was utilized to determine whether the terminations of cutaneous afferents from individual digits are precisely restricted to the appropriate region of the cuneate nucleus or overlap with afferents from adjacent digits. By using the transganglionic transport of horseradish peroxidase (HRP), it was found that, for each digit, the terminal labeling was restricted to the appropriate 150–250‐μm‐wide column that extended rostrocaudally throughout the nucleus. The topographical arrangement of digit input corresponded to the known electrophysiology, with the terminal column for the fifth digit located most medially within the nucleus and those for digits 4 to 1 successively more laterally. Within a column, the density of labeling was greater over cell clusters than between clusters. These results indicate that afferents from adjacent digits do not overlap in the cuneate nucleus. In six animals, the fifth digit was amputated, and 2–4 months later, HRP was injected into the nerves of the fourth digit to determine whether its afferents had sprouted into the denervated fifth‐digit column. The projection pattern from the fourth digit in each of these animals was the same as in normal animals and the same as in the intact contralateral side. These results indicate that the reorganization seen in the cerebral cortex following peripheral deafferentation cannot be attributed to changes in the afferent fiber projections to the cuneate nucleus.