Somatotopy of digital nerve projections to the dorsal horn in the monkey

The dorsal horn projection patterns of finger nerves were investigated in four Macaca mulatta monkeys. Proper digital branches of the median nerves, serving the radial aspect of a digit on each hand, were loaded with wheatgerm agglutinin-horseradish peroxidase complex (WGA:HRP). The distribution of the lectin-enzyme complex was mapped in the right and left dorsal horns. The dorsal horn projections of the digital nerves were localized in segments C6-C8 in laminae I-VI, primarily in laminae I-IV. The wedge-shaped termination zones were somatotopically organized, in agreement with the projections of the digits in cats. The fingers are represented medially, as they are in the cat. This similarity suggests that there is a mediolateral gradient of dorsal horn organization similar to that of the cat, with distal skin represented medially and proximal skin represented laterally. The rostrocaudal trajectory of finger representation, with digit 1 most rostral and digit 5 most caudal, is also in agreement with the organization of hindlimb toe projections in the cat. There was a high degree of bilateral symmetry for homologous nerves, and little overlap of projections from nerves innervating adjacent fingers. The sample size was too small to permit us to assess interanimal variation. These results suggest a similar somatotopy of projections, and presumably of dorsal horn cell somatotopy, in monkey and cat.