Distribution of the tract of Lissauer and the dorsal root fibers in the primate spinal cord.

The tract of Lissauer receives small caliber dorsal root fibers in addition to axons arising from dorsal horn neurons. The termination of Lissauer's tract and dorsal root fibers was examined in the C7 segment of the rhesus monkey spinal cord. The distribution of normal dorsal root afferents was mapped by labelling the C7 dorsal root ganglion with tritiated amino acids, and then compared with the degeneration of C7 dorsal root fibers following an intradural dorsal rhizotomy. To focus on the distribution of the small afferents, the degeneration following a Lissauer tractotomy was compared with the degeneration following dorsal rhizotomy and following selected lesions involving the large afferents. The survival times following the lesions and rhizotomies were varied to facilitate identification of groups of fibers and terminals which might degenerate at different rates. Both large and small diameter dorsal root afferents were found to exhibit the same rostro-caudal topography within the dorsal horn. The C7 root axons and terminals distribute throughout the mid-C7 dorsal horn grey. Proceeding rostrally through C6, the majority of the C7 root fibers ending in laminae I-IV shift to a lateral position. Proceeding caudally through C8, the C7 root fibers shift medially. Few of the small diameter C7 afferents entering via Lissauer's tract extend above C6 or below C8. Large diameter C7 afferents, arising as dorsal column collaterals, can extend several segments above and below C7. Autoradiography revealed label in all dorsal horn laminae, the heaviest always occurring in the substantia gelatinosa. After one day, label was absent over dorsal column and Lissauer's tract axons, suggesting that the label was mainly associated with fine axonal branches or possibly terminals. After six to ten days many axons were labelled and could be traced into the dorsal and ventral horn. Degeneration from the rhizotomies and lesions, as demonstrated with Fink-Heimer and Nauta methods, depended on the survival time. No degeneration products were present before three days. The large afferents begin to degenerate within the dorsal horn after three to four days and mainly terminate in laminae IV-VI; by 12 days they can also be traced into the intermediate and ventral grey. The small afferents, which include those serving pain and temperature sensibility, arise from the tract of Lissauer and distribute to laminae I, II and III. The tract of Lissauer consists of two populations, each containing small afferents. One population degenerates at three to five days and distributes mainly to laminae II and III (substantia gelatinosa); the other degenerates around 12 days and distributes mainly to lamina I and the outer zone of II. It is suggested that the exclusive termination of the small afferents to laminae I, II and III may be correlated with certain unique histochemical properties (e.g., high substance P and high opiate receptor binding levels) of these same dorsal horn areas...