Contralateral termination of primary afferent axons in the sacral and caudal segments of the cat, as studied by anterograde transport of horseradish peroxidase.

Contralateral termination of primary afferent axons in the sacral and caudal segments was studied with the anterograde horseradish peroxidase technique in the cat. The distribution of terminals and branching pattern of terminal fibers were examined on sections incubated in a diaminobenzidine-cobalt chloride medium. The crossed primary afferent axons passing in the dorsal commissure projected to (1) the dorsal gray commissure, (2) small, dorsomedial and ventrolateral parts of laminae III and IV, (3) lamina I, and (4) the Xth area (region dorsal to and around the central canal). In the dorsal gray commissure primary afferent axons formed a dense plexus after taking a tortuous course in mediolateral and rostrocaudal directions. Single axons terminated on both sides, but rarely did they extend outside of this region. The axons terminating in the dorsomedial and ventrolateral parts of laminae III and IV passed through the dorsal gray commissure without giving off collaterals. After reaching laminae III and IV they ascended and/or descended within these laminae and terminated at more rostral or caudal levels. The crossed primary afferent axons passing in the ventral commissure terminated in lamina VIII of the contralateral ventral horn. Some axons terminated in a region ventral to the central canal, and on both sides of lamina VIII.