Propriospinal fibers reaching the lumbar enlargement in the rat.

Propriospinal fibers reaching the lumbar enlargement were investigated in rat by means of retrograde transport of wheat germ agglutinin-horseradish peroxidase conjugate coupled or not coupled with gold particles. Unilateral or medial bilateral injections were done. Identification of projection cells was done by tetramethylbenzidine histochemistry or gold-silver intensification procedures. Unilateral injections resulted in bilateral labeling, with patterns and density related to the spinal segments of origin. Sacral, lumbar and thoracic afferents showed identical patterns. Ipsilateral connections originated laterally from dorsal, intermediate and ventral horns. Contralateral connections originated medially from laminae VII and VIII and laterally from the reticular extension of the neck of the dorsal horn. Cervical afferents were symmetrical, arising from both lamina VIII and the reticular extension of the neck of the dorsal horn. Lamina X projection cells were seen at all levels when injection sites involved this area. Laminae III and IV were almost totally devoid of projection cells. Superficial layer cells (laminae I and II) showed some labeling when injections were situated dorsally. The organization of these tracts in rat is similar to that in cat and monkey. Their origin is discussed in relation to those of long ascending pathways reaching supraspinal levels.