Trajectory of group Ia afferent fibers stained with horseradish peroxidase in the lumbosacral spinal cord of the cat: Three dimensional reconstructions from serial sections

Abstract

A reconstruction was made of the intramedullary trajectory of 23 physiologically identified Ia afferents from cat hind limb muscles (medial gastrocnemius, soleus, plantaris, flexor digitorum-hallucis longus and hamstring). The afferents were stained by intra-axonally injected HRP [horseradish peroxidase]. The axons of these afferents were traced over distances of 5.8 mm to 15.7 mm rostrocaudally. In the dorsal funiculus fibers from all the muscles showed a similar course and similarly bifurcated into an ascending and a descending branch. The mean diameters of stem axons, ascending branches and descending branches were 6.6 .mu.m, 5.8 .mu.m and 3.0 .mu.m, respectively. Within the analyzed lengths of the spinal cord 5-11 collaterals projected from the 2 branches. The distances between adjacent collaterals of the ascending and descending branches averaged 1200 .mu.m and 790 .mu.m, respectively. The collaterals as a rule passed through the medial half of the dorsal horn before they entered the deeper parts of the gray matter. The terminal distribution areas common to all Ia collaterals were: the medial half of the base of the dorsal horn, mainly lamina VI; lamina VII and lamina IX. The numbers of terminals were largest in lamina IX and smallest in lamina VII. . The density of terminals in lamina IX was highest in the homonymous motor cell column. The terminal distribution areas of adjacent collaterals showed no overlap in the sagittal plane. Terminal branches carried 1 bouton terminal and up to 6 boutons en passage with an average of 1.8 terminals per terminal branch. Apparent axosomatic and axodendritic contacts were seen on small-sized and medium-sized neurons in laminae V-VI, medium-sized neurons in lamina VII and large neurons in lamina IX. One motoneuron was contacted by an average of 3.3 terminals. In addition to the common features, Ia collaterals of various muscles of origin showed some differences in their trajectories in the ventral horn, and in their terminations in the gray matter.