Quantitative morphological analysis of spinal motoneurons

Horseradish peroxidase was injected intracellularly into motoneurons responding to cutaneous stimulation of the central foot pad of the hind limb in cats. Three motoneurons were selected for detailed analysis: two excited by foot pad stimulation, and one postsynaptically inhibited by such stimulation. The overall lengths of the dendritic trees of the 3 cells ranged from 15.2 to 20.4 mm; the total surface areas ranged from 0.161 to 0.185 mm2. One cell had 9 primary dendrites, 39 terminal dendrites, and 69 dendritic branches in all. The second cell had 8 primary dendrites, 44 terminal dendrites, and 80 dendritic branches in all. For the third cell, these numbers were 14, 76 and 136. 75.2% of the total dendritic length of one of the cells was accounted for by branches of 3 of its 9 primary dendrites; for the second, 69.1% by 3 of 8; and for the third cell, 47.9% by 3 of 14. In contrast to the marked disparity in overall length of branches of the various primary dendrites, when the lengths of dendrites were analyzed by order of branching, dendritic branches of orders II-V each included greater than 15% of overall dendritic length. All 3 motoneurons displayed an exponential overall loss of total dendritic width with distance from the cell body, as well as an exponential decrease in Rall's dendritic trunk parameter. Four different patterns of branching were observed. Mean estimated electronic dendritic lengths were 1.3 for two cells, and 1.2 for the third. However, 13% to 28% of the dendrites of the 3 cells terminated more than two space constants from the cell body. In spite of striking differences among these cells in first-order morphology, on many of the detailed measures characterizing the dendritic trees of these motoneurons, they proved to be strikingly similar.