Dendritic organization of the human spinal cord: the motoneurons.

The dendritic organization of motoneurons was analyzed with the Golgi stain and a morphometric method in the immature and adult human spinal cord. Each motoneuronal column was found to be characterized by a specific orientation of dendritic trees and by a distinct pattern of dendritic bundling. Ventromedial motoneurons have a pyramidal dendritic tree with numerous, short longitudinal branches and elongated dorsal branches. The latter form thick bundles oriented toward the ventral gray commissure. Longitudinal dendrites form a narrow-meshed dendritic plexus, containing abundant microbundles. Motoneurons of the ventromedial column have fewer primary dendrites and a lower ramification index than other motoneurons. Central motoneurons are predominantly oriented longitudinally. The meshes of the rostrocaudal dendritic plexus are looser and the microbundles are finer. Most transverse dendrites run laterally and participate in dendritic bundles which penetrate into the ventrolateral funiculus. The rostrocaudal dendritic domain of ventrolateral motoneurons is the largest dendritic domain of all spinal neurons. The longitudinal dendritic network contains fine microbundles and appears wide-meshed. Transverse dendrites form lateral or medial dendritic bundles depending upon the position of their perikaryon. Dorsolateral motoneurons differ from other motoneurons by their multipolar organization with a slight preponderance of dorsoventral dendritic spread. Rudimentary lateral dendrite bundles are restricted to marginal neurons. The longitudinal plexuses of motoneuronal dendrites and the verticotransverse dendrite bundles of the ventromedial column are well developed in the 26-28-week-old fetus. In contrast, the horizontotransverse dendrite bundles of central and ventrolateral motoneurons can only be recognized from 36 weeks on. The possible specific functions of the various types of dendrites bundles are examined and a laminar dendroarchitectonic schema of the human cord is proposed.