Branching structure of motoneuron stem dendrites: a study of neck muscle motoneurons intracellularly stained with horseradish peroxidase in the cat.

The branching structure of the stem dendrites of five motoneurons innervating the dorsal neck muscles, biventer cervicis and complexus, was examined in the adult cat using intracellular staining techniques. The dendritic tree of each motoneuron was reconstructed completely and then dissected into several parts, each corresponding to the branches. Twenty-five of the 49 stem dendrites examined had branches which confined to a small region of the territory occupied by the complete dendritic tree. These dendrites were distributed to one of three zones, a ventral zone deep in the ventral horn, a lateral zone dorsolateral to the motor nucleus, and a medial zone dorsomedial to the motor nucleus. The remaining stem dendrites projected to two or more of these zones and occasionally occupied a region almost as large as the territory occupied by the complete dendritic tree. The frequency of each type of stem dendrite, defined according to the number and location of the zones to which they projected, was remarkably consistent from motoneuron to motoneuron. The maximum order of branching reached by stem dendrites of biventer cervicis and complexus motoneurons was variable. Stem dendrites whose maximum order of branching was four or less usually had one major stalk from which emerged simple, unbranched, side processes. The branching structure was not related, in a simple manner, to the dendritic distribution pattern. These results indicate that the distribution and branching structure of motoneuron stem dendrites are organized in a complex, but precise, fashion which could play an important role in the integrative properties of the motoneuron.