Morphology of motoneurons in different subdivisions of the rat facial nucleus stained intracellularly with horseradish peroxidase.

Horseradish peroxidase was injected into single facial motoneurons of the rat. Neurons were identified by antidromic stimulation of either the buccal or the marginal mandibular or the posterior auricular nerve branches. Motoneuronal cell bodies supplying the buccal branch were located in the lateral subdivision of the facial nucleus, those supplying the marginal mandibular branch were in the intermediate subdivision, and those supplying the posterior auricular branch were in the medial subdivision. Eleven motoneurons were reconstructed with a computer-assisted technique. Their soma diameters averaged 20 microns; the average number of primary dendrites was 7.9 and the combined lengths of the dendritic trees averaged 17,650 microns. There was no distinction between the three motoneuron groups in terms of these and other quantitative data. However, on the basis of reconstructed dendritic tree orientation (i.e., dendritic distribution), major differences were observed between motoneurons of the three groups. Dendrites from all groups extended beyond the boundaries of the facial nucleus into the reticular formation. The border between the intermediate and the lateral subdivision was crossed by some dendrites but the overlap was small. In contrast, no dendrite of a motoneuron in the medial subdivision entered the intermediate subdivision and vice versa. The dendritic extent was totally restricted by the borders between these two subdivisions. Outside the Nissl-defined nuclear border, however, dendrites from cells in adjacent subdivisions overlapped. It is concluded that the medial subdivision of the facial nucleus can be distinguished from the intermediate and lateral subdivisions not only by its sharp Nissl-defined border but also by the discrete organization of its dendritic field.