Morphology of cat phrenic motoneurons as revealed by intracellular injection of horseradish peroxidase.

The morphology of phrenic motoneurons (PMs) of adult cat was examined by utilizing the technique of intracellular injection of horseradish peroxidase. Twenty-one cells were reconstructed from serial sections in transverse, sagittal, and horizontal planes. The cell bodies were ellipsoid, with the major diameter oriented parallel to the longitudinal axis of the spinal cord. The dendrites of PMs are not distributed in a radially symmetric fashion, but rather project to four separate fields. The field containing the greatest number of dendrites extends rostrocaudally within the phrenic motor column. This collection of dendrites forms a rostrocaudal bundle in which the dendrites from neighboring PMs lie in close association with one another. The remaining dendrites project dorsolaterally, dorsomedially, and to a lesser extent, ventrally. The dorsolaterally directed dendrites from bundles upon entering the lateral funiculus with the dendrites from other PMs. Several of the dorsomedially directed dendrites cross to the contralateral spinal cord via the anterior commissure or central gray. A wide variety of dendritic spines and appendages was observed. There were not instances in which axon collaterals were observed for the 11 well-stained axons examined. The length of the initial segment of the axon was a function of the distance of the cell body from the ventral funiculus.