Dendritic organization of phrenic motoneurons in the adult rat

The dendritic organization of the phrenic nucleus as a whole was studied after injections of the B-subunit of cholera toxin conjugated to horseradish peroxidase were made into the diaphragm of adult rats. Transverse, sagittal, and horizontal sections through the phrenic nucleus (C3-C5) were incubated according to a modified tetramethylbenzidine HRP technique. The conjugated form of HRP used in this study has a special affinity for the GM1 ganglioside receptors on neuronal cell surfaces. As a result, extensive labeling of the terminal dendritic fields of a large number of phrenic motoneurons occurred simultaneously. The results showed that the majority of the dendrites of phrenic motoneurons were tightly organized rostrocaudally and confined to the boundaries of the column made up of the phrenic cell bodies. In addition, analysis of transverse and horizontal sections revealed dendritic bundles radiating at right angles to the long axis of the cell column in the following directions: dorsolateral into the dorsal half of the lateral funiculus, lateral into the lateral funiculus, ventromedial into the lateral half of the anterior funiculus, ventrolateral into the ventral half of the lateral funiculus, and dorsal into the intermediate gray matter. Some dendritic bundles were measured as far as 900 microns from phrenic cell bodies into the white matter. The horizontal sections also showed that there was a periodicity in the arrangement of the dendritic fascicles in that they were separated by distances ranging from 180 to 250 microns. From the analysis of phrenic dendritic distribution the present results suggest that the majority of synaptic input to phrenic motoneurons occurs within the column of the phrenic cell bodies. In addition, there is evidence to suggest that a synaptic input may also occur directly on distal phrenic dendrites in the lateral and ventral funiculi of the spinal cord white matter.