Recruitment order and dendritic morphology of rat phrenic motoneurons

The detailed morphology of rat phrenic motoneurons (PMs) was studied in 40 electrophysiologically identified cells with intracellular injection of Neurobiotin. In 15 cells, the dendritic trees were fully analyzed by using path-distance analysis, and total surface area and volume were estimated. Based on their relative onset times (ROT; i.e., the time of firing onset relative to the onset of whole phrenic activity), PMs were classified into three types; early recruited (type E; ROT < 10%), late recruited (type L; ROT > 12.5%), and quiescent (type Q; not recruited under normal conditions). Dendrites constituted 93.3% of the surface area of cells and 38.9% of the cell volumes. The number of primary dendrites (nPD) averaged 10.1, and the mean number of terminations was 38.8. The combined diameters of primary dendrites of PMs correlated well with the total dendritic surface area and the number of dendritic terminations. Comparisons among cell types revealed that type Q cells had greater dendritic surface areas and volumes than type E or type L cells. With path-distance analysis, this difference was found to be due to differences between the cell types in the numbers of their dendrites, their combined dendritic lengths, and the number of their branches. The differences between these data and those available for cat motoneurons are discussed. The input resistance of PMs correlated with their total surface area but did not correlate with their somal surface area, indicating that, in rat, PM input resistance is a function of the entire neuronal membrane rather than of the somal surface alone.