Structural and functional characteristics of individual phrenic motoneurons.

Intracellular recording and staining techniques were applied to the study of cat phrenic motoneurons. Spontaneously driven phrenic cells possessed individualistic depolarization and spiking patterns that were a function of the conduction velocity in the different motor axons. Staining of phrenic motoneurons with Procion yellow indicated that fast conducting cells with small slow-wave depolarizations were large in size while slow conducting cells with large depolarizations were small in size. This implicated differences in membrane input resistance between large and small cells, although an unequal distribution of inputs to the individual components could not be discounted. On the average, phrenic motoneurons had a smaller dendritic surface area and smaller dendritic dominance than lumbosacral motoneurons. These factors help to explain the higher membrane resistances and longer time constants of phrenic cells. Phrenic dendrites were found to project in all directions away from the cell body and form ellipsoidal receptive fields that overlapped with other phrenic fields. It is speculated that the close approximation of phrenic dendrites with one another could, in part, be responsible for the high degree of synchronization among the different phrenic units.