Topography in the phrenic motoneuron nucleus demonstrated by retrograde multiple-labelling techniques

A combination of retrograde tracing methods was employed to distinguish populations of motoneurons supplying different motor unit territories in the feline diaphragm. The compatibility of the tracers—horseradish peroxidase, fast or true blue, diamidino yellow, and fluorogold—was first assessed by applying the different tracers concurrently to separate cut branches of hindlimb and neck muscle nerves. On the basis of these initial observations fast blue, fluorogold, and horseradish peroxidase were chosen to compare the distribution of motoneurons whose axons ran in different primary branches of the phrenic nerve. Motoneurons with different target territories were extensively intermixed throughout most of the phrenic motor nucleus. However, motoneurons innervating the sternal and medial costal part of the diaphragm were distributed more densely in the rostral part of the phrenic motor pool, whereas motoneurons serving the lateral costal part were concentrated more caudally. Crural motoneurons were intermingled with costal motoneurons in the middle and caudal portions of the nucleus. Motoneurons within the phrenic nucleus are distributed in clusters. Such clusters commonly contained motoneurons labelled from two or more primary branches. Thus, the highly ordered topography of muscle units in the diaphragm is not mirrored by the intraspinal distribution of phrenic motoneurons.