The longitudinal distributions of both phrenic and medial gastrocnemius motoneurons were quantitatively studied in the cat spinal cord. Both populations of motoneurons were retrogradely labeled by applying horseradish peroxidase (HRP) to the cut central ends of the appropriate peripheral nerves. The longitudinal positions of all labeled motoneurons in each motor column were determined; these data then were used to generate longitudinal distribution histograms and spatial interval distributions (SIDs), the latter being analyzed further by means of power spectra. In three of four cats, longitudinal clustering of phrenic motoneuronal cell bodies was revealed by the presence of a narrow central peak in the SID and the presence of subsidiary peaks. In the fourth cat, only a smaller central peak was observed. Power spectral analysis of the three SIDs having subsidiary peaks revealed that the mean longitudinal distance between clusters was 0.95 mm (range 0.52 to 1.22 mm). The analyses also revealed that on average a phrenic motoneuronal cluster contained 17 motoneurons, and the mean longitudinal length of a cluster was 450 μm. Using single, small-volume injections of HRP into the diaphragm, we concluded that not all the phrenic motoneurons with a a single cluster innervate muscle fibers in a discrete region of the diaphragm. Similar quantitative analyses of the distribution of medial gastrocnemius motoneurons did not reveal clustering in this motor column. We suggest that the fundamental differences in the spatial distributions of motoneurons within these two motor columns may be related to differences in the functional organizations of motoneurons innervating axial versus appendicular musculature, i.e., diaphragm versus medial gastrocnemius.
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