Different neuronal types in transneuronally WGA-HRP-labeled premotor interneurons of the rat spinal cord

Interneurons presynaptic to motoneurons were labeled by retrograde transneuronal transport of WGA-HRP. The tracer was injected either into the quadriceps muscles or into the posterior biceps muscles, thus labeling interneurons presynaptic to the quadriceps motoneurons (QINs) or interneurons presynaptic to posterior biceps motoneurons (PBINs). Statistical cluster analysis of area, perimeter, equivalent diameter and form factor of the labeled interneurons permitted the identification of 4 different types of premotor interneurons in the lumbar enlargement of the rat. Type I are small elongated interneurons which prevail in PBINs. Type II are medium-sized ellipsoidal cells prevailing in the QINs. Type 3 are small ellipsoidal neurons, slightly more frequent in PBINs. Type 4 is the smallest group and it is composed of large multipolar neurons. A different distribution of the 4 morphological neuronal types was found between the population of the QINs and the PBINs and in the laminae of ventral horn for each group.