Similarities and differences in the structure of segmentally homologous neurons that control the hearts of the leech, Hirudo medicinalis.

The neural circuit that controls the hearts in the leech comprises an ensemble of synaptically interconnected cardiac motor neurons (HE cells) and cardiac interneurons (HN cells). Both the HE cells and the HN cells constitute segmentally homologous sets. We have investigated the structure of these neurons by iontophoretic injection of Lucifer Yellow dye. Bilateral pairs of HE cells have been identified in segmental ganglia 3-19 of the nerve cord. Their structure was found to be nearly identical from ganglion to ganglion and from animal to animal. Bilateral pairs of HN cells have been identified in segmental ganglia 1-7 of the nerve cord. Their dendritic structure was found to vary from ganglion to ganglion. These segmental differences among HN cells were observed consistently from animal to animal. Some of the segmental differences in HN cell structure correlate with previously described physiological differences.