Cardiac neurons make up less than 0.01 percent of the cells in heart tissue and are scattered sparsely across the organ. Making the Rarest Cells in the Heart VisibleThe technical obstacle was finding the cells at all. Cardiac neurons were treated as a fairly uniform relay within the parasympathetic system, relaying vagal nerve instructions to the cells that set heart rate. A news analysis in Nature framed the results as challenging the classic view that all cardiac neurons are alike. Any therapy targeting a specific cardiac neuron type would require years of further research and human trials.