Key points• Neurons in the brain express a diversity of ion channels to impart specialized functional properties.• At least three distinct voltage gated calcium channel gene families are known, each of which is thought to produce calcium channels with unique properties, which in turn, differently affect neuronal function.• Here we use a genetic model system to determine which genes are responsible for the calcium currents in an identified motoneuron.• Surprisingly, the same ion channel gene encodes two distinct currents with fundamentally different properties, and the data also suggest that the normal function of this calcium channel gene is affected by the expression of another one.• The results provide insights into the relationship between gene expression and ionic currents, and thus, into the regulation of normal neuronal function. Abstract