Fig. 3. The Malpighian tubule modeled as a single electrical cable (A) and as a double cable (B). In the single cable model, the single core resistance (R_co) is the axial resistance along the length of the tubule. It includes the tubule lumen and epithelial cells. In the double cable model, there are two axial resistances: the gap-junction resistance (R_gj) and the lumen resistance (R_lu). E, electromotive force; R, resistance; a, apical membrane; bl, basolateral membrane; sh, paracellular shunt pathway.