A highly - polarized excitable cell separates sodium channels from sodium - 6 activated potassium channels by more than a millimeter

30 The bioelectrical properties and resulting metabolic demands of electrogenic cells are 31 determined by their morphology and the subcellular localization of ion channels. The electric 32 organ cells (electrocytes) of the electric fish Eigenmannia virescens generate action potentials 33 (APs) with Na currents >10 microamperes and repolarize the AP with Na-activated K (KNa) 34 channels. To better understand the role of morphology and ion channel localization in 35 determining the metabolic cost of electrocyte APs, we used two-photon 3D imaging to determine 36 the fine cellular morphology and immunohistochemistry to localize the electrocytes' ion channels, 37 ionotropic receptors, and Na/K ATPases. We found that electrocytes are highly polarized cells 38 ≈1.5 mm in anterior-posterior length and ≈0.6 mm in diameter, containing approximately 30,000 39 nuclei along the cell periphery. The cell's innervated posterior region is deeply invaginated and 40 vascularized with complex ultrastructural features while the anterior region is relatively smooth. 41 Cholinergic receptors and Na channels are restricted to the innervated posterior region, while 42 inward rectifier K channels and the KNa channels that terminate the electrocyte AP are localized 43 to the anterior region, separated by >1 millimeter from the only sources of Na influx. In other 44 systems submicron spatial coupling of Na and KNa channels is necessary for KNa channel 45 activation. However, our computational simulations showed that KNa channels at a great distance 46 from Na influx can still terminate the AP suggesting that KNa channels can be activated by 47 distant sources of Na influx and overturning a long-standing assumption that AP-generating ion 48 channels are restricted to the electrocyte's posterior face. 49