The charges in electrostatic theory may independently exist at any location in space, but the current sources and sinks of Eq. 1 imply movement of charges through space (i.e., current). The source and sink shown in Fig. 1 may therefore be considered as generators of the fields shown. A physical mechanism underlying the injection into and removal of current from the medium is not required for this hypothetical dipole, nor are there any constraints on the distance between the two poles. In contrast, the currents underlying a real neuronal generator originate in the balance between intra- and extracellular processes associated with the neuronal resting potential, but are only evidenced by flow of current into (depolarization; extracellular sink and negativity) and out of (hyperpolarization; extracellular source and positivity) the cell from the extracellular medium. The necessary circuit closure for these currents is through the intracellular compartment, but is not generally visible to a recording contact in the extracellular medium.
