Equivalent sources for the extracellular potentials of single fibers and bundles

The extracellular field generated by the activity of a single fiber or a bundle of fibers can be considered as arising from equivalent sources distributed in the intracellular space. These equivalent sources can be single or double layer disks. An evaluation of the source density for each single fiber or a fiber in a bundle requires a knowledge of the intracellular and extracellular (interstitial for the bundles) potentials on both sides of the fiber membrane. This can be avoided by substituting the exact expressions for the fields in terms of the transmembrane potential. Such a calculation is rather straight forward for a single fiber, however, for a bundle of tightly packed and numerous fibers it would be easier if the discrete structure is replaced by a bi-domain in which the intracellular and interstitial spaces are considered continua, superimposed in space, separated everywhere by a membrane and characterized by macroscopic conductivities.