Biological tissues have conductive and capacitive properties, but the magnetic susceptibility is essentially that of vacuum. In metals the charge carriers are free electrons, but in biological tissue the charge carriers are ions, e.g., Na+, K+, Cl_, Ca2+, etc. Section 2 described how the membrane current density J has both electric and diffusive contributions. In the extracellular space, the story is more complicated. There is no concentration gradient or diffusive contribution per se, nevertheless, the fields in the extracellular space may be computed by considering the current density to have two contributions:
where JE is the ohmic current that flows in response to the local electric field, and JS is the source current (or impressed current). Within the membrane, (2.1) includes contributions to the current arising from both the transmembrane electric field and the transmembrane concentration gradients for each ion species. For computing fields in the extracellular space, JS is a phenomeno-logical device that subsumes other physical aspects of the problem (Plonsey 1982; Nunez and Srinivasan 2006).
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