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Electrical Structure of Biological Cells and Tissues: impedance spectroscopy, stereology, and singular perturbation theory

Impedance Spectroscopy resolves electrical properties into uncorrelated variables, as a function of frequency, with exquisite resolution. Separation is robust and most useful when the system is linear. Impedance spectroscopy combined with appropriate structural knowledge provides insight into pathways for current flow, with more success than other methods. Biological applications of impedance spectroscopy are often not useful since so much of biology is strongly nonlinear in its essential features, and impedance spectroscopy is fundamentally a linear analysis. All cells and tissues have cell membranes and its capacitance is both linear and important to cell function. Measurements proved straightforward in skeletal muscle, cardiac muscle, and lens of the eye. In skeletal muscle, measurements provided the best estimates of the predominant (cell) membrane system that dominates electrical properties. In cardiac muscle, measurements showed definitively that classical microelectrode voltage clamp could not control the potential of the predominant membranes, that were in the tubular system separated from the extracellular space by substantial distributed resistance. In the lens of the eye, impedance spectroscopy changed the basis of all recording and interpretation of electrical measurements and laid the basis for Rae and Mathias extensive later experimental work. Many tissues are riddled with extracellular space as clefts and tubules, for example, cardiac muscle, the lens of the eye, most epithelia, and of course frog muscle. These tissues are best analyzed with a bidomain theory that arose from the work on electrical structure described here. There has been a great deal of work since then on the bi-domain and this represents the most important contribution to biology of the analysis of electrical structure in my view.