Potential, Impedance, and Rectification in Membranes by David E. Goldman

Since the early recognition of the value of electrical measurements on biological cells and tissues, the resting potentials of many cells have been determined and have been shown to vary with the degree of activity and with the composition of the external medium. When a steady current is passed, the potential takes on a new value. The ratio of the current to the change in potential, namely the conductance, has also been measured for many cells and has also been found to depend on physiological state and environment. Since ion motion is equivalent to an electric current, the conductance may be taken as an index of the "ion permeability." The introduction, by Kohlrausch, of A.C. measurements in place of D.C. has allowed more reliable results to be obtained. Further, one is then able to determine not only the conductance, but the capacitance as well. The use of a range of frequencies of the measuring current (HSber, 1912; Gildemeister, 1919), has resulted in a picture of the cell membrane as electrically equivalent to a parallel resistance-capacitance combination, the capacitance having a constant phase angle less than 90 ° (Philippson, 1921; Cole, 1932). If the magnitude and direction of the current is varied, it is found that the conductance may change. In particular, it may be large with the current in one direction and small with the current in the other and the system then shows electrical rectification. This phenomenon has been studied in Valonia (Blinks, 1930a, c), in the squid giant axon (Cole and Baker, 1941; Cole and Curtis, 1941), and in frog muscle (Katz, 1942). According to the membrane hypothesis of Bernstein, the permeability of the cell membrane increases during activity. Corresponding changes in conductance have been found in many systems and it has recently become possible to study them quantitatively in Nitdla and in the squid giant axon (Cole and Curtis, 1938, 1939). By contrast, the capacitances appear to be remarkably constant and independent of physiological state and environment. They have thus been interpreted as representing the dielectric properties of fixed elements of the membranes to which they are referred while the conductances and potentials depend primarily on ion mobilities and concentrations and their distributions. In so far as any of these properties may involve physical rather than metabolic 37