Impedance of a Small Electrically Isolated Area of the Muscle Cell Surface

A method has been developed permitting measurement of membrane impedance and current, as a function of transmembrane potential, at small, electrically isolated regions of the muscle cell surface without microelectrode impalement. The frequency dependence of the muscle cell membrane capacity found earlier by other methods has been confirmed. The average capacity is 3.5 /zf/cm ~ with a phase angle of 71 ° at 5 kilocycles. The internal phase angle of the complex impedance plot of whole muscle probably does not result from a distribution of fiber diameters and membrane capacities, since it also appears in the present experiments where measurements are confined to a small region of a single fiber. I N T R O D U C T I O N Impedance measurements on whole muscle have suggested that the resting membrane cannot adequately be represented by pure resistive and capacitive structures, but requires non-linear and frequency-dependent elements (Cole and Curtis, 1936; Bozler and Cole, 1935; Cole, 1933, 1932). The present experiments confirm the frequency dependence of the capacity which has been measured by a microelectrode technique obviating the need for cell impalement. This paper describes the method and measurements of resting membrane capacity. A smooth tipped, liquid-filled micropipette (several microns tip diameter) (Fig. 1) is placed against a muscle in such a way that the cell surface under it is electrically isolated except for a leakage resistance path (RI) between tip and cell (Fig. 2). This leakage path may derive in par t from microfibrils (Robertson, 1956) separating the pipette from the resistive membrane. Placed against plastic films the pipettes show nearly perfect sealing with leakage resistance over 108 ohms. Previous use of electrodes to confine electrically a small surface region differed from these experiments in the pipette manufacture (Huxley and Taylor, 1958; Steinman, 1937; Gelfan, 1930; Gelfan and Gerard, 1930; Pratt, 1930; Pratt and Eisenberger, 1919; Taylor, 1925).