Determination of membrane impedance of EDL muscle fibers. Membrane potential responses to injections of square and sinusoidal currents were determined at three inter-electrode distances. This used an experimental setup with a current-injecting and a membrane potential–recording electrode that was described elsewhere (Pedersen et al., 2005). (A) Examples of injected currents (top panel) and the membrane potential responses observed at the three inter-electrode distances (bottom panel). The red lines in the sinusoidal voltage responses show fits to a sinusoidal waveform. The transfer impedance at each of the three locations along the fiber was then determined from the maximum deflection in membrane potential divided by the amplitude of current injection. (B) Magnitudes of the transfer-impedance moduli for the range of frequencies explored in the fiber also used in A. A simple exponential-decaying function was fitted to the recordings, and from the decay constant, the frequency-dependent length constant could be obtained. (C) The average frequency-dependent length constant for five fibers. In addition to the amplitude of the sinusoidal voltage responses, the fitting also extracted the phase of the voltage responses. This phase data were used to calculate a velocity that reflects the speed by which passive currents flow in a muscle fiber. (D) The velocity by which the different sinusoidal currents propagate as a function of the sinusoidal frequency of the current. Also shown is the AP propagation velocity that was determined in the same fibers that were used for membrane-impedance measurements. Average data are presented as means ± SEM.