The presence of a t-system luminal resistance enhances the voltage response to a short duration current similar to the endplate current and increases the velocity of passive current flow in front of a propagating AP. (A) Shows the spatial voltage profiles in both directions away from the point of application of a 1-ms square current pulse, calculated from a convolution of the transfer functions that describe the cable structures in Fig. 2 (left panels) with a 1-ms square current pulse. This was used to mimic the subthreshold events in sarcolemmal AP initiation. The successive lines represent the profile in steps of 100 µs after the initiation of current application. In the far right panel, the voltage profiles at 100, 500, and 900 µs in the three cable structures have been overlaid for comparison. (B) Similarly computed passive sarcolemmal voltage responses 100, 300, and 500 µm in front of a propagating AP (left). These voltage responses have been truncated at 25 mV to depict the initial rise of the passive response in the sink membrane region in front of the propagating AP. (C) Computed passive voltage responses of the sarcolemma, the access resistance, and the t-system membrane to the application of a recorded AP in the lumped cable structure. (D) The passive voltage responses in the distributed cable across the sarcolemma and the t-system membrane shells when a propagating AP is used as input function. Note that in C and D, the passive voltage responses were not truncated at 25 mV and thus extend into a membrane potential that would lead to activation of voltage-gated Na⁺ channels.