Comparison of passive conduction with active AP regeneration. The model with homogenous t-system was stimulated in the central of the 99 compartments at 2 s. (A) Em over time for segments 30–35 (3–3.5 mm from the stimulus) from two simulations: in the first simulation (solid lines), active propagation persisted throughout all fiber segments; in the second (broken lines), conduction block was modeled in compartments 31 and beyond by fixing the permeability of the voltage-gated ion channels to their resting values. This allowed a direct comparison of active and passive signal propagation. The vertical broken lines mark the peaks of these passively conducted AP waveforms. It can be seen that the peaks of the passively conducted signals eventually lag the peaks of the actively propagated APs. (B) The measured peak-to-peak conduction velocity in similar simulations of conduction block, over a range of values of PNa(max), for actively propagated signals (squares); and for the first 100 µm of passive conduction from the final point of active regeneration (triangles). (Inset) Four traces of APs with different relative values of PNA(max) (i, 0.3; ii, 0.5; iii, 1; and iv, 2). The frequency content of these traces of APs was analyzed using Fourier analysis, and their power spectra are shown in (C). Given that APs propagate via the spread of local circuit currents in front of the AP waveform, some elements of such passive current flow must be faster than or as fast as the active propagation. Thus, from the relation of frequency and passive conduction velocity of sinusoidal currents displayed in A, the frequency of a sinusoidal current required to match the AP propagation velocity was determined for the range of PNa(max) values investigated. These frequencies were referred to as equivalent frequencies. (D) This illustrates that the equivalent frequency and the active AP conduction velocity were linearly related and, furthermore, that the equivalent frequency and PNa(max) were log-linearly related.
