Effects of G_M regulation during Phase I and Phase II on simulations of excitation of sarcolemmal APs, their propagation, and t-system excitation. (A) Current flow during endplate excitation of sarcolemmal APs was mimicked by simulating injections of 1-ms square KCl currents in the central segment of the model. To determine the effect of G_M regulation on the current required to excite an AP, a range of different current amplitudes was applied. (A) The membrane potential of the central segment of the model where excitation occurred (the current injections are presented below). Membrane permeabilities of the model for K⁺ and Cl⁻ were adjusted to mimic G_M during control conditions (left), during Phase I (middle), and during Phase II (right). (B) The membrane potential response to a current injection that did not elicit an AP under any of the conditions. Clearly, G_M regulation markedly affects the recovery of membrane potential after cessation of current injection. (C) Membrane potential under the three conditions of control, Phase I, and Phase II during the injection of a current that only triggered an AP during Phase I. (D) The membrane potential when the current amplitude was just sufficient to elicit an AP in Phase II. (E) Sarcolemmal AP (black line) and the membrane potential in the 20 t-system shells 3 mm from the point of AP excitation during control conditions (left), Phase I (middle), and Phase II (right). (F) Similar to E, but with a 50% reduction in maximum permeability for voltage-gated Na⁺ channels.