Figure 7.
The impact of alterations in GM in AP-firing muscle fibers on AP initiation, sarcolemma AP propagation, and t-system excitation are all more pronounced when membrane excitability is depressed and muscles are experiencing elevated extracellular K+. The simulations were designed to mimic additional GM changes resulting from a moderate elevation in extracellular [K+] to 7 mM. (A) The spatial voltage profile in steps of 100 µs of simulated electrotonic potentials similar to endplate potentials when GM in the models was adopted to mimic control conditions (left), conditions during Phase I (middle), and conditions during Phase II (right). (B) Passive sarcolemmal voltage responses to a recorded AP (left) when GM was set to mimic control conditions, Phase I, and Phase II. Colored lines show the initial (to 25 mV) passive sarcolemmal responses 100 and 500 µm in front of the AP. (C) The voltage responses of the sarcolemma (yellow line) 100 µm in front of the AP waveform (black line), and the voltage responses in the 20 t-system shells at this position during control (green), Phase I (red), and Phase II (blue).