Figure 1.

Action potential propagation in a cardiac fiber. (A) Schematic view of three cells in a fiber. In addition to a number of sodium, potassium, and calcium currents in the cell membrane, each cell has non-selective gap junction channels that allow currents to flow down the fiber. The gap junction current, I_gj, allows a depolarized cell to drive electrical activity in a neighboring cell. (B). Equivalent circuit representation of the three cells in (A). Each cell has a capacitance associated with the cell membrane and ionic currents represented by a conductance and a battery for the Nernst potential for the corresponding ion. The inward rectifier potassium current, I_K1, current has a non-linear conductance (see Methods) with a maximal conductance, G_K1, and its reversal potential is the potassium Nernst potential, E_K. The fast sodium current, I_Na, has a time-dependent conductance with the maximal conductance, G_Na, and its reversal potential is the sodium Nernst potential, E_Na. (C) Simulations showing cell membrane potential, V_m, and ion channel currents, I_K1 and I_Na, for a subthreshold stimulus (dashed curves) and suprathreshold stimulus (solid curves). Results shown are for the 5th cell in a fiber of 100 cells with 0.2 ms-duration current stimuli administered at the 5 ms mark to the first 4 cells. A subthreshold stimulus brings V_m to about −60 mV after which V_m declines (dashed curve) back to its resting membrane potential of about −90 mV; the I_K1 waveform due to a subthreshold stimulus is larger than the suprathreshold case (solid curve) because the subthreshold V_ms are more negative and thus less inward rectified. I_Na is partially activated (dashed curve) for a subthreshold stimulus and fully activated (solid curve) for the suprathreshold stimulus; the fully activated I_Na is what triggers the full depolarization of V_m to about +20 Mv.