The code provided is part of a computational model representing the cardiac time-independent inward rectifier potassium current, commonly referred to as \( I_{K1} \). This current plays a crucial role in the cardiac action potential, particularly in stabilizing the resting membrane potential and shaping the final repolarization phase. Below is a biological explanation of the key elements modeled in the code: ### Biological Basis of \( I_{K1} \) Current **Inward Rectifier Potassium Channels (Kir):** The \( I_{K1} \) current is mediated by inward rectifier potassium channels (Kir). These channels are integral to cardiac myocytes and are responsible for allowing potassium ions (K\(^+\)) to flow into or out of the cell. **Role in Cardiac Electrophysiology:** - **Resting Membrane Potential Stabilization:** \( I_{K1} \) ensures that the resting membrane potential of cardiac cells remains close to the equilibrium potential for potassium (ek), typically around -90 mV. This stabilization is crucial for maintaining the refractory status of the cells between action potentials. - **Terminal Repolarization:** During the repolarization phase of the cardiac action potential, \( I_{K1} \) assists in returning the membrane potential back towards its resting level after the plateau phase. **Voltage Dependence:** - The conductance of the \( I_{K1} \) current is voltage-dependent. In the code, this dependence is captured by the exponential term \( \exp(0.07 \times (v + 80)) \) within the formula. This part models how the channel activity changes with membrane potential: more open at hyperpolarized potentials and less active at depolarized potentials—hence "inward rectifier." **Contribution to Ionic Current:** - The potassium current \( ik \) is computed as a function of the membrane potential \( v \), the equilibrium potential for potassium \( ek \), and the maximal conductance \( gK1 \). This reflects the biophysical property of potassium ions moving across the membrane predominantly when the cell is hyperpolarized. ### Conclusion In summary, the code models the \( I_{K1} \) inward rectifier current, crucial for maintaining cardiac electrical stability and aiding in repolarization. By incorporating voltage-dependence and passive ion flow, it captures essential aspects of cardiac ion channel behavior that underpin normal cardiac rhythm maintenance.