The provided code models the time-independent (delayed) potassium current, known as Ik1, which is crucial for the electrical activity of cardiac cells. This current is part of the cardiac action potential, specifically contributing to the repolarization phase, where the cell returns to its resting state after depolarization. ### Biological Basis - **Ion Channel**: Ik1 represents an inward rectifier potassium channel, which plays a key role in stabilizing the resting membrane potential and helping to shape the final phase of cardiac action potentials. It helps maintain the negative membrane potential in cardiac myocytes, particularly prominent in the late phase of repolarization and during diastolic depolarization. - **Potassium (K+) Currents**: The code simulates the flow of K+ ions across the cell membrane. The current (`ik`) is a function of the conductance (`g`), the concentration of extracellular potassium (`ko`), and the difference between the membrane potential (`v`) and the potassium reversal potential (`ek`). The disparity of intra- and extracellular K+ concentrations and electrochemical gradients allows for the movement of K+ ions governed by the channel dynamics, contributing to the cardiac myocytes' electrical behavior. - **Equilibrium Potential (ek)**: This refers to the Nernst potential for potassium, determined by the concentrations of K+ inside and outside the cell. It represents the membrane potential at which there is no net flow of K+ ions, crucial for setting resting membrane potential and influencing the driving force for K+ movement during action potentials. - **Temperature (celsius)**: The temperature is set to a physiological level (37 degrees Celsius), which is essential for modeling ionic kinetics accurately as temperature impacts ion channel conductance properties and the RT product's computation (RT = gas constant R × temperature T), influencing the channel's voltage sensitivity. ### Key Aspects - **Delayed Rectification**: The mathematical expression describes how Ik1 can be sensitive to membrane potential (`v`) with a delay characteristic, reflecting its gating mechanism that exhibits rectification (it allows more inward than outward current), critical for terminating the action potential and setting up the resting potential. - **Factors Affecting Conductance**: The parameters `g` for conductance and `Km1` for michaelis-menten kinetics illustrate how channel conductance is modulated by K+ concentration, impacting the response dynamics under varying physiological and pathological conditions. Overall, the code captures the essence of the Ik1 current's contribution to cardiac electrophysiology, emphasizing how potassium ion flow through specific ion channels supports cardiac action potential morphology and rhythm stability.