The code provided is a computational model of the delayed rectifier potassium (K⁺) current in axonal compartments, based on a model from a study of rodent CA3 pyramidal neurons. This potassium current is an essential component of neuronal excitability and action potential repolarization. Here's a breakdown of the biological elements embedded in the model: ### Biological Basis 1. **Delayed Rectifier Potassium Current (I_Kdr):** - The primary focus of this code is to simulate the delayed rectifier potassium current, I_Kdr, which is crucial for returning the membrane potential to its resting state after an action potential. - This current is generated by potassium ions (K⁺) moving out of the neuron, making the inside of the cell more negative (repolarization). 2. **Ion Channel Dynamics:** - The model incorporates the gating dynamics typical of voltage-gated potassium channels, represented by the **n** variable. This is a common abstraction in Hodgkin-Huxley type models where channel conductance is described by the gating variable raised to a power (here, \( n^4 \)). - The probability of the channel being open (\( n \)) is determined by the kinetics of opening and closing, through the variables \( n_{\text{inf}} \) and \( \tau_n \). 3. **Membrane Potential (Vm) and Ion Involvement:** - The model takes into account the membrane potential difference (Vm) that influences the opening and closing kinetics of the potassium channels. This potential is critical for linking the electrical state of the neuron to the biophysical properties of the channel. - The equilibrium potential for potassium (ek) is read and integrated into the computation of the ionic current, emphasizing the role of potassium ion gradients. 4. **Rate Functions and Channel Kinetics:** - The `rates` procedure in the code calculates the rate constants (\( \alpha_n \) and \( \beta_n \)), which govern the transition between open and closed states of the channel. These are voltage-dependent expressions that reflect how changes in membrane potential modify channel gating. 5. **Biophysical Parameters:** - The **gbar** parameter represents the maximal conductance of the delayed rectifier potassium channel, determining the strength of the current per unit membrane area. - The model incorporates a TABLE statement to store precomputed values across a range of voltages, increasing the efficiency of runtime computations. ### Summary This code models the delayed rectifier K⁺ current, a vital contributor to the repolarization phase of action potentials in neuronal axons. By simulating the voltage-gated dynamics of potassium channels, the model captures important aspects of neuronal excitability, allowing for a detailed exploration of axonal electrical behavior.