The code provided models the SK-type calcium-activated potassium (K\(^+\)) current, which is a critical component in neuronal excitability and signal transduction. Here’s the biological basis of the elements present in the code: ### SK-type Potassium Channels **SK Channels Overview:** The small conductance calcium-activated potassium (SK) channels are integral membrane proteins that contribute to the regulation of the electrical excitability of the neuron. They are activated by intracellular calcium (Ca\(^{2+}\)) and aid in repolarizing the membrane potential after an action potential. **Key Functions:** SK channels are crucial in shaping the afterhyperpolarization (AHP) following an action potential, thus playing a role in the frequency and timing of neuronal firing. These channels are sensitive to intracellular calcium levels, which modulate their opening probability. ### Biological Aspects in the Code **Calcium Activation:** The code models the opening of the SK channels as a function of intracellular calcium concentration (cai). The calcium-binding effect is described by the `zInf` term in the code, which uses a Hill-type equation to represent the steep activation curve dependent on intracellular calcium levels. **Current and Conductance:** - **Ion Species:** The SK current is explicitly modeled to read the potassium reversal potential (`ek`) to determine the driving force for the potassium ion flow across the channel. - **Conductance (gSK_E2):** The channel conductance, which depends on the gating variable (`z`), reflects the channel being open. The conductance is scaled by `gSK_E2bar`, a maximum conductance parameter (also referred to as the channel density in neuron models). **Gating Dynamics:** - **Gating Variable (`z`):** The state of the channel is represented by the gating variable `z`, which transitions between 0 (closed) and 1 (fully open). The dynamics of `z` are governed by the differential equation in the `DERIVATIVE` block, which models the approach to steady-state (`zInf`) modulated by the time constant `zTau`. **Calcium Dependency:** The sensitivity of the SK channels to calcium is captured by the Hill coefficient and half-maximal activation concentration in the `rates` procedure. These parameters allow the model to replicate the biology of SK channel activation under varying calcium concentrations. ### Conclusion Overall, the code serves to reproduce the biophysical behavior of SK-type potassium channels activated by intracellular calcium, emphasizing their role in modulating neuronal signal processing. Understanding and modeling these currents are vital for exploring how neurons integrate and encode information, as well as their influence on neuronal rhythms and network dynamics. The parameters and state variables used in the code reflect the physiological responses of SK channels to fluctuations in intracellular calcium concentrations.