The code provided models the SK-type calcium-activated potassium current, which is part of the cellular electrophysiology of neurons. SK channels are small-conductance, calcium-activated potassium channels that are activated by intracellular calcium ions (Ca²⁺) and enable the flow of potassium ions (K⁺) out of the cell. Here’s a breakdown of the biological basis directly relevant to the code: ### Biological Basis 1. **SK Channels:** - SK channels are a subset of potassium channels that are activated by intracellular calcium concentration rather than changes in membrane potential. They play a key role in regulating neuronal excitability and the firing patterns of neurons. 2. **Calcium Activation:** - The SK channel activation is modeled using the calcium ion (Ca²⁺) concentration inside the cell (`cai`). Calcium ions bind to the channel and cause it to open, allowing K⁺ ions to exit the neuron. 3. **Ionic Currents:** - The code calculates the SK channel potassium current (`ik`), which is influenced by the conductance of the channels (`gSK_E2`) and the difference between the membrane potential (`v`) and the equilibrium potential for potassium (`ek`). The exit of K⁺ ions typically leads to hyperpolarization of the neuron membrane, affecting neuronal firing rates. 4. **Conductance and Gating Variable:** - The conductance of the channel (`gSK_E2`) is derived from a maximal conductance value (`gSK_E2bar`) multiplied by a gating variable (`z`). The gating variable `z` represents the fraction of open SK channels and is governed by calcium concentration through a steady-state variable `zInf`. 5. **Steady-State Activation:** - The `zInf`, steady-state activation level of the gating variable, is a function of the calcium concentration relative to an offset (`offc`) and a slope (`sloc`), modeling the hyperbolic dependency of channel activation on calcium levels. 6. **Time Dynamics:** - The time evolution of SK channel conductance is captured through a first-order kinetic model specified in the `states` block, where `z` dynamically approaches `zInf` over a characteristic time constant (`zTau`). ### Functional Role - **Neuronal Excitability:** SK channels contribute to setting the afterhyperpolarization phase following an action potential. This impacts the width of action potentials and influences spike frequency adaptation, thus modulating repeated firing of neurons in response to synaptic inputs. - **Signal Integration:** Through their sensitivity to intracellular Ca²⁺, SK channels play a crucial role in linking metabolic changes (via calcium signaling) to electrical activity, enabling the modulation of synaptic inputs and plasticity processes. Overall, the implementation in the provided code models how SK-type channels contribute dynamically to neuronal excitability based on calcium concentration changes inside the neuron, reflecting their critical role in neural computation and signaling within the brain.