The provided code snippet appears to be a part of a computational model related to neuronal ion channels, specifically focusing on the dynamics of ion channel gating and its effects on the neuron's excitability. ### Biological Context #### Ion Channels and Gating Ion channels are integral membrane proteins that allow ions to pass through them and play a critical role in the electrical excitability of neurons. These channels have gate mechanisms that open or close in response to various stimuli, such as voltage changes across the membrane. The opening and closing of these channels are crucial for the generation and propagation of action potentials—the electrical impulses used for neuronal communication. #### Left-Shift in Activation Curves The concept of "left-shift" in this context likely refers to the leftward shift of the voltage-dependence of activation curves of ion channels. A left-shift typically implies that a channel becomes activated at a more hyperpolarized (negative) membrane potential than usual. This can increase the excitability of neurons since channels are more readily activated. #### The Parameters: `vLeftShift` and `AC` - **`vLeftShift`**: This parameter likely represents the degree of leftward shift in the voltage-dependence of channel activation. A non-zero value for `vLeftShift` implies that the activation curve of some channel population is shifted to a lower voltage, suggesting an increased tendency for these channels to open and contribute to the neuron's excitability at more negative potentials. - **`AC`**: Standing for "activation coefficient" or a similar concept, `AC` appears to represent the fraction of ion channels that experience this left shift. The fraction is a probability value between 0 and 1. A larger `AC` value suggests a greater proportion of channels are affected by the left shift. ### Biological Implications - An `AC` of 0 implies that no channels are left-shifted, indicating typical channel behavior, with no increased excitability due to a hyperpolarizing shift. - A non-positive `AC` (less than 0) is biologically nonsensical because it would imply a negative or nonexistent fraction of left-shifted channels, contradicting the nature of probabilities and fractions in a biological context. - The presence of a left-shift without a corresponding fraction of channels (`AC = 0`) holding this property is illogical; thus, the code ensures `vLeftShift` is relevant only when `AC` is positive. In summary, this code ensures the physiological plausibility of model parameters being used to simulate ion channel behavior under the influence of altered voltage-dependence, potentially mirroring biological conditions that modify neuronal excitability, such as changes in extracellular environment or pharmacological agents.