### Biological Basis The code provided models a sodium leak current in the Globus Pallidus internus (GPi) neurons. Specifically, it focuses on a TTX-insensitive sodium (Na\(^+\)) current, emphasizing its role in maintaining the resting membrane potential. This current is distinct from the voltage-gated sodium currents typically involved in propagating action potentials. Instead, it is active between spikes, suggesting that it plays a crucial role in stabilizing the neuron's resting state and responsiveness. #### Key Biological Features Modeled: 1. **TTX-Insensitive Sodium Current:** - The modeled sodium leak is resistant to tetrodotoxin (TTX), a common blocker of voltage-gated sodium channels. This implies that the current does not partake in the rapid depolarizations of action potentials, but rather contributes to the resting potential and subthreshold activities. 2. **Contributes to Resting Membrane Potential:** - The sodium leak current aids in setting the resting membrane potential of the neuron. This current can contribute to a depolarized resting state, impacting neuron excitability and readiness to fire. 3. **Voltage Dependence:** - The model allows this current to have some voltage dependence, which is biologically significant as it might affect neuron responsiveness under varying membrane states, thereby influencing the conductance and, consequently, neuronal behavior under certain conditions. 4. **Ion Specificity:** - The model simplifies the current's nature by focusing solely on sodium ions (Na\(^+\)), though in biological settings, other cations may be involved. The concentration gradients of sodium across the membrane play a critical role in determining the direction and magnitude of the current. 5. **Involvement in Neurological Functions:** - Given the location (GPi), this sodium leak current could play a role in motor control and movement regulation, as the GPi is a part of the basal ganglia circuitry involved in these processes. Abnormalities in these currents may contribute to disorders like Parkinson’s disease. In summary, this sodium leak model captures a non-action-potential-conductive sodium current, significant for maintaining the baseline state of the neuron within the GPi, and informs how subtle ion gradients and membrane parameters may shape neuronal excitability and function.