The provided code models a synaptic current with NMDA receptor-like properties, adding specific features to capture the biophysical behavior of these receptors in neural communication. ### Biological Basis **NMDA Receptors:** NMDA (N-methyl-D-aspartate) receptors are a subtype of glutamate receptors expressed in the central nervous system. They are ionotropic receptors that form cation channels, permeable to Na\(^+\), K\(^+\), and notably, Ca\(^{2+}\). These receptors play crucial roles in synaptic plasticity, learning, and memory. 1. **Kinetic Scheme:** - The code uses a double exponential model (`Exp2Syn`) to represent the rise (`tau1`) and decay (`tau2`) dynamics of synaptic conductance. This is important as NMDA receptors have distinct temporal profiles with slower gating kinetics compared to AMPA receptors. 2. **Voltage Gating and Magnesium Block:** - NMDA receptors are known for their voltage-dependent block by Mg\(^{2+}\) ions. At resting membrane potentials, extracellular Mg\(^{2+}\) blocks the NMDA receptor channel pore. Depolarization relieves this block, thereby allowing ion flow. This model captures this feature using a function `vspom`, which describes the dependency of the open probability on membrane potential (`v`) and external magnesium concentration (`extMgConc`). 3. **Conductance State:** - The model uses states `A` and `B` to describe the activation and deactivation of the receptor. This is reflective of the `2-state` kinetic model where `A -> G -> bath`, mimicking the transition of conductance states in NMDA receptors. 4. **Gating Variables:** - The code includes mechanisms to switch the conductance 'on' or 'off' (`isOn`), allowing for control over the receptor's active state. This might represent receptor modulation or pharmacological intervention in a biological context. 5. **Calcium Permeability:** - Though not explicitly parameterized in the code, NMDA receptor's permeability to Ca\(^{2+}\) is a significant biological characteristic. It plays a critical role in cellular signaling pathways and synaptic strength adjustments. ### Conclusion This model aims to simulate the synaptic currents through NMDA receptors by accounting for dual-exponential kinetics, voltage-dependent magnesium block, and dynamic conductance states. These features collectively reflect the crucial role of NMDA receptors in synaptic transmission and neuroplasticity. The implementation highlights the impact of these receptors in mediating slower excitatory postsynaptic potentials and their physiological and pharmacological modulation.