The provided code models the dynamics of synaptic transmission involving two major types of glutamatergic receptors: NMDA (N-methyl-D-aspartate) receptors and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors. These receptors are essential for excitatory synaptic transmission in the central nervous system, particularly in the context of synaptic plasticity, learning, and memory processes. ### Biological Basis: #### **AMPA Receptors:** - **Role:** AMPA receptors mediate fast synaptic transmission and are primarily permeable to cations like sodium (Na+) and potassium (K+). They generate rapid and transient synaptic currents in response to glutamate release. - **Model Representation:** The code includes parameters `tau1` and `tau2` which represent the rise and decay times of AMPA receptor-mediated currents, respectively. The reversal potential `e_AMPA` is set to 0 mV, a typical value since AMPA receptors are non-selective cation channels. The conductance is described with `g_AMPA`, and the synaptic current is computed as `i_AMPA`. #### **NMDA Receptors:** - **Role:** NMDA receptors have slower kinetics and are permeable to calcium (Ca2+), sodium (Na+), and potassium (K+). They are crucial for calcium influx, which can trigger intracellular signaling pathways involved in synaptic plasticity. - **Voltage-dependent Blockade:** NMDA receptors are uniquely characterized by a voltage-dependent magnesium (Mg2+) block, which is modeled in the code using `mgblock(v)`. This aspect ensures that NMDA receptor channels open only during postsynaptic depolarization. - **Model Representation:** The NMDA receptor model involves parameters for `tau_rise` and `tau_decay`, indicating the kinetics of the NMDA receptor-mediated currents. The `e_NMDA` is the reversal potential for NMDA receptors, and `i_NMDA` is the current through these receptors. The code also considers external magnesium concentration `mgo`. - **Calcium Dynamics:** The influx of calcium through NMDA receptors is represented by `ica`, with `ca_ratio` denoting the proportion of total current that is calcium-specific, modeling the biological role of NMDA receptors in calcium signaling. #### **Combined Synaptic Effect:** The code describes a combined synaptic model (`ComboSyn`) that incorporates both AMPA and NMDA receptor contributions. The total synaptic current (`i`) is a sum of currents from both receptor types (`i_AMPA` and `i_NMDA`), reflecting the integrated influence of both receptor types on postsynaptic potentials. Overall, the model attempts to capture important physiological aspects of excitatory synapses, including the rapid signaling by AMPA receptors and the modulatory, calcium-dependent signaling mediated by NMDA receptors. These components are crucial for understanding synaptic integration and plasticity in neuronal networks.