The provided code models the dynamics of synaptic transmission mediated by AMPA and NMDA receptors, incorporating elements of presynaptic short-term plasticity. The model is based on dual-exponential profiles for synaptic conductance and includes mechanisms of short-term synaptic plasticity as observed in certain physiological conditions. ### Biological Basis #### AMPA and NMDA Receptors - **AMPA Receptors (AMPARs):** They are fast excitatory ionotropic receptors that mediate most of the rapid excitatory synaptic transmission in the central nervous system. The code models the AMPAR conductance using dual-exponential kinetics, with specified rise (`tau_r_AMPA`) and decay (`tau_d_AMPA`) time constants. - **NMDA Receptors (NMDARs):** These receptors have slower kinetics and a voltage-dependent magnesium block, allowing calcium influx, which is essential for synaptic plasticity. The NMDA receptor conductance is modeled similarly to AMPA, but with distinct kinetics (`tau_r_NMDA`, `tau_d_NMDA`) and a magnesium-dependent voltage-gating mechanism (`mgVoltageCoeff`). #### Short-term Synaptic Plasticity - **Facilitation and Depression:** The model incorporates short-term synaptic plasticity mechanisms: **facilitation** (increased synaptic strength due to repeated stimulation) and **depression** (reduced synaptic strength). The parameters `Fac` and `Dep` respectively represent the relaxation time constants for facilitation and depression. `Use` reflects the utilization of synaptic efficacy. - **Probability of Release (Pr):** The code calculates `Pr`, the probability of neurotransmitter release, depending on the instantaneous available vesicle probability (`Pv`) and synaptic efficacy (`u`). These are dynamically updated for each synaptic event, reflecting the availability of vesicles and levels of facilitation/depression. #### Conductance and Current Calculations - **Conductance Profiles:** Both AMPA and NMDA receptors' conductance profiles are modeled using state variables (`A_AMPA`, `B_AMPA`, `A_NMDA`, `B_NMDA`). The conductance is computed as the difference between these state variables, scaled by normalization factors. - **Current Calculation:** Synaptic currents (`i_AMPA`, `i_NMDA`) are calculated as the product of conductance, the driving force (difference between membrane potential `v` and reversal potential `e`), and respective gating factors. #### Magnesium Block in NMDA Receptors - The NMDA receptor's `mggate` factor represents the magnesium block's voltage dependency, described by a sigmoidal function. This simulates the characteristic property of NMDA receptors where the conductance is not only time-dependent but also sensitive to the membrane potential due to magnesium blockage. Overall, the code aims to simulate synaptic transmission dynamics, particularly focusing on the complex interaction between AMPA and NMDA receptors and how these are influenced by mechanisms of short-term plasticity specific to presynaptic physiology.