# Biological Basis of the Code The provided code models synaptic transmission by simulating the dynamics of AMPA and NMDA receptors at excitatory synapses, incorporating aspects of presynaptic short-term plasticity. This modeling is based on the biological processes governing synaptic strength and neurotransmission under various conditions. ## Key Biological Components ### AMPA and NMDA Receptors - **AMPA Receptors**: These receptors are ionotropic glutamate receptors responsible for fast synaptic transmission in the central nervous system. The model uses a dual-exponential conductance profile to simulate the fast kinetics of AMPA receptor-mediated currents. - **NMDA Receptors**: Another type of ionotropic glutamate receptor, NMDA receptors have slower kinetic properties and are both ligand and voltage-gated. The voltage dependency is due to the presence of magnesium ions (Mg²⁺) that block the channel at resting membrane potential, as seen in the `mggate` variable, which accounts for voltage-dependent magnesium block. ### Short-Term Synaptic Plasticity The code models presynaptic mechanisms that regulate neurotransmitter release probability, incorporating two forms of short-term synaptic plasticity: - **Facilitation**: This is modeled through the variable `u` and parameter `Fac`. Facilitation reflects a temporary increase in synaptic strength due to prior activity and is thought to result from an increase in presynaptic calcium levels, influencing vesicle release. - **Depression**: Modeled using the variables `Pv` and `Dep`, depression represents a transient decrease in synaptic strength, often due to the depletion of readily releasable vesicles following repeated stimulation. ### Kinetic Parameters and State Variables - **Conductance Dynamics**: The state variables `A_AMPA`, `B_AMPA`, `A_NMDA`, and `B_NMDA` are used to model the kinetics of receptor conductance, taking into account the rise and decay times of the respective currents (`tau_r_AMPA`, `tau_d_AMPA`, `tau_r_NMDA`, `tau_d_NMDA`). - **Normalization Factor**: Each receptor subtype has a normalization factor (`factor_AMPA`, `factor_NMDA`) to ensure the peak conductance matches physiological conditions. ### Synaptic Transmission Mechanics - **Vesicle Release Probability (`Pr`)**: This variable is crucial for determining whether a synaptic event results in neurotransmitter release. The code calculates `Pr` by integrating contributions from facilitation and depression. - **Randomized Synaptic Activation**: The use of random numbers replicates the stochastic nature of synaptic transmission, where the chance of neurotransmitter release varies even under identical input conditions. ## Conclusion Overall, the code captures the complex interplay of receptor dynamics and presynaptic plasticity mechanisms that characterize excitatory synaptic transmission in neuronal networks. By incorporating AMPA and NMDA receptor function and presynaptic short-term plasticity, the model provides a biologically realistic framework for studying synaptic behavior and its modulation under different physiological conditions.