# Biological Basis of the Code The provided code models synaptic transmission with an emphasis on the dynamics of AMPA and NMDA receptors, as well as presynaptic short-term plasticity. Below, I describe the biological components being modeled: ## Synaptic Transmission In the nervous system, synaptic transmission is the process by which signaling molecules, called neurotransmitters, are released by a neuron and activate receptors on a neighboring neuron. ## AMPA and NMDA Receptors - **AMPA Receptors:** These are ionotropic receptors that mediate fast synaptic transmission in the central nervous system. They are gated by the neurotransmitter glutamate and allow the passage of Na\(^+\) and K\(^+\) ions, leading to a postsynaptic depolarization. The code implements dual-exponential kinetics for AMPA receptor conductance, defined by `tau_r_AMPA` (rise time) and `tau_d_AMPA` (decay time). - **NMDA Receptors:** These are also ionotropic glutamate receptors but have distinct properties. They allow the passage of Na\(^+\), K\(^+\), and Ca\(^{2+}\) ions and require both ligand binding (glutamate) and postsynaptic depolarization to become active, due to a magnesium block that needs to be relieved. This block is modeled by the `mggate` equation in the code. NMDA receptors are characterized by slower kinetics, with parameters `tau_r_NMDA` and `tau_d_NMDA`. ## Presynaptic Short-Term Plasticity Short-term plasticity is a mechanism by which the efficacy of synaptic transmission can be transiently increased or decreased based on the recent activity history. - **Facilitation and Depression:** - **Facilitation** is a temporary increase in synaptic strength that occurs when two or more presynaptic action potentials happen in quick succession. It is modeled by the parameters `Fac` (facilitation time constant) and `Use`, where `u` is a dynamic variable representing the neurotransmitter release probability influenced by prior activity. - **Depression** is a transient decrease in synaptic strength, modeled by the parameter `Dep` (depression time constant), representing the depletion of readily releasable vesicles. ## Conductance Profiles and Normalization The dual-exponential profile used for AMPA and NMDA conductances represents the rise and decay phases of synaptic currents following neurotransmitter binding. The normalization factors (`factor_AMPA` and `factor_NMDA`) ensure that the peak conductance reaches a normalized value at the time to peak of the synaptic response, as determined by the kinetic parameters. ## Magnesium Block and Voltage Dependence The `mggate` parameter accounts for the voltage-dependent block of NMDA receptors by extracellular magnesium, a critical factor that distinguishes NMDA receptor behavior from that of AMPA receptors. The blockage is relieved by depolarization, allowing for calcium influx, which is crucial for synaptic plasticity processes like long-term potentiation (LTP). Overall, the code captures key biophysical and kinetic properties of synaptic transmission mediated by AMPA and NMDA receptors, alongside presynaptic factors that dynamically influence the probability and efficacy of neurotransmitter release. This is fundamental for understanding synaptic integration, plasticity, and signal propagation in neural circuits.