The provided code models synaptic transmission incorporating both AMPA and NMDA receptor-mediated currents, along with presynaptic short-term plasticity mechanisms. Let's break down the biological basis for each of these components: ### AMPA and NMDA Receptors - **AMPA Receptors**: These are ionotropic receptors responsible for fast excitatory synaptic transmission in the central nervous system. The code models AMPA receptor conductance using a dual-exponential profile with rise (`tau_r_AMPA`) and decay (`tau_d_AMPA`) time constants. The conductance change is rapid, matching the swift synaptic transmission typical of AMPA receptors. - **NMDA Receptors**: NMDA receptors are also ionotropic but have slower kinetics compared to AMPA receptors, contributing to synaptic plasticity and memory. The code uses two separate time constants for NMDA receptor activation (`tau_r_NMDA` for rise and `tau_d_NMDA` for decay) to model their slower kinetics. An additional complexity in NMDA conductance is the `mggate`, which models the magnesium block that affects NMDA receptor conductance based on membrane potential and extracellular magnesium concentration. ### Presynaptic Short-Term Plasticity - The code incorporates a model for short-term plasticity based on Fuhrmann et al. (2002), which includes both facilitation and depression. - **Facilitation**: This is modeled by adjusting the synaptic efficacy (modeled by `u`) and is influenced by the `Fac` parameter, which represents the time constant for facilitation. If a synapse is used repeatedly in a short period, its efficacy can increase. - **Depression**: Modeled by the parameter `Dep`, representing the time constant for synaptic depression. It reflects the depletion of neurotransmitter release probability over successive stimuli, reducing synaptic efficacy. ### Key Parameters and Their Biological Significance - **`Use`**: Reflects the initial utilization of synaptic efficacy, or how effectively the synapse can release neurotransmitters given an action potential. - **`weight_factor_NMDA`**: Allows tuning of the relative efficacy of NMDA conductance compared to AMPA conductance, acknowledging that different synapses might have varied AMPA/NMDA receptor densities. - **`mg`**: Represents the concentration of extracellular magnesium, which influences the block of NMDA receptors, an important mechanism in activity-dependent synaptic plasticity. ### Conductance Normalization - The code uses normalization factors for both AMPA and NMDA conductances to ensure that the peak conductance matches predetermined values, simulating the natural peak conductance observed in physiological conditions. ### Random Number Generation - A random number generator is included to probabilistically determine whether neurotransmitter release occurs, reflecting the stochastic nature of synaptic transmission. ### Biological Relevance This model captures the essential dynamics of excitatory synaptic transmission, emphasizing the interplay between fast and slow ionotropic receptor kinetics and the modulatory effects of short-term synaptic plasticity. It provides a framework to simulate physiological responses and plastic changes at synapses, which are vital for processes like learning and memory.