The provided code models synaptic transmission mediated by NMDA-type glutamate receptors using a simplified two-state kinetic synapse model from computational neuroscience literature. Below is a breakdown of the biological basis behind this model: ### Biological Basis #### Neurons and Synaptic Connections - **PRE and POST compartments**: The code creates two neuronal compartments representing the presynaptic (PRE) and postsynaptic (POST) sites, often used to simulate the synaptic junction between neurons. - **Passive properties**: Both compartments are equipped with passive electrical properties, including passive membrane conductance, denoted as `g_pas`, and a resting membrane potential, `e_pas`. These are crucial for establishing baseline neuronal excitability. #### Presynaptic Mechanisms - **Presynaptic Action Potentials**: The presynaptic compartment (PRE) includes Hodgkin-Huxley type sodium and potassium channels (`gnabar_hh2` and `gkbar_hh2`) to generate action potentials, key for neurotransmitter release. The `IClamp` introduces a current pulse to trigger these spikes. #### Postsynaptic NMDA Receptors - **NMDA Receptors**: The post-synaptic compartment (POST) incorporates NMDA receptors, crucial for synaptic plasticity and learning. They have a high affinity for glutamate and require both ligand binding (glutamate presence) and membrane depolarization to relieve magnesium block and conduct ions like calcium and sodium. - **Kinetic Model Parameters**: The receptors are modeled using a kinetic scheme governed by parameters: - **`Cmax_NMDA` and `Cdur_NMDA`** define the peak concentration and duration of glutamate presence. - **`Alpha_NMDA` and `Beta_NMDA`** are rate constants for the transition between receptor bound and unbound states. - **`Erev_NMDA`** represents the reversal potential (driving force for ion flow). - **`mg_NMDA`** simulates the extracellular magnesium ion concentration, which usually blocks the receptor in a voltage-dependent manner. For demonstration, `mg_NMDA` is set to zero, eliminating this block to show the receptor behavior without magnesium interference. - **Synaptic Conductance**: `c.gmax` represents the maximum conductance of the NMDA receptor, affecting the strength and duration of the post-synaptic potential. #### Graphical Visualization - The code includes procedures to graphically display key variables, such as the voltage of the pre- and post-synaptic compartments, synaptic conductance changes (`c.R`), neurotransmitter concentrations (`c.C`), and synaptic current (`c.i`), to analyze the synaptic response dynamics. ### Conclusion Overall, this model simulates synaptic transmission through NMDA receptors, incorporating essential pre- and post-synaptic components to study the kinetics of neurotransmitter-receptor interactions and their effect on neuronal signaling. This provides insights into fundamental mechanisms underlying synaptic plasticity and signal propagation in neural circuits.