The provided code snippet is part of a computational neuroscience model that focuses on synaptic transmission, specifically involving NMDA (N-methyl-D-aspartate) and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, both of which are crucial for excitatory neurotransmission in the brain. ### Biological Basis: #### NMDA and AMPA Receptors - **AMPA Receptors**: These receptors are responsible for fast synaptic transmission. They are ligand-gated ion channels that allow the flow of sodium (Na+) and potassium (K+) ions, leading to excitatory postsynaptic potentials. - **NMDA Receptors**: These play a key role in synaptic plasticity, memory formation, and excitatory neurotransmission. Unlike AMPA receptors, NMDA receptors require both ligand binding and membrane depolarization to remove the Mg²⁺ block, allowing Ca²⁺ ions and, to a lesser extent, Na+ and K+ ions to flow through the channel. This influx of Ca²⁺ is critical for downstream signaling pathways involved in synaptic plasticity. #### Key Biological Aspects in the Code: - **Subunit Composition**: The line `str subunit = "NR2B"` indicates that the model is utilizing the NR2B subunit of the NMDA receptor. The composition of NMDA receptor subunits (e.g., NR2A, NR2B) can influence the receptor’s kinetics and pharmacological properties, with NR2B having different decay kinetics compared to NR2A, affecting synaptic plasticity and learning processes. - **Representative Kinetics**: The parameter `NMDAtau2 = (300e-3)/2` sets the decay time constant of the NMDA receptor response. This parameter is critical for determining how long the NMDA receptor remains active after activation. - **Conductance**: `NMDAgmax = 0.94e-9` defines the maximum conductance of NMDA receptors, which impacts the degree of excitatory postsynaptic currents. - **Mg²⁺ Block**: The parameter `Kmg = 3.57` is relevant to the voltage-dependent magnesium block characteristic of NMDA receptors, where the Mg²⁺ ion blocks the channel pore at resting membrane potential and is expelled during depolarization, allowing ion flow. - **Reversal Potential**: `EkNMDA = 0` indicates the intended reversal potential for currents through NMDA receptors, which is typically around 0 mV due to the non-selective permeability to Na⁺, K⁺, and Ca²⁺ ions. - **Goldman-Hodgkin-Katz (GHK) Equations**: The `useAMPANMDAGHKchannels = 0` and `ghk_yesno = 0` parameters suggest that this model does not employ the GHK equations, perhaps indicating a simplified model of ion permeation that does not account for detailed ion concentration gradients. Overall, the code is modeling the biophysical and kinetic properties of the NMDA and AMPA receptor-mediated synaptic transmission, with particular emphasis on the role of the NR2B subunit, which impacts excitatory postsynaptic potentials and contributes to processes like synaptic plasticity and learning.