The provided code attempts to model synaptic transmission and plasticity in pyramidal neurons at excitatory synapses that use both AMPA and NMDA receptors, incorporating local calcium dynamics. Here's a breakdown of the biological basis:

Synaptic Transmission

1. Receptor Types:

   • AMPA Receptor: This is a fast excitatory synaptic ionotropic receptor, which is modeled through variables related to its conductance (e.g., g_ampa, iampa), timing (e.g., Cdur_ampa, AlphaTmax_ampa, Beta_ampa), and reversal potential (Erev_ampa).
   • NMDA Receptor: This is another ionotropic receptor that requires both ligand binding and membrane depolarization to relieve the Mg²⁺ block, allowing calcium ions and other cations to flow through. The NMDA receptor is modeled with similar parameters as the AMPA receptor: g_nmda, inmda, Cdur_nmda, AlphaTmax_nmda, and Beta_nmda.

2. Synaptic Weights:

   • Initial weights (initW) represent the baseline efficacy of the synapses. Separate weights are considered for AMPA (W) and NMDA (W_nmda) receptors.

3. Conductance Change:

   • Both receptors undergo conductance changes (g_nmda, g_ampa) based on their activation state (on_nmda, on_ampa) and synaptic weights.

Synaptic Plasticity

1. Calcium Dynamics:

   • Calcium ions (Ca²⁺) are crucial for synaptic plasticity. The code models a local calcium pool in the synaptic environment (capoolcon), with calcium influx primarily through NMDA receptors (ICa).
   • Modulates synaptic plasticity through a calcium-dependent learning rule (eta function).

2. Plasticity Mechanisms:

   • The hebbian plasticity is achieved through omega, which modulates synaptic weight changes based on calcium concentration relative to two thresholds (threshold1 and threshold2).
   • Long-Term Potentiation (LTP) and Long-Term Depression (LTD) are modeled based on weight changes (dW_ampa) determined by calcium concentration at synapses, with specific learning rules (e.g., lambda1, lambda2) regulating weight adjustments.

3. Weight Constraints:

   • Weight changes are normalized and constrained between a maximum (Wmax) and minimum (Wmin) determined by factors like fmax and fmin.

4. Calcium Buffering and Decay:

   • Calcium concentration dynamics are influenced by the remaining pool of synaptic calcium (Cainf), coupling factors (Afactor, fCa), and decay rates (tauCa).

Additional Aspects

• Equilibrium Potentials:

  • Ion-specific reversal potentials are set for optimal receptor function, especially relevant for NMDA-mediated calcium influx with eca = 120 mV.

• Biophysical Context:

  • This code models electrical currents (inmda, iampa) and synaptic conductance changes in the context of neuronal evoked responses induced by tone stimuli to pyramidal neurons.

In summary, the code models the biophysics of excitatory synaptic transmission in pyramidal cells with a focus on NMDA and AMPA receptors and their roles in synaptic plasticity through calcium-dependent mechanisms.