Biological Basis of the Computational Model

The provided computational code models a synaptic transmission process that incorporates both AMPA and NMDA receptors, with an added focus on NMDA receptor functionality influenced by magnesium (Mg) blocking. This model simulates synaptic activity and plasticity mechanisms that are central to understanding how neurons in the brain communicate and adapt over time. Here, the synaptic transmission is modeled with kinetics and learning rules commonly found in striatal medium spiny neurons.

Key Biological Concepts

Synaptic Transmission and Receptors

1. AMPA Receptors:

   • Biological Role: These are ionotropic glutamate receptors that mediate fast synaptic transmission in the central nervous system. Activation leads to sodium (Na⁺) influx, causing depolarization.
   • Modeling Representation: The kinetics of AMPA receptors are characterized by two-state kinetic schemes with time constants tau1_ampa and tau2_ampa. These values determine the rise and decay of synaptic conductance.

2. NMDA Receptors:

   • Biological Role: These are also glutamate receptors, but they have a dual requirement for activation: binding of glutamate and postsynaptic depolarization to relieve Mg²⁺ blockade. They allow calcium (Ca²⁺) and sodium ions to enter the cell, playing a key role in synaptic plasticity.
   • Modeling Representation: The NMDA receptor kinetics are similarly characterized with time constants tau1_nmda and tau2_nmda. The model also includes a magnesium block (MgBlock function), which is a prominent feature of NMDA receptors that depends on membrane potential.

Synaptic Plasticity

• Long-Term Potentiation (LTP) and Long-Term Depression (LTD):
  • Biological Role: LTP and LTD are mechanisms of synaptic plasticity, representing persistent increases or decreases in synaptic strength, respectively. They are essential for learning and memory.
  • Modeling Representation: In the code, weights (w_ampa, w_nmda) associated with both receptor types can change, governed by intracellular calcium concentrations (ca_nmdai) and learning rules reflected by functions such as pind_LTP and pind_LTD.

Calcium Dynamics

• Role of Calcium:
  • Biological Role: Ca²⁺ influx through NMDA receptors is a crucial signal for initiating synaptic plasticity, linking synaptic activity to downstream intracellular processes.
  • Modeling Representation: The code includes interactions with calcium ions (ca_nmdai, cali), contributing to weight changes of synaptic conductance indicative of plastic changes.

Neurotransmitter and Modulatory Effects

• Glutamate:

  • Biological Role: The primary excitatory neurotransmitter at synapses with AMPA and NMDA receptors. Its concentration influences synaptic activity.
  • Modeling Representation: Glutamate concentration (glu) influences synaptic dynamics and is used in functions like gluind1 and gluind2 that dictate synaptic modulation.

• Dopamine:

  • Biological Role: A neuromodulator involved in reward processing and plasticity.
  • Modeling Representation: Dopamine (dopamine) affects synaptic modification rules, influencing AMPA receptor weights and potentially modulating plasticity.

Conclusion

This code models the fast synaptic response and plastic nature of synaptic connections through AMPA and NMDA receptors. It captures essential aspects of synaptic strength modulation, including the impact of neurotransmitter concentration and neuromodulation, which are central to learning and memory processes in neural circuits. The inclusion of specific biological mechanisms such as Mg²⁺ block in NMDA receptors and calcium dynamics demonstrates the complexity underlying neural connectivity and adaptability.