The code provided models synaptic dynamics in a computational neuronal model, which includes three types of synapses: AMPA, NMDA, and GABA. Each type of synapse is characterized by specific parameters that dictate its biophysical properties and functional roles in neuronal signaling. ### Key Biological Concepts 1. **Neurotransmitter Receptors:** - **AMPA Receptors:** These are responsible for fast excitatory synaptic transmission in the central nervous system. AMPA receptors are glutamate receptors, and their activation leads to the influx of Na⁺ ions causing depolarization. The model specifies parameters such as reversal potential (E_rev), time constants (τ₁ and τ₂), and conductance (Gbar), which together define the kinetics and strength of the synaptic current. - **NMDA Receptors:** These glutamate receptors allow calcium (Ca²⁺) as well as Na⁺ and K⁺ ions to flow through the channel. NMDA receptor activity is voltage-dependent due to a magnesium (Mg²⁺) block that is relieved upon depolarization. The NMDA receptor is critical for synaptic plasticity. The model includes Mg²⁺ parameters (A, B, and C) that modulate this voltage dependency, and `nmdaCaFrac` determines the fraction of NMDA current carried by calcium, reflective of its role in signaling cascades. - **GABA Receptors:** GABA is the primary inhibitory neurotransmitter in the brain. GABA_A receptors (modeled here) are ionotropic receptors causing hyperpolarization by allowing Cl⁻ ions to flow into the neuron. This is reflected in the negative reversal potential. 2. **Synaptic Parameters:** - **E_rev (Reversal Potential):** The voltage at which there is no net ionic flow across the membrane. It determines whether the synaptic input is excitatory (positive E_rev) or inhibitory (negative E_rev). - **Tau (τ):** Time constants τ₁ and τ₂ define the rise and decay times of the synaptic current, respectively. These parameters affect the duration and integration of synaptic signals. - **Gbar (Maximum Conductance):** This parameter reflects the maximum synaptic conductance and influences the strength of synaptic transmission. 3. **Calcium Dynamics:** The NMDA receptor’s ability to allow Ca²⁺ influx links synaptic activity to intracellular signaling pathways, essential for synaptic plasticity mechanisms such as Long-Term Potentiation (LTP). 4. **Magnesium Block:** The NMDA receptor features a voltage-dependent magnesium block, which is important for activity-dependent gating, only relieving at depolarized potentials typically coinciding with coincident pre- and postsynaptic activity—a vital property for Hebbian plasticity. By specifying these biophysical parameters, the model captures the diverse roles of different synaptic receptors in neuronal communication, excitability, and plasticity. Such models help in understanding how different neurotransmitter systems contribute to the dynamics of neural networks.