# Biological Basis of the Computational Model The provided code is part of a computational model aiming to simulate the neural network of the striatum, a critical component of the basal ganglia in the brain. The striatum is involved in various functions such as motor control, action selection, and reward processing. The model specifically includes different types of neurons and their synaptic connections, capturing both intrinsic and extrinsic network dynamics. ## Neuronal Populations The code defines three main neuron types: - **D1 Neurons (D1pop):** Part of the direct pathway, these neurons are called D1 because of their expression of D1 dopamine receptors. They typically facilitate movement initiation by promoting excitatory output to downstream structures. - **D2 Neurons (D2pop):** In contrast to D1, D2 neurons are part of the indirect pathway, expressing D2 dopamine receptors. They generally inhibit movement by reducing excitatory output, thereby working antagonistically to D1 neurons. - **FSI (Fast-Spiking Interneurons) (FSIpop):** These neurons are inhibitory and play a role in regulating the activity of D1 and D2 neurons, contributing to the fine-tuning of motor control. ## Neuron Distribution and Grid The model places these neuron types in a grid-like structure based on specified spacing parameters. These spatial distributions attempt to mimic the physical layout of neurons within the striatum. ## Channel Variability The `chanvar` dictionary specifies variability in ion channel conductance for D1 and D2 neurons. These channels include various potassium (K) and calcium (Ca) channels, which are crucial for neuronal excitability and signal propagation. For example: - **Potassium Channels (Krp, KaF, KaS, Kir):** Involved in repolarizing the cell membrane following action potentials, thereby affecting firing rates and neuron excitability. - **Calcium Channels (CaL13, CaL12, CaR, CaN, CaT):** Important for intracellular signaling, synaptic plasticity, and neurotransmitter release. - **Sodium Channels (NaF):** Generate and propagate action potentials along the axon. - **Calcium-Activated Potassium Channels (BKCa, SKCa):** Link intracellular calcium levels to membrane potential, affecting neuron firing patterns. ## Synaptic Connections The model includes both intrinsic and extrinsic synaptic connections: ### Intrinsic Connections - **Inhibitory GABAergic Synapses:** These connections are present between D1, D2, and FSI neurons, allowing for inhibition within the network. Such reciprocal inhibition is fundamental to the basal ganglia circuitry, enabling precise motor control and the inhibition of competing actions. ### Extrinsic Connections - **AMPA Receptor-Mediated Synapses:** These represent inputs from cortical and thalamic sources to the striatum (CtxSPN and ThalSPN). This input integration is crucial for the striatum's role in receiving and processing environmental cues that guide motor actions. ## Conduction Velocity and Delay The model specifies a conduction velocity for signal propagation (based on real-world findings, such as GABAergic projection velocities) and a minimum synaptic delay, both vital for temporal dynamics in neural signaling. Overall, this code models the striatum as a neural network, capturing the interactions between its major neuronal populations and how they integrate with broader brain inputs, particularly those involved in motor function and control. The model allows for the exploration of how various parameters, such as ion channel variability and synaptic connectivity, could influence striatal network dynamics and function.